Sunday, January 23, 2011

Abscisic Acid as a homeostatic hormonal and osmoregulatory response to drought in Sugar Cane.

By :   P.V.K Lareine.

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ABSTRACT.

The aim of this article is to propose mechanisms involving ABA at the root and leaf level, enabling a high yielding, drought resistant, sugar cane variety (a water-spender under normal conditions)  to avoid / tolerate drought.
The leaf system of the sugar-cane plant (and of all plants in general) can be compared as a water pump. Sugar cane being a considered as a water spender; identifying factors controlling stomata and mesophyll conductance (the pump) is the key to understand homeostasis under drought conditions. Rapid response to drought mediated by Abscissic acid from the root system is the principal factor. It would allow for early stomata closure enabling the plant to save water while maintaining growth during drought. Under severe drought conditions ABA would enable the plant to reduce transpiration by leaf senescence.
Osmoregulation and / or concentration of ABA precursors at root tips seem to be an ideal process involved in the biosynthesis of ABA. The distribution of the root system in the soil is thus an important factor influencing stomatal closure during drought.
Leaf rolling and rapid stomata closure at high water potential suggest the existence of an osmoregulatory process at the leaf level.  Thus, osmoregulation favoured by heat dissipation (qE and other exothermic reactions) and incident light would determine the fine-tuning of stomata closure. Factors influencing osmoregulation, thus stomata closure are:       

1. External temperature (heat).
2. Incident light.
3. Resistance of limit layers to heat dissipation, thus leaf morphology.

Further, let us assume that the capacity for qE is larger in certain high yielding sugar-cane varieties with bigger shoot biomass. Then, all things being equal, such varieties with bigger shoot biomass (leaf No., leaf width etc...) would suffer more from drought (spend all water available quickly) as compared to varieties with lesser shoot biomass, (both without ABA). This effect would be further enhanced for varieties sensible to the yellow spot disease. Thus, for homeostasis (continued growth) in such a drought resistant variety with bigger shoot biomass (and higher capacity for qE) it must both have an adequate root system already in place before drought to do so and also be able to synthesize ABA.
A low yielding variety with relatively lesser shoot biomass would be a natural water saver due to its leaf morphology. However, it would be of lesser agronomic importance.
The adaptation of root morphological characteristics (according to shoot morphology) for homeostasis during drought may thus be genetic and / or induced hormonally.

1. Introduction.

This communication is a complement to research projects carried out on different sugar-cane varieties cultivated in Mauritius. The research was carried out in two parts:

1. First part in Mauritius (at the Mauritius Sugar Industry Research Institute, M.S.I.R.I) on the root system of six sugar-cane varieties.
2. The second part  in Brussels (Université Libre de Bruxelles, U.L.B) on the photosynthetic aspects of drought resistance (PSII) of three contrasted sugar-cane varieties.

It is very important to note that:

1. For further experimentation in Brussels three sugar-cane varieties were chosen according to different root morphological characteristics examined in Mauritius.

2. Two types of drought resistance are considered in Mauritius on sugar-cane:
a. On a physiological basis; according to physiological markers such as leaf elongation,
b. On an agronomic basis; according to the final yield in sugar and cane.

3. In order to prevent any deficiency from occurring prior to and during  experiments in Brussels, the sugar cane plants were regularly fertilized with a mixture of compost, soil and manure before water stress was applied. All leaves were fully dark green before application of water stress and photosynthetic measurements.
The agronomically sensible variety examined during the second part of the experimentation may be adapted for growth in dry coastal regions. Smaller leaves and lesser root biomass would allow for an economic use of water.

The aim of this report is to propose possible mechanisms contributing simultaneously to drought resistance in sugar cane.

The mechanisms proposed involving Abscissic Acid are:

1. At the root level: osmoregulation (Tolerance Mechanism) and/ or concentration of  precursors of ABA.

2. At the leaf level, while keeping the basal meristematic part turgid  for growth (Avoidance):

a. High water potential osmoregulation (avoidance mechanism) at the leaf extremity inducing rapid stomata closure.
            b. Low water potential osmoregulation at the leaf extremity as a form of drought tolerance.

According to the classification of Levitt (1980) and Turner (1986):

1. Drought avoidance is defined as the ability to preserve high leaf water potential, thus maintaining cellular turgidity for growth.
2. Drought tolerance is defined here as the ability to support low cellular water potential during water stress.
The sugar-cane plant is considered here as a “water spender” under normal conditions (Maximov 1929 in Levitt II 1980).

Drought resistance is the result of a combination of factors contributing to maintain growth under conditions of drought.  The mechanisms proposed below would enable certain varieties of sugar cane to maintain (homeostasis) high levels of leaf and stem growth (Mauritius Sugar Industry Research Institute (M.S.I.R.I) Annual Report 1991) if and only if:

1. Before the onset of drought it has developed an adequate root system in order to avoid it (Van Dillewijn 1952).

2. At the onset of/ during drought it is able to favour root growth with respect to shoot growth via hormonal adjustments. Plants adopt different strategies of root growth in order to face drought (Mc intosh 1935 in Van Dillewijn 1952).

3. When at very low water potential the leaf and root cells are maintained turgid for growth through osmoregulation. Physiological, biochemical and morphological processes are thus maintained (Turner 1986).

Under severe water stress the sugar-cane plant ultimately responds by leaf senescence of older leaves so as to reduce its transpiration surface. Leaf senescence on the remaining leaves is an ultimate response to drought in order to reduce the inclination of the leaves w.r.t the incident radiation (Soopramanien et al. 1986).

Rapid growth “resurgence”, another form of drought resistance (O’toole and chang 1979, M.S.I.R.I Ann. Report 1991) is to be correlated with morphological / anatomic characters such as: root diameter, leaf surface area, root length, density of stomata etc... 

2. Osmoregulation at the root level.

2.1 The first leaves as a source of raw materials for the biosynthesis of Abscissic Acid.

During drought the oldest leaves of the sugar-cane plant are the first to suffer from drought as they are unable to maintain the water potential gradient to pump water. A decrease in the water potential of these leaves increases respiration and catabolic processes.
Those processes may cause the pigments (Chlorophylls, Xanthophylls, Anthocyanins etc...)  present on the first leaves to become unstable (Photo 1 see annex).
Senescence of the first  leaves of the sugar cane plant (generally Red/Violet) is generally the first sign observed during drought. Yellowing/ senescence of younger leaves occurs later under severe water stress (Viquera et al. 1983 in Soopramanien et al 1986). Sugar-cane is a graminaceous plant of the same family as that of grass.
The pigments  and products of their catabolism would accumulate at the root level thus  may act  as raw material (xanthoxin) enabling the biosynthesis of Abscisic acid from the Xanthophyll  violaxanthin (Young and Britton, 1990). Under severe drought the sugar-cane plant would maintain 5-6 green leaves while the others (older ones) would undergo senescence to reduce the transpiration surface.
Very Important: After the action of ABA on the green, younger leaves  the residues of the degradation of  proteins and pigments  would accumulate at the root level further contributing to osmoregulation. The water flux/ flow to the leaves  would also be reduced.

2.2 Biosynthesis of Abscissic Acid at the root level.

When water loss via leaf transpiration (demand) exceeds the water absorbed by  the roots (supply), the sugar-cane plant suffers from water stress (Van Dillewijn 1952). The leaf system of the sugar-cane plant can be considered as a water pump. A water potential gradient is maintained between the leaves and the roots so as to maintain the roots at a lower water potential than the soil in order to absorb water.
In sugar-cane ABA is synthesized at the root level (Grantz and Meinzer 1990). How is its biosynthesis induced in the root system? Osmoregulation  seems to be an ideal process involved, as it would occur in parallel:
1. with the concentration of ABA precursors at the root level from the first leaves and,
2. with decreasing soil water potential at the root level.
Three types of stem roots are involved in drought resistance in sugar-cane:
a. Superficial roots, with a lateral (horizontal) extension in the top soil layer,
b. Buttress roots which extend at an angle between 45 and 60 degrees to the vertical  serving to support the plant,
c. Rope Roots extending vertically in the deeper layers of  soil.
Superficial roots extend laterally in the top soil layer (15-20 cm) where the water potential drops  first during drought. They occupy a relatively smaller soil volume as compared to what can potentially be exploited in deeper layers. However under moist conditions they supply the plant with large quantities of water and minerals. Under severe (prolonged) drought conditions the contribution of  superficial roots to drought resistance is negligible (Van Dillewijn 1952). It is thus important to introduce the notion of stock of water in the soil as being the soil volume exploited by the entire root system. The location of superficial roots is important as they may rapidly  sense decreasing  soil water potential. Such location would thus allow for further economic use of the remaining stock of water in the soil.
Morphological characteristics of deeper roots are thus the key to homeostasis during prolonged drought in sugar-cane.
The correlation between osmoregulatory processes (/concentration of ABA precursors at the root level) at the superficial root level and their influence on ABA biosynthesis thus  needs to be further investigated.
Root hydraulic conductance and  stomatal conductance are correlated in sugar-cane (Saliendra and Meinzer 1992). The simultaneous reduction in stomatal (probably mesophyll also) and root hydraulic conductance would be mediated by changes in the composition and flux of materials in the xylem sap (Meinzer, Grantz, and Smit, 1991 in Saliendra and Meinzer 1992).Some by-products (starch, sugars, phenols and benzene) of the degradation of  chlorophylls and anthocyanins (aliphatic and aromatic compounds) from the first leaves are water soluble compounds (K.V Thiemann 1979).
Under drought conditions the necessary xanthophylls and other precursors (xanthoxin) would be present in the roots (section 2.1), triggering of the production of ABA may occur when their concentrations reach a certain level. There may exist critical concentrations of ABA precursors (xanthoxin) triggering  its biosynthesis at the root level.
According to the water potential difference between the leaves (pump) and the root system those precursor concentrations may modulate the intensity of the hormonal action of ABA at the leaf level. During drought an uneven root distribution in the soil   would result in an increase in the concentration of ABA precursors.  The production of ABA would increase consequently. Plant respiration would thus be favoured as was the case during experimentation in pot in Brussels.
Osmoregulation usually occurs seasonally during drought at both the root and leaf levels (Turner 1986). According to the leaf transpiration rates two types of  water flux exist at the root level (Fiscus 1975, in Saliendra and Meinzer 1992) mass flow and osmotic flow;
1. at high transpiration rates: Mass flow,
2. at low transpiration rates : Osmotic flow. 
At low transpiration rates osmotic flow occurs when active solute uptake takes place (Fiscus 1975, in Saliendra and Meinzer 1992).
During experimentation in Brussels  rapid stomatal closure (respiration) was quantified between -7 and -9 bars (both under leaf rolling). Those results are in accordance with previous experimentation on sugar cane where root hydraulic conductance decreased  with stomatal conductance (Saliendra and Meinzer 1992).
Plant respiration and leaf rolling at such high water potential may thus suggest the existence of an osmoregulatory process occurring both at the root and leaf levels. It also gives an indication of the proximity of  the root tips (of superficial roots mostly) to the soil-pot interface during manipulations. An uneven root tip distribution in the pots would cause the root to exploit /pump rapidly all the water at the place where they are concentrated.
After experimentation it was observed that root tips were in contact with the pot walls in certain pots. This may explain why certain standard samples may have been affected by ABA during Pea (see section 4.2.1) measurements. During previous experiments (on plants other than sugar-cane) root development was impeded in different pot conditions. It was suggested that an inhibitory signal was sent from the roots to the shoot to lower stomatal conductance and reduce leaf expansion rate (Richards and Rowe 1977, Peterson et al. 1984, Carmi 1986 and Ruff et al. 1987 in Passioura 1991). In pot conditions the roots may perceive the pot walls as being dry soil. From previous experimentation inhibitory signals were sent to the shoots from the roots as well as by soil hardening as by drying soil (Passioura 1991). It was likely that the signal may be ABA (Zhang and Davies 1990 in Passioura 1991).
After drought osmoregulation would allow better absorption of water by the roots thus contributing to drought resurgence/ recovery.

3. The leaf system as a water pump. Abscissic acid, a switch to stem the water flux.

The leaf system of the sugar-cane plant as a water pump (while saving water) can be considered in two ways:
a) at the plant level: upward movement of water through the stem is maintained when the leaves maintain a more negative water potential w.r.t that of the root system (water potential gradient). A critical water potential (different from the threshold potential described hereunder) may be reached beyond which the basal part is no longer turgid for the elongation process. 
b) at the leaf level: movement of water through the leaf is maintained when the extremity of the leaf is kept at a more negative water potential w.r.t that of the basal part. Thus, while the leaf as a whole is kept at a lower potential than the root system the leaf  basal meristematic part  remains turgid for elongation.
To better understand the following discussion (“dynamics of qE”), the evolution of leaf water potential must be considered as such:
1. at one point, leaf  water potential decreases gradually,
2. according to the decrease in soil water potential  a  water potential gradient is induced along the  leaf. The leaf extremity is kept at a lower (more -ve) water potential w.r.t the basal part.             
“Dynamics” of qE is referred to here as it is perceived by the pea Fluorimeter according to the diffusion of ABA along the leaf (section 3.1). From section 2 the distribution of ABA along the leaf itself would depend upon the water potential difference between  the leaf and the roots.
Leaf water potential and thus leaf osmoregulation is generally referred to here at the leaf  extremity (1/2 to 2/3 part from the extremity).
From the mechanism proposed below, it is to be noted that the action of ABA would be a localized hormonal one:
1. Target: proteins binding the reaction centres to the chlorophylls and proteins in the reaction centres (Protein D1 and 101 KD?).
2. Starting at the extremity towards the basal part of the leaf.
3. Attacking the oldest leaves first.
From the series of results obtained in Brussels PS II heterogeneity was quantified (Pea Fluorimeter) in association with rapid stomatal closure (IRGA) on the resurgent and resistant varieties mainly. The heterogeneity was also quantified together with a drastic increase in antenna size (amount of energy absorbed by the antenna complex and reaction centres). This would have led to photoinhibitory permanent damage to the proteins (D1 and 101Kd) and eventually to the destruction of reaction centres.  Those results indicate that ABA acts at the level of the proteins binding the chlorophylls to the reaction centres. The excess of light energy  absorbed  implies that  chlorophylls and associated proteins have been degraded.

Very Important: Permanent damage to the proteins through ABA (leaf senescence/ abscission) is not to be confused  with activation and deactivation of the reaction centres (see photos 2 and 5 in annex). In the sugar-cane crop, leaf senescence (yellowing) generally occurs under water stress (Viquera et al. 1983 in Soopramanien et al. 1986). All pea Fluorimeter results were bulked for the concept including those of the resurgent variety where photobleaching occurred (section 4.3).  Leaf senescence, photobleaching and leaf burn occurred according to the  localised  hormonal action (at the leaf extremity). 
The fate of Abscissic acid at the leaf level remains to be further studied. Reactions converting it back into xanthophylls after protein degradation may thus result in an increase in the xanthophyll concentration at the leaf level.

3.1 Osmoregulation involving Abscissic acid at leaf level.

The sugar-cane plant exerts little control on its transpiration rate under normal conditions. Early  (rapid) stomatal closure  occurs at high leaf  water potentials (low negative values). It enables the plant to save water. This response to drought is of capital importance for high yielding sugar-cane varieties with higher shoot (stem and leaf) biomass. Leaf rolling (and stomatal closure) at high pressures may be favoured by osmoregulation. This response, similar to leaf senescence is favoured by the bulliform cells of the epidermis (Gascho et al. 1982 in Soopramanien. et al 1986) in order limit water loss and to reduce the amount of light reaching the leaves. Leaf turgor has been reported to play an important role in leaf rolling in sugar-cane (Soopramanien et al. 1986). Leaf rolling may also be associated to a change in the balance of endogenous hormones (Soopramanien et al. 1986).
Two types of osmoregulation can be considered according to the diffusion of ABA along the leaf:
1. at high water potentials.
2. at low water potentials.
Osmoregulation at high water potential would thus be another form of drought avoidance.
At  low water potentials osmoregulation at both the leaf and root levels would maintain cell turgidity (cellular water retention)  which is essential for physiological, biochemical and morphological processes (Turner 1986) such as:
1. Leaf expansion.
2. Stomatal opening
3. Photosynthesis.

Decreasing soil water potential may induce critical ABA precursors concentration in the root tip favouring the biosynthesis of more ABA. A corresponding threshold leaf water potential may thus be reached when the ability of the leaves  to pump water is much reduced. At this threshold leaf water potential drought tolerance occurs.  
The threshold water potential defined here would be a threshold between drought Avoidance and drought Tolerance.

It would vary according to:
1. the intensity of  water stress (concentration of ABA precursors / osmoregulation at the root level).
2. the age of the leaf.

The anatomy of the sugar cane leaf  (the veins of  C4 plants are encased by bundle sheath cells containing chlorophyll) would favour the existence of such a threshold water potential.

Further diffusion of  ABA (at lower potentials) perpendicularly to the leaf  would induce visible leaf senescence.    
During pot experimentation in Brussels water potential, fluorescence (screening parameters) and IRGA measurements  were opposite to field results (homeostasis; M.S.I.R.I Ann. Report 1991). IRGA (and other) measurements were  kept to the green part of the leaves, under conditions of   leaf rolling. Data of stomatal and mesophyll conductance indicated respiration. The resistant variety behaved as a sensible one and vice-versa. This was especially represented by water potential measurements. Rapid stomatal closure (plant respiration) and leaf rolling occurred simultaneously at a water potential  between -7 and  -9 bars for the resistant and resurgent varieties. Stomatal closure without plant respiration may have occurred at higher water potential but was not quantified due to the rapid evolution of  water stress in the pots. Rapidly decreasing  leaf  water potential is to be correlated to the proximity of the root tips (of superficial roots mostly) to the pot walls. For the sensible variety stomatal closure (plant respiration) was quantified at a water potential of  -12  bars, leaf rolling may have occurred earlier. Those results do not imply that ABA did not exist in the sensible variety. Considering the possibility that ABA and leaf rolling may exist on the three varieties examined Pea fluorescence measurements were bulked for all three varieties. At high water potentials pea fluorescence measurements were normal (PSII stability). At low water potential PS II  heterogeneity was quantified for all three varieties. -10 bars was chosen as an indication for the threshold leaf water potential defined above. In the field this value may be higher or lower according to the intensity of the drought prevailing and the age of the leaf. When considered globally IRGA measurements gave an indication of the intensity of the water stress on the three varieties. Thus confirming to a certain extent (from section 2.2 and section 4.2.1) results obtained in Mauritius (on superficial roots mostly).

3.1.1 Possible  mechanisms for rapid stomatal closure (at high water potential).
At the  leaf cellular  level ABA would cause a degradation of proteins D1 and 101 KD (?) associated to the chlorophylls  of the thylacoid membrane (other proteins may also be degraded by ABA). This would lead to a situation of heterogeneity in the photosystems. When chlorophyll molecules are no longer in association with the proteins in the thylacoid membrane it becomes unstable and is converted mainly into starch. Chlorophyll belongs to the family of fats and sugars called aliphatic and is derived from the original starch molecule (K.V Thiemann 1979). What is thus obtained from the degradation of proteins binding the chlorophylls to the reaction centres and in the reaction centre are a) Amino acids, b) Starch, c) Heat  and d) Mg++ ions.

Leaf chlorophyll content would thus be important to drought resistance as:
               
1. it influences osmoregulation.
2. it filters light.

Starch may also be present in granules of the cytoplasm of the leaf cells.

Hereunder are two possible sets of reactions that may occur simultaneously during/ after  the action of ABA proposed above.

Mechanism 1.

Starch present in an acidic medium (proton gradient across the membrane), in the presence of heat may produce sugars with the help of an amylase.
An  accumulation of  sugar in the leaf cells (mesophyll cells etc...) would cause water to be taken out from the  epidermal cells. The guard cells would close by in turn losing water to the epidermal cells which themselves would lose water to adjacent cells   (mesophyll etc...)
To confirm the existence of this mechanism the following must be verified:
a) The acid-base nature of the amino acids resulting from the degradation of proteins.
b) The presence of amylase to convert starch (or other carbohydrates) into sugars.
c) The action of ABA on the proteins binding the chlorophylls to the reaction centres and further degradation of   
    Proteins D1 and 101 Kd.
d) The pH of abscissic acid.

 Mechanism 2.

The magnesium ions may react with the amino acids to form salts neutralizing them. An  accumulation of those salts  in the leaf cells would cause water to be taken out from the  epidermal cells. The guard cells would close by in turn losing water to the epidermal cells.

A fungus (Mycovellosiella Koepkei) causing  the yellow spot disease induces an adverse effect on growth in certain sugar-cane varieties sensible to the disease (M.S.I.R.I Ann. Report 1991). It could be that this fungus induces  stomatal closure through the degradation of proteins via the mechanisms proposed above.

Also, sugar-cane maturation  (accumulation of sugar) is favoured by two factors:
1. Low levels of  nitrogen  in the plant.
2. Drought.
When  both mechanisms above are considered, inflation (increased turgidity) of cells (mesophyll) nearer to the xylem would cause a reduction in mesophyll and stomatal conductance.
When considering the two possible mechanisms above it is important to keep in mind that the principal factor inducing rapid stomatal closure would be the concentration / osmoregulation of ABA precursors at the root level (section 2.2). Its combination with incident light/ ambient heat at the leaf surface and morphological characteristics would modulate the intensity of  stomatal closure (plant respiration).
An increase in leaf temperature and incident radiation would therefore favour stomatal closure. This maybe correlated to previously observed closure of stomata (Cowan and Farquhar 1977 in Turner 1986).
Stomatal reopening may be influenced by the balance of phytohormones (cytokinins) in the plant (Turner 1986).

3.2 Osmoregulation, Abscissic Acid, and qE.

During drought the plant will progressively maintain leaf  water potential lower than that of the roots. In order to save water, it will close its guard cells  when  the evapotranspiration demand is too severe and open them when those conditions are more favourable. Photosynthesis may thus occur principally during  the afternoon or in the morning. In sugar-cane diurnal stomatal closure is an adaptation to drought conditions similar to other features such as leaf rolling, shedding of leaves etc...  (M.S.I.R.I  Drip Irrigation Project 1993).
Chlorophyll acts as a  filter limiting the absorption of light by the chloroplast. Under water stress ABA would attack the proteins associated  to the chlorophylls in the thylacoid membrane. The chlorophyll becomes unstable and degraded, an excess amount of light is trapped by the leaf. The plant would thus tend to eliminate the excess light energy received via photochemical and non-photochemical reactions (Demmig-adams et al 1989, Krause et Behrend 1986 in Horton et al 1994). A proton gradient is created through the thylacoid membrane from which qE (non-photochemical component of elimination of x/s light energy received)  thus heat may be evolved. This mechanism of heat dissipation would protect the photosynthetic apparatus from damage by excess light

During experimentation in Brussels qE was quantified on  (the still green part) of senescing leaves of  the resistant variety only. It was extracted under conditions of leaf rolling between -13 and  -15 bars where stomatal closure (respiration) occurred (Photo 2 see annex). Owing to the morphology of the sugar cane leaves (thick with waxy cuticle) qE was not quantified at higher water potentials but  may exist at the central part of the leaf where osmoregulation takes place. Leaf senescence as a reaction to water stress is common in sugar-cane (Soopramanien et al. 1986). Upon opening (de-rolling) of  the leaves gradual senescence (yellowing) starting at the extremity towards the mid-part  part of the leaf was observed according to the water potential gradient (Photo 2 see annex). Approximately -20 bars at the yellow extremity and approximately -15 bars at the  still green mid part of the leaf. All Pea fluorescence measurements were kept on the bottom green part of the leaves to prevent from any risk of quantifying any  deficiency whatsoever .
A limit for fluorescence measurements on sugar-cane leaves is between -14 and -15 bars.
Severe leaf burn at the thinner leaf extremity and associated localised photobleaching observed on the resurgent variety indicates the presence of qE on the resurgent variety. However it was not quantified as in the resistant variety. Pea and IRGA  measurements were limited to the green thick part of the leaves. The resurgent variety naturally occurring with thicker and larger leaves. Leaf burn as a reaction to water stress is common in sugar-cane (Soopramanien et al 1986).
The leaf morphological characteristics of the sugar-cane plants makes difficult the study of qE unlike Guzmania Monostachia (Horton et al. 1994).The adaptation of the sugar-cane anatomic characteristics for leaf rolling would make further complicated the study of qE. Sugar-cane is a plant  whose leaf anatomy is adapted for C4 type photosynthesis. The veins of  C4 plants are encased by bundle sheath cells containing chlorophyll.
A possible method for further studying qE on sugar-cane would be to grow samples in conditions of reduced light prior to experimentation.
The range -13 and -15 bars was the water potential  at the regions of fluorescence measurements.
qE was quantified nearer to the mid part of the leaf. This would suggest that qE would “move” downwards according to the water potential gradient and leaf senescence.
At the leaf level the limiting factor for saving water while maintaining  photosynthetic activity  is temperature (heat).
Factors influencing leaf temperature when the stomata are closed are:
1. Reduced transpiration.
2. Protein degradation.
3. qE.
4. Excess illumination.

When transpiration is nil (plant respiration) leaf burn may occur.

From mechanism 1 in section 3.1.1 above  could it be that the proton gradient across the thylacoid membrane (associated to qE) creates the necessary acid medium for the conversion of starch  to sugars. Stomatal closure would thus be induced with the help of an enzyme.
Excess light produces heat.
Excess heat (x/s light + qE + protein degradation + other exothermic reactions)  induces excess transpiration.
Excess transpiration would deplete the  stock of water in the soil.

If it is so, then excess light would be “felt” by  the proton gradient. Excess illumination, qE, protein degradation and other exothermic reactions would induce stomatal closure from the mechanisms above (section 3.1.1). During previous observations made on  the sugar-cane crop  (M.S.I.R.I Drip irrigation project 1993) stomatal opening  was strongly influenced by rapidly fluctuating solar radiation.
From the above it is clear that the action of ABA during water stress induces more heat stress. This situation creates a photosynthetic disorder where an excess of light energy is trapped. Through protein degradation the reaction centers are progressively destroyed while heating up. The stomata are closed, the leaf is less able to cool down. The presence of water soluble compounds (osmoregulation) would delay the destruction of reaction centers increasing the resistance of the photosystems to thermal stress (Santarius et al. 1979 in Havaux et al. 1992).
Protection of  remaining active PSII reaction centres from permanent damage would be very important for drought tolerance. At low water potentials at the leaf extremity, cells are able to maintain essential morphological, physiological and biochemical processes to maintain the water flux  while saving water. Thus while the cells at the leaf  extremity are kept turgid at  low leaf water potential (Tolerance / osmoregulation),  the basal part is kept turgid (avoidance) for growth.

From Pea fluorescence measurements, a possible mechanism for maintaining  photosynthetic activity at the leaf extremity (homeostasis) under the action of ABA would be:
1. reduction in the density of reaction centres resulting from protein degradation.
2. drastic increase in the light energy trapped per reaction center to compensate for the decrease in the density of reaction centers.
The photosynthetic efficiency per reaction center increases drastically.
At a water potential between -13 and -15 Bars the quantum yield associated with PSII  photochemistry F(Po) was reduced to  between 70 and 80 %. An approximate reduction to 8% of the density of reaction centres (Do) was  associated to this reduced  F(Po). However, data from IRGA Measurements  indicated plant respiration. This apparent discrepancy  may be accounted for by the substitution of CO2 by O2 as  final electron acceptor (Havaux 1992). Cyclic electron flow round PS I would be another way to relieve electron pressure (Horton 1987 in Horton et al. 1994). qE was probably quantified in association with those measurements and with PSII heterogeneity. It is to be noted that that  Do gives an indication of the chlorophyll content of the chloroplasts. Reduction in Do would occur in parallel with an increase in antenna size. The light energy trapped by the reaction centres would thus increase drastically.
Localised photobleaching and associated leaf burn  occurred in the resurgent variety due to lack of chlorophyll (see section 4.1.2). High values of Tc and  Tp (below) and associated rapid stomatal closure would thus suggest that photobleaching was due to ABA.
Under prolonged drought and with excessive illumination and high temperatures the leaves will progressively senesce and “burn” from the extremity to the basal part. qE would “move” downwards according to the diffusion of ABA along the leaf. Leaf senescence/ abscission  is an adaptation of the sugar-cane plant in order to save water (G.C Soopramanien et al. 1986). From section 2, an adequate root system would enable the remaining active leaves to avoid and tolerate drought while limiting respiration and senescence. qE  “remains at the top  of  the leaves”  (section 3)   homeostasis thus occurs.
Under water stress sugar-cane plants maintain the majority of leaf  elongation when external conditions are more favourable (the evapotranspiration demand is less) i.e at night, afternoon and morning. Stomatas reopen, respiration no longer occurs. It is to be noted that leaf burn occurs under exceptional conditions of high temperatures, light intensity and plant respiration (without leaf rolling) and especially at the thinner leaf extremities. 
Upon confirmation of the osmoregulatory action of ABA; its influence on  stomatal and  mesophyll conductance need to be investigated:
1. at different temperatures,
2. at different light intensities,

Average critical (Tc) and complete denaturation temperatures (Tp) measured on 3 varieties of  sugar cane leaves under normal conditions are:

                               Tc = 49.5°C               Tp = 59.7°C.

Those measurements were made on  leaf segments at a water potential around -5 bars.

Table 1: Average maximum (Tmax.), mean (Tmoy.)  and minimum (Tmin.) greenhouse temperatures before and during experimentation.


Period
Tmax. / °C
Tmoy. / °C
Tmin. /  °C
21 May to 01 June 01
31.9
26.02
20.95
1 June to 19 June
28.2
23.6
19.7
20 June to 1st July
40.0
30.6
23.7


In the table the duration of the experimentation was separated into two distinct  periods (from 1st June to 1st July) of stable  temperatures. Experiments started during the month of June. Temperatures during the month of may were quite stable. It is to be noted that high temperatures prevailed during the last 10 days of the month of June when practically all pea fluorescence measurements were completed.

After  P.A.M measurements in vivo (Tc and Tp) the leaf segments examined turned yellow (permanent damage to the chlorophylls and associated proteins) (photo 3 see annex). These yellow segments were observed  to “move” upwards  according to growth (leaf elongation) occurring from the basal  meristematic part of the leaf.

Tc and Tp were measured on segments of  the drought Resistant/ Tolerant variety under a combination of water and heat stress. Rapid desiccation was induced to about -20 +/- 3 bars during and after P.A.M fluorimeter measurements. Segments were collected on standards which were not supposed to have been affected by ABA.
 At  such low pressures (-20 +/- 3 bars) rolling of the leaf segments occurred as compared to High water potential leaf rolling discussed above (section 3.1).

Tc readings were measured at two sensibilities of the P.A.M fluorimeter.
Values are as follows:
a. 1st set  of readings  Tc:  52.33 °C  and  Tp: 60.7 °C; Tc increased by  2.17°C.
b. 2nd set of readings  Tc:  52.98 °C Tc increased by  2.83°C.

Increase in Tc is lower than  previous data (5°C) on tomato and potato (Havaux 1992) recorded under the combination of  desiccation and heat stress.

The increase in Tc may originate from (Havaux 1994, Havaux 1992):

1. The presence of water soluble compounds and metal cations in the stroma. Accumulation of thermoprotective compounds in the stroma may have resulted from the action of ABA at the leaf level. Induction of ABA in the leaf may have resulted from an uneven distribution of the superficial  roots  in the pot.  
2. An increase in metal cation or proton concentration in the stroma due to rapid desiccation.
3. Increased thermostability of  the PSII due to stabilization of lipid-protein interaction. Heat stress before  Tc would strengthen the interaction between PS II  proteins and their lipid environment (as opposed to the proposed action of ABA above). The increase in Tc would imply that the PS II had been stable during manipulation. The plants did not suffer from heat hardening before as may have been suggested by the abrupt increase in temperature in the greenhouse (Table 1). No heat hardening seem normal given the high values of Tc and Tp (49.5 ºC  and   59.7ºC) P.A.M measurements were conducted during / after the period of abrupt increase in temperature. 

The combination of rapid desiccation and heat stress increased the resistance to heat. The contribution of osmoregulation to the 2.83 increase in Tc  is unknown. Increased resistance to heat as a result of osmoregulation is important as the action of ABA under water stress would induce more heat stress. This is important for drought tolerance. The adaptation of the sugar-cane anatomic characteristics for leaf rolling would make complicated the study of the contribution of osmoregulation in the rise in Tc. Sugar-cane is a plant  whose leaf anatomy is adapted for C4 type photosynthesis and leaf rolling. The veins of C4 plants are encased by bundle sheath cells containing chlorophyll. Critical  (Tc) and complete denaturation (Tp)  temperatures therefore need to be measured in vivo under the action of ABA according to the “dynamics of qE”. Given the anatomy of the sugar-cane leaf it is very difficult to determine the exact contribution of osmoregulation in the 2.83 increase in Tc. It is highly probable that under drought (ABA/osmoregulation), in the natural habitat the increase in Tc is higher than the 2.83 °C  increase observed in Brussels.

3.3 Drought avoidance, Abscissic Acid and qE.

3.3.1 High water potential osmoregulation as a form of drought avoidance.

Drought avoidance occurs when the plant is able to maintain growth (at the basal meristematic region) with a higher water potential (low negative values). During drought, water flux through the leaf is maintained when the extremity of the leaf is kept at a more negative water potential w.r.t that of the basal meristematic part of the leaf and w.r.t the root system. The basal part of the leaf is kept turgid so as to maintain the leaf elongation process. Thus while globally reducing leaf water potential to maintain the water flux, stomatal closure at the leaf extremity would enable the plant to prevent wastage of water when the evapotranspiration demand is higher. High water potential osmoregulation is another form of drought avoidance.
From the discussions in section 2.2, 3.1,3.1.1 and 3.2.1 above a resistant, high yielding variety with high capacity for qE and sensible to the yellow spot disease can only maintain high levels of leaf elongation if and only if it has an adequate root system. Thus high water potential osmoregulation as a form of drought avoidance may be the main form of drought resistance in the case of  such a drought resistant variety. A certain form of elasticity in stomatal opening and closure can thus be maintained.
An adequate root system may allow the resistant variety to maintain the leaf water potential below or near the threshold potential described above (Sections 2.2 and 3.1). 
The results obtained in Brussels are opposite to what is observed in the field. Further, from section 2.2, 3.1,3.1.1 and 3.2.1 the correlation between late maturation of the resistant variety and the distribution of its root system needs to be further investigated.

4. Anatomic and morphological adaptations in relation with abscissic acid.

Under normal conditions (before drought) the sugar cane plant (“water spender”) exerts little control on its transpiration rate (thus its stock of water in the soil). This is partly due  to its high leaf (shoot) to root biomass ratio. Rapid stomatal  closure implies a rise in leaf  temperature and a reduction in gaseous exchanges. From section 3.1.1 above, an increase in leaf  temperature would favour rapid stomatal closure. Larger leaf surface area would increase the resistance of limit layers to heat dissipation favouring a rise in leaf temperature, stomatal closure and plant respiration. This effect would be even more pronounced in sugar-cane varieties which have a greater capacity for qE and sensible to the yellow spot disease. In Guzmania Monostachia, species adapted for growth in adverse environments exhibit  extents of qE thrice as in the species used for laboratory experiments (Horton et al. 1994). 
If certain sugar-cane varieties have a greater capacity for qE, such varieties with larger leaf surface area and sensible to the yellow spot disease would have a greater extent (section 3.1.1) of stomatal closure (plant respiration) during drought. Therefore, considering the discussion above  (section 2.2)  a stress tolerant /resistant sugar-cane variety must have an adequate root system to prevent plant respiration (thus reduce the intensity of the water stress and favour homeostasis).
This communication is a complement to research projects carried out on different sugar-cane varieties cultivated in Mauritius. The first part of the study in Mauritius was carried out on  the morphological characteristics of the root system of six varieties of Sugar-cane. Results  on the root system were  bulked for superficial and buttress roots. Superficial roots contributed to the majority of the results as compared to buttress roots. No root from the rope system was observed.
Results obtained at the M.S.I.R.I therefore do not give sufficient  indication of the distribution of the root system in the deeper layers of soil due to:
a. insufficient data from buttress roots and no data from the rope system,
b. impedance in the development of the superficial roots.  Such impedance would have induced the superficial roots to support the plant and reduced the expression (development) of buttress roots.
However a comparison has been attempted between varieties. When superficial and buttress roots were bulked marked differences were observed between the varieties. Three possible conclusions can be drawn from results obtained then:
1. Conditions of experimentation may have favoured an increase in  the root to shoot ratio of  the varieties examined. The hormonal factor may be more pronounced in some varieties.
2. Given that conditions were the same for the six varieties examined results may give an indication of  the contribution of the genetic factor to root morphological traits. Assuming that the hormonal factors affecting root growth are the same for all varieties examined.
3. The genetic characters  were already expressed before the growth of the varieties was impeded by the bags. Results thus give an indication of  both the influence of the hormonal and  genetic factors  that have influenced the growth of  the six varieties  examined.
Three sugar cane varieties out of the six  were then selected according to root morphological characteristics for the second part of the study in Brussels (photosynthesis).
When grown in greenhouse conditions in Brussels the selected varieties demonstrated leaf and stem morphological traits as follows (photo 4 see annex):
a. Resurgent: large and thick,
b. Resistant: intermediate, more comparable to the resurgent variety,
c. Sensible: opposite to the resurgent variety.

Photosynthetic results obtained in Brussels (Plant respiration) are opposite to those obtained from field trials in Mauritius (Homeostasis: M.S.I.R.I Ann. Report 1991).
Considering both sets of results and the discussions in  sections 2 and 3 above, the following conclusions may be derived:
 1. The action of ABA alone is not sufficient to explain homeostasis during prolonged drought for high yielding resistant varieties with higher shoot biomass (with higher capacity for qE ? and sensible to the yellow spot disease ).
2. It would appear that homeostasis for those varieties can be attained under prolonged drought if  its root system is well developed during and before drought. The genetic factor influencing deep root development before drought would thus seem  important for drought avoidance.
3.  Hormonal adjustments  influencing the root to shoot biomass ratio is also an important factor for homeostasis. They would allow for the development of deeper roots.          
The distribution of the root system of the sugar-cane varieties during and before drought therefore seem essential for homeostasis in the field.

4.1  Leaf anatomic and morphological adaptations.

4.1.1 High water potential leaf rolling as an osmoregulatory and hormonal adaptation to favour root growth w.r.t shoot growth.

The photosynthetic experiments leading to the discussion/ proposals above were made under conditions of leaf rolling occurring on the three varieties of sugar-cane (resistant, sensible and resurgent) examined. This response, similar to leaf senescence is favoured by the bulliform cells of the epiderm (Gascho et al. 1982 in Soopramanien et al. 1986) in order limit water loss and to reduce the amount of light reaching the leaves. The bulliform cells are especially adapted to lose their water to adjacent mesophyll cells? etc.... Leaf rolling (and stomatal closure) at high pressures may be favoured by Osmoregulation .  This response, similar to leaf senescence is favoured by the bulliform cells of the epidermis (Gascho et al. 1982 in Soopramanien. et al 1986) in order limit water loss and to reduce the amount of light reaching the leaves. Leaf turgor has been reported to play an important role in leaf rolling in sugar-cane (Soopramanien et al. 1986). Leaf rolling may also be associated to a change in the balance  of  endogenous hormones (Soopramanien et al. 1986).
During the greenhouse studies in Brussels a drought cycle prior to photosynthetic experimentation was carried out. This previous drought cycle may have favoured an increase in the root to shoot ratio of the resurgent and resistant varieties as examined Mauritius. From section 2 this would have contributed to the severe water stress suffered by the resurgent and resistant varieties during manipulations. Thus the results obtained on the leaf system (IRGA and PEA) may to some extent confirm results obtain on the root system in Mauritius (where the growth of the root system was impeded).

4.1.2  Adaptation of leaf morphological characteristics according to ecoclimatic regions.

Permanent protein degradation (section 3) implies that after several drought cycles on the same leaf, leaf rolling no longer occurs at low water potential. Rapid stomatal closure mediated by ABA implies a rise in leaf temperature and a reduction in gaseous exchange with the surroundings. Therefore the transpiration rate is thus essentially determined by shoot (leaf and stem)  morphological characteristics independently from leaf rolling.
From the above (section 3.2), adaptation of  S. Cane  leaf morphological characteristics to drought (independently from leaf rolling) is directly correlated to the leaf inclination w.r.t incident radiation. The action of ABA being from the leaf extremity to the basal part, morphological characteristics  need to be selected to optimize heat dissipation while minimizing plant respiration, leaf senescence and  leaf burn.
This would help maintain leaf elongation (homeostasis) as qE (and other exothermic reactions) would stay at the distal part of the leaf. Movement of qE downward would indicate a reaction to water stress to reduce the leaf transpiration surface without maintaining leaf elongation (homeostasis).
Consider  the leaf subdivided into three different segments w.r.t the stem : a. basal proximal meristematic and non-meristematic nearly upright part, b . the intermediate inclined (approx. 45°) part, c. the distal segment perpendicular to incident radiation.
Three types of segments can thus be defined: 1. thick, large 2. intermediate 3 thin rigid .
From resurgent  (adapted to high RH, relatively lower temperatures) to coastal (sea-level, sunny/dry ecoclimatic regions, higher temperatures) varieties, leaf adaptation to ecoclimatic regions would present different combinations of these three theoretical segments. Accordingly,  leaf chlorophyll contents under normal conditions would be an important factor  in varietal selection.
The comparison of density of stomata also need to be further investigated.

4.2 Root morphological and anatomic adaptation to drought.

Abscissic acid induces early stomatal closure enabling the plant to avoid / tolerate drought by an economic use of the remaining stock of water in the soil (section 2.2). However, a sugar-cane plant would be unable to maintain high rates of leaf elongation during prolonged drought if it was to depend solely on its superficial roots (Van Dillewijn 1952). This, even under the influence of  ABA, it would eventually senesce.
From sections 2.2 and 3.2 the elasticity in stomatal opening and closure would be maintained (thus homeostasis) if the leaf water potential is below or exceeding within a certain range the threshold potential. A critical leaf water potential may be thus be reached where senescence occurs; the plant is no longer able to pump water.  
Varieties with higher evapotranspiration demand (larger leaf and stem width), sensible to the yellow spot disease and possibly with higher capacity for qE will therefore need to have a higher supply (root distribution) for homeostasis under prolonged drought. An adequate stock of water in the soil (section 2.2) would prevent critical respiration, senescence and eventually  leaf burn of the remaining active leaves (section 2.1). The water flux is thus maintained through the leaf, the basal part is kept turgid while saving water homeostasis thus occurs.    
The adaptation of root morphological characteristics for exploitation of a larger volume of soil may thus be induced hormonally and genetically.

Very Important: The discussions on the root system in sections 4.2.1 and 4.2.2 are only valid for drought resistance if the development of deeper roots follow the same trend as for bulked results (superficial and buttress) in Mauritius. It is assumed throughout this communication that the morphological characteristics of deeper roots (hormonal or genetic, conditional for homeostsis ) follows the trends described  in results from experiments at the M.S.I.R.I. Bigger biomass of superficial roots may contribute to drought resistance but is insufficient to explain homeostasis during prolonged drought. Further experimentation is required to compare the morphological characteristics of deep roots ( rope and buttress) in the varieties examined.

4.2.1 Genetically induced root morphological characteristics.

From the discussion above, several results indicate that the genetic factor contributes to homeostasis:

1. Largest root diameters observed on the resistant and resurgent varieties as compared with the sensible one. Those measurements correspond to contrasted leaf width of the varieties examined in Brussels.
2. Largest root numbers observed on the resistant and resurgent varieties as compared with the sensible one.
3. Increase in Tc  (P.A.M Fluorimeter measurements), possibly resulting from osmoregulation due to uneven root distribution.
4. Certain standard (PEA) Fluorimeter readings were similar to water stressed readings probably indicating the induction of ABA in the standards due to its root distribution (Section 2.2). Leaf damage (veins and bundle sheath cells) during transportation of pots and sensibility to the yellow spot disease may have contributed to those results.
5. All three varieties suffered a drought cycle before experimentation in Brussels. The resurgent and resistant variety suffered more severe water stress as compared to the sensible one. This was indicated by rapid decrease in water potential measurements, IRGA and PEA Measurements. Assuming that the hormonal factor favouring root growth is the same for the three varieties, those results would imply that the resurgent variety and resistant varieties had a  genetically more developed root system before the two drought cycles were performed in Brussels.

When considering results collected on the root system in Mauritius it is highly probable that in pot conditions in Brussels the absorbing part of the root system of the resurgent and resistant varieties were situated nearer to the interface soil-pot as compared to the sensible variety.
Considering this, it is advisable that further photosynthetic experimentation be performed in field conditions.

4.2.2 Hormonally induced root morphological characteristics.

Under drought conditions the sugar-cane plant will favour the development of deeper roots w.r.t superficial ones.
The development of the root system has been impeded during both sets of experimentation either in Brussels or in Mauritius.
The three varieties chosen from experimentation in Mauritius were subjected to a water stress cycle prior to manipulations in Brussels. During manipulations (second drought cycle) leaf rolling (high water potential) was observed on the three varieties examined. From section 4.1.1 this previous drought cycle may have induced hormonal readjustments favouring an increase in the root to shoot ratio of the resurgent and resistant varieties as examined Mauritius. From section 2 this would have contributed to the severe water stress suffered by the resurgent and resistant varieties during manipulations. Thus the results obtained on the leaf system (IRGA and PEA) may to some extent confirm results obtain on the root system in Mauritius (where the growth of the root system was impeded). The action of ABA and hormones favouring an increase in root to shoot biomass ratio on the sensible variety is not to be ignored.
The morphological plasticity of the root white active (absorbing) part is a well-known characteristic of the sugar-cane plant. The absorbing part is often seen to shape up according to obstacles met (soil compaction, rocks etc...) preventing it from pumping water.   
The correlation between plasticity/ elasticity of the  white absorbing active part of the sugar-cane plant and  possible osmoregulation (section 2.2) needs to be further examined. It would allow for the hormonal development of roots in the deeper layers of soil under drought conditions.
Since only the extremity (active part) of the root system absorbs water its distribution according to the soil water potential gradient / content during (tolerance and avoidance) and before (avoidance) drought conditions  needs to be further investigated.

4.3  Root and leaf morphological characteristics and drought “resurgence”.

During experimentation the following observations were made on certain samples of the drought resurgent variety only (photo 5 see annex) under water stress:

1. Bleaching,
2. No leaf rolling,
3. Permanent damage after bleaching at the leaf extremity occurred (with severe leaf burn on certain samples).          
4. Rapid growth resurgence afterwards from the leaf basal part gave the impression that the leaves greened again (from white). Sugar-cane is a graminaceous plant of the same family as that of grass.

Other samples (majority) of the resurgent variety which did not bleach showed leaf rolling, standards did not bleach.
Experimentation was performed during summer when daylength increased. Those samples showing leaf burn and which bleached (resurgent water stress only) were found nearer to the greenhouse wall  and may thus have received less sunlight during growth. This position (near the wall) and after two cycles of drought may have resulted in a lack of chlorophylls (etioplasts) essential for osmoregulation and thus prevented leaf rolling. Lack of chlorophyll (etioplasts) and the thinning of the leaves would have favoured qE (Horton et al. 1994) thus leaf burn.
Localised photobleaching associated to leaf burn (qE ?), high values of Tc and Tp (section 3.2) ,  and rapid stomatal
closure observed on water stressed samples of the resurgent variety would suggest that photobleaching was associated to ABA.
Leaf chlorophyll content would thus be important to drought resistance as:

1. it influences osmoregulation.
2. it filters light.

It is to be noted that when stomata (through the mechanisms presented above) are closed the leaf is less able to cool down by transpiration. Several factors combine causing leaf “burn” (from the extremity to the basal part):

1. decreased transpiration/ respiration.
2. Degradation of proteins.
3. qE .
4. Excess illumination.
5. No leaf rolling, high leaf surface area thus high  resistance of limit layers to heat dissipation.
6. Thinning of the leaf extremity.

Large leaf surface area of the resurgent variety may thus contribute to its sensibility during drought.
During the drought season the sugar cane crop may face several cycles of water stress. From sections 3.1 and 3.2 above the sensibility of the resurgent variety during drought may be associated to its leaf morphological characteristics, possibly high capacity for qE and sensibility to the yellow spot disease. The correlation between drought recovery, morphological and anatomical parameters and adaptation to different ecoclimatic regions may thus be very important for the final yield in cane and sugar (all other factors being equal: 1. ABA/osmoregulation, 2. hormonal  adjustments increasing the ratio of root to shoot biomass , etc...).
 When recovering from water stress the remaining sugar-cane leaves still suffer from heat stress while the effects of osmoregulation are reduced. This would lead to a situation where it will have to evaporate more water in order to cool down due to:
 1. protein degradation.
2. “residual” qE.
3. excess illumination.

After drought, sugar-cane varieties with favourable morphological traits (larger leaf surface area, more developed root system, etc...) would thus be able to pump water more efficiently. An estimated evaporation of 100 to 200 gm of water is necessary for each gm of dry matter synthesized (Fauconnier et Bassereau 1990).

This may result from:
               
1. excess transpiration,
2. increased photosynthetic efficiency (reduced density of reaction centres),
3. increased root biomass to shoot biomass  ratio due to hormonal adjustments,
4. osmoregulation at the root level,
5. density of stomata.

The growth of sugar-cane varieties sensible to the  “Yellow Spot” disease is severely affected during water stress/drought. However the pumping capacity of their leaf system may be increased when recovering from water stress. This results in rapid growth resurgence especially for varieties with favorable morphological traits.
Results obtained in Mauritius on the root system of the resistant variety was comparable to that of resurgent variety. Drought resurgence on this variety was established during trials carried out under conditions of severe drought in the western part of the island. The western part of the island is a region where drought prevails together with high temperatures and also subjected to high rates of solar irradiation. If results obtained on the root system (M.S.I.R.I) can be extended to deep roots then the resurgent variety would behave as a resistant one in regions where drought occurs under milder climatic conditions  (lower temperatures and intensity of solar radiation).

5. Conclusions.
In the light of the osmoregulatory mechanisms described above (linked with Abscissic acid), the raw materials / molecules for the biosynthesis of ABA are obtained from the catabolism of the pigments (xanthophylls, anthocyanins etc…) found on the first leaves (red, yellow etc…) of the sugar cane plant. These precursors translocate / accumulate in the roots and enable the biosynthesis of ABA.
At the leaf level, the degradation of proteins binding the chlorophylls in the thylacoid membrane by ABA will provide the necessary conditions and solutes (amino acids, ions, sugars, heat and PH) for osmoregulatory stomata closure. An increase in leaf temperature and incident light radiation would therefore favour stomata closure, especially in the middle of the day.
An increase in water stress intensity results in an increase in the quantity of ABA precursors translocating to the root system. The quantity of ABA moving from the root system to the leaves through transpirational pull will also increase. Homeostasis (i.e. continued growth or avoidance) can only be maintained for a high yielding variety if and only if it has an adequate root system. This happens under field condition for the resistant variety, where it is able to use water economically at the onset of drought and avoid drought for longer periods.
Under conditions of prolonged severe drought, the resistant variety exhibits drought tolerance. Its penultimate response is leaf senescence. The adaptation of root morphological characteristics (according to shoot morphology) for homeostasis during drought may thus be genetic and / or induced hormonally.
Therefore, results obtained from photosynthetic experiments in Brussels confirm data collected from experiments on the root system of the three varieties in Mauritius. It is obvious that the three varieties selected have different morphological characteristics due to genetic differences. Due to its genetic differences, the resistant variety avoids drought for longer periods than the other varieties in field conditions. The resurgent variety recovers rapidly from drought due to its larger leaves and thicker roots.
From the mechanisms described above, protein degradation by ABA results in stomata closure. Photosynthetic efficiency can be maintained at 70-80% during drought w.r.t normal with only 8% of reaction centres. Varieties with an ideal blend of chlorophyll binding proteins that have different sensibilities to ABA will be able to avoid drought and maintain homeostasis better.

Drought avoidance in sugar cane is therefore the combination of several factors namely:

1.        Osmoregulatory stomata closure induced by ABA enabling economic use of water.
2.        Genetically or hormonally induced morphological adaptation of the root system.
3.        An ideal blend of chlorophyll binding proteins having different sensibilities to ABA.

Furthermore the synthesis of new chlorophyll binding proteins induced by ABA has to be investigated.




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