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The aim of this study was to evaluate the physiological responses of three genotypes of sugarcane Saccharum spp. to water deficit in the initial growth phase. Genotypes IACSP94-2094, SP87-365 and IACSP96-2042 submitted to water deficit in the initial growth phase by suspension of watering until photosynthesis reached values near zero. The experimental design was randomized, and the causes of variation were genotype and water condition. Drought caused a reduction in gas exchange in the genotypes, but genotype IACSP96-2042 which was a greater reduction in the values of gas exchange and showed damage to the photochemical apparatus. In contrast, genotype IACSP94-2094 showed characteristics of tolerance to water deficit in this developmental stage, both for evaluations of gas exchange in evaluations that demonstrate their photochemical characteristic of tolerance to water deficit. The SP 87-365 genotype has a characteristic intermediate because there was a difference between that of the control plants and plants with water deficit on photosynthetic rates compared with other genotypes. The genotypes under drought IACSP there was an increase in soluble sugars, and in IACSP96-2042 this increase is due to the increase in sucrose. This genotype also had an increase in leaf proline content. The only genotype that showed a reduction in dry mass of stem was IACSP96-2042, however under ideal conditions of water availability this genotype accumulated more dry weight of stems than the others.
Sugarcane plants present significant decreases in biomass production due to drought, reaching reductions around 35% on dry matter and 33% in total leaf area when water deficit occurs in parallel with high evaporative demand (Inman-Bamber, 2004; Sato et al., 2010). According to Machado et al. (2009), the drought sensitive of sugarcane may be verified when water deficit occurs during the initial growth phase and plants are tillering.
Water stress affects many aspects of plant metabolism, mainly photosynthesis (Chaves et al., 2008). As CO2 uptake is reduced, biomass production is affected as well. Photosynthesis is limited by decreases in stomatal conductance under water deficit conditions, the first line of defense activated even before reductions in leaf water content (Yordanov et al., 2003). Besides stomatal limitation, photosynthesis is also down-regulated by decreases in total soluble protein, chlorophyll content, photochemical and biochemical efficiencies, indicating a metabolic limitation under severe stressful conditions (Flexas et al., 2004; Lawlor & Tezara, 2009; Ghannoum, et al., 2003).
Some plant species subjected to water deficit have significant changes in leaf carbohydrate content, which may be related to the activation of responses to cope with such adverse environmental condition (Griffin et al., 2004). A regulatory role of sucrose on stomata has been reported, with stomatal closure being detected when there is high sucrose content in the apoplast of stomata guard cells. Under water deficit, sucrose accumulation may occur in plants experiencing reduced growth and increases in efficiency of water use are consequences of low stomatal conductance (Lu et al., 1997). Reduction in starch content due to low photosynthesis and high respiration may increase the content of soluble sugars, aminoacids and organic acids under drought conditions (Lee et al., 2008). In fact, increase of leaf osmolyte content is an important strategy to maintain high leaf water potential under limiting water conditions (Silveira et al., 2009).
The physiological responses commented above have been reported in many studies and plant species, with large differences when comparing crop species and varieties (Galmes et al., 2007; Sofo et al., 2009; Graça et al., 2010). Such natural source of variation should be explored for increasing our knowledge about the main physiological traits related to drought tolerance. High use efficiency of natural resources such as water is an important issue currently under debate due to IPCC predictions on water availability in agricultural lands (IPCC,2007).
Concerning sugarcane, the breeding programs have launched many varieties for cultivation in dry lands during the last ten years (Landell et al., 2004, 2005); however, the physiological mechanisms responsible for better crop performance and yield are poorly understood. In such context, the aim of this study was to test the hypothesis that drought tolerant sugarcane genotype IACSP94-2094 is less sensitive to water shortage during early growth phase, being this response a consequence of fine stomatal control on leaf gas exchange and maintenance of leaf hydration when compared to the sensitive genotype IACSP96-2042. In addition, the physiological responses of SP87-365 genotype will be studied under water deficit.
MATERIAL AND METHODS
Plant material and water deficit treatment
Three genotypes of sugarcane (Saccharum spp.) were used in this study. The IACSP94-2094 genotype is considered drought tolerant and the IACSP96-2042 genotype is sensitive to water deficit (Machado et al., 2009). After 42 days of bud planting in plastic pots, one plant of each genotype was transferred to a container (0.6 m3) inside a greenhouse. There were eight containers, with each container having the three genotypes. Plants were grown in soil fertilized according to the recommendations for sugarcane cultivation (van Raij et al., 1996) and maintained with the main stem and three tillers.
Sugarcane plants were subjected to two treatments: well-hydrated condition with soil water potential around -20 kPa maintained through irrigation (control) when the soil water potential was lower than -30 kPa; and water deficit induced by water withholding during the initial growth phase. The water deficit treatment began 90 days after bud planting and ended when the first plant presented null values of CO2 assimilation. During this time, soil water content (q, % w/w) was monitored by the gravimetric method, with soil samplings collected around 600 h at a depth of 0.2 m. Soil rehydration was promoted at the evening of the day in which null values of CO2 assimilation was recorded (maximum water deficit), which occurred after 115 days of bud planting (DAP).
During the experiment, the average air temperature was 19.7 Â°C (40.7 °C maximum and 4.1 Â°C minimum), whereas the average soil temperature was 22.3 Â°C (34.3 °C maximum and 12.6 Â°C minimum) in well-watered conditions and 23.6 Â°C (33.5 °C maximum and 7.8 Â°C minimum) in drought treatment. Inside greenhouse, the maximum photosynthetic active radiation (Q) registered during the experimental period was around 2000 mmol m-2 s-1.
The leaf water potential (Ïˆ) was measured by the psycrometric method, using a microvoltmeter model HR-33T (Wescor, Logan UT, USA). Immediately after excision, leaf discs (diameter of 0.6 cm) were placed in sample chambers model C-52 (Wescor, Logan UT USA). Leaf samples were collected at predawn (6:00 h) on the day of maximum water deficit. Samples were excised in a median region of fully expanded leaves exposed to solar radiation.
Leaf gas exchange was measured using an infrared gas analyzer model Li-6400 (Licor, Lincoln NE, USA). The following physiological variables were evaluated: CO2 assimilation (PN), transpiration (E) and stomatal conductance (gS). The cumulative reductions in PN (CRPN) and E (CRE) due to drought were calculated during the experimental period. These differences are represented in Figure 2 by hatched areas. Air CO2 concentration during the measurements was 390Â±13 mmol mol-1, while Q was artificially fixed by the Li-6400 at 2000 mmol m-2 s-1. Evaluations were taken considering the natural variation of relative humidity and air temperature throughout the day, in intervals of one to two days and between 1400 and 1500 h. Before, during and after the imposition of water deficit, measurements were taken in the first fully expanded leaf with visible ligule (leaf +1), at the middle third of the leaf blade.
Measurements of chlorophyll fluorescence emission were performed with a modulated fluorometer PAM-2000 (Heinz Walz GmbH, Effeltrich, Germany). The basal (FO) and maximum (FM) fluorescence signals were monitored in leaf tissues after dark adaptation (30 min). The instantaneous (FS') and maximum (FM') fluorescence signals were evaluated in leaf tissues adapted to light. The FM and FM' signals were determined after a pulse saturation (l < 710 nm, PFFD ~ 10000 mmol m-2s-1, 0.8s), whereas the minimum fluorescence signal (FO') was sampled after the excitation of photosystem I with far red light (l = 735 nm, PFFD < 15 Wm-2,3.0s). Based on the above signals, some photochemical variables were estimated: potential [FV/FM=(FM-FO)/FM] and effective [DF /FM'=(FM'-FS')/FM'] quantum efficiency of photosystem II (PSII), the nonphotochemical quenching [NPQ=(FM-FM')/FM'] and the apparent electron transport rate [ETR'S = Q x DF /FM' x 0.4 x 0.85] (McCormick et al., 2008a). The photochemical activity was evaluated at the day of maximum water deficit, under Q of 450 mmol m-2 s-1, leaf-to-air vapor pressure difference (VPDL) of 1.3Â±0.3 kPa and leaf temperature (TL) of 24.5Â±1.7 °C.
For leaf carbohydrate evaluations, samples were collected after gas exchange measurements on the day of maximum water deficit. Leaf samples were frozen at 78,5 °C and then dried in an oven (60 Â°C) with forced circulation until constant weight. Soluble carbohydrates (SS) and sucrose (Suc) were extracted and purified using a solution of methanol:chloroform:water (MCW 12:5:3, v/v) according Bielesk & Turner (1966). The quantification of SS was done by the phenol-sulfuric method (Dubois et al., 1956), whereas Suc by the method described by van Handel (1968). The determination of Sta content was done by the enzymatic method described by Amaral et al. (2007). The total nonstructural carbohydrate content (NSCC) was calculated as NSCC=SS+Sta.
The proline content in leaves was evaluated by the classical method of Bates et al. (1973), using the same procedure of extraction and solution purification as described for carbohydrates.
Experimental design and data analysis
The causes of variation were genotype (IACSP 94-2094, IACSP96-2042 and SP87-365) and water condition (control and water deficit). The experiment was arranged in a complete random design, and the data subjected to the analysis of variance (ANOVA). The mean values (n=3 or 4) were compared by the Tukey test (p<0.05) when significant difference was detected.
Soil moisture and leaf water potential
Soil water moisture was maintained near at field capacity (around 29%, w/w) in control treatment throughout the experimental period, being significantly reduced by water withholding. After 25 days of treatment (90 to 115 DAP), soil moisture reached 16% at the day of maximum water deficit. At this time, the leaf water potential (y) was reduced due to water shortage in SP87-365 and IACSP96-2042 genotypes, varying around -1.8 MPa at predawn (Figure 1). Non-significant changes were noticed in y of IACSP94-2094 due to water withholding (Figure 1).
Leaf gas exchange and photochemistry
The water deficit affected leaf gas exchange of all genotypes (Figure 2), causing reductions on stomatal conductance (gS), CO2 assimilation (PN) and transpiration (E). However, the drought sensitive was not the same, with the IACSP96-2042 genotype being more affected by drought (Figure 2c,f,i). At the day of maximum water deficit, IACSP94-2094, SP87-365 and IACSP96-2042 exhibited reductions in PN around 46%, 65% and 69%, whereas gS was reduced around 58%, 62% and 72% in IACSP96-2042, respectively. Regarding transpiration, differences were less pronounced with plants showing reductions due to drought between 56% and 64% (Figure 2).
Stomatal sensitive to water deficit was also different among sugarcane genotypes. The IACSP94-2094 genotype was the first one to show gS reduction, with stomata presenting an aperture 68% lower than that one found under well-hydrated conditions at 104 DAP (Figure 2a). The other genotypes showed partial stomatal closure due to drought at 106 (SP87-365) and 108 DAP (IACSP96-2042).
The consequences of water deficit were also evaluated considering the integrated-reduction of PN and E throughout the experimental period, as indicated by dashed lines in Figure 2. The largest reduction of PN was observed in IACSP96-2042 (~34%), whereas the lowest reduction was found in IACSP94-2094 (~21%). Regarding E, the integrated-reductions ranged from 22% in SP87-365 to 35% in IACSP96-2042.
After soil rehydration, the recoveries of gS, PN and E were variable depending on sugarcane genotype. In general, the genotype IACSP96-2042 showed slower recovery of gas exchange as compared to IACSP94-2094 and SP87-365 (Figure 2).
With exception of IACSP94-2094, there were decreases in FV/FM of SP87-365 and IACSP96-2042 on the day of maximum water deficit, being IACSP96-2042 more sensitive to drought (Figure 3a). Water deficit affected DF /F'M and ETR'S only in IACSP96-2042, with a reduction of 42% in both photochemical variables (Figure 3b,c). NPQ was not affected by water deficit in IACSP94-2094 and SP87-365, whereas IACSP96-2042 exhibited increases under water stress conditions (Figure 3d).
Leaf carbohydrate and proline
Drought caused increases of soluble sugars content (SS) in IACSP96-2042 and IACSP94-2094 (Figure 4a). Leaf sucrose content (Suc) of IACSP94-2094 was not affected by drought stress, which promoted decrease of Suc in SP87-365 and a large increase in IACSP96-2042 (Figure 4b). Comparing genotypes, significant differences were noticed only under well-watered conditions, where SP87-365 had the highest Suc content and IACSP96-2042 the lowest one.
Starch content was affected by water deficit only in SP87-365, with plants showing reduction around 50% (Figure 4c). On the other hand, water withholding increased nonstructural carbohydrate content (NSCC) in IACSP94-2094 and IACSP96-2042 (Figure 4d). Under well-hydrated conditions, SP87-365 had the highest NSCC when compared to the others, which did not vary due to water deficit (Figure 4d).
At the day of maximum water deficit, leaf proline content increased in SP87-365 and IACSP96-2042 (Figure 4e), with the latter showing the highest values. Absolute value of proline in IACSP96-2042 was about 2.3 to 2.7 times higher than the other genotypes under water deficit.
The difference between SS and Suc represents the content of reducing sugars, such as hexoses (glucose and fructose). From the data of SS and Suc, we can infer that the drought caused considerable increase in the content of hexoses in leaf IACSP94-2094 (8.9 times) and SP87-365 (3.9 times). Genotype IACSP96-2042 showed the smallest variation in leaf content of sugars, with stressed plants showing an increase of 1.9 times.
4.1 Leaf water potential, soluble carbohydrates and proline
The suspension of irrigation caused severe water stress in genotypes SP87-365 and IACSP96-2042 with y ranging between -1.73 and -1.85 MPa predawn (Figure 1). In this situation water cell elongation is stopped (Inman-Bamber & De Jager, 1986). Regarding the tolerant genotype, we observed the absence of significant variation in y IACSP94-2094 (Figure 1), this lack of variation may be due to plant acclimation to growth inhibition or loss of leaf area to limit water use (Chaves et al., 2008). The y can be retained, due to accumulation of solutes (Inman-Bamber & Smith, 2005) and proline (Molinari et al., 2007).
Besides y remain unchanged due to water deficit, genotype IACSP94-2094 showed no change in proline content under drought conditions (Figure 4e). This finding is at odds with those reported in the literature of plants tolerant to water scarcity (Inman-Bamber et al., 2005). Typically, the increase of the proline is related to an osmotic adjustment mechanism and osmoprotectant (Molinari et al., 2007), which was not present in IACSP94-2094.
In the case of sugar, it was noted increased foliar carbohydrate content in plants in water stress condition (Figure 4). According to Zhou & Yu (2010), these changes are related to activation of responses to cope with this adverse environmental condition, so to assist in the maintenance of cell water relations. The genotype IACSP94-2094 showed an increase in SS content under drought (Figure 4a), but did not provide significant changes in proline content (Fig. 4e). In the other hand the genotype IACSP96-2042 showed a significant increase in SS and content of proline (Figure 4a, e). The increase in SS in the IAC materials and the increase of Suc in the genotypes IACSP96-2042 was not correlated with the degradation of starch, in other hand can be correlated in case of SP87-365 genotype, since there was no reduction in carbohydrate content of the water deficit treatment (Figure 4a-c). It is known that increasing the content of molecules osmoregulators, mainly K +, soluble carbohydrates and amino acids are related to the reduction of osmotic potential cellular (Molinari et al., 2007; Zhou & Yu, 2010), leading to a reduction in total water potential. However, genotype IACSP94-2094 showed no significant reduction y under drought, while the other genotypes sensitive to drought IACSP96-2042 and SP87-365 (Machado et al., 2009) presented a variation of carbohydrate and proline (Figures 1 and 4).
The accumulation of soluble carbohydrates in leaves during water deficit is considered a plant response to maintain hydration of the shoot and also protect enzymes and membrane system through the stabilization of proteins and lipids (Lawlor, 2002; Yordanov et al. 2003). In genotype IACSP94-2094, there is evidence of significant accumulation of some soluble sugars, not Suc in water stress conditions (Figure 4a, b). This conclusion is valid because the levels remained stable at Suc sheets IACSP94-2094 on the condition of water restriction.
4.2 Effect of drought on photosynthesis, gas exchange and photochemical activity
The water deficit reduced stomatal conductance (gS) in the three genotypes (Figure 2a-c). This response is expected in plants subjected to water stress, and is considered one of the first strategies to prevent excessive dehydration of leaves (Inman-Bamber and Smith, 2005; Yordanov et al., 2003). However, the genotypes were differentially affected by drought when considered gS, and in general the genotype IACSP94-2094 the least affected and genotype IACSP96-2042 the most adversely affected by drought (Figure 2a-c). Besides the genotype IACSP94-2094 remain the stomata closed for at least the maximum water deficit (Figure 2a), it showed early closure of stomata in comparison to other genotypes in water deficit situation. Probably, these two aspects (maintenance and stomatal sensitivity to water stress) should be involved with this genotype increased tolerance to water deficit.
The rapid stomatal closure in sugarcane in water stress conditions is a desirable characteristic and is related to the efficient signaling between roots and leaves (Inman-Bamber et al. 2005; Smit & Singels, 2006). In this study, genotype IACSP94-2094 showed early closure of stomata remained unchanged y due to water deficit compared to other genotypes (Figures 1 and 2). These results are in agreement with Smith et al. (2005), which reported that genotypes tolerant to drought have reduced gS even at high values of y compared to sensitive genotypes.
The reduction of stomatal aperture in water stress conditions may be caused by changes in the turgidity of guard cells of stomata (Liu et al., 2003). However, this justification could be applied only to genotypes IACSP96-2042 and SP87-365 since the reduction in gS to genotype IACSP94-2094 occurred even at high y in water stress conditions (Figures 1 and 2). The lower decrease in stomatal aperture in IACSP94-2094 (~ 58%) compared to IACSP 96-2042 (~ 72%) can be related to the maintenance of growth (Machado et al., 2009) by maintaining photosynthesis in condition water restriction.
Regarding signage, it can be inferred that the variation in gS in the genotype IACSP94-2094 was governed only by chemical aspects (chemical signaling) since the shoot did not change the y (hydraulic signaling) at the time of maximum water deficit. Another type of chemical signaling refers to the increased contents of Sac in plant tissues, especially in the stomata guard cells, and consequent reduction in stomatal opening (Lu et al., 1997). So the greater stomatal closure detected in genotype IAC96-2042 could be a consequence of the significant increase Suc in water stress conditions (Figures 2c and 4b).
The reduction of stomatal opening caused a significant decrease in E in the three genotypes in water stress conditions (Figure 2 g-i). Therefore, can consider that the reduction of water use by reduction of E was a successful strategy in the three genotypes. However, the decreases in gS induced by drought also affected PN, as expected and widely reported in the cultivated species (Chaves et al. 2008; Singels et al., 2005). At first, the effects of lower gS in photosynthesis would result in the reduction of CO2 concentration in the leaf mesophyll, being called diffusive limitation (Niinemets et al., 2009). With the worsening drought, the biochemical reactions of photosynthesis could be affected, there are limitations to stomatal and non stomatal origin in conditions of maximum stress (Flexas et al., 2009). Machado et al. (2009) found significant reductions in PN/CI in sugarcane after 43 days under water deficit in the initial growth phase, indicating the occurrence of non-stomatal limitation of photosynthesis in the water deficit susceptible genotype. As in other phenological phases were detected significant reductions in both genotypes after 15 and 14 days under water deficit. According Bota et al. (2004), the decrease in activity of Rubisco is the major cause of decreased photosynthesis, and this decreased activity of Rubisco and/or the contents of RuBP are found at the end of the drought cycle. Lawlor (2002) comments that the lower synthesis and/or RuBP regeneration due to reduced production of ATP are the major factors related to reduced photosynthesis in plants exposed to water deficit. However, these changes occur only in conditions of severe stress, and photosynthesis is regulated primarily by the limitations of diffusive origin (Lawlor, 2002).
On the day of maximum water deficit, plants of the genotypes and SP87-365 and IACSP96-2042 showed leaf y measured at predawn below -1.7 MPa in drought conditions (Figure 1) and probably were reductions in this variable throughout days. In corn plants with y around -2.0 MPa, there was biochemical limitation of photosynthesis by reducing the activity of carbonic anhydrase, phosphoenolpyruvate carboxylase and of RuBP carboxylase (Prakash & Rao, 1996). Inman-Bamber & Smith (2005) reported that the metabolic limitation of photosynthesis begins when the leaf tissue of sugarcane y reach below -1.2 MPa. In a study conducted in four species of grasses, Ghannoum et al. (2003) concluded that water stress reduced plant photosynthesis due to limitations to biochemical with y from -2.8 MPa.
The susceptibility of PN to drought was higher in IACSP96-2042 and lowest in IACSP94-2094 (Figure 2d-f). Besides being less affected by water deficit, genotype IACSP94-2094 also showed rapid and complete recovery of gas exchange after rehydration of the soil (Figure 2). While SP87-365 had good recovery, the genotype IACSP 96-2042 showed slow and incomplete recovery gS and PN even one week after rehydration of the soil (Figure 2). Flexas et al. (2006) commented that both the speed and the extent of recovery are affected by oxidative stress caused by severe drought, rapid recovery was observed in plants with high antioxidant system activity and high content of antioxidants. In fact, oxidative stress in genotype IACSP96-2042 is also suggested by the results of proline (Kingston-Smith & Foyer, 2000). However second Molinari et al. (2007) proline can reduce oxidative damage in the plant.
Regarding the photochemical aspect of photosynthesis, the greatest reduction in FV/FM in IACSP96-2042 (Fig. 3b) confirms its greater sensitivity to water deficit. This assertion is reinforced by the reduction of DF/F'M and ETR'S and increased NPQ observed only in genotype IACSP96-2042 (Figure 3). The decrease of FV/FM, DF/F'M and ETR'S in IACSP96-2042 could be motivated by the reduction in chlorophyll content (Akram et al., 2007), both which were found to vary with the SPAD index between 34, 9 Â± 1.5 and 50.6 Â± 3.5 (data not shown).
Our results are partly in disagreement with the work of Silva et al. (2007), in which there was agreement between the reduction in chlorophyll content and FV/FM in genotypes of sugarcane more susceptible to drought. However, attention should be paid to the effects of the interaction between drought and high temperature on the photochemical activity, observed in a study conducted by Silva et al. (2007) and could cause significant reductions in FV/FM. The increase on NPQ in the genotype IACSP96-2042 could be an indirect consequence of the high sensitivity of PN to water deficit. The lower consumption of products of photochemical activity (ATP and NADPH) by the PN spike down the electrochemical potential gradient in the membranes of thylakoids, which in turn cause an increase of quenching non-photochemical, indicated by NPQ (Horton et al. , 1996), or even reduction of DF/F'M (Baker & Rosenqvist, 2004). This regulatory mechanism tries to maintain the efficiency of photosystem II and was recognized as a strategy of photoprotection (Horton et al., 1996, Yordanov et al., 2003) and photosynthesis in short-term adaptation to adverse conditions (Biswal & Biswal, 1999).
The absence of significant changes in FV/FM in the genotype IACSP94-2094 even in conditions of drought and the period of highest atmospheric demand (afternoon) could be related to morphological changes in the leaf. The coiling of the leaves was a phenomenon observed in this genotype, even in conditions of water availability. According Flexas & Medrano (2003), changes in orientation of the leaf can cause minor solar energy and thus prevent excess energy in times when the use of it is compromised. This mechanism maintains the photochemical activity, which could be indicated by no reduction in FV/FM in the genotype IACSP94-2094 under drought conditions. The leaf curling is a mechanism has been identified in sugarcane, but with great genotypic variation (Inman-Bamber and Smith, 2005).
Since that ETR's was affected only by IACSP96-2042 drought (Figure 3), it is assumed that ATP synthesis was reduced only in this genotype. This result confirms the high tolerance of the photochemical apparatus to drought (Lawlor, 2002; Yordanov et al., 2003). Since considering the variable ETR'S as a global measure of the photochemical performance, changes in photochemical activity observed in plants under drought was insufficient to limit PN in the genotypes IACSP94-2094 and SP87-365, with the ratio ETR'S/PN ranging between 07 and 19 mmol mmol-1 and being above the theoretical minimum.
The decrease in PN due to drought did not reduce the amount of leaf carbohydrates in the three genotypes at the time of maximum water deficit (Figure 4d). Instead, the drought caused an increase in NSCC on IACSP96-2042 and IACSP94-2094, and is basically due to the increase in SS (Figure 4a, d). The basic difference between these two genotypes would be IACSP94-2094 accumulated more soluble carbohydrates than Suc, while the increase in SS in the genotype IACSP 96-2042 was due to increases in the content of Suc in water stress conditions (Figure 4). The trehalose may be a sugar accumulated in condition of water stress IACSP94-2094, since this disaccharide was accumulated in sugarcane plant tolerant to drought, causing stability of cellular structure (Zhang et al., 2006). Moreover, the decrease of PN in IACSP 94-2094 and IACSP96-2042 under drought could be a consequence of reduced expression of genes related to photosynthesis. This possibility is based on the fact that the accumulation of hexoses present in an inhibitory retro effect in PN on sugarcane plant (McCormick et al., 2008b).
In this study, the accumulation of Suc in water stress conditions contrary to the reported by Inman-Bamber & Smith (2005), suggesting that the synthesis of sucrose is maintained even at y less than -1.8 MPa (Figures 1 and 4b). Recently, Fresneau et al. (2007) observed increased activity of sucrose phosphate synthase in plants subjected to water deficit, which would explain the increase in Suc in the IAC genotypes (Figure 4b). Still, one must consider that the increase in Suc may have been caused by lower demand from sinks (McCormick et al., 2006) in IACSP96-2042, once the growth of this genotype was reduced under drought conditions.
The tolerant genotype IACSP94-2094 presents early stomatal closure in response to water deficit, keeping the leaf water potential when the water deficit is imposed at the initial stage of development. The accumulation of soluble carbohydrates - than not sucrose - is also related to better performance in limiting condition. The best balance of water in the plant causes the stomata remain closed at least during the drought, allowing the plants to show a smaller reduction in CO2 assimilation. These responses were responsible for the maintenance of growth even in conditions of low water availability.
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Figure 1: Leaf water potential * (MPa) in three genotypes of sugarcane kept under well-hydrated (control) or subjected to water deficit by watering suspension. Measurements take off predawn on the day of maximum leaf water stressed (115 DAP). Each bars represent average values (n = 3) Â± standard error.
Figure 2: Temporal variation of stomatal conductance (gS), CO2 assimilation rate (PN) and transpiration (E) in the genotypes of sugarcane IACSP94-2094 (a, d, g), SP87-365 (b, e, h) and IACSP96-2042 (c, f, i) on condition of well-hydrated (C, â-) or under water stressed by watering suspension (WS, â-¡). Measurements taken between 14:00 and 15:00, each symbol being the mean value (n = 4) Â± standard error. The hatched area indicates the difference between the two treatments during the experimental period. The arrow indicates the time of rehydration of the soil (115 DAP).
Figure 3: The potential quantum efficiency of photosystem II (FV/FM, a), and effective of photosystem II (DF/FM',in b), apparent electron transport (ETR'S, in c) and nonphotochemical quenchinq (NPQ, in d) in three genotypes of sugarcane in conditions of well-hydrated (WD) or under water stressed by watering suspension (WS). Measurements performed on the day of maximum water deficit (115 DAP), between 14:00 and 15:00 h. Each histogram represents the mean (n = 4) Â± standard error.
Figure 4: Content of soluble sugars (SS, in a), sucrose (Suc, in b), starch (Sta, in c), nonstructural carbohydrate content (NSCC, in d) and proline (Proline, in e) in the genotypes of sugarcane in well-hydrated (WH) or under water stressed by watering suspension (WS). Evaluations conducted on the day of maximum water deficit and collected in leaves +1 at 14:00 h. Each histogram represents the mean (n = 4) Â± standard error.