Tropical Agricultural Research & Extension 12(2):2009 *Corresponding author:chandrajithdesilva@yahoo.com VARIETAL VARIATION IN STOMATAL CONDUCTANCE, TRANSPIRATION AND PHOTOSYNTHESIS OF COMMERCIAL SUGARCANE VARIETIES UNDER TWO CON- TRASTING WATER REGIMES ALC De Silva1* and WAJM De Costa2 1Division of Agronomy, Sugarcane Research Institute of Sri Lanka, Uda Walawe 2Department of Crop Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka Accepted: 6th October 2009 ABSTRACT The study was conducted to evaluate some physiological characters of commercial sugarcane varieties under different growing conditions in Sri Lanka. A field experiment was conducted at the Sugarcane Research Institute, Uda Walawe (6o21’N latitude, 80o48’E longitude and 76 m altitude) using eight sug- arcane (Saccharum hybrid L.) varieties grown under irrigated and rainfed conditions in a split plot de- sign. Stomatal conductance (gs), instantaneous transpiration rate (E1) and photosynthetic rate per unit leaf area (Pn) were measured. Canopy stomatal conductance (gc), instantaneous canopy transpiration rate (Ec) and transpiration efficiency (Pn/E1) was calculated. The behaviour of gs in many respects to the moisture availability and growing stage was similar to the responses seen in Pn. Water deficit signifi- cantly reduced gs, E1 and Pn. Recovery of gs and Pn from water stress with rainfall was quite rapid un- der rainfed conditions. The varieties Co775, SL8306, SL7103 and SL88116 which had higher Pn and Pn/ E1, and lower gs, E1, gc and Ec showed comparatively superior physiological performances under rainfed conditions. Water conservation through lowering stomatal conductance, both at the individual leaf and canopy level, and higher photosynthetic rate were identified as some physiological mechanisms respon- sible for drought tolerance of sugarcane. Key words: Sugarcane, Stomatal conductance, Transpiration, Photosynthesis, Water regimes INTRODUCTION Sugarcane is one of the extreme types C4 plant and it can have extraordinarily high rates of Pn, found in crop plants (Irvine 1967; 1975 and 1983). Moreo- ver, Pn of sugarcane does not show light saturation even at full sunlight because of their CO2 concen- trating mechanism. Therefore, even under optimum conditions, the stomatal conductance (gs) of sugar- cane is lower than that of the C3 crops. Therefore, sugarcane could maintain a higher rate of Pn under full sunlight with lower rates of canopy transpira- tion rate (Ec) to increase the water use efficiency (De Costa 2000). The light saturated maximum Pn in commercial varieties of sugarcane ranges from 31 to 53μmol CO2m -2s-1 (Irvine 1967 and Roberts et al. 1990). Moreover, it is at a maximum during the grand growth stage and tends to decrease during late grand growth and maturation phases of sugar- cane (Gascho and Shih 1983). However, water availability affects the rate of Pn. Gascho and Shih (1983) recorded that positive Pn occurs in leaves of sugarcane at or below the wilting point. However, the amount of Pn was much less than in plants with an adequate water supply. Moreover, Roberts et al. (1990) revealed that Pn, gs and leaf extension growth of sugarcane were very low under rainfed conditions particularly during dry periods. However, differences in Pn and gs between well watered and water stressed was not established early enough under stress and were not as con- sistent as for cell extension growth. Moreover, he pointed out that the effect of rainfall and recovery of Pn, gs and leaf extension growth occurred rapidly in rainfed cane except in cases where cane had only senescing leaves. All the above physiological varia- bles showed a greater level of activity than was ob- served in cane receiving regular irrigation. The behaviour of gs is in many respects was similar to the responses seen in Pn. The gs responds to the onset of stress at about the same value of wa- ter stress as Pn and after prolonged stress very low gs are observed. Maximum values of gs of around 400mmol m-2s-1 were observed on well irrigated cane, in full radiation but with only moderate va- pour pressure deficit (Roberts et al. 1990 and Grantz et al. 1987). Therefore, measurements of gs made directly by porometers could be used as a means of selecting drought tolerant varieties (Roberts et al. 1990). This could apply to varieties which conserve water by stomatal closure or alter- natively maintain a high gs and moderate leaf water potential by more efficient or deeper rooting pat- terns. Bull and Glasziou (1975) recorded that in all cases investigated any reduction in Pn was accom- panied by increased stomatal resistance. Moreover, the decrease of Pn in sugarcane under water stress was caused by both stomatal and non-stomatal fac- Paper presented at the 2nd National Symposium, Faculty of Agriculture, University of Ruhuna tors, initially mainly by stomatal closure, followed by non-stomatal factors as stress became severe (Du et al. 1996). Therefore, the objective of this study was to evaluate the stomatal limitation of Pn in commercial sugarcane varieties in Sri Lanka and thereby identifying high yielding varieties under well watered and water stressed conditions and as well as drought tolerant traits of sugarcane which could be used for the hybridization programme to produce better hybrids of sugarcane for different sugarcane growing environments in Sri Lanka. METHODOLOGY A field experiment was conducted at the Sugarcane Research Institute (SRI), Uda Walawe (6o21’N lati- tude, 80o48’E longitude and 76 m altitude) where the annual average rainfall is about 1450 mm with a distinctly bimodal distribution (Panabokke 1996). The mean annual temperature ranges from 220C - 320C. The average evaporation from a free water surface is about 5mm per day (Sanmuganathan 1992). The soil has been classified as Ranna series of Reddish Brown Earth (RBE), great group of Rhodustalfs (order Alfisols, suborder Ustalfa), sandy clay loam texture (De Alwis and Panabokke 1972; Anon 1975), and moderately well drained with a pH of 6.5 - 6.7. The bulk density of the soil ranges from 1.59 – 1.85gcm-3 (Sithakaran 1987). The respective soil water contents at saturation, field capacity and permanent wilting point are 30%, 20% (10kPa) and 8% (1500kPa), respectively (Sanmuganathan 1992). The experiment was conducted as a two-factor factorial with 16 treatment combinations, composed of two main plot treatments as ‘irrigated’ (‘well- watered’) and ‘rainfed’ (‘water-stressed’) and eight commercial sugarcane (Saccharum hybrid L.) vari- eties (i.e. SL7103, SL7130, SL8306, SL8613, SL88116, SLI121, M438/59 and Co775) as subplot treatments, in a split plot design. The irrigated treat- ment received 2m3 of water per irrigation at 5-10 day intervals to maintain the soil water potential in the top 1m above -0.05MPa. One meter deep trenches were made between irrigated and rainfed plots to avoid the lateral movement of water. Each treatment combination was replicated thrice. Plot size was 9m x 8.22m, containing 6 furrows at 1.37m spacing. The sugarcane was planted and maintained under recommended procedures (Anon 1991). Stomatal conductance (gs) and instantaneous transpiration rate (El) per unit leaf area were meas- ured in leaves of top, middle and bottom parts of the canopy layers using a steady-state porometer (LI-1600, LI-COR, Inc. LTD., Lincoln, USA) at 6 and 9 months after planting (MAP). The measure- ments were done between 09:30 and 14:30h. Cano- py stomatal conductance (gc) and instantaneous canopy transpiration rate (Ec) were computed by summing the products of mean leaf gs and partial leaf area index in the three canopy layers (Squire and Black 1981). Gas exchange studies were carried out using a portable photosynthetic system (LI –6400 and LI – 6200, LI-COR, Inc. LTD., Lincoln, Nebraska, U.S.A.) comprising a LI 6200 gas analyser with a LI 6250 computer software. The apparatus was cal- ibrated prior to measurements as described by Welles (1986). Leaf chamber in one litre capacity was adjusted to expose an area of 20cm2 of the leaf. Atmospheric air was drawn into the system, by keeping the leaf chamber open. Then, a portion of sugarcane leaf was clamped into the leaf chamber. Measurements were taken when a steady decline in CO2 concentration in the chamber was observed. CO2 assimilation rate, stomatal conductance, tran- spiration rate, internal leaf CO2 concentration, air CO2 concentration and photosynthetically active radiation (PAR) were monitored. The relative hu- midity (RH%) of the system was maintained at 65- 70% during the measurement by adjusting the air flow rate through the magnesium perchlorate desic- cant. Measurements were taken at 6 MAP, 8 MAP and 11 MAP on six days and two sessions per day. Photosynthetic (CO2 assimilation) rate in all the leaves from top to bottom of the stalk in each varie- ty was measured to identify the potential photosyn- thetic capacity and effective and ineffective leaves in the stalk. Transpiration efficiency (Pn/El) of each and every experimental plot was calculated as the ratio between instantaneous photosynthetic rate (Pn) and transpiration rate (El) to determine the effi- ciency of water transpire through the stomata while photosynthesizing. Significance of treatment differences was tested by analysis of variance (ANOVA). Means were sep- arated by the least significant difference (LSD). Correlations between variables were determined by simple linear correlation analysis. The SAS statisti- cal computer package was used to analyse the data. RESULTS AND DISCUSSION Impacts of variation in water regimes on stomatal conductance and transpiration There were significant water regime x variety interaction effects on stomatal conductance and instantaneous transpiration rate in terms of both individual leaves in the top leaf layer (gs, El) and the whole canopy (gc, Ec). Measure- ments taken from 159 and 167 DAP in Septem- ALC DE SILVA AND WAJM DE COSTA :VARIATION IN PHYSIOLOGICAL CHARACTERS OF SUGARCANE 98 Tropical Agricultural Research & Extension 12(2):2009 ber 2002, which fell within the period when the crops were experiencing a prolonged and con- tinuous soil moisture depletion, showed the sig- nificant varietal variation of gs and El within and between water regimes (Table 1). Moreover, soil water deficits significantly (p<0.001) reduced gs and El in all varieties tested during prolonged and continuous soil moisture depletion from 159 and 167 DAP. It reduced the average gs to the lowest value of 0.8mm s-1, which confirmed the findings of Inman-Bamber and De Jager (1986) that the gs of sugarcane reached a minimum of about 0.5 to 1.0mm s-1 when midday leaf water potential de- creased by about -1.3 to -17 MPa at different water stress cycles. In the present study, the lowest gs and El under rainfed conditions were observed in SL7103 and SL8306 respectively which conserving moisture during the drought. The variety SL88116 which had the highest biomass production showed the highest gs and El under both irrigated and rain- fed conditions (Table 1). Measurements taken at 322 DAP during the short dry spell in February 2003 after the Maha season rainfall, show the varietal variation on gs, El, gc and Ec within and between water regimes and the inter- action effect of water regime x variety (Tables 2 and 3). Moreover, at 322 DAP, soil water deficit significantly reduced gs, El, gc and Ec in a majority of varieties. Notably, SL8613 and SL7130 showed significantly greater gs and El under rainfed condi- tions. The variety SL88116 which had the highest biomass production under both conditions (De Sil- va 2007) showed the highest gs, El, gc and Ec under Table 3 Canopy stomatal conductance (gc) and in- stantaneous canopy transpiration rate (Ec) ± stand- ard error at 322 DAP in different sugarcane varieties under irrigated and rainfed conditions. 99 Table 1 Mean stomatal conductance (gs) and instan- taneous transpiration rate (El) ± standard error per unit leaf area at 159 and 167 DAP in different sugar- cane varieties under irrigated and rainfed conditions. Table 2 Mean stomatal conductance (gs) and instan- taneous transpiration rate of top leaves (El) ± stand- ard error at 322 DAP in different sugarcane varie- ties under irrigated and rainfed conditions. Variety gs (cm s-1) El (μg cm-2 [leaf area] s-1) Irrigated Rainfed Irrigated Rainfed SL88116 0.299± 0.03 0.119±0.02 6.062±0.36 2.185±0.27 Co775 0.225±0.02 0.117±0.03 4.298±0.14 2.068±0.30 SL8306 0.233±0.02 0.068±0.01 4.508±0.23 1.374±0.19 SL8613 0.188±0.02 0.079±0.02 3.699±0.25 1.454±0.22 SL7130 0.243±0.02 0.064±0.02 4.488±0.23 1.381±0.20 M438/59 0.212±0.02 0.072±0.01 4.601±0.33 1.527±0.22 SL7103 0.233±0.02 0.063±0.01 4.854±0.43 1.401±0.26 SLI121 0.294±0.04 0.064±0.01 5.608±0.56 1.442±0.21 Mean 0.241±0.01 0.081±0.01 4.765±0.13 1.604±0.09 LSDv 0.067 0.044 0.955 0.663 LSDw 0.013 0.233 Note: LSDv = LSD (p=0.05) for varietal comparisons within a water regime; LSDw = LSD (p=0.05) for com- parison of mean values between water regimes. Variety Mean stomatal conductance of top leaves, gs, (cm s-1) Instantaneous transpira- tion rate of top leaves, El, (μg cm-2 [leaf area] s-1) Irrigated Rainfed Irrigated Rainfed SL88116 0.195± 0.06 0.069±0.01 6.163±1.16 1.798±0.31 Co775 0.142±0.04 0.057±0.01 5.028±1.70 1.765±0.41 SL8306 0.173±0.02 0.103±0.02 4.387±1.10 3.498±0.11 SL8613 0.079±0.03 0.108±0.03 2.793±0.66 3.912±0.10 SL7130 0.072±0.01 0.132±0.02 1.792±0.13 4.588±1.29 M438/59 0.177±0.01 0.095±0.02 4.178±0.18 1.992±0.21 SL7103 0.172±0.04 0.077±0.02 4.647±0.88 2.183±0.52 SLI121 0.147±0.03 0.112±0.03 4.457±0.68 2.613±0.76 Mean 0.145 0.094 4.181 2.794 LSDv 0.100 0.060 2.706 1.885 LSDw 0.028 0.814 Note: LSDv = LSD (p=0.05) for varietal comparisons within a water regime; LSDw = LSD (p=0.05) for com- parison of mean values between water regimes. Variety gc (cm s-1) Ec (μg cm-2 [land area] s-1) Irrigated Rainfed Irrigated Rainfed SL88116 1.251±0.35 0.279±0.06 37.93±8.70 10.25±3.82 Co775 0.941±0.19 0.344±0.10 32.53±9.26 9.78±2.15 SL8306 0.899±0.14 0.420±0.07 31.19±5.50 12.28±1.74 SL8613 0.719±0.44 0.500±0.11 18.49±10.62 15.79±3.39 SL7130 0.559±0.17 0.669±0.12 14.04±3.92 22.65±7.75 M438/59 0.624±0.11 0.328±0.07 17.41±2.70 7.59±1.29 SL7103 0.704±0.14 0.296±0.05 18.40±3.76 7.75±0.94 SLI121 1.090±0.35 0.349±0.10 34.93±7.56 8.32±2.55 Mean 0.838 0.386 25.21 11.33 LSDv 0.673 0.256 19.743 8.620 LSDw 0.189 5.379 Note: LSDv = LSD (p=0.05) for varietal comparisons within a water regime; LSDw = LSD (p=0.05) for com- parison of mean values between water regimes. Variety Pn (μmol m-2 s-1) gs (mol m-2 s-1) Irrigated Rainfed Irrigated Rainfed SL88116 24.7±3.1 16.7±2.5 0.775±0.12 0.541±0.10 Co775 26.5±4.4 20.9±2.5 0.639±0.08 0.407±0.03 SL8306 28.1±3.4 27.3±2.0 0.726±0.06 0.373±0.03 SL8613 25.1±3.2 15.5±2.2 0.783±0.15 0.378±0.03 SL7130 26.0±3.9 21.1±3.1 0.531±0.06 0.370±0.03 M438/59 27.2±3.4 10.4±1.9 0.763±0.11 0.336±0.02 SL7103 25.5±2.7 15.2±2.4 0.695±0.07 0.428±0.05 SLI121 23.0±5.3 17.2±3.4 0.490±0.12 0.329±0.07 Mean 26.0 18.1 0.69 0.40 LSDw 2.90 0.06 Table 4 Mean net photosynthetic rate (Pn) and sto- matal conductance (gs) ± standard error of all leaves in the canopy during the period from 180 – 183 DAP in different sugarcane varieties under irrigated and rainfed conditions. Note: LSDw = LSD (p=0.05) for comparison of mean values between water regimes. irrigated conditions and the lowest gc and the se- cond lowest gs and El under rainfed conditions. The variety Co775 which had second highest biomass production under both conditions recorded lowest gs and El under rainfed conditions whereas lowest Ec was recorded in M438/59. SL7103 observed se- cond lowest gc and Ec under rainfed conditions (Table 2 and 3). When yields under both water regimes were considered, cane yield of this experiment showed significant positive correlations with gs (r = 0.49 with p=0.05), El (r = 0.51 with p=0.04), gc (r = 0.80 with p=0.0002) and Ec (r = 0.74 with p=0.0001). This indicated that greater stomatal opening and efficient water use are pre-requisites for increasing overall sugarcane yields in this environment. On the other hand, cane yield under rainfed conditions showed moderate negative correlations with gs (r = -0.53 with p=0.18), El (r = -0.30 with p=0.47) and gc (r = -0.22 with p=0.60). This indicated that water conservation mechanisms (i.e. lowering of gs and El) are needed in a variety to achieve higher yields under rainfed conditions. For example, the variety SL88116 which showed the highest rainfed cane yield had the second lowest gs and El and lowest gc under rainfed conditions at 322 DAP. Conversely, SL8613 which had the lowest rainfed cane yield had the second highest El, gc and Ec under rainfed conditions. Moreover, the present study confirmed that water conservation under drought conditions is of major importance in obtaining a satisfactory yield of sugarcane (De Silva 2007). As drought is common in many sugarcane growing areas and irri- gation is often not possible, it is important to con- sider reducing transpiration and thereby reducing consumptive water use. Impacts of variation in water regimes on sto- matal conductance, transpiration and photosyn- thesis During prolong dry period from 180-183 DAP, soil water deficits significantly (p<0.05) reduced Pn and gs in all varieties tested except in SL8306 (Table 4). The behaviour of gs in majority of tested varieties was similar to the responses seen in Pn. However, SL8306 recorded the highest Pn under both water regimes and the lowest reduction of Pn due to water stress among all varieties tested under rainfed con- ditions. SL88116 had the second highest and the highest gs within the irrigated and rainfed regimes respectively. The Pn varied among tested varieties from 23-28µmol m-2s-1 and 10-27µmol m-2s-1 under irrigated and rainfed conditions respectively (Table 4). At 256 DAP, Pn/El showed significant varietal vari- ation under rainfed conditions and water regime x variety interaction (Table 5). Both at 256 DAP (during the Maha season rainfall) and at 340 DAP [(end of the Maha season rainfall) (late grand growth and maturation stages)], a majority of varie- ties tested recorded greater Pn, El and gs, and lower Pn/El under rainfed conditions than irrigated condi- tions with the few exceptions (Tables 5 and 6). The difference in these variables between irrigated and rainfed conditions at these stages are due to the dif- ferent developmental stages of the crops in spite of differential water availability under the two condi- tions. Because of the delayed development of the rainfed crops (De Silva and De Costa 2004 and De ALC DE SILVA AND WAJM DE COSTA :VARIATION IN PHYSIOLOGICAL CHARACTERS OF SUGARCANE 100 Variety Pn (μmol CO2 m -2 s-1) El (mol H2O m-2 s-1) Pn/El (μmol CO2 mol-1 H2O transpired) Irrigated Rainfed Irrigated Rainfed Irrigated Rainfed SL88116 22.4±3.4 20.4±0.5 1.8±0.3 2.4±0.1 12.3±0.2 8.7±0.4 Co775 28.3±2.7 21.7±0.5 3.3±0.1 1.8±0.3 8.6±0.6 13.4±2.9 SL8306 17.9±1.5 24.2±1.3 1.9±0.0 2.1±0.2 9.6±1.0 11.7±0.5 SL8613 8.4±1.5 23.3±1.4 0.9±0.0 2.5±0.1 9.6±1.6 9.1±0.2 SL7130 17.5±0.8 28.4±0.6 1.4±0.4 2.4±0.1 14.0±4.0 12.0±0.7 M438/59 12.0±0.9 27.6±0.3 0.8±0.1 3.2±0.1 16.5±3.0 8.5±0.3 SL7103 21.1±1.8 23.8±1.7 1.5±0.3 2.6±0.0 15.2±2.4 9.0±0.6 SLI121 18.7±3.1 26.9±0.7 2.2±0.3 2.7±0.1 8.4±1.2 10.0±0.6 Mean 18.3±1.3 24.5±0.6 1.7±0.2 2.5±0.1 11.8±0.9 10.3±0.5 LSDv 6.0 3.0 0.6 0.4 6.1 3.3 LSDw 1.7 0.2 1.7 Note: LSDv = LSD (p=0.05) for varietal comparisons within a water regime; LSDw = LSD (p=0.05) for comparison of mean values between water regimes. Table 5 Mean net photosynthetic rate (Pn), instanta- neous transpiration rate (El) and transpiration effi- ciency (Pn/El) ± standard error at 256 DAP in differ- ent sugarcane varieties under irrigated and rainfed conditions. Variety Pn (μmol CO2 m -2 s-1) El (mol H2O m-2 s-1) Pn/El (μmol CO2 mol- 1 H2O transpired) Irrigated Rainfed Irrigated Rainfed Irrigated Rainfed SL88116 21.2±2.3 32.0±2.2 4.3±0.4 5.8±0.1 5.0±0.7 5.5±0.5 Co775 29.5±1.1 22.8±1.6 5.0±0.4 4.5±0.6 5.9±0.3 5.1±0.3 SL8306 21.4±0.3 29.3±1.3 4.3±0.1 5.8±0.1 5.0±0.1 5.1±0.2 SL8613 17.8±0.6 27.2±3.5 3.2±0.1 5.6±0.6 5.5±0.3 4.9±0.1 SL7130 15.0±2.8 30.4±0.9 3.5±0.4 5.7±0.2 4.2±0.3 5.4±0.3 M438/59 19.0±1.2 22.9±2.3 3.6±0.3 4.4±0.4 5.3±0.1 5.2±0.5 SL7103 22.0±2.3 14.2±1.6 4.3±0.2 3.1±0.1 5.2±0.8 4.5±0.4 SLI121 17.9±1.2 27.3±2.7 4.3±0.2 5.0±0.4 4.2±0.5 5.4±0.1 Mean 20.5±1.0 25.8±1.3 4.1±0.1 5.0±0.2 5.0±0.2 5.1±0.1 LSDv 5.1 6.9 0.8 1.2 1.4 1.0 LSDw 2.0 0.4 0.4 Note: LSDv = LSD (p=0.05) for varietal comparisons within a water regime; LSDw = LSD (p=0.05) for comparison of mean values between water regimes. Table 6 Mean net photosynthetic rate (Pn), instanta- neous transpiration rate (El) and transpiration effi- ciency (Pn/El) ± standard error at 340 DAP in differ- ent sugarcane varieties under irrigated and rainfed conditions. Tropical Agricultural Research & Extension 12(2):2009 Silva 2007), at the time of measuring photosynthe- sis, leaves of the rainfed crop were probably at a more metabolically active stage than the leaves of the irrigated crop, which were near maturation. Pn is at a maximum during the grand growth stage and tends to decrease during late grand growth and mat- uration phases (Gascho and Shih 1983). In agreement with current findings, Roberts et al. (1990) pointed out that the effect of the rainfall and recovery of all above physiological variables show a greater level of activity under rainfed condi- tions than observed in cane receiving regular irriga- tion. In the present study, Co775 recorded the high- est Pn, El and gs both at 256 and 340 DAP and high- est Pn/El at 340 DAP under irrigated conditions whereas it had the lowest gs and E1, and the highest Pn/El under rainfed conditions at 256. At 340 DAP; SL88116 recorded the highest Pn, El, gs and Pn/El under rainfed conditions. The lowest E1 and gs were observed in SL7103 under rainfed conditions. Moreover, varieties SL8306 (both at 256 and 340 DAP), Co775 (at 256 DAP) and SL88116 (at 340 DAP) showed greater Pn/El under rainfed condi- tions than irrigated conditions [(Tables 5 and 6) (Data of simultaneous measurements of gs at 256 and 340 DAP are not shown)]. Greater Pn/El could be an important trait in drought resistant varieties. The gs is a key parame- ter that control both Pn and El because of the central position of stomata in the leaf gas exchange path- way (De Costa 2000). Sensitivity of stomata to wa- ter stress contributes to drought tolerance of a vari- ety. More sensitive stomata could conserve more water until yield formation. Varieties with less sen- sitive stomata may be able to maintain Pn at a high- er rate and may produce a higher yield under inter- mittent drought but it does not persist for a long period (Ludlow and Muchow 1990). However, ac- curate determination of Pn, El, gs and Pn/El under field conditions are difficult because it is difficult to impose a specific level of water stress on plants under open field conditions. Above variables in sugarcane respond quickly to unpredictable rainfall that occurs at any time during the dry spells as it is an indeterminate type long duration crop. CONCLUSIONS The study showed that there is adequate varietal variation in the evaluated physiological characters under the different growing conditions. Great dif- ferences existed between irrigated and rainfed con- ditions in the characters and varied significantly with the time and stage of crop growth. Therefore, it is required to evaluate above characters at vary- ing levels of water stress on plants during grand growth stage in accurate determination of physio- logical response of determining yield and drought resistance. Among the eight varieties tested, there was no single variety in which all above characters performed at favourable levels under different con- ditions. Different characters were responsible for higher performances in different varieties under different conditions. Therefore, we recommend to use above characters which have shown significant correlations with cane yield under different condi- tions for hybridisation programmes to produce hy- brids in which several characters are combined at favourable levels for different sugarcane growing conditions in Sri Lanka. ACKNOWLEDGEMENTS This research was funded by the Sugarcane Re- search Institute of Sri Lanka. 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