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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 (6 o 21'N latitude, 80 o 48'E longitude and 76 m altitude) using eight sugarcane (Saccharum hybrid L.) varieties grown under irrigated and rainfed conditions in a split plot design. Stomatal conductance (g s), instantaneous transpiration rate (E 1) and photosynthetic rate per unit leaf area (P n) were measured. Canopy stomatal conductance (g c), instantaneous canopy transpiration rate (E c) and transpiration efficiency (P n /E 1) was calculated. The behaviour of g s in many respects to the moisture availability and growing stage was similar to the responses seen in P n. Water deficit significantly reduced g s , E 1 and P n. Recovery of g s and P n from water stress with rainfall was quite rapid under rainfed conditions. The varieties Co775, SL8306, SL7103 and SL88116 which had higher P n and P n / E 1 , and lower g s , E 1 , g c and E c 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 responsible for drought tolerance of sugarcane.

The objective of this study was to investigate the effects of soil moisture deficits on the growth of root system and its effect on soil moisture utilization in sugarcane to identify relationship of root growth with drought tolerance of sugarcane varieties. An experiment with eight sugarcane varieties under rain-fed and irrigated conditions in a split-plot design was conducted from 2002 to 2007 at the Sugarcane Research Institute, Uda Walawe, Sri Lanka (6°21 0 N, 80°48 0 E). Root length densities (RLD) and soil moisture contents at different depths of the 1 m soil profile were measured. The variety SL 88 116 showed the highest RLD values of 1.49 in top (0-30 cm), 0.33 in middle (30-60 cm) and 0.65 cm cm -3 in entire (0-100 cm) layers of soil profile, and SL 83 06 showed the highest RLD of 0.14 cm cm -3 in the bottom (60-100 cm) layer of soil profile under rain-fed conditions. RLD of all varieties except SL 88 116 under irrigation were significantly (P \ 0.05) greater (15-63%) than under rain-fed conditions. The rain-fed cane yield showed a significant (P \ 0.05) positive correlation with RLD in the middle soil layer. Varieties with higher root length densities in the 30-60 cm soil layer survive better during significant water deficit periods in the top soil layer (0-30 cm), and such varieties produce high sugarcane yields in the rain-fed environments of Sri Lanka.

Increased atmospheric CO 2 and consequent increases in temperature are two prominent features of climate change, a major challenge to crops. Here, our objectives were to determine: (1) the responses of sugarcane during the first 90 days of elevated CO 2 (ECO 2 ) and elevated temperature (ETem), both individually and together, and (2) the genotypic variation of these responses. Eight varieties were grown both in open-top chambers in a factorial combination of ambient/ECO 2 concentrations (344-351/777-779 ppm) and ambient/ETem (34.9-35.6/36.6-38.4 C) and in open fields. Significant treatment  variety interaction effects were observed on leaf net photosynthetic rate (A n ), stomatal conductance (g s ), transpiration rate (E l ), and instantaneous transpiration efficiency (T E ). In most varieties, ECO 2 alone did not affect A n, but the combination of ECO 2 and ETem decreased A n . ECO 2 decreased g s and E l while increasing T E in all varieties. These effects were amplified when ETem was combined with ECO 2 . ETem alone had variable effects on A n and g s depending on variety, while it increased E l and did not affect T E in a majority of varieties. Germination, tillering and stem diameter were not affected by treatments and did not show varietal variation. Leaf water potential, chlorophyll (spad), leaf area, and aboveground dry weight per plant showed varietal variations but were not affected by treatments. The variable responses to ETem and the significant genotypic variation to ECO 2 and elevated temperature (ETem) observed in this work, both individually and together, demonstrate a considerable scope to breed sugarcane varieties for a future high-CO 2 and warmer climate.

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