Photosynthesis of the rice plant under water stress

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Abstract
Leaf-resistance values sharply decreased after sunrise, remained low during daytime, and increased sharply toward sunset. This general pattern was modified by weather conditions and soil-water stress. On a cloudy day, leaf resistance remained low (3-5 s/cm) during daytime regardless of the soil-water regime. No increase in leaf resistance equivalent to “midday closure of stomates” was observed under flooded conditions on a sunny day. Under water stress and on a sunny day, leaf resistance began to increase in the morning or afternoon, depending on the degree of water stress. Both the light intensity at which the leaf photosyn-thetic rate reached light-saturation and the maximum value of the photosynthetic rate became lower as water stress became more severe. A simulation model revealed that crop photosynthesis was almost identical (17-18 g·m-2·day-l) for both water-stressed and well-watered crops on a cloudy day when solar radiation was 200 cal·cm-2day-l. On a sunny day, when solar radiation was 600 cal·cm-2day-l, however, crop photosynthesis under water stress reached its maximum at about 8 a.m., after which it declined with time and became almost zero after 12 noon. Under flooded conditions, diurnal changes in crop photosynthesis were similar to those of incident solar radiation. As a result, crop photosynthesis was about 33 g·cm-2day-l under flooded conditions and only about 10 g·cm-2day-l under water stress. Thus, crop photosynthesis under water stress was greater by 70% on a cloudy day than on a sunny day. This suggests that a partly shaded environment favors crop growth under water stress.