Photosynthesis of the rice plant under water stress
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Leaf-resistance values sharply decreased after sunrise, remained low during daytime, and increased sharply toward sunset. This general pattern was modified by weather conditinos 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 equaivalent 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 photosynthetic 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^<-1>) for both water-stressed and well-watered crops on a cloudy day when solar radiation was 200 cal・cm^<-2>・day^<-1>. On a sunny day, when solar radiation was 600 cal・cm^<-2>・day^<-1>, 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・m^<-2>・day^<-1> under flooded conditions and only about 10 g・m^<-2>・day^<-1> 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.
- Soil Science and Plant Nutrition
Soil Science and Plant Nutrition 22(2), 169-180, 1976-06-26
Japanese Society of Soil Science and Plant Nutrition