Changes in respiration,dry-matter production and its partition in rice (Oryza sativa L.) in response to water deficits during the grain-filling period

A simple, physiologically based model was devised and used for estimating the respiration rate and the overall conversion efficiency of photosynthate into the grain dry-matter in dehydrated plants relative to well watered controls. The model described mathematically the partitioning of assimilate produced by current photosynthesis and of assimilate stored previously between the grain and those plants parts other than grain (the “straw”). Using data obtained from the dry-matter analysis and CO₂ gas exchange measurements, the model gave us two independent estimates of the respiration rate and the overall conversion efficiency; one for the plants given a prolonged dark period and the other for those grown in a normal light and dark cycle. The rate of dark respiration increased with mild water stress: 4.3 mg g⁻¹ day⁻¹ in control plants with leaf water potential of around −0.4 MPa and 11.3 mg g⁻¹ day⁻¹ in dehydrated plants with leaf water potential of around −1 MPa, when both the control and dehydrated plants were left in the dark for ten days. Similar values were obtained for plants in a normal light and dark cycle: 5.6 in well watered and 8.1 mg g⁻¹ day⁻¹ in the stressed plants. Accordingly, the overall conversion efficiency (the ratio of grain dry-matter against the gross carbohydrate input to the construction and maintenance processes) was 0.7 to 0.8 for the well hydrated control and 0.4 to 0.5 for moderately dehydrated plants. With increasing water deficits, however, the respiration rate decreased: 4.8 mg g⁻¹ day⁻¹ when plants were severely stressed (below −3 MPa in midday leaf water potential). The decrease in straw dry weight alone overestimates dry-matter partition of the stored assimilate in the straw into grain by 20 to 30% in well watered plants and the error increases to 50 to 60% in more dehydrated plants.