Diurnal and Developmental Changes in Canopy Gas Exchange in Relation to Growth in Transplanted and Direct-Seeded Flooded Rice

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Abstract
Transplanted and direct-seeded flooded rice were compared in a field experiment using identical planting density and geometry. Leaf area index (LAI), plant dry weight, and tiller number were determined at 7-14 d intervals from seeding to maturity. Canopy CO2 and H2O exchange were measured using a mobile depletion-chamber system which requires 1-1.5 minutes per measurement. The canopy CO2 assimilation rates confirmed the plant dry matter accumulation observed. Diurnal measurements of net CO2 assimilation and night respiration indicated a mild midday/afternoon depression that depended on the atmospheric vapour pressure deficit (VPD). Light response of canopy CO2 assimilation exhibited light saturation at full daylight when LAI was lower than 1. No light saturation was observed at higher LAI. Transplanting shock in transplanted rice reduced net assimilation rates and delayed foliage expansion and tillering by 15 days. Crop development was retarded by 7 days. Uninhibited growth of direct-seeded rice during the vegetative stage led to superior biological yield and tiller number at maturity while grain yield was equal to that of transplanted rice. Potential yield increase in direct-seeded flooded rice is discussed on the basis of growth kinetics and assimilate source/sink relationships.