Heterosis for Photosynthesis, Dry Matter Production and Grain Yield in F₁ Hybrid Rice (Oryza sativa L.) from Thermo-Sensitive Genic Male Sterile Line Cultivated at Different Soil Nitrogen Levels

Abstract

This study was conducted to determine the effects of soil nitrogen levels (0.22, 0.44 and 0.88gN pot^-1) on the heterosis for photosynthetic characters, dry matter production and grain yield in F₁ hybrid rice obtained from thermo-sensitive genic male sterile line. Net CO₂ exchange rate (CER) and other related characters viz., SPAD reading, leaf N content, soluble protein content and ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activity increased significantly with increasing N level in all the F₁ hybrid and parental cultivars. With increasing N level, the number of tillers, leaf area, root weight, wholeplant dry weight and grain yield also increased. Under the low N level, the F₁ hybrid did not show heterosis for most photosynthetic characters. However, at high N level, heterosis for CER over both the male parent and the mean of parents was ascribed to high leaf N content and several characters related to CER. The F₁ hybrid exhibited substantial heterosis over the male parent for whole-plant dry weight, number of panicles per plant, number of spikelets per panicle and grain yield in all N levels. Among the yield components, the strengthening of heterosis for number of filled grains per panicle was the major cause of greater heterosis for grain yield in the F₁ hybrid at high N level.