Productivity of rice plant in relation to photosynthesis, photorespiration and translocation

Leaf photosynthesis, respiration and translocation of photosynthates for grain filling were studied in eleven rice (Oryza saliva L.) varieties, in an attempt to find the basis of varietal difference in rice plant productivity. Leaf photosynthetic rates as measured by ¹⁴CO₂ feeding to the standing crops at flowering showed considerable varietal differences. The low yielding tall varieties were not necessarily less efficient than the high yielding dwarf varieties. However, no direct correlation of relative photosynthetic efficiency with either the grain yield or the biological yield in different varieties was observed. The higher glycollate oxidase activity and also a higher ¹⁴CO₂ outburst from the ¹⁴CO₂-fed flag leaves of tall varieties indicate their higher photorespiratory activity compared with the dwarf ones. Althouth the contribution of preflowering reserve carbohydrates for grain filling is appreciable in tall varieties, they are inferior in transporting the post-flowering photosynthates to the developing grains compared to the high yielding dwarf ones.