The effect of planting pattern on the rate of photosynthesis and related processes during ripening in rice plants

Abstract

Rice plants ( Oryza sativa L.) planted in such a manner that each hill contains one plant (pattern I) produce more dry matter than plants planted such that each hill contains three plants (pattern III), particularly during the ripening stage. As one of the causal factors of the difference in dry matter production, the rate of photosynthesis and related physiological processes were investigated in direct-sown rice plants cultivated in a submerged paddy field after heading. The rate of photosynthesis was similar at full heading for plants from patterns I and III. When the rate of photosynthesis decreased during the period of ripening, plants from pattern I maintained higher rates of leaf photosynthesis than plants from pattern III. This was primarily related to the maintenance of higher leaf photosynthetic activity in pattern I plants. Plants from pattern I maintained higher leaf levels of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and nitrogen during the ripening stage than plants from pattern III. Close liner relationships were observed between the levels of Rubisco and the rate of photosynthesis as well as between the levels of nitrogen and Rubisco. Nitrogen accumulation in aboveground parts and partitioning of nitrogen to leaves maintained significantly higher levels during the ripening stage in plants from pattern I than plants from pattern III. This resulted in the higher leaf levels of nitrogen in pattern I plants during the ripening stage. Plants from pattern I had a significantly greater number of crown roots and root length densities, and maintained relatively higher cytokinin fluxes from roots to shoot during the ripening stage than plants from pattern III. The free cytokinin, trans-zeatin was responsible for the larger total flux for the cytokinins in pattern I plants. These root characteristics may also contribute to the maintenance of the higher levels of Rubisco and nitrogen in leaves in plants from pattern I.