Distribution of assimilates to each organ in rice [Oryza sativa] plants exposed to a low temperature at the ripening stage, and the effect of brassinolide on the distribution

Previously, we found that promotion of ripening by brassinolide(BR) in rice plants is attributable to the rapid translocation of assimilates from leaves to the panicle (Fujii and Saka, 1992). In this study, we examined the effect of BR treatment on the distribution aspects of carbohydrates (starch, sucrose, glucose and fructose) in upper organs including the panicle of the rice plant (cv. Nipponbare) grown in a low temperature condition(20 deg C in the daytime / 13 deg C at night) in a phytotron under natural daylight in summer. Rice plants in a greenhouse in summer were sprayed with BR(2*10**-8 M and 2*10**-9 M) twice at around the heading stage, and were then kept in the phytotron and in the greenhouse to examine the carbohydrate contents in leaf blades, leaf sheaths, culms, hulls and hulled grains at the milk-ripe and dough-ripe stages. At milk-ripe stage, the starch content in the plants grown in the greenhouse without BR treatment was overwhelmingly high in hulled grains with a low content in culm, leaf sheaths and hulls, whereas starch was hardly detected in the leaf blade. At the dough-ripe stage, starch was more accumulated in hulled grains. Sucrose content at the milk-ripe stage was low in hulls, but high in leaf sheaths and culms, especially in leaf blades. However, at the dough-ripe stage, a large amount of sucrose was accumulated in culms, and only a small amount in leaf blades. The content of glucose and fructose was 10 to 20% of that of sucrose in each organ, and did not differ so much between the two stages of ripeness. In the plants grown in the phytotron, the total amount of starch was markedly lower and the contents of sucrose, glucose and fructose were considerably higher than those in the greenhouse. The marked decrease of starch content in the phytotron was found in hulled grains while the content in leaf sheaths, culms and leaf blades increased. On the other hand, the increase of sucrose content resulted from the increase in vegetative organs with the decrease in hulled grains. An increase of glucose and fructose were also found in leaf organs at the milk-ripe stage. BR decreased the starch content in leaf sheaths and culms, and increased the contents of starch and sucrose in hulled grains in both temperature conditions. The contents of starch and sucrose in hulls were not influenced so much by BR. Neither did BR largely influence the contents of glucose and fructose. However, BR treatment strikingly increased the 1,000-grain weight and the percentage of ripened grains in the rice plant in both of the greenhouse and phytotron.