Carbon and Nitrogen Transport during Grain Filling in Rice Under High-temperature Conditions

Rice (Oryza sativa cv. Koshihikari) seedlings were grown in a sandy dune soil in pots with a basal dressing of N (0.5 g N), P and K. Two N treatments were applied, a +N treatment in which a top dressing of N-15-labeled 0.5 g N was supplied on July 20 and a -N treatment in which no additional fertilizer was supplied. During the grain-filling stage from August 6 to 13, plants were subjected to one of three temperature treatments; controlled low temperature, LT (day/night 28/23 degrees C), controlled high temperature, HT (35/30 degrees C) and uncontrolled glasshouse temperature, UT (day/night averages, 38/26 degrees C). All plants were then exposed to (CO2)-C-13 for 1 h on August 11 in a growth chamber at 25 degrees C. On August 13, all plants were harvested and the C-13 and N-15 abundances and starch and sugar concentrations in the ears, shoots and roots were determined. The C-13 content of the ear was lower in UT than in LT irrespective of the +N or -N treatment. The translocation of N-15 to the ears was also slightly depressed in UT compared with LT. Under high-temperature conditions (HT and UT), the starch content per plant was reduced for -N, but for +N, it was not significantly different among the temperature treatments. A high accumulation of sucrose was observed in all plant parts under UT conditions. It is suggested that extreme high day temperatures during the grain-filling period may reduce starch synthesis in the grains and, especially so under N-deficient conditions. High temperatures also induce an accumulation of sucrose and a decrease in carbon and nitrogen transport from the shoots to the ears via the phloem.