Growth Responses of Rice Plant to Environmental Conditions : I. The effects of air-temperatures on the growth at vegetative stage

Abstract

A japonica var. Norin-17 and a indica var. IR-8 were grown in the growth chambers at 35-30, 30-25, 25-20, 20-15, 15-10°C day (12 hr)-night (12 hr) temperatures, respectively, under natural light intensity. Three times of sampling were taken at 10 days interval. The results obtained are as follows. 1) Plant height and leaf emergence rate were higher with the higher temperatures. The optimum temperature for tillering was a little lower than for plant height and dry matter production, the tillering ability (tiller number/leaf number of main stem) being highest at 25-20°C. 2) The dry matter production was greatest at 30-25° at the early growth period for both varieties, being followed by 25-20, 35-30, 20-15 and 15-10°C in order. At the later stages, the growth at 25-20° gradually increased, subsequently becoming highest in IR-8. 35-30° seemed to be too high for growth and the lower temperatures than 20- 15° significantly decreased growth. In the regime of this experiment, the optimum for growth were 25-30° day and 20-25° night temperatures. 3) T/R and LB/LS ratios decreased with the decrease of temperature, with the exception of 15-10° where T/R ratio was higher. 4) Leaf number and leaf area per plant changed parallel with the dry matter production. The density thickness of leaf increased with decrease of temperature. The dry matter percentage of leaf was low at the optimum temperature for growth. 5) RGR of rice plant was controlled firstly through RLGR and secondly through NAR, both being declined with the advancement of growth especially sharply when the temperature increased. 6) The growth of IR-8 was greater than Norin-17, probably due to its greater seed size and greater seedling at the start of experiment, since RLGR and NAR during the temperature treatments were not significantly higher in the former variety.