EFFECTIVE HEAT UNIT SUMMATION AND BASE TEMPERATURE ON THE DEVELOPMENT OF RICE PLANT .1. A METHOD DETERMINING BASE TEMPERATURE AND ITS APPLICATION TO THE VEGETATIVE DEVELOPMENT OF RICE PLANT

A new method determining base temperature (BT) was investigated, and temperature reaction of rice plant (o. sativa L.) in the vegetative phase was analized on the concept of the effective heat unit summation (EHUS). Periods of leaf emergence were observed on some rice varieties grown under three controlled temperatures. Relation between periods of a particular leaf emergence and temperature summations for the periods was expressed by a linear equation. In a temperature range in which the above relation is effective, constancy both of EHUS and BT was ascertained. In a linear regression equation which expresses a relationship between periods of development and temperature summations for the periods, BT and EHUS could be shown as the regression coefficient and the constant term, respectively. Theoretical BT and EHUS, therefore, can be determined quite easily in this manner if at least two temperature regimes are adopted in an experiment. This method was applied to the development of entire leaves on the stem. Theoretical BT for the lower leaves was 6∼7°C, but it increased toward the upper leaves reaching its maximum of 12°C for the uppermost leaf. Rice plants in later stage of vegetative growth therefore seemed to require higher temperatures to develop themselves. BT's of japonica upland rice were obviously lower than those of japonica lowland rice, suggesting their advantageous growth in cool conditions. Some experimental formulas were presented for the estimation of specific periods such as from the infant stage of seedling to heading and from the stage of reduction division in pollen mother cell to heading, using EHUS and BT as fixed values, and assumed mean air temperature and number of leaves on the main stem as variables. © 1990, CROP SCIENCE SOCIETY OF JAPAN. All rights reserved.