抄録
In order to identify the factors which have played a significant role in the increase of rice yield, comparative growth analysis was attempted for several cultivars of rice plant which have been cultivated in the past or currently being cultivated in the south-western part of Japan (Kyushu district). Dry matter production of old and new types of rice cultivars was compared in the field. The old types refer to the ear-weight-type with long culms, vs. the new types or ear-number-type with short culms. The results obtained are as follows: During the growth period before heading, dry matter production was higher in thc old types than in the new types of cultivars (Table 2). This difference was due to the production of dry matter during the ear initiation stage. Dry matter increase was positively correlated with canopy height in this stage (r=0.858**) (Fig. 2). Further analysis indicated that the canopy of the old types consisted of lower leaf area density as compared to that of the new types of cultivars (Fig. 3). As a result, dry matter increase showed a significant negative correlation with the leaf area density (r=-0.852**) (Fig. 4), suggesting a possible correlation between CO2 diffusion and a vertical distribution of leaf area in the canopy. During the growth period after heading, dry matter increase showed a significant positive correlation with the leaf area index (LAI) and net assimilation rate (NAR)(Fig. 5). The new types which showed higher LAI and NAR produced a larger amount of dry matter as compared to the old types of cultivars (Fig. 5). Also, the new types had higher nitrogen content in the leaf blade and lower decrease in thc photosynthetic rate (p0) and leaf area during ripening stage than the old types of cultivars (Fig. 6, Table 4). The old types of cultivars experienced lodging during the middle and later ripening stages whereas red-kernel rice, regarded as a very old type of cultivar, showed lodging at heading, due to the presence of a long culm. In order to increase dry matter production and hence grain yield, the role of canopy height should be reevaluated in relation to dry matter production both before and after heading.
