THE EFFECTS OF CULTIVATION DENSITY ON THE PERCENT OF HEAD MILLED RICE AND SOURCE TO SINK RATIOS OF MID-SEASON HYBRID RICE IN EASTERN AND SOUTHERN SICHUAN PROVINCE
川东南高温伏旱区杂交中稻超稀栽培对稻米整精米率的影响与组合间库源结构的关系

Many studies on grain quality as affected by rice cultivation density have been conducted on one or two rice varieties, but there are few studies on varieties of hybrid midseason-rice. The hybrid midseason-rice grown in southern and eastern Sichuan Province has very low grain quality, especially the percentage of head milled rice, because of high temperatures and summer droughts. This paper explores the effects of low density cultivation on the percentage of head milled rice and source to sink ratios of varieties of hybrid midseason-rice, and it provides a theoretical and practical basis for improved cultivation. The experiment was conducted with eighteen mid-season rice hybrids grown at two densities, a low density cultivation and a traditional cultivation system, in a randomized block design with 4 replications using two hybrids (a big-panicle type hybrid `Gangyou 22' and a small-panicle type hybrid `II you 7') with 3 different treatments of grain or leaf cut. By using variance, correlation and regression analysis, the results indicated that the percentage of head milled rice increased in hybrids with small-medium panicles under low density cultivation but not for the hybrids with big-panicles. This was because of the significant decrease in the leaf to grain ratio and grain filling rate of hybrids for small-middle panicle under low density cultivation, as compared to the traditional cultivation system, resulting in an increase in the grain unit weight and percentage of head milled rice. The leaf to grain ratio and the grain filling rate of big panicle hybrids were low in traditional cultivation, and decreased significantly under low density cultivation, causing a decrease in the grain unit weight and percentage of head milled rice. These results were produced because of an imbalance in the leaf to grain ratios. Hybrids with less than 177 spikelets per panicle under cultivation using traditional densities can adapt to the low density cultivation in the eastern and southern regions of Sichuan Province.