Higher leaf area growth rate contributes to greater vegetative growth of F1 rice [Oryza sativa] hybrids in the tropics

The early vigor of hybrid rice (Oryza saliva L.) developed in temperate areas has been mainly attributed to its higher tillering rate. The objective of this study is to identify factors that contribute to greater vegetative growth of F sub1 hybrid rice recently developed in the tropics. We analyzed data from field experiments conducted from 1994 to 1997 in both dry and wet seasons at the International Rice Research Institute farm. Each experiment had one to four pairs of comparisons between best hybrids and best conventional varieties. Parameters used for comparison were tillering rate, leaf area growth rate (LAGR), and crop growth rate (CGR) between transplanting (TR) and midtillering (MT) and also between MT and panicle initiation (PI). In most cases, the hybrids exhibited greater CGR than the conventional varieties, either between TR and MT or between MT and PI or both. The difference in CGR was associated with the difference in LAGR between hybrids and conventional varieties. However, the tillering rate of hybrids was significantly lower than or equal to that of conventional varieties. Therefore, the higher LAGR, independent from tillering, contributed to greater CGR of recently developed F sub1 hybrid rice in the tropics. The difference in LAGR and CGR between hybrids and conventional varieties was not associated with tillering. The higher LAGR of the F sub1 hybrids than that of the conventional varieties could be attributed to their thinner leaf blades.