Abstract
Hybrid rice is proved to be effective in increasing yield, but lodging, the permanent displacement of stems from the vertical instance, often limits its yield in high-yielding environments. The present study was conducted to determine the genotypic variation in lodging resistance and lodging related morphological traits among hybrid and inbred check varieties, and to identify the key morphological traits associated with lodging resistance. Lodging behavior of 16 rice genotypes including 12 hybrids and four inbreds were studied in randomized complete block design at the International Rice Research Institute farm during 2004 wet season (WS) and 2005 dry season (DS). Experiments were conducted to find out the effect of genotypes and nitrogen management in split-pot design to improve lodging resistance of hybrid rice during 2005 WS and 2006 DS. Micro plot experiments were conducted in randomized complete block design to find the effect of time of artificial lodging on yield reduction and the effect of silicon on lodging resistance of hybrid rice. Yield and yield components were determined at maturity. Growth analyses were conducted at different growth stages to determine crop growth rate (CGR), leaf area index (LAI), and specific leaf weight (SLW). Culm characters related to lodging were determined at 30 days after flowering and degree of lodging was determined at maturity. IR76712H produced the highest yield (7.6 t ha⁻¹), followed by IR75217H and Magat, which produced 7.5 t ha⁻¹ in WS; in DS IR79118H produced the highest yield (9.17 t ha⁻¹) followed by IR73855H and SL-8H, which produced 8.88 t ha⁻¹ and 8.78 t ha⁻¹, respectively. The hybrids exhibited greater CGR than the inbreds at any growth stages. The hybrids showed higher LAI, which might be attributed to greater CGR. The highest SLW was observed in the inbreds compared with the hybrids indicating that the hybrids have thinner leaves. Large genotypic differences in lodging related morphological traits were observed. Among these traits, dry weight unit⁻¹ length, breaking resistance and lodging index (bending moment / breaking resistance × 100) of lower internodes were significantly correlated with visual score of lodging. Several hybrids demonstrated high lodging resistance although their average plant height was over 120 cm. The lodging related morphological traits and visual score of lodging in these hybrids were comparable with some of the non-hybrid check varieties with strong lodging resistance. These results suggest that increases in dry weight unit⁻¹ length and breaking resistance of lower internodes are primary targets for reducing lodging index, thus improving overall lodging resistance of hybrid rice in a breeding program. In general, hybrids showed higher translocation of nonstructural carbohydrate (NSC) from culm and higher accumulation of NSC in the grain compared with inbred rice. Lower amount of nitrogen application at the early vegetative stage slightly increased the lower internode dry weight and subsequently increased internode breaking resistance. But nitrogen management in high yielding environment had little effects on the improvement of lodging resistance of hybrid rice. Grain yield was reduced when lodging occurred at early grain filling stage. Application of silicon improved some growth and some lodging characters of hybrid rice but it did not significantly increase the lodging resistance. These results suggest that the effective applicable approach for increasing lodging resistance in hybrid rice is to develop varieties with increased lower internode dry weight unit⁻¹ length with higher breaking resistance.