Drought is a major constraint to rice (Oryza sativa L.) production in rainfed and poorly irrigated environments. Identifying genomic regions influencing the response of yield and its components to water deficits will aid our understanding of the genetic mechanism of drought tolerance (DT) of rice and the development of DT varieties. Grain yield (GY) and its components of a recombinant inbred population developed from a lowland rice and an upland rice were investigated under different water levels in 2003 and 2004 in a rainout DT screening facility. Correlation and path analysis indicated that spikelet fertility (SF) was particularly important for grain yield with direct effect (P=0.60) under drought stress, while spikelet number per panicle (SN) contributed the most to grain yield (P=0.41) under well-watered condition. A total of 32 quantitative trait loci (QTLs) for grain yield and its components were identified. The phenotypic variation explained by individual QTLs varied from 1.29% to 14.76%. Several main effect QTLs affecting SF, 1,000-grain weight (TGW), panicle number (PN), and SN were mapped to the same regions on chromosome 4 and 8. These QTLs were detected consistently across 2 years and under both water levels in this study. Several digenic interactions among yield components were also detected. The identification of genomic regions associated with GY and its components under stress will be useful to improve drought tolerance of rice by marker-aided approaches.

The role of calcium (Ca) in the cycle of phosphorus in water environment depends on the speciation of Ca in sediments, and reliable Ca sequential extraction method is useful for obtaining information about Ca transport processes from sediment to water. Experiments have been carried out to develop a simple but robust Ca sequential extraction method for identification of Ca portioning in sediments. The sequential extraction protocol was modified for extractants, extracting time, concentration and pH of the reagents in the first three steps of Tessier sequential procedure. With optimum conditions, there were 1:25 sediment to solution ratio and 3 h of extracting time shaken with NH 4 AC (1 mol L -1 , pH 7.0) (exchangeable Ca fraction); 1:25 sediment to solution ratio and extracted with NH 4 AC (1 mol L -1 , pH 5.0) for 5 h (carbonatebound Ca fraction); and 1:30 sediment to solution ratio and extracted with NH 2 OHÁHCl (0.5 mol L -1 , pH 1.5) for 6 h (Fe and Mn oxides bound Ca fraction). The recovery for Tessier procedure ranged form 35.6 to 71.1 %, while the recovery for the modified Tessier procedure ranged form 46.1 to 86.9 %. These results revealed that Tessier procedure underestimated the proportion of Ca in exchangeable fraction. In sediments, Ca was mainly exchangeable and carbonate bound, which was considered as the most liable fractions.