| Raising yield potential in rice depends largely on morphological improvement and heterosis utilization. Rates of grain yield advances through plant breeding programs have apparently declined in recent years. It may partly be because that understanding of morphological and physiological bases for crop growth and yield are rather limited. New information and methodologies such as QTL analysis are employed to approach the grain yield problem and search yield genes in cereals. In the study, a recombinant inbred population derived from ZS97x MH63 cross (F10, F11) was employed for morpho-physiological analyses and molecular dissection of yield-related traits in 1999 and 2000 at International Rice Research Institute (IRRI). The main results are as follows:1 Strong seedling vigor are desirable for crop establishment in rice. Germination rate and seedling growth were measured to quantify seedling vigor Total amylase activity, a-amylase activity, reducing sugar content, root activity, and seed weight were determined. Overall, correlation was observed between the seedling vigor traits (germination rate, seedling dry weight, and root maximum length etc) and physiological traits (amylase activity, root activity, reducing sugar content etc). QTL analysis reveals that the intervals of RZ403-C1087-RG393 on chromosome 3, C1447-M26-C246 and C734b-RG360-R3166 -R830 on chromosome 5, and C952-Waxy-C1496 on chromosome 6 are the four main chromosomal regions controlling seedling vigor. Several QTLs for amylase activities, reducing sugar content, and root activity were localized in the similar region as the QTLs for seedling vigor, respectively. The results show that it is common that single marker, QTL, and digenic interaction have pleiotropic effects, suggesting that pleiotropy of detected single marker, QTL, and digenic interaction should be the molecular basis for relationship among amylase and seedling vigor. Six interactions between QTLs within a trait were detected (one for root dry weight, one for total amylase activity, two for reducing sugar content, two for seed size).2 Plant height, tiller number, and heading date are agronomic traits of importance. We mapped quantitative trait locus (QTLs) for plant height (at seedling stage, at 35 days after transplanting, and at maturity stage) and tiller number (at 35 days after transplanting, and at maturity stage), and for heading date using a recombinant inbred (RI) population. Negative correlations between plant height and tiller number were observed at two growth stages (35 days after transplanting and at maturity stage). Heading date significantly correlated with tiller number and plant height at maturity. At p^O.Ol, a total of 65 single markers associated with studies traits but the seedling height were detected simultaneously in 1999 and 2000. Some markers were observed to be associated with at least two traits. A total of 32 QTLs were detected, 11 of which were identified simultaneously in two years. Several QTLs had pleiotropic effects. Three QTLs for seedling height were located in the similar regions with QTLs for 1000-grain weight, and significantly positive correlation between seedling height and 1000-grain weight were observed, suggesting that seedling height may be closely associated with seed size. The interval of RG424-RZ667 showed its large effects on heading date, plant height and tiller numbers at maturity. Delay of heading date for approximately 6 days due to the effect of the region may result in a reduction in living tiller numbers at maturity by one and an increase in ultimate plant height by approximately 6 cm. A total of 549 digenic interactions for all traits but the seedling height were detected simultaneously in two years by two-way ANOVA. Among them, 55 combinations had pleiotrpic effects by simultaneously affecting two or more traits. The results show that different QTLs and digenic interactions affect plant height and tiller number at different developmental stages, respectively, suggesting selective expression of genes for...
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