QTL ANALYSIS TO STUDY THE ASSOCIATION BETWEEN LEAF SIZE AND ABSCISIC-ACID ACCUMULATION IN DROUGHTED RICE LEAVES AND COMPARISONS ACROSS CEREALS

Plants accumulate abscisic acid (ABA) under droughted conditions. Gene tic variation in the accumulation of ABA in detached and partially deh ydrated leaves of rice has previously been reported, and this was foun d to be associated with variation in leaf size (smaller leaves made mo re ABA). Correlation analysis failed to distinguish clearly between a causal relationship between the two traits and close genetic linkage b etween loci controlling the traits. Here we present a detailed genetic analysis of ABA accumulation in detached and partially dehydrated ric e leaves, using a population of F-2 plants generated from the lowland x upland cross IR20 (high-ABA) x 63-83 (low-ABA) which was mapped with RFLP and AFLP markers. Several highly significant quantitative trait loci (QTLs) for ABA accumulation and leaf weight were identified. Only one of the minor QTLs for ABA accumulation (accounting for only 4% of the phenotypic variance) was coincident with any QTLs for leaf size s uch that the high-ABA allele was associated with smaller leaves. This analysis, therefore, showed that the association previously found betw een ABA accumulation and leaf size was probably largely due to genetic linkage and not to a direct effect of leaf size on ABA accumulation o r vice versa. Because of the importance of ABA accumulation in regulat ing responses of plants to drought stress and the effects of plant siz e on the rate of development of stress, QTLs for drought-induced ABA a ccumulation, leaf size and tiller number were compared between rice an d wheat. In particular, a possible location in rice was sought for a h omoeologue of the major wheat vernalization responsive gene, Vrn1, as this gene is also associated with major effects on leaf size, tiller n umber and ABA accumulation in wheat. The likelihood of homoeologous lo ci regulating ABA accumulation, leaf size and tiller number in the two crops is discussed.