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An F1 hybrid rice combination from a cross between a thermo-sensitive genic male sterility (TGMS) '103S' line and an upland rice 'IR17525' were investigated for physiological response traits under drought stress and recovery at the heading time by comparing with its parents. The F1 and parental plants were planted in pots in a net house at different times for synchronizing flowering. At the stage of 5-3 days before heading, water was withdrawn from the pot. When the soil water potential in the pot reached -60 kPa, pho- tosynthetic rate and dry matter accumulation were measured. Recovery traits including photosynthetic rate and dry weight of panicle were measured at 5 days interval until harvesting. The results showed that the photosynthetic rate was lower in the F1 hybrid than that in the parents during drought treatment, in contrary it was higher during recovery stage. The grain yield was maintained in the F1 hybrid under drought treatment as much as under the control condition, whereas it significantly reduced as drought treatment in the male parent. The grain yield was higher in the F1 hybrid than that in the male parent under the well- irrigated condition, and even much higher under drought treatment. It is suggested that such advantageous characters, including the shorter ripening duration, the longer panicle exertion, the higher recovery photo- synthetic rate as well as the faster of dry matter translocation into panicle at the ripening stage, contribute drought tolerance in rice. Thus, the F1 hybrid rice 103S/IR17525 would be a promising rice with high yield potential under drought stress.

The 57 strains of Xanthomonas oryzae pv. oryzae (Xoo) collected from rice growing countries of India, Indonesia, Malaysia, Thai, Taiwan and Philippines were characterized by using polymerase chain reac- tion fingerprinting and virulence analysis. The strains of Xoo were grouped into 13 races on the basis of their pathogenicity to international differential lines. Distribution of races was quite specific to the country. Three strains from India and two strains from Indonesia were virulent to cultivars containing the bacterial blight resistance gene xa5, while most strains from other countries were avirulent to xa5. The strains from India showed high virulence and broad range of pathogenicity, in contrast to those from Malaysia. Two vari- eties of containing Xa21 and xa5 gene, respectively, were resistant to almost Asian Xoo strains. Our Xoo collections from Asian countries were also divided into 4 genetic groups by clustering statistics on the basis of the results from PCR-based RFLP with IS1112 primers. A partial relationship was found among the genetic groups, countries and races, suggesting that strategies that target regional resistance breeding and gene deployment are feasible. The results of this study will facilitate the further understanding of the popu- lation structure of Xoo in Asia.

structed to infer the phylogenetic relationships of the materials. The stability of the nodes in the tree was tested by the bootstrap analysis. All accessions except two could be distinguished from each other by at least one DNA marker. The dendrogram revealed 2 well‐distinguished groups, namely, Group I and II. Group I corresponded Japonica. It was further divided into two subgroups Ia and Ib. The subgroup Ia contained Japanese Japonica accessions and all the accessions in the subgroup Ib were Myanmar accessions clearly differentiated from the subgroup Ia. Indica accessions were contained in Group II. It comprised small clusters. Bootstrap analysis was performed to determine the confidence levels of the forks in the dendrogram. In the resulting consensus tree, only 6 forks had bootstrap values above 80%. The grouping of the UPGMA tree and the majority‐rule consensus tree were comparable in the subgroups Ia, Ib and IIe. However, the Indica subgroups, IIa, IIb, IIc and IId were not significantly differentiated.