The effects of cross-tolerance to oxidative stress and drought stress on rice dry matter production under aerobic conditions

Oxidative damage occurring in plant cells under drought stress is a known cause of reduced plant primary production. Decreasing oxidative damage through oxidative stress tolerance is expected to confer drought stress tolerance. In this study, we estimated cross-tolerance to oxidative stress and drought stress for breeding populations and analyzed the effects of the cross-tolerance on dry matter production in field experiments. For a total of 91 rice genotypes, including 72 backcross lines (BCLs), cross-tolerance was estimated from the first principal component score (PCS1) derived from a principal component analysis using a data set with a parameter of chlorophyll fluorescence and cell membrane stability index in both the oxidative and the drought stress treatments as the factors. The higher cross-tolerance was represented by the higher PCS1, and generally the values of PCS1 were segregated in the BCLs, suggesting that cross-tolerance is a heritable trait that can be improved by crossbreeding. The effects of positive and negative PCS1 on dry matter production under flooded and aerobic conditions were tested in field experiments. The decrease in dry matter production under aerobic conditions was smaller for the positive PCS1 genotypes. However, these genotypes also showed a lower stomatal conductance and smaller shoot biomass, especially under flooded conditions. We concluded that cross-tolerance is a useful trait for improving dry matter production, especially under severe drought stress. In view of the trade-offs between cross-tolerance and dry matter production, it is important to develop rice varieties with an optimal level of cross-tolerance for a target environment characterized by drought stress

Mild drying of sandy soil can physically limit the uptake of phosphorus by rainfed lowland rice in northeast Thailand

<p>Poor response of rice to phosphorus (P) fertilization and low phytoavailability of soil P have been reported in sandy rainfed fields in northeast Thailand. In order to evaluate the effects of mild soil drying on the uptake of P by rainfed lowland rice, we carried out nutrient omission trials for nitrogen (N) and P at Ubon Ratchathani Rice Research Center under rainfed and flooded conditions. The surface soil was classified as sandy loam. To avoid severe soil drying and drought stress in the rainfed field, soil water potential at a depth of 20 cm was maintained at the field capacity (> −20 kPa) by flush irrigation. The effects of flooding and drying on the soil properties were also evaluated in the laboratory using soils with diverse textures in and around the center. In the field experiments, the above-ground biomass of rice plants (RD6) did not respond significantly to P fertilization in the rainfed field, although it responded positively to N fertilization. Root length in the surface 10 cm under the rainfed condition was significantly smaller than that under the flooded condition due partly to the increased soil hardness upon drying, but this could not quantitatively explain the large discrepancy of P uptake observed between the rainfed and flooded conditions. Under the rainfed condition, the P uptake did not increase significantly, even when the concentration of soil Bray P was tripled by transferring the surface soil from the flooded to the rainfed field. From the laboratory experiments, it was further suggested that soil P was supplied mainly by diffusion and that the effective diffusion coefficient for P can become less than one-tenth of the value in the flooded field when the sandy soil with clay at around 10% dried to −100 kPa. Our results suggest that the uptake of P by the rainfed lowland rice grown in sandy soil can be limited physically by mild soil drying that reduces the supply of P to roots by diffusion rather than the chemical extractability of soil P.</p

Magnaporthe oryzae populations adapted to finger millet and rice exhibit distinctive patterns of genetic diversity, sexuality and host interaction

In this study, host-specific forms of the blast pathogen Magnaporthe oryzae in sub-Saharan Africa (SSA) were characterised from distinct cropping locations using a combination of molecular and biological assays. Finger millet blast populations in East Africa revealed a continuous genetic variation pattern and lack of clonal lineages, with a wide range of haplotypes. M. oryzae populations lacked the grasshopper (grh) element (96%) and appeared distinct to those in Asia. An overall near equal distribution (47–53%) of the mating types MAT1-1 and MAT1-2, high fertility status (84–89%) and the dominance of hermaphrodites (64%) suggest a strong sexual reproductive potential. Differences in pathogen aggressiveness and lack of cultivar incompatibility suggest the importance of quantitative resistance. Rice blast populations in West Africa showed a typical lineage-based structure. Among the nine lineages identified, three comprised ~90% of the isolates. Skewed distribution of the mating types MAT1-1 (29%) and MAT1-2 (71%) was accompanied by low fertility. Clear differences in cultivar compatibility within and between lineages suggest R gene-mediated interactions. Distinctive patterns of genetic diversity, sexual reproductive potential and pathogenicity suggest adaptive divergence of host-specific forms of M. oryzae populations linked to crop domestication and agricultural intensificatio