Our results demonstrate that photorespiration is strongly involved in NH(3) emission by rice leaves and suggest that differences in AER between cultivars result from their different GS activities, which would result in different capacities for reassimilation of photorespiratory NH(3). The results also suggest that NH(3) emission in rice leaves is not directly controlled by transpiration and stomatal conductance.

In rice (Oryza sativa L.), seeds exposed to heat stress during grain filling exhibit delayed germination because of DNA methylation levels at promoters of abscisic acid (ABA, a germination-inhibiting hormone) catabolism genes and α-amylase (starch-hydrolyzing enzyme) genes, affecting their expression levels. Cold atmospheric plasma is known as an innovative and sustainable energy that has positive effects on the growth and development of many plant species. We, therefore, treated seeds that matured under heat stress with cold plasma and found that subsequent germination was significantly restored; genes involved in ABA biosynthesis (OsNCED2 and OsNCED5) were downregulated, whereas genes involved in ABA catabolism (OsABA8′OH1 and OsABA8′OH3) and α-amylase genes (OsAmy1A, OsAmy1C, OsAmy3B, and OsAmy3E) were upregulated. Cold plasma treatment caused significant hypermethylation of the OsNCED5 promoter and hypomethylation of OsAmy1C and OsAmy3E promoters, which matched their expression patterns. We suggest that cold plasma treatment can significantly improve the germination of rice seeds affected by heat stress by affecting epigenetic regulation.

High temperature during grain filling considerably reduces yield and quality in rice (Oryza sativa L.); however, how high temperature affects seed germination of the next generation is not yet well understood. Here, we report that seeds from plants exposed to high temperature during the grain filling stage germinated significantly later than seeds from unstressed plants. This delay remained even after dormancy release treatments, suggesting that it was not due to primary seed dormancy determined during grain filling. In imbibed embryos of heat-stressed seeds, expression of abscisic acid (ABA) biosynthesis genes (OsNCEDs) was higher than in those of control seeds, whereas that of ABA catabolism genes (OsABA8′OHs) was lower. In the aleurone layer, despite no change in GA signaling as evidenced by no effect of heat stress on OsGAMYB gene expression, the transcripts of α-amylase genes OsAmy1C, OsAmy3B, and OsAmy3E were significantly down-regulated in heat-stressed seeds in comparison with controls. Changes in promoter methylation levels were consistent with transcriptional changes of ABA catabolism-related and α-amylase genes. These data suggest that high temperature during grain filling results in DNA methylation of ABA catabolism-related and α-amylase gene promoters, delaying germination of heat-stressed seeds.

Rice is believed to show photosynthetic symmetry between adaxial and abaxial leaf sides. To verify this, we re-examined dorsoventral asymmetry in photosynthesis, chlorophyll fluorescence and anatomical traits in flag leaves of two Oryza sativa cultivars that differ in nitrogen (N) response and in leaf angle: 'Akenohoshi', a cultivar that can adapt to low-N (LN), with low leaf angle (more erect leaves), and 'Shirobeniya', a cultivar that is unable to adapt to LN, with higher leaf angle. Plants were grown under standard-N (SN) and LN conditions. LN leaves of both cultivars became more erect than SN, but LN Akenohoshi still had more erect ones than Shirobeniya. Contrary to results of previous studies, leaves of both cultivars showed an asymmetry in photosynthetic rate between adaxial and abaxial sides (higher on the adaxial side) under SN. SN leaves of both cultivars showed lower susceptibility to photoinhibition on the adaxial side than on the abaxial side. However, leaves of Akenohoshi showed less asymmetry in these traits under LN than under SN, whereas leaves of Shirobeniya had similar degrees of asymmetry in these traits under both SN and LN. Both cultivars also showed dorsoventral asymmetry in anatomical traits of mesophyll tissue regardless of N level, but the degree of asymmetry was lower in LN Akenohoshi. These data reveal that rice leaves exhibit dorsoventral asymmetry in photosynthetic and anatomical features, and that the degree of asymmetry varies with cultivar and N level. It is suggested that lower leaf angles (particularly in Akenohoshi) in the presence of LN represent a light acclimation to prevent photoinhibition.

BackgroundHigh water use efficiency is essential to water-saving cropping. Morphological traits that affect photosynthetic water use efficiency are not well known. We examined whether leaf hairiness improves photosynthetic water use efficiency in rice.ResultsA chromosome segment introgression line (IL-hairy) of wild Oryza nivara (Acc. IRGC105715) with the genetic background of Oryza sativa cultivar ‘IR24’ had high leaf pubescence (hair). The leaf hairs developed along small vascular bundles. Linkage analysis in BC5F2 and F3 populations showed that the trait was governed by a single gene, designated BLANKET LEAF (BKL), on chromosome 6. IL-hairy plants had a warmer leaf surface in sunlight, probably due to increased boundary layer resistance. They had a lower transpiration rate under moderate and high light intensities, resulting in higher photosynthetic water use efficiency.ConclusionIntrogression of BKL on chromosome 6 from O. nivara improved photosynthetic water use efficiency in the genetic background of IR24.Electronic supplementary materialThe online version of this article (doi:10.1186/s12284-017-0158-1) contains supplementary material, which is available to authorized users.