Cultivar differences in the grain protein accumulation ability in rice (Oryza sativa L.)

AbstractThe demand for rice grain protein content (GPC) differs in different regions of the world. Despite large differences in GPC among cultivars, evaluation of the effects of genotype on GPC is difficult because GPC is influenced not only by cultivar traits (such as nitrogen uptake ability, sink size and heading date) but also by the environment. We hypothesized that grain protein accumulation ability (GPA) also affects GPC. The objective of this study was to clarify the differences in GPA among six lodging-tolerant, high-yielding Japanese cultivars: Bekoaoba, Habataki, Takanari, Hokuriku193, Momiroman, and Akenohoshi. To produce a wide variation in nitrogen availability per unit sink capacity (Nav), we used nitrogen topdressing at heading and spikelet-thinning treatment. In each cultivar, we found a logarithmic relation between GPC and Nav: GPC=A×Ln(Nav)+B, where A is the regression coefficient and B is a constant. A highly significant difference in regression coefficients among cultivars was found (P<0.01). The regression coefficient was considered to be a measure of GPA; it varied from 0.969 in Bekoaoba to 1.820 in Takanari. This relation suggests that GPC is determined by Nav and GPA and that the environment affects GPC through Nav. GPA is a good criterion for evaluation of the effects of genotype on GPC. Nitrogen harvest index was highly significantly explained by multiple regression with GPA and the ratio of sink capacity to total dry matter production as independent variables, suggesting the influence of GPA on plant nitrogen dynamics during the grain-filling period. Therefore, it would be useful to determine the cultivars’ GPA values for optimizing nitrogen management

A quantitative measure for assimilate partitioning efficiency in rice (Oryza sativa L.)

AbstractGrain filling in rice is determined by the balance between sink size and source supply. Partially filled or unfilled spikelets is produced when source supply per spikelet is insufficient. Cultivars with high assimilate partitioning efficiency, which produce fewer partially filled spikelets, would be preferable because the grains of partially filled spikelets have low market value. However, it is unclear whether there is a genotypic difference in assimilate partitioning efficiency, and there is no quantitative measure for this parameter. The objective of this study was to develop such a measure from the relation between grain-filling percentage (GFP) and percentage of filled spikelets (PFS) of cultivars with various grain-filling abilities. GFP is a dry-weight-based grain filling degree, i.e. the ratio of total hulled grain yield to the sink capacity. PFS is a spikelet-number-based grain filling degree, i.e. the proportion of the number of filled spikelets to the total number of spikelets per area. We investigated the factors associated with partitioning efficiency based on grain weight distribution during grain filling. We also determined the filling potential of empty spikelets, which are indistinguishable from unfertilized ones, under limited assimilate supply. Cultivars with a large (indica-dominant Takanari, japonica-dominant Momiroman) or moderate sink size (indica Kasalath, japonica Koshihikari) were grown in a paddy field and defoliated at full heading to limit assimilate supply. Grain weight distribution was investigated at 10, 20, and 30days after anthesis. Yield, yield components, and shoot dry matter were measured at maturity. We found an exponential relation between GFP and PFS, which can be transformed to Ln(PFS)=B +A×GFP, with regression coefficients A varying among cultivars. A lower regression coefficient shows a higher assimilate partitioning efficiency and production of fewer partially filled spikelets. In indica and indica-dominant cultivars, which have lower regression coefficients, the difference in the weight of fast- and slowly-growing grains was larger than in japonica and japonica-dominant cultivars, suggesting an association between assimilate partitioning efficiency and the initial grain growth rate of fast-growing superior spikelets. We found that empty spikelets caused by low assimilate supply were fertilized and had a filling potential. In conclusion, the results suggest the regression coefficient between GFP and Ln(PFS) differs among cultivars with different grain filling abilities, and could be used as a quantitative measure of assimilate partitioning efficiency in rice breeding

Effects of Assimilate Supply and High Temperature during Grain-Filling Period on the Occurrence of Various Types of Chalky Kernels in Rice Plants (Oryza sativa L.)

The objective of this study was to clarify the effect of assimilate supply and high temperature on the occurrence of chalky kernels, i.e. milky white, white back, basal white and white belly kernels. Rice cultivars Koshihikari and Takanari, contrasting in number of spikelets in a panicle were used. After heading, sink-source manipulation was imposed on plants, through changing supply of assimilates to spikelets by shading or panicle clipping. Plants with each sink-source manipulation were subjected to temperature treatments, i.e., high temperature and ambient temperature, using a temperature-gradient chamber. Percentage of various types of chalky kernels was examined with the treatment for each cultivar. High temperature treatment increased milky white and white back kernels while no significant effect of temperature was observed on the percentage of white belly and basal white kernels. Effects of sink-source manipulation on the occurrence of chalky kernels varied with the type of chalky kernels. Although sink-source manipulation had no effect on white back and basal white kernels, it had a significant effect on the percentage of milky white and white belly kernels, which indicates the association of assimilate supply with the occurrence of these types of chalky kernels. A close hyperbolic relation was observed between the rate of assimilate supply and the percentage of milky white kernels, suggesting that milky white kernels are caused by assimilate deficit during the initial half period of grain filling. The higher percentage of milky white kernels at a given rate of assimilate supply at a high temperature implied that the high temperature during the grain-filling period increases the assimilate demand to avoid the occurrence of milky white kernels. This is presumably because the high temperature during the grain-filling period accelerates grain growth especially in inferior spikelets

Dynamics of Abscisic Acid Levels during Grain-Filling in Rice : Comparisons between Superior and Inferior Spikelets

In a rice panicle, the superior spikelets display higher growth rates than the inferior spikelets during the initial phase of grain-filling. To better understand the regulatory mechanism of this phenomenon, we examined the dynamics of endogenous abscisic-acid (ABA) levels and the effects of shading on the dynamics during the grain-filling period in superior and inferior spikelets. While ABA content in the superior spikelets increased rapidly after flowering, that in the inferior spikelets increased slowly and reached the maximum later than in the superior spikelets. Shading significantly exaggerated the inherently different ABA dynamics between the superior and inferior spikelets. We concluded that ABA levels are correlated with the dry matter accumulation patterns of spikelets. These results support our hypothesis that ABA is involved in assimilate partitioning among spikelets in a panicle during the initial phase of grain-filling of rice

The effects of nitrogen uptake before and after heading on grain protein content and the occurrence of basal- and back-white grains in rice (Oryza sativa L.)

Chalky rice (Oryza sativa L.) grains are induced by high temperature during the grain-filling period. Plant nitrogen status also affects the occurrence of basal- and back-white grains (BBWG). The objective of this study was to elucidate the relations between nitrogen availability per spikelet during the grain-filling period (NGF) and each of the percentage of BBWG and grain protein content (GPC). We further compared the effect of the components of NGF determined before heading (NBH) and after heading (NAH) on BBWG and GPC. We grew the rice cultivar ‘Koshihikari’ in pots in 2012 and 2013, and top-dressed nitrogen at the panicle formation and heading stages, under two (2012) or three (2013) temperature regimes during the grain-filling period. GPC was explained well by NGF, but BBWG was not. BBWG was best explained in a multiple-regression equation by mean air temperature after heading and by NBH and NAH. The partial correlation coefficients for NBH were 1.6 and 3.0 times those for NAH in 2012 and 2013, respectively. On the other hand, in a multiple-regression equation for GPC, the partial correlation coefficients for NBH were 0.91 and 0.71 times those for NAH in 2012 and 2013, respectively. These results suggest that rice grains are most sensitive to plant nitrogen status before heading for BBWG but after heading for GPC, and that there is an optimal timing for nitrogen top-dressing that would maximize the reduction in BBWG per unit increment of GPC

Effects of high temperature and shading on grain abscisic acid content and grain filling pattern in rice (Oryza sativa L.)

Abscisic acid (ABA) is a key factor regulating starch biosynthesis genes and is involved in assimilate partitioning to individual spikelets. The objective of this study was to clarify the effects of high temperature and shading during grain filling on grain ABA content and the grain filling pattern of spikelets located at different positions in a panicle. We grew the rice cultivar ‘Koshihikari’ in pots in 2009 under two temperature regimes and two light conditions during grain filling. We periodically measured grain dry weight and grain ABA content (pmol per grain) and concentration (pmol per grain dry weight). Shading increased a grain weight difference between superior and inferior spikelets while high temperature decreased the difference regardless of light condition. High temperature decreased ABA content and concentration in grains. There was a close correlation between mean grain ABA content and mean grain-filling rate averaged over the first half of grain filling

Varietal Difference in the Occurrence of Milky White Kernels in Response to Assimilate Supply in Rice Plants (Oryza sativa L.)

We examined the association of assimilate supply in the occurrence of milky white kernels in three cultivars with different percentages of milky white kernels in the field condition: ‘Hatsuboshi’, ‘Koshiibuki’ and ‘Koshihikari’. Five days after heading, the plants were placed in four controlled-environment chambers with either a high or low night temperature and elevated or normal [CO2] supply. Plants in each chamber were either defoliated with only flag leaf remaining, flag leaf and second leaf remaining or left intact (control). The percentage of each type of chalky kernel was examined. The percentage of milky white kernels was increased by defoliation and decreased by elevated [CO2], associated with assimilate supply. No association was observed between assimilate supply and white back or basal white kernels. The percentage of milky white kernels was negatively correlated with assimilate supply at a high night temperature in all cultivars. At a low night temperature, there was a clear threshold of assimilate supply, over which the percentage of milky white kernels was nearly zero. Cultivar differences were observed in the relation between the percentage of milky white kernels and assimilate supply. In conclusion, we found a varietal difference in the occurrence of milky white kernels in response to assimilate supply. In the cultivars used in this study, ‘Hatsuboshi’ was more sensitive to the low assimilate supply than ‘Koshihikari’