Grain Filling In Rice Research Articles

Post-anthesis development of inferior and superior spikelets in rice in relation to abscisic acid and ethylene

The purpose of this study was to test the hypothesis that the interaction between abscisic acid (ABA) and ethylene may be involved in mediating the post-anthesis development of spikelets in rice (Oryza sativa L.). Two rice genotypes were field-grown, and the changes of ABA, ethylene, and 1-aminocylopropane-1-carboxylic acid (ACC) levels in spikelets during grain filling and their relationships with endosperm-division and grain-filling rates were investigated. The results showed that earlier-flowering superior spikelets exerted dominance over later-flowering inferior spikelets in endosperm cell-division and grain-filling rates. The two genotypes behaved the same. Later-flowering spikelets had higher levels of ethylene and ACC than earlier-flowering spikelets. The ethylene evolution rate was significantly and negatively correlated with the cell division and grain filling rates. By contrast to ethylene, later-flowering spikelets contained a lower ABA content/concentration and showed a low content ratio of ABA to ACC than earlier-flowering ones. The cell-division and grain-filling rates were significantly and positively correlated with both ABA contents and the ratio of ABA to ACC. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at an early grain-filling stage significantly increased endosperm cell division rate and cell number, grain-filling rate, and grain weight of inferior spikelets. Application of ethephon (an ethylene-releasing agent) or fluridone (an inhibitor of carotenoid synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene mediate endosperm cell-division and grain-filling in rice. A higher ratio of ABA to ethylene in rice spikelets is required to maintain a faster grain-filling rate.

Time-related mapping of quantitative trait loci controlling grain-filling in rice (Oryza sativa L.)

Grain-filling is a crucial process that determines final grain yield in rice (Oryza sativa L.). To understand the genetic basis of dynamics of grain-filling, quantitative trait locus (QTL) analysis was conducted using time-related phenotypic data on grain-filling collected from a population of 155 recombinant inbred lines (F12), derived from a cross between Milyang 23 and Akihikari. Two QTLs detected on chromosomes 8 and 12 were strongly associated with increased filling percentage per panicle. These QTLs were not linked with those controlling spikelet numbers per panicle. This result confers the possibility of improving grain-filling together with an enlargement of sink size. The QTL for filling percentage per panicle on chromosome 8 exactly overlapped that for non-structural carbohydrate (NSC) content in the culm and leaf sheaths during grain-filling, and the Milyang 23 allele associated with increased grain-filling percentage per panicle was associated with decreased NSC content. Therefore, this QTL may be directly involved in NSC translocation from the culm and leaf sheaths to panicle. In addition, the Milyang 23 alleles of QTLs associated with greater spikelet number per panicle on chromosomes 1 and 6 were also related with a reduction in NSC content in the culm and leaf sheaths during grain-filling. These results indicate that NSC dynamics during grain-filling is partly dependent on sink size. NSC accumulation in the culm and leaf sheaths at the heading stage was mainly controlled by different genetic regulations from NSC dynamics during grain-filling. Nitrogen dynamics during grain-filling may also be involved in carbohydrate dynamics.

Expression Patterns of Genes Encoding Carbohydrate-metabolizing Enzymes and their Relationship to Grain Filling in Rice (Oryza sativa L.): Comparison of Caryopses Located at Different Positions in a Panicle

In rice, caryopses located at the base of the panicle have a lower growth rate than those at the tip of the panicle. The former and latter types of caryopses are called inferior and superior caryopses, respectively. Taking the different growth rate into consideration, sugar status and the expression of genes encoding carbohydrate-metabolizing enzymes in inferior caryopses were compared with those in superior caryopses. During the first 5 d after flowering, superior caryopses elongated rapidly, but inferior caryopses did not. At this phase, inferior caryopses had a low ratio of hexose to sucrose, high activity of acid invertase and the absence of the expression of the genes encoding the above enzymes except for two isoforms of cell wall invertase, OsCIN4 and INV1, in comparison with superior caryopses. At the start of caryopsis elongation in both superior and inferior caryopses, the hexose/sucrose ratio increased accompanied by gene expression of vacuolar invertase (INV3), sucrose synthase (RSus1) and ADP-glucose pyrophosphorylase (AGP-L2: D50317). Furthermore, the genes related to endospermal starch accumulation were expressed highly with the decrease in the hexose/sucrose ratio after its peak. Based on the comparison of superior and inferior caryopses, the possible mechanism of grain filling in rice is discussed.

Remobilization of carbon reserves in response to water deficit during grain filling of rice

Delayed senescence in rice plants caused by heavy use of nitrogen results in much non-structural carbohydrate (NSC) left in the straw and leads to low harvest index. This study was designed to determine if a moderate water deficit imposed during grain-filling could enhance plant senescence and thus, lead to more remobilization of NSC stored in vegetative tissues to grain. Two lodging-resistant rice cultivars, Wuyujing 3 (japonica) and Yangdao 4 (indica), which usually show delayed senescence, were grown in the field. Two levels of nitrogen, either normal (5 g N m −2, NN) or high amounts (10 g N m −2, HN) were applied at heading. Two levels of soil–water potential ( ψ soil), well-watered (WW, ψ soil=0) and water deficit stressed (WS, ψ soil=−0.05 MPa), were imposed 9 days after anthesis (DAA) until maturity. Leaf water potential of both cultivars markedly decreased at mid-day under WS treatments but completely recovered by early morning. Chlorophyll (Chl) concentrations and photosynthetic rate (Pr) of the flag leaves declined faster under WS than under WW treatments, indicating that the water deficit enhanced senescence, while HN greatly delayed it. The reduction of NSC in the culm and sheath was substantially enhanced by the WS treatment, NSC remobilization being increased by 23.8–27.1% at NN and 19.6–36.7% at HN, respectively, compared to the WW treatment. The contribution of remobilized NSC to grain, partitioning of fed 14 C from the flag leaves into grain, and harvest index were all significantly increased by WS at either NN or HN. WS also shortened the grain-filling period by 2.9–5.5 days at NN and 5.7–7.4 days at HN, and increased grain-filling rate at NH and NN by 0.18–0.29, and 0.31–0.37 mg per day per grain, respectively, when compared to WW treatments. The percentage of ripened grains, grain weight and grain yield under WS treatments were not significantly reduced at NN, but increased significantly at HN. Water applied to WS during the water-withholding period was only 64–70% of that to WW treatments. We conclude that the early senescence induced by a moderate water deficit during grain-filling period can enhance the remobilization of stored assimilates and accelerate grain-filling of rice. Such enhancement may improve grain yield in cases where plant senescence is unfavorably delayed by heavy use of nitrogen. This practice would also contribute to water-saving in rice production.

Effects of Different Levels of Salinity on Grain-filling in Rice

Effects of Different Levels of Salinity on Grain-filling in Rice

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.

Effect of Water Stress on Grain Filling in Rice

Water stress, by affecting the rate of grain filling, decreased yield per panicle of rice by 29–40% in the small-seeded variety IR747(L) and by 19–32% in the large-seeded variety PP6R-13-12-3. It had little effect on the duration of grain filling. High nitrogen level increased culm+sheath and leaf blade weights substantially in both varieties, but decreased starch concentrations (determined for IR747(L) only) at flowering in the culm+sheath significantly. Relatively more starch remained in the culm+sheath of the water-stressed plants at maturity. The balance sheet suggested that plants grown at a low nitrogen level were more dependent on “reserve starch” and that the percentage of reserve starch that went to the panicle was higher in the stressed plants than in the unstressed plants. High nitrogen level increased the nitrogen concentration in the culm+sheath at flowering by about 80%. At harvest, the concentration decreased substantially irrespective of nitrogen levels and varieties. Relatively more nitro...

The Effects of Water Stress on the Grain-filling in Rice

Grain growth, translocation of assimilates to the ear and photosynthesis are important processes to contribute to the formation of grain during the period of grain-filling in cereal crops. The objective of this investigation was to identify, for rice, which of these three proecesses is most affected by the water stress developed in grain-filling Perlod. We grew rice plants (Oryza Sativa, L. cv. Nipponbare) singly in 4 litter pots under outdoor conditions. The pots were kept flooded untill 15 days after anthesis. Then we transferred plants in a vinyl-covered house and stopped watering half the plants (deslccated). The rest of the plants were kept watered (control). Just before treatment, plants had their tillers removed to have four culms of approximately the same age. Plant water status was determined in terms of relative water content (RWC), being supplemented by measurements of the stomatal aperture and the transpiration rate. Plants were harvested before and after treatment, and dry weight was determined for unhuIled rices, rachis, top internode, lower internodes, leaf blades, leaf sheaths, crowns and roots. Progressive decline in RWC was more evident in plants subjected to soil desiccation. The change in stomatal aperture and transpiration rate with time also indicate that more severe stress developed in desiccated plants. But, there was little difference in dry weight increase in unhulled rice over 10 day period between control and desiccated plants, though the increase of total dry matter was reduced significantly in desiccated plants. This was accompanied by the greater decrease in dry weight of non-grain parts in desiccated plants. We therefore conclude that the water stress during the rapid growth of grain has little effect on grain growth as well as translocation in rice, while it reduces the rate of dry matter production considerably.