Estimation of Photosynthetic Induction Is Significantly Affected by Light Environments of Local Leaves and Whole Plants in Oryza Genus.

Photosynthetic induction and stomatal kinetics are acknowledged as pivotal factors in regulating both plant growth and water use efficiency under fluctuating light conditions. However, the considerable variability in methodologies and light regimes used to assess the dynamics of photosynthesis (A) and stomatal conductance (gs) during light induction across studies poses challenges for comparison across species. Moreover, the influence of stomatal morphology on both steady-state and non-steady-state gs remains poorly understood. In this study, we show the strong impact of IRGA Chamber Illumination and Whole Plant Illumination on the photosynthetic induction of two rice species. Our findings reveal that these illuminations significantly enhance photosynthetic induction by modulating both stomatal and biochemical processes. Moreover, we observed that a higher density of smaller stomata plays a critical role in enhancing the stomatal opening and photosynthetic induction to fluctuating light conditions, although it exerts minimal influence on steady-state gs and A under constant light conditions. Therefore, future studies aiming to estimate photosynthetic induction and stomatal kinetics should consider the light environments at both the leaf and whole plant levels.

Increased panicle nitrogen application improves rice yield by alleviating high-temperature damage during panicle initiation to anther development.

The grain yield is closely associated with spikelet fertility in rice (Oryza sativa L.) under high temperatures, and nitrogen (N) plays a crucial role in yield formation. To investigate the effect of panicle N application on yield formation under high temperatures at the panicle initiation stage, two rice varieties [Liangyoupeijiu (LYPJ, heat susceptible) and Shanyou63 (SY63, heat tolerant)] were grown and exposed to high daytime temperature (HT) and control temperature (Control) during the panicle initiation stage. Low (LPN) and high (HPN) panicle N applications were conducted. HT markedly decreased the yields by 87% at LPN and 48% at HPN in LYPJ and 31% at LPN and 36% at HPN in SY63. The decrease in grain yield under HT was primarily attributed to the decline in spikelet fertility, HPN increased spikelet fertility. HT resulted in the abnormal development of anthers, which included disordered, enlarged, and broken anther wall layers, degraded and irregularly shaped microspores, delayed tapetum degradation, less vacuolated microspores per locule, abnormal and aborted pollen grains; however, HPN improved the development of anthers under HT, particularly in LYPJ. A high rate of evapotranspiration resulted in an approximately 1°C decrease in panicle temperatures at HPN compared with that at LPN in both varieties under HT. Overall, these results demonstrate that the increased panicle N application favors normal anther development in LYPJ by decreasing the panicle temperature, which results in high pollen viability and spikelet fertility, and consequently less yield loss under HT.

Ratoon Rice Cropping Mitigates the Greenhouse Effect by Reducing CH4 Emissions through Reduction of Biomass during the Ratoon Season.

The ratoon rice cropping system (RR) is developing rapidly in China due to its comparable annual yield and lower agricultural and labor inputs than the double rice cropping system (DR). Here, to further compare the greenhouse effects of RR and DR, a two-year field experiment was carried out in Hubei Province, central China. The ratoon season showed significantly lower cumulative CH4 emissions than the main season of RR, the early season and late season of DR. RR led to significantly lower annual cumulative CH4 emissions, but no significant difference in cumulative annual N2O emissions compared with DR. In RR, the main and ratoon seasons had significantly higher and lower grain yields than the early and late seasons of DR, respectively, resulting in comparable annual grain yields between the two systems. In addition, the ratoon season had significantly lower global warming potential (GWP) and greenhouse gas intensity-based grain yield (GHGI) than the main and late seasons. The annual GWP and GHGI of RR were significantly lower than those of DR. In general, the differences in annual CH4 emissions, GWP, and GHGI could be primarily attributed to the differences between the ratoon season and the late season. Moreover, GWP and GHGI exhibited significant positive correlations with cumulative emissions of CH4 rather than N2O. The leaf area index (LAI) and biomass accumulation in the ratoon season were significantly lower than those in the main season and late season, and CH4 emissions, GWP, and GHGI showed significant positive correlations with LAI, biomass accumulation and grain yield in the ratoon and late season. Finally, RR had significantly higher net ecosystem economic benefits (NEEB) than DR. Overall, this study indicates that RR is a green cropping system with lower annual CH4 emissions, GWP, and GHGI as well as higher NEEB.

Varietal responses of root characteristics to low nitrogen application explain the differing nitrogen uptake and grain yield in two rice varieties.

Rice root characteristics are tightly associated with high-efficient nitrogen uptake. To understand the relationship of root plastic responses with nitrogen uptake when reducing nitrogen application for green rice production, a hydroponic experiment and a soil pot experiment were conducted under high (HN) and low (LN) nitrogen applications, using two rice (Oryza sativa L.) varieties, NK57 and YD6, three nitrogen absorption traits (total nitrogen accumulation, net NH4 + influx on root surface, nitrogen uptake via apoplasmic pathway) and root characteristics were investigated. In comparison with HN, LN significantly reduced nitrogen absorption and grain yield in both varieties. Concomitantly, there was a decrease in total root length, root surface area, root number, root volume, and root cortical area under LN, while single root length, root aerenchyma area, and root lignin content increased. The expression of OsAMT1;1 and OsAMT1;2 down-regulated in both varieties. The findings revealed that YD6 had smaller reduction degree for the three nitrogen absorption traits and grain yield, accompanied by smaller reduction degree in total root length, root surface area, root cortical area, and expression of the two genes under LN. These root characteristics were significantly and positively correlated with the three nitrogen absorption traits and grain yield, especially under LN. These results indicate that a large root system, lower reduction degree in several root characters, and high expression of OsAMT genes in YD6 explains its high nitrogen accumulation and grain yield under reduced nitrogen application. The study may provide rationale for developing varieties with low nitrogen fertilizer requirements for enabling green rice production.

Heat Stress Decreases Rice Grain Weight: Evidence and Physiological Mechanisms of Heat Effects Prior to Flowering.

Heat stress during the preflowering panicle initiation stage seriously decreases rice grain weight in an invisible way and has not been given enough attention. The current review aims to (i) specify the heat effects on rice grain weight during the panicle initiation stage compared with the most important grain-filling stage; and (ii) discuss the physiological mechanisms of the decreased rice grain weight induced by heat during panicle initiation in terms of assimilate supply and phytohormone regulation, which are key physiological processes directly regulating rice grain weight. We emphasize that the effect of heat during the panicle initiation stage on rice grain weight is more serious than that during the grain-filling stage. Heat stress during the panicle initiation stage induces alterations in endogenous phytohormones, leading to the inhibition of the photosynthesis of functional leaves (source) and the formation of vascular bundles (flow), thus reducing the accumulation and transport of nonstructural carbohydrates and the growth of lemmata and paleae. The disruptions in the "flow" and restrictions in the preanthesis "source" tissue reduce grain size directly and decrease grain plumpness indirectly, resulting in a reduction in the final grain weight, which could be the direct physiological causes of the lower rice grain weight induced by heat during the panicle initiation stage. We highlight the seriousness of preflowering heat stress on rice grain weight, which can be regarded as an invisible disaster. The physiological mechanisms underlying the lower grain weight induced by heat during panicle initiation show a certain novelty because they distinguish this stage from the grain-filling stage. Additionally, a number of genes that control grain size through phytohormones have been summarized, but their functions have not yet been fully tested under heat conditions, except for the Grain Size and Abiotic stress tolerance 1 (GSA1) and BRASSINOSTEROID INSENSITIVE1 (OsBRI1) genes, which are reported to respond rapidly to heat stress. The mechanisms of reduced rice grain weight induced by heat during the panicle initiation stage should be studied in more depth in terms of molecular pathways.

Phloem Unloading in Developing Rice Caryopses and its Contribution to Non-Structural Carbohydrate Translocation from Stems and Grain Yield Formation.

Phloem unloading plays an important role in photoassimilate partitioning and grain yield improvements in cereal crops. The phloem unloading strategy and its effects on photoassimilate translocation and yield formation remain unclear in rice. In this study, plasmodesmata were observed at the interface between the sieve elements (SEs) and companion cells (CCs), and between the SE-CC complex and surrounding parenchyma cells (PCs) in phloem of the dorsal vascular bundle in developing caryopses. Carboxyfluorescein (CF) signal was detected in the phloem of caryopses, which showed that CF was unloaded into caryopses. These results indicated that the SE-CC complex was symplasmically connected with adjacent PCs by plasmodesmata. Gene expression for sucrose transporter (SUT) and cell wall invertase (CWI), and OsSUT1 and OsCIN1 proteins were detected in developing caryopses, indicating that rice plants might actively unload sucrose into caryopses by the apoplasmic pathway. Among three rice recombinant inbred lines, R201 exhibited lower plasmodesmal densities at the boundaries between cell types (SE-CC, SE-PC and CC-PC) in developing caryopses than R91 and R156. R201 also had lower expression of SUT and CWI genes and lower protein levels of OsSUT1 and OsCIN1, as well as CWI activity, than R91 and R156. These data agreed with stem non-structural carbohydrate (NSC) translocation and grain yields for the three lines. The nitrogen application rate had no significant effect on plasmodesmal densities at the interfaces between different cells types, and did not affect CF unloading in the phloem of developing caryopses. Low nitrogen treatment enhanced expression levels of OsSUT and OsCIN genes in the three lines. These results suggested that nitrogen application had no substantial effect on symplasmic unloading but affected apoplasmic unloading. Therefore, we concluded that poor symplasmic and apoplasmic unloading in developing caryopses might result in low stem NSC translocation and poor grain yield formation of R201.

On-farm comparison in grain quality between main and ratoon crops of ratoon rice in Hubei Province, Central China.

BACKGROUND: While ratoon rice has been increasingly practiced by farmers recently in China, on-farm performance in grain quality of main and ratoon crops in the mechanized rice ratooning system is less studied and remains poorly understood. Therefore, a multi-location on-farm survey was conducted to collect rice grain samples from farmers' fields to determine grain quality of main and ratoon crops of ratoon rice at 12 locations across Hubei Province, central China, in 2016. RESULTS: On average, milled and head rice percentage in the ratoon crop was 70.2% and 65.7%, which was significantly higher than in the main crop, whereas chalky grain percentage and grain chalkiness in ratoon crop (10.1% and 2.8%, respectively) were significantly lower than those in the main crop (36.6% and 14.2%, respectively). The differences in these quality traits between the two crops were consistent at all locations. Averaged across 12 locations, scores of translucency and gel consistency were significantly lower but amylose content and alkali spreading value were significantly higher in the ratoon crop than in the main crop, with the difference between the two crops varying in gel consistency by location. CONCLUSION: Overall, grain quality, especially milling and appearance of the ratoon crop, was superior to the main crop in the mechanized rice ratooning system. As a result, this study emphasizes the potential role of the rice ratooning system in other regions with a similar biophysical background producing high-quality rice. © 2022 Society of Chemical Industry.

Stem small vascular bundles have greater accumulation and translocation of non-structural carbohydrates than large vascular bundles in rice.

Phloem unloading and loading are associated with stem non-structural carbohydrates (NSCs) accumulation and remobilization in rice (Oryza sativa L.). Four rice recombinant inbred lines (R032, R191, R046, and R146) derived from a cross between Zhenshan 97 and Minghui 63 were used to investigate the contributions of stem large and small vascular bundles (SVBs) to NSCs accumulation and translocation. Before heading, the parenchyma cells in stem cortex tissues (PCs) surrounding SVBs had higher starch density than those surrounding large vascular bundles (LVBs). Moreover, the protein levels of sucrose transporters (SUTs), cell wall invertase, sucrose synthase, and adenosine diphosphate glucose pyrophosphorylase, as well as the phloem plasmodesma densities were higher in SVBs than those in LVBs. After heading, starch density decreased more in PCs surrounding SVBs than in LVBs. Also, the protein levels of SUTs, α-amylase, sucrose phosphate synthase and sucrose synthase, the phloem plasmodesma densities in SVBs were higher than those in LVBs. The correlations of the number and total cross-sectional area of SVBs with mass and contribution to yield of transferred NSCs were higher than those of LVBs. Our results suggest that SVBs may have higher contributions to pre-anthesis stem NSCs accumulation and post-anthesis translocation than LVBs, which is potentially attributed to the high level of protein and enzyme involved in stem unloading and loading via apoplastic and symplastic pathways.

Effects of contrasting N supplies on leaf photosynthetic induction under fluctuating light in rice (Oryza sativa L.).

Nitrogen (N) is one of the most important nutrients for crop growth and yield formation, as it is an important constituent in a large amount of proteins, cell walls, and membranes related to photosynthesis. Recently, increasing studies have suggested the important roles of photosynthetic induction and stomatal movement under fluctuating light in regulating plant carbon assimilation and water use efficiency. How leaf N content affects photosynthetic induction remains uncertain. Here, we observed a significantly faster photosynthetic induction with the increasing supply of N under fluctuating light conditions. Photosynthetic induction was mainly limited by biochemical processes but not stomatal opening after a stepwise increase in irradiance across different N supplies. Higher N supply enhanced photosynthetic efficiency under constant and fluctuating light conditions but reduced leaf intrinsic water use efficiency (WUEi ). This study is mainly focused on clarifying the crucial limitation of photosynthetic induction under different N treatments, which may facilitate the improvement of photosynthetic efficiency under complicated environments in the future.

Stomatal Ratio Showing No Response to Light Intensity in Oryza.

Stomata control carbon and water exchange between the leaves and the ambient. However, the plasticity responses of stomatal traits to growth conditions are still unclear, especially for monocot leaves. The current study investigated the leaf anatomical traits, stomatal morphological traits on both adaxial and abaxial leaf surfaces, and photosynthetic traits of Oryza leaves developed in two different growth conditions. Substantial variation exists across the Oryza species in leaf anatomy, stomatal traits, photosynthetic rate, and stomatal conductance. The abaxial stomatal density was higher than the adaxial stomatal density in all the species, and the stomatal ratios ranged from 0.35 to 0.46 across species in two growth environments. However, no difference in the stomatal ratio was observed between plants in the growth chamber and outdoors for a given species. Photosynthetic capacity, stomatal conductance, leaf width, major vein thickness, minor vein thickness, inter-vein distance, and stomatal pore width values for leaves grown outdoors were higher than those for plants grown in the growth chamber. Our results indicate that a broad set of leaf anatomical, stomatal, and photosynthetic traits of Oryza tend to shift together during plasticity to diverse growing conditions, but the previously projected sensitive trait, stomatal ratio, does not shape growth conditions.

Abnormal anther development leads to lower spikelet fertility in rice (Oryza sativa L.) under high temperature during the panicle initiation stage.

BACKGROUND: Decreased spikelet fertility is often responsible for reduction in grain yield in rice (Oryza sativa L.). In this study, two varieties with different levels of heat tolerance, Liangyoupeijiu (LYPJ, heat susceptible) and Shanyou63 (SY63, heat tolerant) were subjected to two temperature treatments for 28 days during the panicle initiation stage in temperature/relative humidity-controlled greenhouses: high temperature (HT; 37/27 °C; day/night) and control temperature (CK; 31/27 °C; day/night) to investigate changes in anther development under HT during panicle initiation and their relationship with spikelet fertility. RESULTS: HT significantly decreased the grain yield of LYPJ by decreasing the number of spikelets per panicle and seed setting percentage. In addition, HT produced minor adverse effects in SY63. The decreased spikelet fertility was primarily attributed to decreased pollen viability and anther dehiscence, as well as poor pollen shedding of the anthers of LYPJ under HT. HT resulted in abnormal anther development (fewer vacuolated microspores, un-degraded tapetum, unevenly distributed Ubisch bodies) and malformation of pollen (obscure outline of the pollen exine with a collapsed bacula, disordered tectum, and no nexine of the pollen walls, uneven sporopollenin deposition on the surface of pollen grains) in LYPJ, which may have lowered pollen viability. Additionally, HT produced a compact knitted anther cuticle structure of the epidermis, an un-degraded septum, a thickened anther wall, unevenly distributed Ubisch bodies, and inhibition of the confluent locule, and these malformed structures may be partially responsible for the decreased anther dehiscence rate and reduced pollen shedding of the anthers in LYPJ. In contrast, the anther wall and pollen development of SY63 were not substantially changed under HT. CONCLUSIONS: Our results suggest that disturbed anther walls and pollen development are responsible for the reduced spikelet fertility and grain yield of the tested heat susceptible variety, and noninvasive anthers and pollen formation in response to HT were associated with improved heat tolerance.

Estimating the yield stability of heat-tolerant rice genotypes under various heat conditions across reproductive stages: a 5-year case study.

Heat events during the reproductive stages of rice plants induce great yield losses. Cultivating heat-tolerant varieties is a promising strategy for guaranteeing grain security under global warming scenarios. Most heat-tolerant rice genotypes were identified under heat during the flowering stage, but it is unclear whether these currently screened heat-tolerant rice genotypes maintain stable high grain yields when heat stress occurs during the other reproductive stages. In the present study, two notable heat-tolerant rice cultivars, Nagina22 and Shanyou63, and one typical heat-sensitive cultivar, Liangyoupeijiu, were evaluated for their yield response and yield stability under heat treatments during the panicle initiation, flowering, and grain filling stages during 2010-2014. Our results revealed that rice cultivars respond differently to heat stress during different reproductive stages. Nagina22 was the most tolerant to heat stress during the flowering and grain filling stages but was susceptible during panicle initiation; Shanyou63 was the most tolerant to heat stress during panicle initiation and grain filling and was moderately tolerant to heat stress during the flowering stages. Genotype and genotype-by-environment interaction biplot yield analysis revealed that Shanyou63 exhibited the highest stability in high grain yield, followed by Nagina22, and Liangyoupeijiu exhibited stable low grain yield when experiencing heat stress across the three reproductive stages. Our results indicate that the heat tolerance of different rice cultivars depends on the reproductive stage during which heat stress occurs, and the effects manifest as reductions in grain yields and seed setting rates. Future efforts to develop heat-tolerant varieties should strive to breed varieties that are comprehensively tolerant to heat stress during any reproductive stage to cope with the unpredictable occurrence of future heat events.

A newly isolated bacterium Comamonas sp. XL8 alleviates the toxicity of cadmium exposure in rice seedlings by accumulating cadmium.

Cadmium (Cd) is a typical heavy-metal highly accumulating in crops and drinking water, thus posing a severe health risk for human health. In this study, we firstly isolated 24 Cd-resistant bacteria from the heavy-metals contaminated soil at Daye Iron Ore, in which Comamonas sp. XL8 exhibited a high resistance and strong accumulation capacity to Cd. After absorption, Comamonas sp. XL8 could biosynthesize intracellular Cd-nanoparticles (CdNPs), which has not been reported in characteristics of Comamonas genus before. We found that the gene expressions of cadA and bmtA related to Cd transportation and binding in strain XL8 were significantly upregulated with Cd exposure, suggesting that genes cadA and bmtA may contribute to the formation of CdNPs. Of particular note, the co-inoculation of Comamonas sp. XL8 and rice seedlings (Oryzae sativa L.) significantly decreased the oxidative stress-induced by Cd accumulation and subsequently alleviated toxicity of Cd exposure. Our results reveal the biochemical process of Cd accumulation in Comamonas sp. XL8 by forming CdNPs, showing that it has great potential for effective bioremediation of environmental Cd exposure.

Effect of Stomatal Morphology on Leaf Photosynthetic Induction Under Fluctuating Light in Rice.

Plants are often confronted with light fluctuations from seconds to minutes due to altering sun angles, mutual shading, and clouds under natural conditions, which causes a massive carbon loss and water waste. The effect of stomatal morphology on the response of leaf gas exchange to fluctuating light remains disputable. In this study, we investigated the differences in leaf stomatal morphology and photosynthetic induction across twelve rice genotypes after a stepwise increase in light intensity. A negative correlation was observed between stomatal size and density across rice genotypes. Smaller and denser stomata contributed to a faster stomatal response under fluctuating light. Plants with faster stomatal opening also showed faster photosynthetic induction and higher biomass accumulation but lower intrinsic water use efficiency ( i WUE) under fluctuating light. Moreover, stomatal morphology seemed to have less effect on the initial and final stomatal conductance, and there was a minimal correlation between steady-state and non-steady-state stomatal conductance among different rice genotypes. These results highlight the important role of stomatal morphology in regulating photosynthetic efficiency and plant growth under fluctuating light conditions. To simultaneously enhance leaf i WUE when improving the photosynthetic efficiency under fluctuating light, it may be necessary to take biochemical processes into account in the future.

A hot-blast warming facility for simulating global warming in low-stature crop systems and its application case to assess elevated temperature effects on rice in Central China.

BACKGROUND: To study the impact of climate warming on crops, it is crucial to have a warming equipment suitable for their field environment. A facility is needed that can provide suitable combinations of different temperatures at reasonable cost for large plots. RESULTS: Here, an additional field warming facility option named the hot-blast warming facility (HBWF), which comprised heaters, blowers, wind breaks, and a control board was developed. An application case based on HBWF was carried out to assess elevated temperature effects on rice in Central China during 2015 and 2016. We tested four elevated temperature treatments on four rice cultivars under paddy field conditions and measured yield and its components. Heating convection air directly, the facility could increase the temperature of the rice canopy up to 1-2 °C, which could properly simulate global warming. Considering the costs, the HBWF reduced the operating costs because of its relatively lower power consumption (0.164 kW/m2), which was 80% lower than that of Free Air Temperature Increase. Our results demonstrate that the HBWF could build a 25 m2 homogeneous heating area and had little effect on the relative humidity under a paddy field environment. Warming treatments significantly reduced the grain yield by 4.4-22.7% in 2015, and 30.8-61.9% in 2016, compared to the control. The main contribution to the significant decrease of the grain yields was the decrease in seed setting rate. Moreover, a reduction of 1000-grain weight led to the decline in grain yield. The increasing ranges of the temperature simulated by HBWF were stable in different years, however, whether the elevated treatments demonstrated significant difference on rice growth mainly decided by the basic atmospheric temperature (as the control) during the growth period. CONCLUSIONS: The new warming facility is suitable for field trials to assess elevated temperature combinations and provides an extra equipment option for use in elevated temperature research in the future.

Optimizing nitrogen management to balance rice yield and environmental risk in the Yangtze River's middle reaches.

Currently, the urgency of balancing rice production and environmental risk from nitrogen (N) fertilization is gaining scientific and public attention. As such, a field experiment was conducted to investigate the rice yield and the fate of applied-15N for Yangliangyou 6 (a two-line hybrid cultivar) and Lvdaoq 7 (an inbred cultivar) using 10 combinations of N rates and splitting ratios in the middle reaches of the Yangtze River. The results showed that N application primarily affected fertilizer N loss to the environment, followed by plant N absorption, but had little effect on grain yield. Generally, there was no significant increase in grain yield and N accumulation in the aboveground plant when N inputs surpassed 130 or 170 kg ha-1. Fertilizer N residue in soil peaked at approximately 48 kg ha-1 at an N rate of 170 kg ha-1 for both varieties; however, a sharp increase of fertilizer N loss occurred with further incrementally increasing N rates. Although a higher ratio of panicle-N fertilizer together with a lower ratio of tillering-N fertilizer at rates of 130, 170, and 210 kg ha-1 had no grain yield benefit, it promoted aboveground N accumulation and plant N accumulation derived from fertilizer, and it reduced the amount of N residue in soil and N loss to the environment. Overall, reducing tillering-N ratios and increasing panicle-N ratios at an N rate between 130 and 170 kg ha-1 using fertilizer rates of 90-0-40 kg ha-1 and 90-40-40 kg ha-1 N at basal-tillering-panicle initiation stages could reduce the adverse environmental risks of chemical N from rice production without sacrificing rice yield.

Low Nitrogen Application Enhances Starch-Metabolizing Enzyme Activity and Improves Accumulation and Translocation of Non-structural Carbohydrates in Rice Stems.

More than 4 billion inhabitants in Asia depend on rice for 35-60% of the calories consumed in their diets, but new rice cultivars frequently do not reach expected yields because of poor rice grain filling. Here, we quantified the activities of enzymes involved in starch metabolization in rice to investigate the mechanisms regulating the accumulation and translocation of stem non-structural carbohydrates (NSC) under different levels of nitrogen fertilizer application. A pot experiment was conducted using two rice cultivars, Liangyoupeijiu (LYPJ) and Shanyou63 (SY63), under high and low nitrogen applications. Compared with high nitrogen application (HN), low nitrogen application (LN) increased stem NSC concentration before the heading stage and NSC translocation during the grain filling stage; concomitantly, LN significantly shortened the active grain filling period and increased the grain filling rate in superior spikelets. Compared with the LYPJ cultivar, SY63 exhibited a higher grain weight, higher grain filling percentage, and higher stem NSC concentration before heading and greater NSC translocation after heading. During the period between panicle initiation and heading, the activities of adenosine diphosphate-glucose pyrophosphorylase (AGP), starch synthase (StS), and starch branching enzyme (SBE), all enzymes involved in starch synthesis, increased under the LN treatment and positively correlated with increases in stem NSC. During grain filling, the activities of enzymes involved in starch-to-sucrose conversion [α-amylase, ß-amylase, and sucrose phosphate synthase (SPS)] increased under the LN treatment and positively correlated with stem NSC remobilization. Overall, the investigated enzymes exhibited higher activities in SY63 than in LYPJ. Our results suggest that low nitrogen increases the activities of AGP, StS, SBE, α-amylase, ß-amylase, and SPS, leading to increased accumulation and remobilization of stem starch and NSC in SY63. We conclude that calculated reductions in nitrogen application and the choice of an appropriate cultivar may improve rice grain yields via enhanced stem NSC accumulation and translocation, thereby reducing the costs and increasing the sustainability of rice production.

The Effect of Storage Condition and Duration on the Deterioration of Primed Rice Seeds.

Seed priming is a successful practice to improve crop establishment under adverse environment. However, reduced longevity of primed rice (Oryza sativa L.) seeds during storage limited the adoption of this technique. Present study investigated the effect of temperature, relative air humidity (RH) and oxygen on the longevity of primed rice seeds in a range of 60 days storage. In addition, the biochemical and morphological mechanisms associated with deterioration of primed seeds during storage were explored. Three types of priming treated rice seeds and one non-primed control were stored under (1) low temperature-vacuum (LT-V), (2) room temperature-vacuum (RT-V), (3) room temperature-aerobic-low RH (RT-A-LH) and (4) room temperature-aerobic- high RH (RT-A-HH) for 0, 15, 30, 45, and 60 days. The results showed that storage of seeds under different conditions for 15-60 days did not influence the longevity of non-primed rice seeds. Meanwhile, the viability of primed rice seeds did not reduce when stored under LT-V, RT-V, and RT-A-LH, but was significantly reduced under RT-A-HH. Under vacuum condition, the increases of storage temperature (30°C) did not reduce the longevity of primed seeds. Likewise, the oxygen did not influence the longevity of primed rice seeds stored under low RH. Nevertheless, increase of RH significantly reduced the viability of primed seeds stored for 15-60 days. Reduced starch metabolism, the consumption of starch reserves in rice endosperms, the accumulation of malondialdehyde and the decreases of antioxidant enzyme activities might be associated with the deterioration of primed rice seeds during storage. In conclusion, storage of primed seeds under high RH condition beyond 15 days is deteriorative for germination and growth of rice. The primed rice seeds are recommended to store at vacuum or low RH or low temperature condition to ensure good crop establishment.

The Effect of Season-Long Temperature Increases on Rice Cultivars Grown in the Central and Southern Regions of China.

Rice production is challenged by the asymmetric increases in day and night temperatures. Efforts are required to improve our understanding of the impact of climate change on rice production. To this end, 2-year experiment was conducted to evaluate the response of mid-season rice growth in the central and southern regions of China to elevated temperatures. Four replicates of four widely planted indica rice cultivars (Huanghuazhan: HHZ; Shanyou63: SY63; Yangliangyou6: YLY6; Liangyoupeijiu: LYPJ) were subjected to four elevated-temperature treatments (control: ambient temperature; NW: night-time warming; DW: daytime warming; AW: all-day warming) generated by an open-top hot-blast system under field conditions. This apparatus causes an ~2°C increase in the rice canopy temperature. Of all the elevated-temperature treatments, AW was the most devastating treatment for all rice cultivars, negatively affecting nearly all of investigated parameters, including grain yield and its components, dry matter accumulation, biomass, and harvest index (HI). The AW treatment decreased the grain yield by 11-35% and 43-78% in 2015 and 2016, respectively. No significant reduction in the grain yield was observed in the DW and NW treatments in 2015. However, the grain yield was decreased in DW and NW treatments by 20-52% and 18-55%, respectively, in 2016. Furthermore, the temperature-driven degradation of pollen viability, the number of pollen grains adhering to the stigma and pollen germination on the stigma caused spikelet sterility and thereby decreased the grain yield. The YLY6 and SY63 cultivars performed better than the HHZ and LYPJ cultivars with respect to grain yield and its components in all elevated-temperature treatments in both years. However, 42.97 and 61.01% reductions still occurred for the SY63 and YLY6 cultivars, respectively, in the AW treatment in 2016. The above results suggested that the elevated temperature may cause a noteworthy reduction in the productions of these widely planted genotypes in central and southern regions of China. To ensure the security of rice production in this region in an expected global warming environment, currently planted varieties will need to be replaced by heat-resistant varieties in the future.

Limitation of Unloading in the Developing Grains Is a Possible Cause Responsible for Low Stem Non-structural Carbohydrate Translocation and Poor Grain Yield Formation in Rice through Verification of Recombinant Inbred Lines.

Remobilisation of non-structural carbohydrates (NSC) from leaves and stems and unloading into developing grains are essential for yield formation of rice. In present study, three recombinant inbred lines of rice, R91, R156 and R201 have been tested for source-flow-sink related attributes determining the nature of NSC accumulation and translocation at two nitrogen levels in the field. Compared to R91 and R156, R201 had lower grain filling percentage, harvest index, and grain yield. Meanwhile, R201 had significantly lower stem NSC translocation during grain filling stage. Grain filling percentage, harvest index, and grain yield showed the consistent trend with stem NSC translocation among the three lines. In comparison with R91 and R156, R201 had similarity in leaf area index, specific leaf weight, stem NSC concentration at heading, biomass, panicles m-2, spikelets per panicle, remobilization capability of assimilation in stems, sink capacity, sink activity, number and cross sectional area of small vascular bundles, greater number and cross sectional area of large vascular bundles, and higher SPAD, suggesting that source, flow, and sink were not the limiting factors for low stem NSC translocation and grain filling percentage of R201. However, R201 had significant higher stem and rachis NSC concentrations at maturity, which implied that unloading in the developing grains might result in low NSC translocation in R201. The results indicate that stem NSC translocation could be beneficial for enhancement of grain yield potential, and poor unloading into caryopsis may be the possible cause of low stem NSC translocation, poor grain filling and yield formation in R201.