Breeding efforts to mitigate damage by heat stress to spikelet sterility and grain quality

Global warming is predicted to aggravate the risk of unstable crop production. It is of great concern that damage to rice spikelet sterility and grain quality will increase, resulting in yield and economic losses. To secure the global food supply and farmers’ income, the development of rice cultivars with heat resilience is a pressing concern. Regarding spikelet sterility, rice cultivars with heat tolerance at different growth stages have been identified in recent years. The early-morning flowering (EMF) trait is effective in heat escape because it shifts the time of day of flowering to earlier in the morning when it is cooler. Although varietal differences are very small, there are some genetic resources for EMF in wild rice accessions. Regarding heat-induced grain chalkiness, heat-tolerant japonica cultivars for mitigating white-back type of chalky grains (WBCG) were found. Quantitative trait loci for heat tolerance at flowering, EMF, and for WBCG in grain quality have been mapped on the rice chromosomes. Further genetic efforts have been successfully connected to the development of near-isogenic lines for each trait with tagged molecular markers. These breeding materials are quite unique and useful in facilitating marker-assisted breeding toward the development of heat-resilient rice in terms of spikelet sterility and grain quality.     

Research Progress on Heat Stress of Rice at Flowering Stage

Global warming has caused frequent occurrence of heat stress at the flowering stage of single-season rice in the Yangtze River region of China, which results in declines of spikelet fertility and yield in rice. Rice flowering stage is the most sensitive period to high temperatures, and therefore, the key for heat stress happening is the flowering stage coinciding with high temperature, which causes spikelet fertility decreasing in heat-sensitive varieties, and is the major factor for heat injury differences among various rice planting regions. With the development of rice breeding, temperature indexes for heat stress has been converted from daily maximum temperature of 35 °C to 38 °C with the stress duration of more than 3 d. During the flowering stage, anther dehiscence inhibition and low pollen shedding onto the stigma are two main reasons for spikelet fertility reduction under high temperatures. At panicle initiation stage, high temperatures aggravate spikelet degeneration, and destroy floral organ development. Various types of rice varieties coexist in production, and indica-japonica hybrid rice demonstrates the highest heat resistance in general, followed by indica and japonica rice varieties. In production, avoiding high temperature is the main strategy of preventing heat stress, and planting suitable cultivars and adjustment of sowing date are the most effective measures. Irrigation is an effective real-time cultivation measure to decline the canopy temperature during the rice flowering stage. We suggested that further study should be focused on exploring heat injury differences among different rice variety types, and innovating rice-planting methods according to planting system changes in rice planting regions with extreme heat stress. Meanwhile, high temperature monitor and warning systems should be improved to achieve optimal heat stress management efficiencies.     

Heat-Induced Floret Sterility of Hybrid Rice (Oryza sativa L.) Cultivars under Humid and Low Wind Conditionsin the Field of Jianghan Basin,China

Abstract

Projected global warming is expected to increase the occurrence of heat-induced floret sterility (HIFS) in rice. However, there are few field-scale studies that could aid in predicting the potential risks to rice yield and developing countermeasures against yield losses. The aim of this study was to elucidate the factors that induce floret sterility under high temperature conditions during the flowering season in the field condition in China. Studies were conducted in irrigated paddy fields with the regional hybrid-rice cultivars grown in Jianghan Basin where air temperature is not so high during the flowering season but HIFS frequently occurs. The microclimate, panicle temperature, floret sterility, pollination, and size of dehiscence formed at the base of anthers were investigated. Significant losses in seed set were observed under the high temperature condition. Although the maximum atmospheric temperature was approximately 35ºC, the relative humidity was very high (around 70% at the time of maximum temperature), with low wind speeds, occasionally below 1 m s¹. Under such conditions sunlit panicle temperature exceeded atmospheric temperature by as much as 4ºC. Moreover, the anthers of some cultivars exhibited short basal dehiscence, and the dehiscence length was positively correlated with the percentage of sufficiently pollinated florets (r=0.859, P<0.05, n=7) and with seed set (r=0.827, P<0.05, n=7) across the cultivars. The results suggest that the combination of hot, humid, and windless climatic conditions with short basal dehiscence of anthers induces HIFS in hybrid rice grown in this region.     

The Difference in Sterility due to High Temperatures during the Flowering Period among Japonica-Rice Varieties

Summary

The objectives of this study are to compare the floret sterility induced by a high temperature given in the daytime during the flowering period among nine japonica rice varieties, and to clarify the mechanism of the high-temperature-induced sterility. Nine japonica rice varieties were subjected to 35.0, 37.5 or 40.0°C day- temperature conditions (1000-1600) for six consecutive days using sun-lit phytotrons, and the percentage of fertility, pollination and germinated pollen grains on the stigmas were examined. The temperature that caused 50% sterility varied with the variety, and a difference of approximately 3.0°C was observed between the most tolerant and susceptible varieties. Under the 37.5°C day-temperature condition, the percentage of florets with 10 or more germinated pollen grains was roughly coincident with the percent fertility, but under the 40° C day-temperature condition, it was higher than the percent fertility. Many of the florets with less than 10 germinated pollen grains had less than 20 total pollen grains on their stigmas under both temperature conditions. From these results, we concluded that sure pollination under high-temperature conditions is an important factor and that the high-temperature tolerance of the processes following pollen germination is also required for fertility under excessively high temperature conditions.     

Species,ecotype and cultivar differences in spikelet fertility and harvest index of rice in response to high temperature stress

Spikelet fertility (seed-set) is an important component of yield that is sensitive to high temperature. The objectives of this research were (a) to quantify the effects of high temperature on spikelet fertility and harvest index of rice; (b) to determine if there were species, ecotype, and/or cultivar differences in response to high temperature; and (c) to understand the reasons for lower and/or differential spikelet fertility and harvest index of rice cultivars at high temperatures. Fourteen rice cultivars of different species (Oryza sativa and Oryza glaberrima), ecotypes (indica and japonica) and origin (temperate and tropical) were exposed to ambient and high temperature (ambient + 5 °C) at Gainesville, Florida. High temperature significantly decreased spikelet fertility across all cultivars, but effects varied among cultivars. Based on decreases in spikelet fertility at high temperature, cultivar N-22 was most tolerant, while cultivars L-204, M-202, Labelle, Italica Livorna, WAB-12, CG-14 and CG-17 were highly susceptible and cultivars M-103, S-102, Koshihikari, IR-8 and IR-72 were moderately susceptible to high temperature. There were no clear species or ecotype differences, as some cultivars in each species or within ecotypes of tropical and temperature origin were equally susceptible to high temperature (for example M-202 temperate japonica, Labelle tropical japonica, CG-14 O. glaberrima, and WAB-12 interspecific). Decreased spikelet fertility and cultivar difference at high temperature were due mainly to decreased pollen production and pollen reception (pollen numbers on stigma). Lower spikelet fertility at elevated temperature resulted in fewer filled grains, lower grain weight per panicle, and decreased harvest index. There is a potential for genetic improvement for heat tolerance, thus it is important to screen and identify heat-tolerant cultivars. Spikelet fertility at high temperature can be used as a screening tool for heat tolerance during the reproductive phase.     

Screening for Spikelet Fertility and Validation of Heat Tolerance in a Large Rice Mutant Population

A total of 10 000 M4 individuals in Jao Hom Nil (JHN) mutant population was treated with high temperature (40 ℃ to 45 ℃) during the day time (6 h) from the booting to the harvesting stages, and ambient temperature (33 ℃ to 35℃) was used as the control. The results of screening and yield trials found that the mutant line M9962 had a high spikelet fertility of 78% under heat stress. In addition, the other mutant lines, including M3181 and M7988, had a spikelet fertility of approximately 70%. However, the JHN wild type, Sin Lek, RD15 and RD33 had very low spikelet fertility of 34%, 14%, 9% and 4%, respectively. The lower spikelet fertility at an elevated temperature resulted in a dramatic decrease of filled grain and contributed to a loss in 100-grain weight. M9962 is a potential genetic stock for use in a heat tolerance breeding programme. In addition, spikelet fertility at high temperature was representative of heat tolerance and can be used as a screening trait for heat tolerance during the reproductive phase on a large scale.     

Modeling the effects of N application on growth,yield and plant properties associated with the occurrence of chalky grains of rice

The objective of this study was to propose a model for explaining rice responses to a wide range of N application rates in various growth attributes associated with the occurrence of chalky grains. We improved the sub-model for N uptake process of a previous rice model which was originally developed for explaining genotypic and environmental variations in the whole growth processes, considering the difference in the rate of N loss from the plant-soil system between indigenously supplied soil mineral N and fertilizer N. A total of 80 growth datasets of cultivar ‘Koshihikari’ grown at Shiga prefecture, Japan, in 2010 was utilized for the calibration and validation of the model. The rice growth model well explained the above-ground biomass growth (RMSD = 78.7 g m^?2) and rough dry grain yield (RMSD = 83.2 g m^?2) for the validation data-set, simultaneously. The simulated carbohydrate content available per single spikelet was negatively correlated with the observed percentage of the milky-white grain which includes white-cored grain (r = ?.77, p < .001) for all the data-sets of calibration and validation. On the other hand, the observed percentage of the sum of white-back and white-base grains was closely correlated with the simulated plant N content available per single spikelet (r = ?.59, p < .001). It was suggested that the present rice growth model would rationally explain the effects of N application on the occurrence of the chalky grains through the dynamic change of the carbohydrate content and plant N content available per single spikelet.     

Effects of the number of pollen grains on cold tolerance at the booting stage in rice lines with QTLs for cold tolerance

This study was conducted to clarify the relationship between the number of pollen grains per anther and spikelet fertility under low temperature conditions in the rice cultivars and lines including lines with quantitative trait loci (QTL) for cold tolerance, adapted to the Tohoku region of northern Japan. Cold-water treatment decreased anther length, the number of pollen grains per anther, and spikelet fertility in all cultivars and lines. The number of pollen grains was proportional to anther length in all cultivars and lines and under all temperatures. Spikelet fertility decreased with decreasing the number of pollen grains in cold-water treatments at 18.5 and 19.3?°C. ‘Ouu 415’, with the qLTB3 QTL for cold tolerance, had 28% more pollen grains and 9% higher spikelet fertility than the recurrent parent, ‘Hitomebore’, in the 18.5?°C cold-water treatment, suggesting that qLTB3 increased both parameters. Lines with the qCTB8 QTL for cold tolerance had significantly more pollen grains in two of the three years and significantly higher spikelet fertility in all three years in the 19.3?°C cold-water treatment, suggesting that qCTB8 reinforced cold tolerance. Ctb1, a proposed cold-tolerance QTL, had no effect on the number of pollen grains or spikelet fertility. In conclusion, the cultivars and lines with more pollen grains had a higher cold tolerance. Some of QTLs were inferred to increase the number of pollen grains and reinforce cold tolerance.     

Suppressive Effects of Low Seed-Soaking Temperatures on Germination of Long-Term-Stored Rice Seeds

Abstract

Abstract: We investigated the effects of soaking temperature and duration on the germinability of seeds of rice (Oryza sativa L., cv. Koganemochi, Gohyakumangoku, and Koshihikari) that had been stored for a long period. The germinability of the seeds soaked at 5ºC for 5 d was markedly lower than that of seeds soaked at 12ºC for 5 d. The germinability of the seeds soaked at 5ºC for 24 hr was not increased by subsequent soaking at 12ºC for 4 d. On the other hand, the germinability of the seeds soaked either at 12ºC for 24 hr or at 30ºC for 80 min was similar to that of seeds soaked at 12ºC for 5 d, even when followed by treatment at 5ºC. Thus, the soaking temperature during the first 24 hr was most important for the germination of rice seeds that had been stored for a long period. Western blotting analysis revealed characteristic expression patterns of α-amylase isoforms in cultivars correlating with the germinability after soaking at a low-temperature.     

优质籼型不育系浙农8010A的选育与应用

不育系浙农 ８０１０ Ａ 育性败育彻底、开花习性好、异交率高、配合力强,尤其是其低直链淀粉含量对改善杂交籼稻的米质有广阔的应用前景。     

Agronomic Performance of F1 Hybrids of Rice (Oryza sativa L.) in Japonica-Indica Crosses

Summary

Generally F₁ hybrids from crosses between japonica and indica rice show variable degrees of sterility. Previous studies revealed that among the indica rice, the Aus type has sexual affinity with japonica cultivars giving higher fertility in F₁ hybrids. Therefore, we made an effort to obtain higher grain yield by producing F₁ hybrids between japonica and Aus-type indica. A field experiment was conducted to investigate the heterosis in these F₁ hybrids for dry matter accumulation, spikelet fertility, yield and yield attributes, to find cross combinations with higher grain yield, and examined the relationships of grain yield with some agronomic characters. F₁ hybrids flowered earlier than their parent cultivars. Heterosis for dry matter accumulation was positive at the panicle initiation stage, at heading and at maturity. The average heterosis for number of panicles, grain and straw yield per plant, number of spikelets per panicle and 1000-grain weight was positive. A few F₁ hybrids showed positive heterosis for number of filled grains per panicle, spikelet fertility and harvest index; but the average heterosis for these traits was negative. Among the yield components, a higher number of panicles per plant and higher number of spikelet per panicle contributed to higher grain yield in F₁ hybrids. A significant and positive relationship existed between spikelet fertility and grain yield per plant. The higher grain yield was attributed to higher dry matter production rather than higher harvest index in F₁ hybrids, although low spikelet fertility limited yield potential in some F₁ hybrids.     

Enclosed stigma contributes to higher spikelet fertility for rice(Oryza sativa L.) subjected to heat stress

With global warming, rice plants may be subjected to heat stress more regularly during the heatsensitive flowering stage, causing spikelet sterility and grain yield loss.Stigma exsertion is considered to increase pollen reception and promote female reproductive success.The aim of this study was to investigate the role of stigma exsertion on spikelet fertility at high temperatures.Five rice cultivars(Liangyoupeijiu, Shanyou 63, Huanghuazhan, Nagina 22, and IR64) with differing degrees of stigma exsertion were cultivated and exposed to high temperature at anthesis.Heat-tolerant cultivars did not always show a high percentage of spikelets with exserted stigmas, and vice versa.Irrespective of the presence of more pollen grains on exserted stigmas, spikelets with exserted stigmas did not show greater spikelet fertility than spikelets with fewer exserted stigmas or hidden stigmas under heat stress.GA3 application augmented the percentage of spikelets with exserted stigmas;however, it did not increase spikelet fertility under heat stress.Spikelet fertility of whole panicles was negatively correlated with the percentage of spikelets with exserted stigmas, but positively with that with hidden stigmas.Viability of the hidden stigmas was less reduced than that of exserted stigmas under heat stress, suggesting that hidden stigmas have an advantage in maintaining viability.Heat stress delayed anther dehiscence and reduced the viabilities of both exserted stigmas and pollens, thereby causing low spikelet fertility.Together, these results suggest that high spikelet fertility does not depend on stigma exsertion and that enclosed stigma generally contributes to higher spikelet fertility and heat tolerance under high-temperature conditions during flowering in rice.     

Effects of yield-related QTLs SPIKE and GPS in two indica rice genetic backgrounds

Recent progress in rice genomics has promoted the identification of quantitative trait loci (QTLs) associated with yield and its related traits. SPIKE, a QTL controlling spikelet number per panicle, and GPS, a QTL controlling leaf photosynthesis rate, were identical to NAL1. To assess the effect of SPIKE/GPS on yield potential, we compared DNA sequences of these alleles and conducted yield experiments in the field of Japan using the near-isogenic lines NIL-SPIKE (allele from Daringan in IR 64 genetic background), NIL-GPS (allele from Koshihikari in Takanari genetic background), and a chromosome segment substitution line, SL2115 (allele from Koshihikari in IR 64 genetic background). Despite the two SNPs in the promoter regions between Koshihikari and Daringan, both alleles were effective to increase the number of spikelets per panicle both in Takanari and IR 64 backgrounds. However, the extent of the increase was smaller and unstable in Takanari background than that in IR 64 background. In addition, SPIKE/GPS improved percentage of filled spikelets only in the IR 64 background. These results suggest that the effects of SPIKE/GPS alleles are similar but are affected by the difference of the genetic backgrounds. Because the increasing effect of spikelets number per panicle was canceled by the decrease of the number of panicles, which seems to be affected by environmental factors, none of NIL-SPIKE, SL2115, or NIL-GPS significantly out-yielded their parental cultivars. These results indicate the importance to consider genetic backgrounds and QTL-environment interaction toward the future use of SPIKE/GPS.     

Genetic studies for breeding of rice cultivars with superior grain appearance and lodging resistance from the rice cultivar ‘Emi-no-kizuna’

ABSTRACT

We conducted a quantitative trait locus (QTL) analysis of grain appearance (GA) and agronomic traits of rice, using 128 recombinant inbred lines derived from ‘Emi-no-kizuna’ and ‘Tomohonami’. We detected two promising QTLs associated with GA: qGA4 on chromosome 4 and qGA8 on chromosome 8. qGA4 contributed highly to the greater percentage of perfect grains of the Emi-no-kizuna genotype. In the same region, we detected other QTLs associated with panicle number and spikelet number per panicle. In near-isogenic lines (NILs) in which Emi-no-kizuna alleles were introgressed in the genomic region of only the semi-dwarf 1 (sd1) locus (NIL_1) and both the sd1 locus and qGA8 (NIL_2), respectively, the percentage of perfect grains was significantly higher and the percentages of milky white, basal white, and white back grains were significantly lower than in Tomohonami; and the percentages of milky white and white back grains of NIL_2 were significantly lower than those of NIL_1. These results suggest that introgression in the sd1 region could improve GA, and that the addition of qGA8 could further improve GA. The culm lengths of the NILs were significantly shorter than that of Tomohonami, indicating improved lodging resistance. Grain weight of NIL_2 was significantly smaller than that of NIL_1, suggesting that the effect of qGA8 could be pleiotropic, or the gene that underlies qGA8 could be linked with genes associated with grain weight.

Abbreviations: ANOVA: analysis of variance; AT20: mean air temperature in the 20 days after heading; BW: basal white grain; CL: culm length; DAH: days after heading; GA: grain appearance; GW: 1000-grain weight; LOD: logarithm of odds; MW: milky white grain; NIL: near-isogenic line; PG: perfect grain; PL: panicle length; PN: panicle number; PTSN: putative total spikelet number; PVE: percentage of phenotypic variation explained; QTL: quantitative trait locus; RIL: recombinant inbred line; SN: spikelet number per panicle; SNP: single nucleotide polymorphism; WB: white back grain     

【Short Report】Identification of Chromosome Regions Affecting Leaf Area with Rice Chromosome Segment Substitution Lines

Abstract

The aim of this study was to obtain the information for improvement in source ability via expansion of a flag leaf area in rice (Oryza sativa L.). We identified chromosome regions affecting a trait (CRATs) for leaf area and confirmed their effects with two sets of chromosome segment substitution lines developed from indica landraces Kasalath or Nona Bokra as a donor under japonica cultivar Koshihikari genetic background. We detected 4 and 8 CRATs that increased leaf area by Kasalath and Nona Bokra alleles, respectively. With the Nona Bokra allele, CRAT on chromosome 7 (LA7) increased leaf area by 54% comparing to Koshihikari. LA7 maintained the contents of Rubisco and photosynthetic rate and as a result its “estimated” source ability of a flag leaf was increased by 23%. The effect of LA7 was stable under different environmental conditions, and might be useful for breeding rice with higher source ability and yield.     

Correlation and Regressive Model Between Spikelet Fertilized Rate and Temperature in Inter-Subspecific Hybrid Rice

To study the sensitivity of inter-subspecific hybrid rice to climatic conditions, the spikelet fertilized rate (SFR) of four types of rice including indica-japonica hybrid, intermediate hybrid, indica and japonica were analyzed during 2000-2004. The inter-subspecific hybrids showed lower SFR, and much higher fluctuation under various climatic conditions than indica and japonica rice, showing the inter-subspecific hybrids were sensitive to ecological conditions. Among 12 climatic factors, the key factor affecting rice SFR was temperature, with the most significant factor being the average temperature of the seven days around panicle flowering (T7). A regressive equation of SFR-temperature by T7, and a comprehensive synthetic model by four important temperature indices were put forward. The optimum temperature for inter-subspecific hybrids was estimated to be 26.1-26.6℃, and lower limit of safe temperature to be 22.5-23.3℃ for panicle flowering, showing higher by averagely 0.5℃ and 1.7℃, respectively, to be compared with indica and japonica rice. This suggested that inter-subspecific hybrids require proper climatic conditions. During panicle flowering, the suitable daily average temperature was 23.3-29.0℃, with the fittest one at 26.1-26.6℃. For an application example, optimum heading season for inter-subspecific hybrids in key rice growing areas in China was as same as common pure lines, while inferior limit for safe date of heading was about a ten-day period earlier than those of common pure lines.     

Effects of Soil Temperature on Growth and Root Function in Rice

Abstract

The objective of this study was to clarify the effects of soil temperature in the stage from late tillering to panicle initiation (SI) and during the grain-filling stage (SII) on grain setting, dry matter production, photosynthesis, non-structural carbohydrate (NSC), xylem exudation and abscisic acid (ABA) levels in rice (Oryza sativa L. cv. Koshihikari). Rice plants were exposed to four different soil temperatures during SI or SII: 17.5, 25, 31.5 and 36.5°C (ST18, ST25, ST32 and ST37, respectively). The yield, yield components, grain filling and quality in SI were negatively influenced by high soil temperature of 37°C. On the other hand, there was no significant difference in those characters among temperature treatments in SII. The root/shoot ratio was smallest in the ST37 plants in both SI and SII, mainly due to their lighter root weight. At 7 days after initiation of treatment (DAT) in both SI and SII, the photosynthetic and xylem exudation rate tended to increase slightly as soil temperature increased up to 32°C. At 21 DAT, however, the photosynthetic rate was lowest in ST37, with concurrent decrease of diffusion conductance and SPAD value. In addition, decrease of NSC concentration in stem and xylem exudation rate, and increase of ABA level in leaves and xylem exudate were observed in ST37 plants at 21 DAT. These results suggested that high soil temperature before heading especially influenced yield, grain quality and plant growth. Possible mechanisms of the effect of soil temperature are discussed.     

Responses of Yield Characteristics to High Temperature During Flowering Stage in Hybrid Rice Guodao 6

By sowing at different dates during 2005 and 2006 both in paddy fields and greenhouse, a super hybrid rice combination Guodao 6 and a conventional hybrid rice combination Xieyou 46 (as control) were used to analyze the differences in heat injury index, seed setting rate, grain yield and its components. Guodao 6 showed more stable yield and spikelet fertility, and lower heat injury index than Xieyou 46. Further studies indicated that the spikelet sterility is positively correlated with the average daily temperature and the maximum daily temperature, with the coefficients of 0.8604 and 0.9850 (P<0.05) respectively in Guodao 6. The effect of high temperature injury on seed setting caused by maximum daily temperature was lower than that by average daily temperature during the grain filling stage.     

超级杂交稻国稻6号对开花结实期高温热害的反应

以超级杂交稻国稻6号（内2优6号）为材料,于2005-2006年在杭州分期播种,通过不同齐穗期条件下的产量及产量构成因子变异研究,分析开花结实期自然高温对灌浆结实的影响,同时在温室条件下设置可控极值高温（40～42℃）测定热害指数。国稻6号对开花结实期自然高温或设计极值高温的反应明显较对照协优46钝感,两者在小穗育性和热害指数上的差异达显著水平（P<0.05）。试验结果还表明国稻6号随花期日平均温度升高小穗不育率增加,尤其与日最高温度的关系更为密切,两者的相关系数分别为0.8604和0.9850（P<0.05）,表明日最高温度对小穗育性伤害更烈。而灌浆结实期日最高温度对国稻6号结实率的影响不如日平均温度,相关系数分别为0.7352和0.9317（P<0.05）。     

水杨酸减轻高温抑制水稻颖花分化的作用机理研究

水稻颖花原基分化期高温可抑制颖花分化,导致每穗粒数下降,最终会影响水稻产量的形成。以杂交稻甬优12为材料,探讨高温下不同浓度水杨酸(salicylic acid, SA)对水稻颖花分化的影响,并揭示其作用机制。研究表明,常温下适量SA促进水稻颖花分化,但高浓度SA严重降低每穗粒数。然而高温下,10~50000 μmol/L SA处理每穗粒数均高于对照。无论常温或高温,100 μmol/L水杨酸处理效果最好,其每穗粒数及产量均显著高于对照。SA增强水稻耐热性表现在三个方面:1)高温下SA处理的叶片实际荧光量子效率及净光合速率均高于对照;2)高温下SA处理的颖花SOD、POD及CAT活性显著高于对照,而MDA含量则显著低于对照;3)无论高温或常温,SA处理的颖花ZR及IAA含量均高于对照,尤其在高温下,差异达显著水平。