Carbon balance and source‐sink metabolic changes in winter wheat exposed to high night‐time temperature

Carbon loss under high night‐time temperature (HNT) leads to significant reduction in wheat yield. Growth chamber studies were carried out using six winter wheat genotypes, to unravel postheading HNT (23°C)–induced alterations in carbon balance, source‐sink metabolic changes, yield, and yield‐related traits compared with control (15°C) conditions. Four of the six tested genotypes recorded a significant increase in night respiration after 4 days of HNT exposure, with all the cultivars regulating carbon loss and demonstrating different degree of acclimation to extended HNT exposure. Metabolite profiling indicated carbohydrate metabolism in spikes and activation of the TriCarboxylic Acid (TCA) cycle in leaves as important pathways operating under HNT exposure. A significant increase in sugars, sugar‐alcohols, and phosphate in spikes of the tolerant genotype (Tascosa) indicated osmolytes and membrane protective mechanisms acting against HNT damage. Enhanced night respiration under HNT resulted in higher accumulation of TCA cycle intermediates like isocitrate and fumarate in leaves of the susceptible genotype (TX86A5606). Lower grain number due to lesser productive spikes and reduced grain weight due to shorter grain‐filling duration determined HNT‐induced yield loss in winter wheat. Traits and mechanisms identified will help catalyze the development of physiological and metabolic markers for breeding HNT‐tolerant wheat.     

Reduction in seed set upon exposure to high night temperature during flowering in maize

High temperature reduces crop production; however, little is known about the effects of high night temperature (HNT) on the development of male and female reproductive organs, pollination, kernel formation and grain yield in maize (Zea mays L.). Therefore, a temperature-controlled experiment was carried out using heat-sensitive maize hybrid and including three temperature treatments of 32/22°C (day/night; control), 32/26°C and 32/30°C during 14 consecutive days encompassing the flowering stage. When exposed to 30°C night temperature, grain yield and kernel number reduced by 23.8 and 25.1%, respectively, compared with the control. The decrease in grain yield was mainly because of the lower kernel number rather than change in kernel weight under HNT exposure around flowering. No significant differences in grain yield and kernel number were found between 22 and 26°C night temperatures. HNT had no significant effects on the onset of flowering time and anthesis-silking interval but significantly reduced time period of pollen shedding duration and pollen viability, and increased leaf night respiration. Different from high daytime temperature, HNT had no lasting effects on daytime leaf photosynthesis, biomass production and assimilate transportation. From the perspective of source–flow–sink relationship, the unchanged source and flow capacities during daytime are supposed to alleviate the adverse effects on sink strength caused by HNT compared with daytime heat stress. These new findings commendably filled the knowledge gaps concerning heat stress in maize.     

Rice yield formation under high day and night temperatures—A prerequisite to ensure future food security

Increasing temperatures resulting from climate change dramatically impact rice crop production in Asia. Depending on the specific stage of rice development, heat stress reduces tiller/panicle number, decreases grain number per plant and lower grain weight, thus negatively impacting yield formation. Hence improving rice crop tolerance to heat stress in terms of sustaining yield stability under high day temperature (HDT), high night temperature (HNT), or combined high day and night temperature (HDNT) will bolster future food security. In this review article, we highlight the phenological alterations caused by heat and the underlying molecular-physiological and genetic mechanisms operating under different types of heat conditions (HDT, HNT, and HDNT) to understand heat tolerance. Based on our synthesis of HDT, HNT, and HDNT effects on rice yield components, we outline future breeding strategies to contribute to sustained food security under climate change.     

Grain growth and endosperm cell size under high night temperatures in rice (Oryza sativa L.)

BACKGROUND AND AIMS: High night temperatures are more harmful to grain weight in rice than high day temperatures. Grain growth rate and growth duration were investigated to determine which was the cause of the decrease in final grain weight under high night temperatures. Endosperm cell number and cell sizes were also examined to determine which might cause the decrease in final grain weight. METHODS: Rice plants were grown outdoors in plastic pots and moved at heading time to three temperature-controlled glasshouses under high night temperature (HNT; 22/34 degrees C), high day temperature (HDT; 34/22 degrees C) and control conditions (CONT; 22/22 degrees C). Grains were sampled periodically, and the time-course of grain growth was divided into rate and duration by logistic regression analysis. Endosperm cell numbers and cell sizes were analysed by digitalized hand-tracing images of endosperm cross-sections. KEY RESULTS: The duration of grain growth was reduced by high temperature both day and night. However, the rate of grain growth was lower in HNT than in HDT. The number of cells in endosperm cross-sections in HNT was similar to that in HDT, and higher than that in CONT. The average cell area was smaller in HNT than in either CONT or HDT. The differences in average cell areas between HNT and HDT were greater at distances 60-80 % from the central point of endosperm towards the endosperm surface. CONCLUSIONS: The results show that HNT compared with HDT reduced the final grain weight by a reduction in grain growth rate in the early or middle stages of grain filling, and also reduced cell size midway between the central point and the surface of endosperm.     

Physiological and biochemical characterization of NERICA‐L‐44: a novel source of heat tolerance at the vegetative and reproductive stages in rice

The predicted increase in the frequency and magnitude of extreme heat spikes under future climate can reduce rice yields significantly. Rice sensitivity to high temperatures during the reproductive stage is well documented while the same during the vegetative stage is more speculative. Hence, to identify and characterize novel heat‐tolerant donors for both the vegetative and reproductive stages, 71 rice accessions, including approximately 75% New Rice for Africa (NERICAs), were phenotyped across field experiments during summer seasons in Delhi, India, and in a controlled environment study at International Rice Research Institute , Philippines. NERICA‐L‐44 (NL‐44) recorded high seedling survival (52%) and superior growth and greater reproductive success exposed to 42.2°C (sd ± 2.3) under field conditions. NL‐44 and the heat‐tolerant check N22 consistently displayed lower membrane damage and higher antioxidant enzymes activity across leaves and spikelets. NL‐44 recorded 50–60% spikelet fertility, while N22 recorded 67–79% under controlled environment temperature of 38°C (sd ±1.17), although both had about 87% fertility under extremely hot field conditions. N22 and NL‐44, exposed to heat stress (38°C), had similar pollen germination percent and number of pollen tubes reaching the ovary. NL‐44 maintained low hydrogen peroxide production and non‐photochemical quenching (NPQ) with high photosynthesis while N22 avoided photosystem II damage through high NPQ under high‐temperature stress. NL‐44 with its reproductive stage resilience to extreme heat stress, better antioxidant scavenging ability in both vegetative tissue and spikelets and superior yield and grain quality is identified as a novel donor for increasing heat tolerance at both the vegetative and reproductive stages in rice.     

Effects of Asymmetric Heat on Grain Quality During the Panicle Initiation Stage in Contrasting Rice Genotypes

Heat stress during the panicle initiation stage hinders the formation of rice grains. It is speculated that heat exposure during the panicle initiation stage could influence grain quality in rice. To obtain preliminary knowledge on the effects of asymmetric heat on rice grain quality during the panicle initiation stage, four rice genotypes (Shanyou63, Liangyoupeijiu, Huanghuazhan, and Nagina22) were subjected to three heat treatments, i.e., high daytime temperature (HDT; 37.9 °C/24.5 °C), high nighttime temperature (HNT; 30.9 °C/30.5 °C), the combination of high daytime and nighttime temperature (HDNT; 38.5 °C/31.0 °C), and a control (CK, 31.5 °C/24.0 °C) in temperature-controlled greenhouses for 15 days during the panicle initiation stage. The milling and appearance qualities, which are crucial for commercial value, were studied. Heat treatments significantly reduced the amounts of brown rice, milled rice, and head rice and the grain length, grain width, chalky grain amount, and grain chalkiness in the rice genotypes Liangyoupeijiu, Nagina22, and Huanghuazhan during the panicle initiation stage, and the largest reductions in grain quality were frequently observed under HDNT treatment. The milling and appearance qualities in genotype Shanyou63 were negligibly affected by heat treatments and thus were regarded as tolerant to heat, and the rice genotypes Liangyoupeijiu, Huanghuazhan, and especially Nagina22 were susceptible to heat during the panicle initiation stage. We concluded that heat stress during panicle initiation impacted the milling and appearance qualities in rice, and differences existed among rice genotypes. The underlying mechanisms of the effects of heat on rice grain quality need further study.

     

Effect of elevated CO2 and high temperature on seed-set and grain quality of rice

Hybrid vigour may help overcome the negative effects of climate change in rice. A popular rice hybrid (IR75217H), a heat-tolerant check (N22), and a mega-variety (IR64) were tested for tolerance of seed-set and grain quality to high-temperature stress at anthesis at ambient and elevated [CO(2)]. Under an ambient air temperature of 29 °C (tissue temperature 28.3 °C), elevated [CO(2)] increased vegetative and reproductive growth, including seed yield in all three genotypes. Seed-set was reduced by high temperature in all three genotypes, with the hybrid and IR64 equally affected and twice as sensitive as the tolerant cultivar N22. No interaction occurred between temperature and [CO(2)] for seed-set. The hybrid had significantly more anthesed spikelets at all temperatures than IR64 and at 29 °C this resulted in a large yield advantage. At 35 °C (tissue temperature 32.9 °C) the hybrid had a higher seed yield than IR64 due to the higher spikelet number, but at 38 °C (tissue temperature 34-35 °C) there was no yield advantage. Grain gel consistency in the hybrid and IR64 was reduced by high temperatures only at elevated [CO(2)], while the percentage of broken grains increased from 10% at 29 °C to 35% at 38 °C in the hybrid. It is concluded that seed-set of hybrids is susceptible to short episodes of high temperature during anthesis, but that at intermediate tissue temperatures of 32.9 °C higher spikelet number (yield potential) of the hybrid can compensate to some extent. If the heat tolerance from N22 or other tolerant donors could be transferred into hybrids, yield could be maintained under the higher temperatures predicted with climate change.     

Carbon dioxide responsiveness mitigates rice yield loss under high night temperature

Increasing night-time temperatures are a major threat to sustaining global rice (Oryza sativa L.) production. A simultaneous increase in [CO₂] will lead to an inevitable interaction between elevated [CO₂] (e[CO₂]) and high night temperature (HNT) under current and future climates. Here, we conducted field experiments to identify [CO₂] responsiveness from a diverse indica panel comprising 194 genotypes under different planting geometries in 2016. Twenty-three different genotypes were tested under different planting geometries and e[CO₂] using a free-air [CO₂] enrichment facility in 2017. The most promising genotypes and positive and negative controls were tested under HNT and e[CO₂] + HNT in 2018. [CO₂] responsiveness, measured as a composite response index on different yield components, grain yield, and photosynthesis, revealed a strong relationship (R² = 0.71) between low planting density and e[CO₂]. The most promising genotypes revealed significantly lower (P < 0.001) impact of HNT in high [CO₂] responsive (HCR) genotypes compared to the least [CO₂] responsive genotype. [CO₂] responsiveness was the major driver determining grain yield and related components in HCR genotypes with a negligible yield loss under HNT. A systematic investigation highlighted that active selection and breeding for [CO₂] responsiveness can lead to maintained carbon balance and compensate for HNT-induced yield losses in rice and potentially other C₃ crops under current and future warmer climates.

Active selection for carbon dioxide responsiveness in rice and other C₃ crops can mitigate yield loss induced by high night temperature.     

Genome‐wide expression quantitative trait locus studies facilitate isolation of causal genes controlling panicle structure

The morphology of rice (Oryza sativa L.) panicles is an important determinant of grain yield, and elucidation of the genetic control of panicle structure is very important for fulfilling the demand for high yield in breeding programs. In a quantitative trait locus (QTL) study using 82 backcross inbred lines (BILs) derived from Koshihikari and Habataki, 68 QTLs for 25 panicle morphological traits were identified. Gene expression profiling from inflorescence meristems of BILs was obtained. A combination of phenotypic QTL (pQTL) and expression QTL (eQTL) analysis revealed co‐localization between pQTLs and eQTLs, consistent with significant correlations between phenotypic traits and gene expression levels. By combining pQTL and eQTL data, two genes were identified as controlling panicle structure: OsMADS18 modulates the average length of the primary rachis and OsFTL1 has pleiotropic effects on the total number of secondary rachides, number of grains per panicle, plant height and the length of flag leaves. Phenotypes were confirmed in RNA interference knocked‐down plants and overexpressor lines. The combination of pQTL and eQTL analysis could facilitate identification of genes involved in rice panicle formation.     

Improving the performance of short‐duration basmati rice in water‐saving production systems by boron nutrition

Over exploitation of groundwater and decreasing canal water resources are threating the productivity of conventional rice production systems in Asia which is the main rice bowl. Therefore, strategies are needed to produce more rice with less water in the shortest possible duration without compromising the yield to feed the increasing world population. Panicle sterility is one of the major obstacles in wide‐scale adoption of water‐saving rice production systems. Boron (B) deficiency, in water‐saving rice production systems, has been identified as a possible reason for panicle sterility. This 2‐year field study was aimed to investigate the potential of pre‐optimised boron application through various methods in improving the productivity of short‐duration basmati rice (Shaheen Basmati) in water‐saving production systems, as delivered through seed priming (0.1 mM boron), foliar spray (200 mM boron) or soil application (1 kg boron ha^?1), while hydropriming and no boron application were taken as control. Boron nutrition, by either way, improved the growth, water relations, morphology, yield‐related traits, panicle fertility, grain yield, grain quality and grain boron contents of short‐duration basmati rice; nonetheless, boron application as seed priming was superior and cost effective with maximum marginal rate of return. In conclusion, boron nutrition through seed priming is cost effective and may help improving the productivity, quality, and boron grain contents in short‐duration basmati rice under water‐saving production systems.     

Comparative analysis of panicle proteomes of two upland rice varieties upon hyper-osmotic stress

Drought is a major environmental factor that limits the yield of rice dramatically. Upland rice is now regarded as a promising rice cultivar in water saving agriculture. Two varieties of upland rice Zhonghan 3 and IR29 were used to compare the physiological and proteomic responses to hyper-osmotic stress induced by 15% polyethyleneglycol (PEG) at the reproductive stage. Osmotic stress affected the growth development and caused the loss of production especially the grain yield. IR29 was more tolerant to PEG than Zhonghan 3 as shown by less yield loss under osmotic stress conditions. Comparative proteomic analysis of the panicle suggested that the up-regulation of glycolysis related proteins and defense proteins may contribute to the better osmotic tolerance in IR29.     

两个特用稻的营养供应水平与~(32)P、~(14)C分布和谷产量的关系

香稻(竹香粘)的分蘖数多,其分蘖期根系吸收~(32)P和~(32)P分布到分蘖,以高、中肥和高N,高K肥处理的为高;~(14)C-葡萄糖同化物分布在分蘖比黑米稻(黑优粘)和常规稻“双桂36”的高,但灌浆期和黄熟期~(14)C分布在稻穗较少.低肥处理的稻谷产量较高.而在高、中肥条件下结实率降低,实粒数减少,成穗率稍低,谷产量也比“黑优粘”和“双桂36”低. “黑优粘”在高肥、高N条件下的分蘖多,成穗率高、结实粒数也较多,稻谷产量比“竹香粘”高,在高、中肥、高 N高 P条件下,~(32)P分布于分蘖多,分蘖的~(14)C分布则以高 N,高 K肥条件下的多;开花和灌浆用稻穗的~(14)C分布皆以高 K肥水平的为高。“黑优粘”是耐肥和需钾肥品种,高肥、高K可提高其稻谷产量,但仍比“双桂36”低.     

Restricting the above ground sink corrects the root/shoot ratio and substantially boosts the yield potential per panicle in field‐grown rice (Oryza sativa L.)

Rice has shallow, weak roots, but it is unknown how much increase in yield potential could be achieved if the root/shoot ratio is corrected. Removing all tillers except the main one, in a japonica (Sakha 101) and an indica (IR64) rice cultivar, instantly increased the root/shoot ratio from 0.21 to 1.16 in Sakha 101 and from 0.16 to 1.46 in IR64. Over 30 days after detillering, the root/shoot ratios of the detillered plants decreased to 0.49 in Sakha 101 and 0.46 in IR64 but remained significantly higher than in the controls. The detillered plants showed two‐ or fourfold increase in the main tiller fresh weight, as a consequence of more positive midday leaf relative water content (RWC), and consistently higher rates of stomatal conductance and photosynthesis, but not transpiration, compared with the controls. The enhanced photosynthesis in Sakha 101 after detillering resulted from both improved water status and higher Rubisco contents whereas in IR64, increasing the Rubisco content did not contribute to improving photosynthesis. Detillering did not increase the carbohydrate contents of leaves but prevented starch depletion at the end of grain filling. The leaf protein content during vegetative and reproductive stages, the grain filling rate, the number of filled grains per panicle were greatly improved, bringing about 38.3 and 35.9% increase in the harvested grain dry weight per panicle in Sakha 101 and IR64, respectively. We provide evidence that improving the root performance by increasing the root/shoot ratio would eliminate the current limitations to photosynthesis and growth in rice.     

Growth and CO2 assimilation of lowland rice in response to timing and method of N fertilization

In order to understand more clearly the dynamics of rice (Oryza sativa L.) yield development in relation to N fertilization, a field experiment was conducted in Laguna, Philippines. The soil, a Maahas clay (Aquic Hapludalf), was flooded, puddled, and then planted with 20-day-old seedlings of IR64 rice. Treatments were six combinations of N fertilizer timing and method: (1) 0 N control; (2) prilled urea broadcast at 15 and 60 days after transplanting (DAT) (BR-LATE); (3) prilled urea injected with a spring auger applicator at 15 DAT and broadcast at 60 DAT (INJ-LATE); (4) prilled urea broadcast and incorporated at 0 DAT and broadcast at 40 DAT (BR-EARLY); (5) urea super granules (USG) manually deep-placed at 3 DAT and prilled urea broadcast at 40 DAT (DP-EARLY); and (6) USG manually deep-placed at 3 DAT (DP). Except for the control, all treatments received a total of 58 kg N ha^-1. Yield results were consistent with those of other experiments, namely, DP had the highest yields, the early-split treatments (BR-EARLY and DP-EARLY) were second best, followed by the late-split treatments (BR-LATE and INJ-LATE), with the control having the lowest yield. Sequential harvest results showed that the advantages of DP in terms of dry matter assimilation, tillering, and leaf area index (LAI) were expressed later in the season. For all treatments, midday net CO₂ assimilation (Ac) peaked around 48 DAT, approximately panicle initiation. Grain yield was highly correlated with midday Ac at panicle initiation and flowering but not at other growth periods. Rates of midday Ac and night respiration depended strongly on shoot N content. We conclude that N application method and timing should be designed to give high shoot N content at panicle initiation and flowering, and that DP satisfied this requirement best among the treatments tested.     

Drought tolerance in rice: morphological and molecular genetic consideration

Rice is one of the most important food crop drastically affected by drought in lowland rice ecosystem. Dissecting out the traits of importance and genomic regions influencing the response of drought tolerance and yield traits on grain yield will aid the breeders to know the genetic mechanism of drought tolerance of rice leads to the development of drought tolerant varieties. Grain yield and its components on drought situation of recombinant inbred population (IR 58821/IR 52561) were investigated under lowland managed stress situation in 2003 and 2004 by given importance to the relative water content. Water deficit resulted in significant effect on phenology and grain yield. Best lines were selected for further varietal development programme. Variability studies showed the traits viz., days to 70% relative water content, leaf rolling, leaf drying, harvest index, biomass yield and grain yield offer high scope for improvement for drought tolerance by way of simple selection technique. Correlation and path analysis indicated that, to harness high yielding combined with drought tolerance breeders should give selection pressure on relative water content, panicle length, grains per panicle, harvest index, biomass yield, root/shoot ratio and root length in positive direction, and low scores of leaf rolling, leaf drying and drought recovery rate. Analysis of quantitative trait loci for drought tolerance, yield and its components allowed the identification of 38 regions associated with both drought tolerant and yield traits. Out of these, 18 were closely linked with DNA markers could be used for marker assisted selection in breeding for drought tolerance in rice. Pleiotropism and G × E effects interaction were noticed in some of the traits. Parent IR 58821 contributed favorable alleles for the entire drought related and most of the yield component traits. Identification of traits of importance and their nature of relationship by morphological and molecular level under lowland condition will be useful to improve drought tolerance of rice.