Effect of two-week heat stress during grain filling on stem reserves,senescence, and grain yield of European winter wheat cultivars

To examine the extent to which heat stress during grain filling impacts on the development and yield of winter wheat (Triticum aestivum L.), a 3-year field experiment was conducted on a loess soil with high water holding capacity in the North German Plain. Thirty-two mostly European winter wheat cultivars were exposed to heat stress in a mobile foil tunnel with maximum air temperatures of 45.7, 45.4, and 47.2°C in 2015, 2016, and 2017, respectively. The 14-day post-anthesis heat stress treatment caused an average 57.3% grain yield reduction compared to a close-by non-stressed control. The proportion of green crop area after the heat stress phase varied from 7% to 98% in 2016 and from 37% to 94% in 2017. The green crop area percentage did not significantly correlate with grain yield, indicating that the delayed senescence of stay-green phenotypes offers no yield advantage under terminal heat stress. The water soluble carbohydrate (WSC) concentration of the stems at crop maturity varied between 6 and 92 g/kg dry matter, showing that the genotypes differed in their efficiency at using the stem carbohydrate reserves for grain filling under heat stress. The stem WSC concentration correlated positively with the beginning of anthesis (r = 0.704; p < .001) but negatively with the grain yield (r = −0.431; p < .05). For heat tolerance breeding, the stem reserve strategy, i. e. the rapid and full exhaustion of the temporary carbohydrate storage therefore seems more promising than the stay-green strategy.     

Application of natural plant extracts improves the tolerance against combined terminal heat and drought stresses in bread wheat

Drought and heat are among the main abiotic stresses causing severe damage to the cereal productivity when occur at reproductive stages. In this study, ten wheat cultivars were screened for combined heat and drought tolerance imposed at booting, heading, anthesis and post‐anthesis stages, and role of the foliage applied plant extracts was evaluated in improving the performance of differentially responding wheat cultivars under terminal heat and drought stresses. During both years, wheat crop was raised under ambient temperature and 70% water holding capacity (WHC) till leaf boot stage. The plant extracts (3% each) of sorghum, brassica, sunflower and moringa were foliage applied at booting, anthesis and post‐anthesis stage; and after one week of application of these plant extracts, combined heat and drought was imposed at each respective stage. Heat and drought stresses were imposed at each respective stage by placing pots in glass canopies with temperature of 4 ± 2°C above than the ambient temperature in combination with drought stress (35% WHC) until maturity. Combination of drought and heat stresses significantly reduced the performance of tested wheat cultivars; however, stress at the booting and heading stages was more damaging than the anthesis and post‐anthesis stages. Cultivars Mairaj‐2008 and Chakwal‐50 remained green with extended duration for grain filling, resulting in the maintenance of number of grains per spike and 100‐grain weight under stress conditions and thus had better grain yield and water‐use efficiency. However, in cultivars Fsd‐2008, and Shafaq‐2006, the combined imposition of drought and heat accelerated the grain filling rate with decrease in grain filling duration, grain weight and grain yield. Foliar application of all the plant extracts improved the wheat performance under terminal heat and drought stress; however, brassica extract was the most effective. This improvement in grain yield, water‐use efficiency and transpiration efficiency due to foliage applied plant extracts, under terminal heat and drought stress, was owing to better stay‐green character and accumulation of more soluble phenolics, which imparted stress tolerance as indicated by relatively stable grain weight and grain number. In crux, growing of stay‐green wheat cultivars with better grain filling and foliage application of plant extracts may help improving the performance of bread wheat under combined heat and drought stresses.     

Growth and senescence characteristics associated with tolerance of wheat-alien amphiploids to high temperature under controlled conditions

Tolerance of wheat (Triticum aestivumL.) to high temperature might be improved by introducing alien genes from amphiploids. Our objectives were to determine responses of synthetic hexaploid and octaploid amphiploid wheats to high temperature and evaluate their potential usefulness for developing improved cultivars. Thirty synthetic hexaploids from durum wheat (T. turgidum L.) × Aegilops tauschii Cos. Accessions and four octaploid amphiploids from Chinese Spring wheat × different grasses were grown at 20/15 and 30/25 ^°C day/night during maturation. Tolerance was ascertained by two measures of senescence, leaf chlorophyll content and grain filling duration, plus grain yield and its components. Leaf chlorophyll was measured after 10 and 15 days of treatment, and grain yield was determined at maturity to calculate the heat susceptibility index(HSI), a gauge of the reduction in yield at high temperature of each line relative to all other lines. Chlorophyll content, grain filling duration, yield, and kernel weight were highly negatively correlated with HIS of the hexaploid amphiploids at30/25 ^°C, but grain yield was positively correlated with HSI at20/15 ^°C. The hexaploid lines might be useful for improving wheat for regions where stress from high temperature occurs frequently. Chlorophyll content and grain filling duration also were highly negatively correlated with HSI of the octaploid lines, but they would be less directly useful for improving wheat because the kernel number was reduced greatly due to unbalanced meiotic chromosomal segregation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Remobilization of Nitrogen and Carbohydrate from Stems of Bread Wheat in Response to Heat Stress during Grain Filling

When wheat (Triticum aestivum L.) is grown under heat-stress conditions during grain filling, preanthesis stored total non-structural carbohydrates (TNC) and nitrogen (N) could serve as alternative source of assimilates. This study was performed to evaluate wheat genotypes for their ability to accumulate and remobilize TNC and N stored in their stem to support grain filling under heat stress. Eighteen genotypes were used for N remobilization study while nine of them were used for TNC remobilization study. They were grown in pots and placed in a vinyl house with the maximum temperature kept below 30 °C. Five days after anthesis (5DAA), half of the pots were taken to phytotrons where temperature was gradually increased and the maximum was set at 38 °C. Grain yield and grain weight decreased by about 35 % under heat stress. Significant differences were found among genotypes in percentage reduction in grain yield, grain weight, grain filling duration and harvest index because of heat stress. The N and TNC concentrations of the stem at 5DAA were significantly different among genotypes. Heat stress significantly reduced the N remobilization efficiency of most of genotypes. However, heat stress significantly increased TNC remobilization efficiency and significant variation were observed among genotypes. N remobilization efficiency across treatments significantly correlated with grain yield, grain weight, harvest index and grain filling duration. TNC at 5DAA negatively correlated with N at 5DAA and harvest index, but the TNC remobilization efficiency under heat stress positively correlated with mainstem grain yield, grain weight and harvest index. The rate of chlorophyll loss from flag leaf positively correlated with N and TNC remobilization efficiencies under heat stress suggesting a link between leaf senescence and remobilization efficiency. The results indicate that heat stress negatively affected grain yield, its components and N remobilization while it increased TNC remobilization because of the increasing demand for resources.     

Grain yield of wheat (Triticum aestivum L.) under long‐term heat stress is sink‐limited with stronger inhibition of kernel setting than grain filling

The aim of the study was to investigate source‐sink relations of wheat under continuous heat stress and to identify bottle necks of yield formation. A pot experiment was conducted in two climatic chambers exposing wheat plants (Triticum aestivum L. cv. Thasos) either to day/night temperatures of 20/20°C (control conditions) or of 30/25°C (heat stress) during the whole vegetation period in the absence of plant water deficit. Plants were harvested at four phenological stages: three‐node stage (DC 33), start of flowering (DC 61), grain filling (DC 75) and maturity (DC 94). Heat stress shortened the development phases of the plants and caused a significant decrease in total above‐ground biomass between 19% and 41%. At grain filling and at maturity, the reductions in total shoot biomass mainly resulted from grain yield depressions by 77% and 58%, respectively. The ear number per plant was significantly higher under heat stress in comparison with the control, at maturity it was more than doubled. On the contrary, under heat stress, the kernel number per ear was strongly decreased by 83% and 75% during grain filling and at maturity, respectively. The decrease in individual kernel weight was 23% at maturity. Thus, the heat‐stressed plants were able to strongly increase the number of ear‐bearing tillers which were able to set only a small number of kernels, yet these kernels showed good grain filling. The harvest index (HI) of heat‐stressed plants was significantly reduced by 36% (control: HI = 50.1% ± 0.4, heat: HI = 32.2% ± 0.9***). The plants in the stress treatment adapted to the adverse conditions by less biomass production which presumably allowed a higher transpiration without an increase in total water consumption. Nevertheless, under heat stress, the water use efficiency (WUE_grain) was strongly decreased by 62% as a result of a small grain yield. In ears and grains, the sucrose, glucose and fructose concentrations were not significantly different between control and heat stress at start of flowering and during grain filling. Thus, the supply of assimilates was not restricted (no source limitation). Sink capacity was reduced by heat stress, as lesser and smaller kernels were produced than in the control. Concerning sink activity, the sink‐limiting step during kernel set is probably the active transport of hexoses across the plasma membrane into the developing kernels, which could also affect grain filling. This needs to be investigated in more detail in further studies.     

Does post‐anthesis heat stress affect plant phenology,physiology, grain yield and protein content of durum wheat in a semi‐arid Mediterranean environment?

Increasing air temperature due to changing climate is projected to decrease the length of the growing season, hasten vegetative development and maturation, and ultimately affect yield of many C3 crops. Previous multilocation trials highlighted strong relationships between thermal trends in the interval “beginning of flowering‐end of grain filling” and grain yield, and protein content in durum wheat (Triticum turgidum subsp. durum (Desf.) Husn.). With the aim to confirm these relationships, nine durum wheat genotypes, including old (Capeiti 8, Senatore Cappelli and Trinakria) and modern (Amedeo, Arcangelo, Mongibello, Simeto, and Svevo) varieties and a Sicilian landrace (Russello) were grown at three different sites representing a climate gradient in Sicily, Italy. Moreover, the effect of post‐anthesis heat stress on these durum wheats was further investigated in two contrasting environments: open‐field (control—C) and greenhouse heat stress (HS). HS shortened the interval “beginning‐end of flowering” of 1.5 days across genotypes, and the “end of flowering‐beginning of grain filling” and maturation of 4.9 days, with a range of 1 day in Arcangelo to 11 days in Cappelli. Advances in main phenophases significantly decreased kernel weight (KW) and grain yield (GY), whereas grain protein content (PC) increased. As expected, a negative relationship was observed between GY and PC, while positive relationships were found for GY and grain‐filling duration (GFD), and GY and KW. Genotypes responded differently to heat stress, as evidenced by the net photosynthesis, transpiration rate, instantaneous water use efficiency and dry matter accumulation in kernels. Genotypes were ranked according to the heat susceptibility index (HSI): Amedeo, Arcangelo, Capeiti 8, Svevo and Trinakria resulted heat‐tolerant. These varieties were characterized by an early trigger of dry matter accumulation in kernels under HS (Amedeo, Arcangelo and Trinakria), or showed similar length of the GFD (Capeiti 8) between environments. The multilocation trial confirmed a negative relationship between maximum temperatures and grain yield, and a positive relationship between minimum temperatures and protein content during grain–filling periods. Research focusing on agronomic strategies, phenology and breeding for tolerance to heat stress is of strategic importance to cope with the detrimental effect of global warming in semi‐arid climates.     

Exogenous application of allelopathic water extracts helps improving tolerance against terminal heat and drought stresses in bread wheat (Triticum aestivum L. Em. Thell.)

The allelopathic water extracts (AWEs) may help improve the tolerance of crop plants against abiotic stresses owing to the presence of the secondary metabolites (i.e., allelochemicals). We conducted four independent experiments to evaluate the influence of exogenous application of AWEs (applied through seed priming or foliage spray) in improving the terminal heat and drought tolerance in bread wheat. In all the experiments, two wheat cultivars, viz. Mairaj‐2008 (drought and heat tolerant) and Faisalabad‐2008 (drought and heat sensitive), were raised in pots. Both wheat cultivars were raised under ambient conditions in the wire house till leaf boot stage (booting) by maintaining the pots at 75% water‐holding capacity (WHC). Then, managed drought and heat stresses were imposed by maintaining the pots at 35% WHC, or shifting the pots inside the glass canopies (at 75% WHC), at booting, anthesis and the grain filling stages. Drought stress reduced the grain yield of wheat by 39%–49%. Foliar application of AWEs improved the grain yield of wheat by 26%–31%, while seed priming with AWEs improved the grain yield by 18%–26%, respectively, than drought stress. Terminal heat stress reduced the grain yield of wheat by 38%. Seed priming with AWEs improved the grain yield by 21%–27%; while foliar application of AWEs improved the grain yield by 25%–29% than the heat stress treatment. In conclusion, the exogenous application of AWEs improved the stay green, accumulation of proline, soluble phenolics and glycine betaine, which helped to stabilize the biological membranes and improved the tolerance against terminal drought and heat stresses.     

Reduction in kernel weight as a potential indirect selection criterion for wheat grain yield under terminal heat stress

Heat is an important abiotic stress during wheat (Triticum aestivum L.) grain‐filling in South Asia. A study was undertaken to determine effectiveness of selection for reduction in 1000‐kernel weight (TKWR) under heat stress to increase grain yield. Selection was made for low and high TKWR and selected progenies were evaluated in timely and late seeded trials at two locations in Nepal in 2003. One thousand kernel weight (TKW), biomass yield, grain yield, harvest index (HI), grain‐filling duration (GFD) and area under spot blotch progress curve per day (AUDPC/day) were examined. The low and high TKWR groups did not differ significantly for TKW, biomass yield, grain yield, HI, days to heading, GFD and AUDPC/day under timely seeding. However, low TKWR lines showed higher TKW, biomass yield, grain yield, HI, and GFD and lower AUDPC/day than the lines with high TKWR under late seeding. Realized heritability estimates for TKWR ranged from 0.68 to 0.85. The findings show that selection for low TKWR could be used as an indirect selection criterion to identify high grain yielding lines under terminal heat stress.     

The Effect of Late-terminal Drought Stress on Yield Components of Four Barley Cultivars

Barley (Hordeum vulgare L.) is an important winter cereal crop grown in the semiarid Mediterranean, where late‐terminal drought stress during grain filling has recently become more common. The objectives of this study were to investigate the growth performance and grain yield of four barley cultivars under late‐terminal drought stress under both glasshouse and field conditions. At grain filling, four barley cultivars (Rum, ACSAD176, Athroh and Yarmouk) were exposed to three watering treatments: (1) well‐watered [soil maintained at 75 % field capacity (FC)], (2) mild drought stress at 50 % FC, (3) severe drought stress at 25 % FC in the glasshouse experiment and (1) well‐watered (irrigated once a week), (2) mild drought (irrigated once every 2 weeks), (3) severe drought (non‐irrigated; rainfed) in the field. As drought stress severity increased, gross photosynthetic rate, water potential, plant height, grain filling duration, spike number per plant, grain number per spike, 1000‐grain weight, straw yield, grain yield and harvest index decreased. In the glasshouse experiment, the six‐row barley cultivars (Rum, ACSAD176, and Athroh) had higher grain yield than the two‐row barley cultivar (Yarmouk), but the difference was not significant among the six‐row cultivars under all treatments. In the field experiment, Rum had the highest grain yield among all cultivars under the mild drought stress treatment. The two‐row cultivar (Yarmouk) had the lowest grain yield. In general, the traditional cultivar Rum had either similar or higher grain yield than the other three cultivars under all treatments. However, the yield response to drought differed between the cultivars. Those, Rum and ACSAD176, that were capable of maintaining a higher proportion of their spikes and grains per spike during drought also maintained a higher proportion of their yield compared with those in well‐watered treatment. In conclusion, cultivar differences in grain yield were related to spike number per plant and grain number per spike, but not days to heading or grain filling duration.     

适度土壤干旱对贪青小麦茎鞘贮藏性糖运转及籽粒充实的影响

选用春性小麦品种扬麦158和扬麦11为材料,设置出穗后高氮和正常氮两个水平及土壤水分胁迫处理(WS),以正常浇水为对照(WW),研究适度土壤干旱对贪青迟熟小麦籽粒灌浆和茎鞘贮藏性碳水化合物分配的影响。结果表明,在始穗期施用过量氮肥(HN)导致小麦贪青迟熟,主要表现为灌浆速率和产量降低,茎鞘中滞留大量贮藏性糖。在灌浆     

Drought stress‐induced changes in starch yield and physiological traits in potato

Drought stress is an important limitation for potato (Solanum tuberosum L.) production as potato depends on appropriate water availability for high yields of good quality. Therefore, especially in the background of climate change, it is an important goal in potato breeding to improve drought stress tolerance. In this study, 34 European starch potato cultivars were evaluated for drought stress tolerance by growing under well‐watered and long‐term drought stress conditions in rainout shelters in 2 years’ pot trials. Besides yield, six physiological traits, that is free proline content, osmolality, total soluble sugar content, chlorophyll content (SPAD), cell membrane stability and crude protein content, were determined in leaves sampled during vegetative growth and during flowering to investigate their association with drought tolerance. ANOVA revealed significant treatment effects for all physiological traits and increased genotypic effects at flowering. The sensitivity of physiological traits to drought was significantly higher during flowering than during vegetative growth. Drought stress decreased starch yield significantly (p < .001), on average by 55%. Starch yield was significantly influenced by genotype and genotype × treatment interactions. Stress tolerance index (STI) calculated from starch yield ranged from 0.26 (sensitive) to 0.76 (tolerant) with significant genotype effects (p ≤ .001). STI correlated positively with cell membrane stability (r = .59) and crude protein content (r = .38) and negatively with osmolality (r = ?.57) and total soluble sugar content (r = ?.71). These contrary correlations suggest a dual adaptation strategy in potato under long‐term drought stress conditions including increased membrane stability combined with an increased osmolality due to an increased soluble sugar content.     

不同小麦品种籽粒灌浆特性的研究

以黄淮南片主产麦区广泛种植和新近育成推广的12个小麦品种为供试材料,对籽粒灌浆特性进行研究.结果表明:籽粒平均灌浆速度及最大灌浆速度均与百粒重、籽粒体积和籽粒最大体积呈极显著的正相关;籽粒灌浆持续期仅与饱满指数呈显著的正相关,而与百粒重无显著相关.供试品种的籽粒平均灌浆速度和最大灌浆速度,均以郑州853、陕农7859为最快,灌浆持续期最短,属晚花基因型品种;冀麦5418和徐州21的灌浆持续期最长,属早花基因型品种.还探讨了黄淮麦区南片小麦籽粒灌浆模式.     

Infrared Thermal Imaging as a Rapid Tool for Identifying Water‐Stress Tolerant Maize Genotypes of Different Phenology

The main task of this research was to evaluate canopy temperature and Crop Water Stress Index (CWSI) by assessing genotype variability of maize performance for different water regimes. To that end, three hundred tropical and subtropical maize hybrids with different phenology in terms of date of anthesis were evaluated. The influence of phenology on the change in canopy temperatures and CWSI was not equal during the three dates of measurement. At the end of vegetative growth (82 days after sowing, DAS) and at the blister stage (DAS 97), a high significant difference in temperatures and CWSI (P < 0.001) were obtained between the early‐ and late‐maturity genotypes. During anthesis (DAS 89), phenology had a significant effect (P < 0.01) only for the well‐watered genotypes, while under water‐stress conditions, no differences were found between early and late genotypes in terms of canopy temperature and CWSI. High significant differences (P < 0.001) in stomatal conductance (g_s) between early and late genotypes for different treatments were observed. A relationship (R² = 0.62) between g_s and canopy temperature was obtained. Under a water‐stress canopy, temperature was measured at anthesis, which was negatively correlated with grain yield of the early (r = ?0.55)‐ and late (r = ?0.46)‐maturity genotypes in the water‐stressed condition.     

花后短暂高温对小麦籽粒蛋白质含量的影响及其生理机制

采用人工气候室控温, 研究花后短暂高温对弱筋小麦扬麦9号和中筋小麦扬麦12籽粒蛋白质含量的影响及其生理机制。结果表明,开花至花后33 d,35℃以上高温处理使籽粒蛋白质含量显著高于对照, 温度越高上升越显著;花后33 d以后遇高温胁迫对蛋白质含量影响相对较轻。籽粒灌浆前期,花后6~8 d 35℃以上高温胁迫对蛋白质含量的影响最大,花后1~3 d处理影响次之;在灌浆中后期,花后19~21 d高温胁迫对蛋白质含量影响较大,花后36~38 d处理的影响较小,且灌浆前期籽粒蛋白质含量受高温胁迫的影响较灌浆中后期大。剑叶硝酸还原酶（NR）和谷氨酰胺合成酶（GS）活性以25℃处理较高,随温度的升高而降低, 与籽粒蛋白质含量呈极显著负相关。高温胁迫导致粒重下降,籽粒蛋白质含量相对上升。花后短暂高温胁迫导致弱、中筋小麦籽粒蛋白质品质变劣。     

开花期复水对受旱冬小麦的补偿效应研究

以冬小麦品种北农6号为材料,研究了开花期复水对小麦生长的影响。结果表明,前期受旱程度不同的植株开花期恢复供水后,其株高、单株叶面积、生物量及产量等都超过相应的干旱对照,表现出明显的补偿生长效应。同时,各复水处理与对照相比,分配到冠部的干物质比例均增加,R/S下降。中度水分胁迫后充分供水的处理,可以在少减     

抽穗和灌浆早期高温对耐热性不同籼稻品种产量的影响及其生理原因

选用4个耐热性不同的籼稻品种,分别于抽穗(始穗后0~10 d)和灌浆早期(始穗后11~20 d)进行高温(白天温度>33℃)处理,以同期自然温度(白天温度<30℃)为对照,研究高温对产量的影响及其生理机制。结果表明,与对照相比,高温处理显著降低热敏感品种的花粉可育率、受精率,而耐热品种与对照无显著差异。高温胁迫明显降低热敏感品种的结实率,导致产量显著下降,且抽穗期高温处理影响大于灌浆早期处理。高温胁迫显著增加耐热品种黄华占叶片的抗氧化酶活性,对热敏感品种影响不大。高温胁迫显著降低热敏感品种籽粒ATP酶活性,耐热品种结果则相反。高温处理增加两类品种叶片温度和丙二醛(MDA)含量,降低根系活力和叶片光合速率,耐热品种增降的幅度显著小于热敏感品种。在高温胁迫下较低的叶片温度,较强的根系活力和抗氧化保护系统能力及较高的籽粒ATP酶活性是耐热性品种保持较高产量的重要生理原因,也是耐热品种的重要生理特征。     

Physiological responses to water stress and yield of winter wheat cultivars differing in drought tolerance

Moderate to severe drought (water stress) at the reproductive stage is common in the U.S. Southern High Plains (SHP), where wheat is grown as a major winter crop. The objective of this study was to better understand the physiological basis of drought tolerance in elite wheat cultivars. A 2‐yr glasshouse study was conducted using three cultivars (TAM 111, TAM 112 and TAM 304) and two water treatments (wet: adequate water and dry: water‐limited). Overall, TAM 111 and TAM 112 used more water for cumulative evapotranspiration (ET) and had more tillers and greater root mass and shoot mass compared to TAM 304. In the dry treatment, TAM 112 had 67% and 81% more grain yield than TAM 111 and TAM 304, respectively. Water use efficiency for grain (WUEg) and water use efficiency for biomass (WUEbm) were also greater in TAM 112 compared to the other cultivars in the dry treatment. The flag leaves in TAM 112 at mid‐grain filling stage (about 15 days after flowering) had lower stomatal conductance (Gs), intercellular CO₂ concentration (Ci), transpiration rate (Tr) and net photosynthetic rate (Pn), but higher photosynthetic water use efficiency (PWUE) than TAM 111 and TAM 304 under water stress. This study demonstrated a distinct role of gas exchange parameters in response to drought, and TAM 112 was more efficient than TAM 111 and TAM 304 in evolving physiological mechanisms to adapt to water stress.     

Mapping quantitative trait loci associated with grain filling duration and grain number under terminal heat stress in bread wheat (Triticum aestivum L.)

Terminal heat stress has the potential negative impact on wheat production across the world, especially in South Asia. Under the threat of terminal heat stress, wheat genotypes with stay green trait would suffer from high temperature stress during their long grain filling duration (GFD). The genotypes with short GFD would be advantageous. To identify quantitative trait loci (QTL) for heat tolerance, a RIL population of K 7903 (heat tolerant) and RAJ 4014 (heat sensitive) wheat genotypes was investigated under timely and late‐sown conditions. Heat susceptibility index of GFD, yield components and traits under late‐sown condition were used as phenotypic data for QTL identification. Stable QTLs associated with these traits were identified on chromosomes 1B, 2B, 3B, 5A and 6B. The LOD value ranged from 2.9 to 5.0 and the corresponding phenotyping variation explained ranged from 12.0–22%. QTL for heat susceptibility index for the grain filling duration were colocalized with QTL for productive tillers under late sown and GFD under late‐sown condition on chromosomes 1B and 5A, respectively. These genomic regions could be exploited for molecular wheat breeding programmes targeting heat tolerance.     

Wheat Cultivars Adapted to Post-Heading High Temperature Stress

The existence of genetic variation in wheat for tolerance to high temperature stress has been reported but cultivars released for a particular production system often are not characterized. The objective of this study was to identify and describe the characteristics of wheat cultivars adapted to production systems with risks of high temperature during the post‐heading period. Fifteen diverse wheat cultivars and one unreleased genotype were evaluated at the Texas A&M University Agricultural Research and Extension Center, Uvalde, TX, during two seasons characterized by daily maximum temperatures as high as 36 °C. Measurements during both seasons included days to heading, days to physiological maturity and grain yield. Large and significant (P < 0.05) grain yield differences were measured among cultivars within each season. Yield varied between 2979 and 4671 kg ha^?1 in the first season and between 1916 and 5200 kg ha^?1 in the second season. Late planting in the second season delayed heading date resulting in the post‐heading period to coincide with periods of high temperatures. Cultivars that headed early, in general, yielded better than cultivars that headed later within each season with yield reduction of 35.3 kg ha^?1 in the first season and 91.0 kg ha^?1 in the second season for every 1 day delay in heading after mid‐March. Early‐heading cultivars outperformed later‐heading cultivars because of two distinct advantages: the early‐heading cultivars had longer post‐heading and, therefore, longer grain filling period than the later‐heading cultivars. In addition, early‐heading cultivars completed a greater fraction of the grain filling earlier in the season when air temperatures were lower and generally more favourable. The advantage of earlier‐heading cultivars was also manifested in the amount of green leaves retained to anthesis. Earlier‐heading cultivars produced fewer total leaves per tiller but retained more green leaves and lost fewer leaves to senescence at anthesis than later‐heading cultivars. The results suggest that early heading is an important and effective single trait defining wheat cultivars adapted to production systems prone to high temperature stress during the post‐heading period.     

Effect of Heat Stress on the Photosynthetic Characteristics in Flag Leaves at the Grain‐Filling Stage of Different Heat‐Resistant Winter Wheat Varieties

Heat stress has become an increasingly important factor in limiting wheat yields. In northern China, high temperature (>30 °C) during the grain filling is one of the major constraints in increasing wheat productivity. We used two winter wheat (Triticum aestivum L.) cultivars with different sensitivities to heat stress (Jimai 22 ‘JM22’, low sensitivity and Xinmai 26 ‘XM26’, high sensitivity) to study the various aspects of photosynthetic characteristics during the grain filling stage under heat stress. The results showed that photosynthesis rates (P_n) in flag leaves of XM26 decreased faster than in JM22 under heat stress during the grain‐filling stage. P_n decreased more rapidly under heat stress than without stress, by up to 69.9% and 59.3%, respectively, at 10 days following heat stress (10 DAS). This decline of P_n was not caused by heat‐induced stomatal limitation, but rather by a decline in Rubisco activity and a functional drop in photosystem II (PSII). After heat stress, the grain yield of JM22 decreased by 6.41%, but XM26 decreased by 11.43%, when compared with their respective controls. Heat stress also caused an alteration of mesophyll cell ultrastructure. Injury caused by heat stress to organelles in XM26 was more severe than JM22. Moreover, the JM22 cultivar showed some self‐repair capacity following heat stress injury. These results indicate that declines in photosynthetic performance caused by heat stress were cultivar‐dependent. Compared with XM26, the JM22 cultivar had superior heat stability in terms of PSII function and carboxylation activity, both of which are susceptible to heat stress.