Effects of High Temperature on Key Enzymes Involved in Starch and Protein Formation in Grains of Two Wheat Cultivars

High temperature is a major determinant of grain growth and yield formation in wheat. The present study was undertaken to investigate the effects of high temperature regimes on the activities of key regulatory enzymes involved in starch and protein accumulation in grains of two winter wheat (Triticum aestivum L.) cultivars Yangmai 9 and Xuzhou 26 with different protein contents. Four day/night temperature regimes of 34 °C/22 °C, 32 °C/24 °C, 26 °C/14 °C and 24 °C/16 °C were established after anthesis, resulting in two daily temperature levels of 28 °C and 20 °C and two diurnal temperature differences of 12 °C and 8 °C. The activities of glutamine synthase (GS) in flag leaves and glutamate pyruvic aminotransferase (GPT), sucrose synthase (SS), soluble starch synthase (SSS) and granule‐bound starch synthase (GBSS) in grains were measured during the periods of grain filling. High temperature reduced both content and yield of starch in grains, while enhanced protein content and reduced protein yield in grains. High temperature significantly enhanced the activities of SS and GBSS on 14 days after anthesis (DAA). High temperature affected SSS slightly in Yangmai 9, but reduced SSS activity markedly in Xuzhou 26 on 14 DAA. However, at the middle and late stages of grain filling, high temperature reduced the activities of SS, GBSS and SSS significantly in the two wheat cultivars. High temperature reduced GPT activity in grains in the two wheat cultivars, but reduced GS activity in flag leaves of Yangmai 9 and enhanced GS activity of Xuzhou 26 on 14 DAA. In addition, under the same high temperature level, SS activity was higher at 34 °C/22 °C, whereas the activities of SSS and GBSS were higher at 32 °C/24 °C. Also, diurnal temperature differences affected GPT and GS activities differently between the two cultivars. Under optimum temperature level, the activities of key enzymes for starch and protein synthesis were higher at 26 °C/14 °C. The activities of SS, SSS and GBSS significantly correlated with starch accumulation in grains, except for GBSS activity to starch content on 14 DAA. GPT activity was positively correlated with protein yield, and GS activity was negatively correlated with protein yield on 14 DAA, while the activities of both GPT and GS were negatively related to protein content in grains.     

1‐Methylcyclopropene (1‐MCP)‐Induced Alteration in Leaf Photosynthetic Rate,Chlorophyll Fluorescence,Respiration and Membrane Damage in Rice (Oryza sativa L.) Under High Night Temperature

High night temperature (HNT) can induce ethylene‐triggered reactive oxygen species production, which can cause premature leaf senescence and membrane damage, thereby affecting production, consumption and transfer of photosyn‐thates, and yield. The 1‐methylcyclopropene (1‐MCP) can competitively bind with ethylene receptors and decrease ethylene effects. The objective was to determine the effects of HNT and 1‐MCP on leaf photosynthetic rate (P_N), chlorophyll fluorescence, total chlorophyll (TC), respiration, membrane damage, pollen germination, spikelet fertility (SF) and yield of rice hybrid ‘XL723’. Plants were grown under ambient night temperature (ANT) (25 °C) or HNT (30 °C) with or without 1‐MCP treatment. Application of 1‐MCP was at the boot stage. The decrease in yield (11 %) under HNT was associated with decreased P_N (4 %), stomatal conductance (8 %), quantum yield (11 %) TC (23 %) and SF (5 %) and increased respiration (74 %), F_o/F_m (increase in thylakoid membrane damage; 11 %) and membrane damage (leaf electrolytic leakage; 57 %). The 1‐MCP‐treated plants grown under HNT showed increased yield (17 %), which was associated with increased P_N (10 %), stomatal conductance (30 %), quantum yield (9 %), TC (37 %) and SF (11 %) and decreased respiration (39 %), F_o/F_m (5 %) and membrane damage (18 %). Plants grown under HNT showed increased grain chalkiness (154 %) compared with plants grown under ANT.     

Effects of Night Temperature,Spikelet Position and Salicylic Acid on Yield and Yield‐Related Parameters of Rice (Oryza sativa L.) Plants

Periodic episodes of heat stress and seasonally high night temperatures (HNT) are predicted to occur more frequently in the current changing weather environment. These events affect aspects of crop growth and development, including oxidative‐stress damage, reducing crop yield and quality. Salicylic acid (SA), a naturally occurring phenolic compound, associated with thermo‐tolerance, prevents oxidative damage by detoxifying superoxide radicals and altering antioxidant capacity in plants. A study was conducted to determine the effects of HNT and SA on multiple rice yield‐determining parameters with special emphasis on effects of HNT and SA at different spikelet positions in the panicle. Plants were grown under ambient night temperature (27 °C) or HNT (32 °C) in the greenhouse. They were subjected to HNT through use of continuously controlled infrared heaters, starting from 2000 h to 0600 h. The HNT did not affect productive tillers, main‐stem panicle length or number of primary branches per panicle; however, reduced yield resulted from significant negative effects on spikelet fertility (SF), grain length and width. The grains located at the base of the panicle showed decreased fertility, length and width. Application of SA increased antioxidant capacity, thereby preventing damage to membranes, hence increasing yield by predominantly affecting SF in rice plants.     

Frost response in wheat and early detection using proximal sensors

Frost can significantly reduce production of field crops grown in Mediterranean‐type environments, where significant economic losses for Australian wheat occur annually. If non‐destructive sensors could make rapid, spatial assessment of frost damage, this could limit economic losses through timely management decisions. This paper reports on a methodology for imposing frost treatments to wheat under field conditions and the utility of canopy reflectance data for detecting early crop damage. Purpose‐built chambers using stepped additions of dry ice allowed for a range of frost scenarios to be tested when applied at wheat heading and anthesis. For frost treatments applied at anthesis, grain number and yield were reduced by 8.8% and 7.2%, respectively, for every degree Celsius below zero (down to ?4°C). This effect was additive over two consecutive nights. For cold load equivalent, there was a 2.2% and 1.9% reduction in grain number and yield, respectively, per °C hr (below 0°C). For wheat, spectral indices PRI and NDVI (reflectance) and FRF_G and SFR_G (fluorescence) showed significant relationships, with cold load applied for heading treatments. Next steps include targeting frost intensities equivalent to cold loads between 20 and 80°C hr and testing the utility of these proposed indices in a commercial paddock setting.     

Increased Night Temperatures During Cotton's Early Reproductive Stage Affect Leaf Physiology and Flower Bud Carbohydrate Content Decreasing Flower Bud Retention

Higher than optimum temperatures during cotton's (Gossypium hirsutum L.) growing season is a common occurrence in cotton‐growing areas around the world with negative consequences on productivity. According to climate projections, night temperatures are expected to increase more than day temperatures. The objective of this study was to monitor and record the effects of higher than optimum night temperatures during cotton's early reproductive stage on physiological (photosynthesis, respiration and membrane damage) and biochemical (leaf adenosine triphosphate (ATP) levels, as well as glutathione reductase (GR) content and soluble carbohydrate concentrations of the leaf subtending the flower buds. Number, dry weight, carbohydrate concentrations and GR levels of flower buds were also measured at the end of the experiment. Growth chamber experiments were conducted using cotton cultivar ST 5288 B2RF and treatments consisted of normal day/night temperatures (32/24 °C) and high night temperatures (32/30 °C) for 7 days at squaring (approximately 4 weeks after planting). The results indicated that high night temperatures had an immediate effect on leaf respiration rates and membrane damage by significantly increasing them compared to the control and a similar pattern was observed on leaf photosynthesis and ATP levels that were markedly decreased. Leaf GR levels were also substantially increased under conditions of high night temperatures, in contrast to flower bud GR content which remained unaffected. High night temperatures had a significant effect on leaf carbohydrate concentrations resulting in significant decreases in hexose, sucrose and starch levels. Nevertheless, flower bud sucrose content was substantially increased under conditions of high night temperatures, while hexose content was decreased and starch concentrations remained unaffected. A detrimental effect of elevated night temperatures on the number of flower buds per plant and on the dry weight of flower buds was observed, and it was concluded that high night temperatures had a negative effect on cotton flower bud production due to disruptions on flower bud carbohydrate metabolism as a result of the insufficient GR response.     

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.     

A strategy of ideotype development for heat-tolerant wheat

Heat stress significantly limits yield in many wheat-growing areas globally including north-western NSW. While various traits linked to high-temperature tolerance have been identified, the combination of traits that optimize the heat tolerance of wheat has not been established in most environments. A total of 554 genotypes were evaluated in the field at different times of sowing in north-western NSW for three consecutive years to develop a heat-tolerant wheat ideotype for this environment. The later sown experiments were exposed to higher temperatures at the critical reproductive and grain-filling stages of development. The impact of high temperature was greatest at anthesis, and eventual grain yield was reduced by between 4% and 7% with every 1°C rise in average maximum temperature above the optimum of 25°C. High temperature reduced yield, plant height, grain weight and days to anthesis and maturity, and increased the percentage of screenings and grain protein content. Genotypes that produced higher yield under heat stress had shorter days to flowering and maturity, higher NDVI during grain filling, greater chlorophyll content at the milk stage of grain fill, taller plants, greater grain weight and number, and lower screenings compared with the benchmark cultivar Suntop. The genotype closest to the predicted heat-tolerant wheat ideotype identified from trait ranges had 79.6% similarity.     

Effect of high temperature on pollen morphology,plant growth and seed yield in quinoa (Chenopodium quinoa Willd.)

Quinoa (Chenopodium quinoa Willd.) has gained considerable attention worldwide during the past decade due to its nutritional and health benefits. However, its susceptibility to high temperatures has been reported as a serious obstacle to its global production. The objective of this study was to evaluate quinoa growth and pollen morphology in response to high temperatures. Pollen morphology and viability, plant growth and seed set, and several physiological parameters were measured at anthesis in two genotypes of quinoa subjected to day/night temperatures of 22/16°C as a control treatment and 40/24°C as the heat stress treatment. Our results showed that heat stress reduced the pollen viability between 30% and 70%. Although no visible morphological differences were observed on the surface of the pollen between the heat‐stressed and non‐heat‐stressed treatments, the pollen wall (intine and extine) thickness increased due to heat stress. High temperature did not affect seed yield, seed size and leaf greenness. On the other hand, high temperature improved the rate of photosynthesis. We found that quinoa has a high plasticity in response to high temperature, though pollen viability and pollen wall structure were affected by high temperatures in anthesis stage. This study is also the first report of quinoa pollen being trinucleate.     

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.     

Evaluation of Usefulness of Daily Mean Temperature Studies on Impact of Climate Change

The impact of global warming on rise in temperature in different regions has often been expressed as a change in mean temperature (T_mean). The recent results suggest that this change could be both in diurnal and interannual temperatures. Therefore it is important to assess the impact of diurnal variation with the same mean temperature on crop plants for understanding the impact of climate change on agriculture, and also assess the possibility of genetic variation in adaptation. The present study in wheat (Triticum aestivum) varieties examines the effect of varying T_max and T_min, while maintaining the same mean temperature on phenology, growth and productivity. The mean temperatures examined are 18 °C with T_max/T_min combination of 18/18, 20/16, 22/14 and 24/12 °C. These wheat varieties differed considerably in their response to varying T_max and T_min with respect to days to ear emergence, anthesis, biomass accumulation and grain yield. The wheat variety HD2329, a popular Mexican dwarf high yielding cultivar showed maximum adaptation in the temperature combinations examined. The results suggest the need to refine the crop ideotypes in the context of the changing global scenario. This may require detailed experimental studies on various phenological phases. Such studies would help in assessing genotypes which may be having adaptation and thus identify the potential donors for further improvement of crops.     

Differential heat sensitivity of two cool-season legumes,chickpea and lentil,at the reproductive stage,is associated with responses in pollen function,photosynthetic ability and oxidative damage

Increasing temperatures are adversely affecting various food crops, including legumes, and this issue requires attention. The growth of two cool-season food legumes, chickpea and lentil, is inhibited by high temperatures but their relative sensitivity to heat stress and the underlying reasons have not been investigated. Moreover, the high-temperature thresholds for these two legumes have not been well-characterised. In the present study, three chickpea (ICCVO7110, ICC5912 and ICCV92944) and two lentil (LL699 and LL931) genotypes, having nearly similar phenology with respect to flowering, were grown at 30/20°C (day/night; control) until the onset of flowering and subsequently exposed to varying high temperatures (35/25, 38/28, 40/30 and 42/32°C; day/night) in a controlled environment (growth chamber; 12 hr/12 hr; light intensity 750 µmol m⁻² s⁻¹; RH-70%) at 108 days after sowing for both the species. Phenology (podding, maturity) was accelerated in both the species; the days to podding declined more in lentil at 35/25 (2.8 days) and 38/28°C (11.3 days) than in chickpea (1.7 and 7.1 days, respectively). Heat stress decreased flowering–podding and podding–maturity intervals considerably in both the species. At higher temperatures, no podding was observed in lentil, while chickpea showed reduction of 14.9 and 16.1 days at 40/30 and 42/32°C, respectively. Maturity was accelerated on 15.3 and 12.5 days at 38/28°C, 33.6 and 34 days at 40/30°C and 45.6 and 47 days at 42/32°C, in chickpea and lentil, respectively. Consequently, biomass decreased considerably at 38/28°C in both the species to limit the yield-related traits. Lentil was significantly more sensitive to heat stress, with the damage—assessed as reduction in biomass, reproductive function-related traits (pollen viability, germination, pollen tube growth and stigma receptivity), leaf traits such as membrane injury, leaf water status, photochemical efficiency, chlorophyll concentration, carbon fixation and assimilation, and oxidative stress, appearing even at 35/25°C, compared with 38/28°C, in chickpea. The expression of enzymatic antioxidants such as superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and non-enzymatic antioxidants declined remarkably with heat stress, more so in lentil than in chickpea. Carbon fixation (assessed as Rubisco activity) and assimilation (assessed as sucrose concentration, sucrose synthase activity) were also reduced more in lentil than in chickpea, at all the stressful temperatures, resulting in more inhibition of plant biomass (shoot + roots), damage to reproductive function and severe reduction in pods and seeds. At 38/28°C, lentil showed 43% reduction in biomass, while it declined by 17.2% in chickpea at the same time, over the control temperature (30/20°C). At this temperature, lentil showed 53% and 46% reduction in pods and seed yield, compared to 13.4% and 22% decrease in chickpea at the same temperature. At 40/30°C, lentil did not produce any pods, while chickpea was able to produce few pods at this temperature. This study identified that lentil is considerably more sensitive to heat stress than chickpea, as a result of more damage to leaves (photosynthetic ability; oxidative injury) and reproductive components (pollen function, etc.) at 35/25°C and above, at controlled conditions.     

Independent and Combined Effects of Soil Warming and Drought Stress During Anthesis on Seed Set and Grain Yield in Two Spring Wheat Varieties

Increase in soil temperature together with decrease in soil moisture during anthesis of spring wheat (Triticum aestivum L) crops is predicted to occur more frequently in a future climate in Denmark. The objective of this study was to investigate the responses of two Danish spring wheat varieties (Trappe and Alora) to soil warming (H), drought (D) and both (HD) during anthesis. The plants were grown in pots in a climate‐controlled glasshouse. In H, the soil temperature was increased by 3 °C compared with the control (C). In both D and HD treatments, the plants were drought‐stressed by withholding irrigation until all of the transpirable soil water had been depleted in the pots. Results showed that, particularly under D treatment, Alora depleted soil water faster than Trappe. In both varieties, flag leaf relative water content (RWC) was significantly lowered, while spikelet abscisic acid (ABA) concentration was significantly increased by D and HD treatments. Compared with the C plants, D and HD treatments significantly reduced ear number, ear to tiller ratio, shoot biomass, grain yield, harvest index and seed set but hardly affected tiller number and 1000‐kernel weight, whereas H treatment alone only decreased shoot biomass and reduced seed set. When analysed across the varieties and the treatments, it was found that the reduction in seed set was closely correlated with the increase in spikelet ABA concentration, indicating that D and HD treatments induced greater spikelet ABA concentrations might have caused seed abortion. It was concluded that the grain yield reduction under D and HD treatments during anthesis in spring wheat is ascribed mainly to a lowered seed set and wheat varieties (i.e. Alora) with more dramatic increase in spikelet ABA concentration are more susceptible to D and HD treatment.     

Independent and Combined Effects of High Temperature and Drought Stress During Grain Filling on Plant Yield and Chloroplast EF‐Tu Expression in Spring Wheat

High temperature and drought stress are among the two most important environmental factors influencing crop growth, development and yield processes. These two stresses commonly occur in combination. Objectives of this research were to investigate the independent and combined effects of high temperature and drought stress during grain filling on physiological, vegetative and yield traits and expression of a chloroplast protein synthesis elongation factor (EF‐Tu) of wheat (Triticum aestivum L.). Two spring wheat cultivars (Pavon‐76 and Seri‐82) were grown at control temperatures (CT; day/night, 24/14 °C; 16/8 h photo/dark period) from sowing to heading. Thereafter, one half of the plants were exposed to high temperature stress (HT; 31/18 °C in Exp. 1 and 34/22 °C in Exp. 2), drought stress (withholding water), or a combination of both HT and drought stress. There were significant influences of HT and/or drought stress on physiological, growth and yield traits. There was no cultivar or cultivar by temperature or cultivar by drought interaction effects on most traits. The decreases in leaf photosynthesis were greater at HT compared with drought alone throughout the stress period, and the combination of HT and drought had the lowest leaf photosynthetic rates. Overall, HT or drought had similar effects (about 48–56 % decrease) on spikelet fertility, grain numbers and grain yield. High temperature decreased grain numbers (by 56 % averaged across both experiments) and individual grain weight (by 25 %), while, respective decreases due to drought were 48 % and 35 %. This suggests that the grain numbers were more sensitive to HT and grain weights to drought for the range of temperatures tested in this research. The interaction between HT and drought stress was significant for total dry weights, harvest index and spikelet fertility, particularly when HT stress was severe (34/22 °C). The combined effects of HT and drought were greater than additive effects of HT or drought alone for leaf chlorophyll content, grain numbers and harvest index. High temperature stress and the combination of HT and drought stress but not drought stress alone resulted in the overexpression of EF‐Tu in both spring wheat cultivars.     

Effects of Mild Temperature Stress on Grain Quality and Root and Straw Nitrogen Concentration in Malting Barley Cultivars

Abstract It is a challenge to obtain the appropriate protein concentration in cereals for the intended end‐use. This study examined ambient temperature effects on two spring malting barley cultivars (Henley and Tipple) grown in soil or in solution culture with controlled nitrogen supply in daylight chambers with low temperature (day 18 °C, night 12 °C), and high temperature (23 °C/17 °C) to/after anthesis. In soil‐grown plants, high temperature to anthesis resulted in higher grain nitrogen amount (GNA), grain nitrogen concentration (GNC) and straw nitrogen concentration (SNC). In plants grown in solution, high temperature to anthesis resulted in lower GNA and higher GNC. A temperature rise of 1 °C during the growing period in solution cultivation increased GNC, root nitrogen concentration (RNC) and SNC, by 1.20, 1.35 and 0.33 mg g^?1, respectively. In solution culture, GNC was positively correlated with RNC and SNC (P < 0.01). Cv. Henley had higher GNC but lower SNC than cv. Tipple. Cv. Henley was more stable in grain size and cv. Tipple in GNC. The results showed that temperature has a direct effect on GNC. Accounting for temperature fluctuations up to the latest possible nitrogen fertilisation occasion can therefore help when deciding appropriate nitrogen supply for intended end‐use.     

Winter Soil Warming Exacerbates the Impacts of Spring Low Temperature Stress on Wheat

The increase in global mean air temperature is likely to affect the soil temperatures in agricultural areas. This study aims to study the effects of winter soil warming on the responses of wheat to low temperature stress in spring. Wheat plants were grown under either normal or increased soil temperature by 2.5 °C for 82 days in winter. The physiological and yield responses of the plants to a 2‐day low temperature stress (4/2 °C in the day/night) at jointing stage were investigated. After exposing to low spring temperature, the plants that had experienced winter soil warming showed lower leaf and root water potential, lower oxygen scavenging capacity and poor photosynthetic performance as compared with the plants grown under normal soil temperature during winter. WL plants had significantly lower sugar content in shoot than the CL plants, which might have contributed to their higher susceptibility to low temperature. In addition, winter soil warming exacerbated the negative effects of low spring temperature on grain yield.     

Variation in dormancy duration of the U.K. wheat cultivar Hornet due to environmental conditions during grain development

Dormancy of wheat grains, the property conferring sprouting resistance, is affected by environmental conditions experienced during grain development. We investigated the hypothesis that short dormancy duration in U.K. wheat grain (thus a high risk of post-maturity sprouting) is related to weather conditions, i.e. high temperatures during grain development. Four wheat varieties were grown at four sites ranging from the far south to the far north of the country in the years 1995–1997,ensuring different temperature and rainfall conditions during grain development. This paper focuses on one variety, Hornet, which has a high sprouting resistance rating. Other varieties gave similar results. Serial laboratory germination tests (seven days, 20°C) at 100°C-dayintervals were used to measure dormancy duration, which was assessed from logistic curves fitted to the data. During the experiment the mean temperatures during grain development differed by over4°C, due to the site × year effect. Significant effects (p>0.05) of site and year (i.e. weather) on dormancy were found, when definitions of dormancy duration of D_A (number of days from anthesis to 50% germination in seven days at 20°C) or D_P (number of days from physiological maturity at 45% grain moisture to 50% germination) were used. Dormancy was markedly shorter in the hot, dry year 1995 compared to the cooler, wetter years 1996 and 1997. A relationship, as postulated by Belderok, between accumulated temperature during the dough stage of grain filling and dormancy duration was not found. However, a relationship of dormancy duration to the mean temperature during grain development was found, with short dormancy periods occurring after high mean temperatures were experienced. This revised version was published online in August 2006 with corrections to the Cover Date.     

Daylength,Temperature and Solar Radiation Effects on the Phenology and Yield Formation of Spring Durum Wheat

Future food security will depend on crop adaptation to changing environments. We studied the limitations imposed by daylength, temperature and solar radiation on wheat yield in eight field experiments conducted at contrasting northern latitudes and involving 42 adapted spring durum wheat genotypes of divergent phenology, and reduced or without photoperiod sensitivity. Air temperatures averaged from sowing to anthesis (SA) increased from northern to southern sites, while daylength and minimum temperatures from anthesis to maturity (grain filling, GF) followed the opposite trend, due to differences in the latitude of sites. The site effect explained 96 % of the variation in the number of days SA, which was much smaller in southern sites. Average minimum daily temperatures above 6.9 °C before anthesis and below 10.8 °C during GF accompanied by photoperiods during GF of less than 14.2 h resulted in less than 14 000 kernels m^?2, which was the threshold below which kernel number limited yield. Radiation during GF lower than 1.8 kJ kernel^?1 day^?1 limited kernel weight, which was then a constraint to the achievement of yield potential.     

Impact of Humidity on Temperature-Induced Grain Sterility in Rice (Oryza sativa L)

High temperature‐induced grain sterility in rice is becoming a serious problem in tropical rice‐growing ecosystems. We studied the mechanism of high temperature‐induced grain sterility of different rice (Oryza sativa L) cultivars at two relative humidity (RH) levels. Four varieties of Indica and Japonica rice were exposed to over 85 % RH and 60 % RH at 36/30 °C, 34/30 °C, 32/24 °C and 30/24 °C day/night air temperatures from late booting to maturity inside sunlit phytotrons. Increasing both air temperature and RH significantly increased spikelet sterility while high temperature‐induced sterility decreased significantly with decreasing RH. Neither Indica nor Japonica rice types were superior to the other in the response of their spikelets to increased air temperature and RH. Increased spikelet sterility was due to increased pollen grain sterility which reduced deposition of viable pollen grains on stigma. Reduction in sterility with decreased RH was more due to decreased spikelet temperature than to air temperature. Thus the impact of RH should be considered when interpreting the effect of high temperature on grain sterility. Spikelet fertility was curvilinearly related to spikelet temperature. Grain sterility increased when spikelet temperature increased over 30 °C while it became completely sterile at 36 °C. The ability of a variety to decrease its spikelet temperature with decreasing RH could be considered as avoidance while the variability in spikelet sterility among varieties at a given spikelet temperature could be considered as true tolerance.     

灌浆期高温对两种品质类型小麦品种籽粒淀粉合成关键酶活性的影响

以徐州26（高蛋白含量）和扬麦9号（低蛋白含量）2个不同品质类型小麦品种为材料，利用人工气候室模拟小麦花后高温条件，研究了灌浆期高温对籽粒淀粉积累及淀粉合成关键酶活性的影响。结果表明，与适温处理（20℃）相比，高温（28℃）显著降低了籽粒中总淀粉及支链淀粉的含量，而对直链淀粉含量的影响较小，导致支/直链淀粉的比例显著降低。高温提高了灌浆初期小麦籽粒中蔗糖合成酶（SS）和结合态淀粉合成酶（GBSS）活性，但明显降低了灌浆后期SS、GBSS和可溶性淀粉合成酶（SSS）活性。品种间相比，灌浆初期高温处理提高了扬麦9号籽粒GBSS活性，但显著降低了徐州26籽粒SSS活性，说明高温抑制2种类型小麦籽粒淀粉合成的酶学机制不同。此外，不同昼夜温差处理（8℃和12℃）间比较发现，高温下两品种籽粒淀粉含量没有明显差异，适温下两品种淀粉含量以温差较大的处理含量较高。灌浆中后期，高温下SS活性随温差增大而升高，SSS和GBSS则以温差小的处理较高；适温下3种关键酶活性均以温差大的处理为高。总之，灌浆期温度较温差对小麦籽粒淀粉合成的影响大。