Short Periods Of High Temperature Research Articles

Effects of extreme temperature events on the parasitism performance ofDiadegma semiclausum, an endoparasitoid ofPlutella xylostella

AbstractWith climate change, the occurrence of extreme weather events such as heatwaves is expected to increase. Extreme temperature events are short periods of high temperature (i.e., >95th percentile of temperature distribution), and their occurrences are difficult to predict. These extreme temperature events pose great challenges to living organisms, especially ectotherms such as insects whose body temperature is greatly influenced by environmental temperature. Temperature is known to play an essential role in driving insect performance as well as inducing behavioral changes. Exposure to extreme temperatures causes physiological damage leading to potential long‐lasting effects on insect performance. We aimed at assessing the impact of direct (i.e., current) and indirect (previously experienced) exposure to extreme temperature on the oviposition success ofDiadegma semiclausumHellén (Hymenoptera: Ichneumonidae).Diadegma semiclausumis a larval endoparasitoid ofPlutella xylostellaL. (Lepidoptera: Plutellidae), a worldwide pest of brassicaceous crops. Oviposition success, measured as the percentage of parasitizedP. xylostella, was significantly lower at 33 °C than at 25 °C. However, previous exposure to 33 °C did not significantly impact the egg‐laying performance ofD. semiclausum, indicating that, after a heatwave, the egg‐laying performance can recover. Neither the direct nor indirect exposure to extreme temperature alteredD. semiclausumhost‐instar preference. These results indicate that temperature directly impactsD. semiclausumparasitism performance, although more detailed research is needed to disentangle direct physiological mechanisms from behavioral responses. Overall, these study results confirm others, suggesting that control ofP. xylostellabyD. semiclausumis likely to be impaired by more frequent heatwaves predicted under climate change scenarios.

Fifteen years in a global warming hotspot: changes in subtidal mobile invertebrate communities

Temperate subtidal reefs are increasingly exposed to gradual warming and short periods of high temperatures (marine heatwaves; MHWs). These pressures can directly and indirectly affect the mobile invertebrate communities on these reefs. We investigate changes in mobile invertebrate communities from benthic surveys 15 yr apart (1999-2001 vs 2016-2019), spanning a 4° latitudinal gradient in Western Australia (30-34°S), expecting the biggest changes to the communities in the lower latitudes, where the cumulative effect of MHWs and warming is largest. The urchinsCentrostephanus tenuispinus(warm temperate affinity) andPhyllacanthus irregularis(cool temperate affinity) showed trends toward opposite responses over time; whileP. irregularisdensities declined (non-significantly at all sites),C. tenuispinusdensities increased (significantly at one of 3 sites). The magnitude of the responses appeared to decrease with increasing latitude, withC. tenuispinusrecorded in significantly higher densities only at the lowest latitude location. Neither the densities, nor change in density over time of cool temperate gastropodLunella torquatusand urchinHeliocidaris erythrogrammareflected a latitudinal gradient, suggesting other localized factors play a larger role in determining abundances of these species. However, size distributions ofL. torquatuspopulations at the lower latitude locations were clearly impacted by the 2011 MHW, while those at higher latitudes remained relatively consistent. Overall, the biggest changes over time were seen at the warm edge of the temperate ecosystem. As temperatures continue to rise, the magnitude of these changes is not only expected to increase, but also to occur at higher latitudes.

Exposure to heat stress during flowering period reduces flower yield and pyrethrins in Pyrethrum (Tanacetum cinerariifolium)

AbstractHeat stress is one of the major limitations to crop productivity worldwide. Global warming effects are expected to increase the number of hot days and increase the probability and intensity of heat stress events. Short periods (3–5 days) of heat stress with maximum temperatures exceeding 35°C often occur during late spring and early summer in some pyrethrum growing regions of Australia. These heat stress events usually coincide with pyrethrum flowering period. Pyrethrum is a perennial herbaceous plant which is commercially grown for extraction of pyrethrins which accumulate in the achenes of the flowers and are used as a natural insecticide. This experiment was conducted to understand the effects of timing of short periods of heat stress on flower development and pyrethrum yield. Plants were subjected to short periods of high temperature treatments (12 hr at 35–40°C) for three consecutive days at three flower maturity stages (early, mid, late). Control plants were grown at ambient temperature (10–25°C) throughout the flowering period. Exposure of pyrethrum plants to short periods of high temperature during the flowering period caused a significant reduction in the flower and pyrethrin yield. This was associated with the reduction in flower size and accelerated flower senescence. Exposure of pyrethrum plants to heat stress significantly increased the rate of flower development resulting in a shorter flowering period. Overall, plants grown under control treatment showed slower rate of flower development and longer duration flowering period. This resulted in longer duration of pyrethrin accumulation and higher yield of pyrethrins per flower. Timing and duration of heat stress significantly influenced pyrethrin yield per flower. Heat stress caused more severe yield reductions at early flowering than later in the flowering period. Research focusing on agronomic strategies, phenology and breeding for tolerance to heat stress is therefore important to cope with future climate changes and to obtain maximum pyrethrin yield.

High Temperatures during Flowering Reduce Fruit Set in Rabbiteye Blueberry

After nearly a decade of development, the scale of blueberry (Vaccinium sp.) cultivation has increased, particularly in south China; however, this region is becoming increasingly challenged by temperature changes during the flowering phenophase. Understanding the effects of temperature on pollen germination and pollen tube growth in blueberry is thus important. Using the rabbiteye blueberry (V. ashei) ‘Brightwell’, different temperature treatments were carried out during open pollination and cross-pollination with the pollen from rabbiteye blueberry ‘Gardenblue’ in field, greenhouse, and controlled temperature experiments over two consecutive years. The differences in pollen germination, pollen tube dynamics, and ovule viability following different treatments were analyzed, and the critical temperatures were calculated using quadratic and modified bilinear equations to quantify the developmental responses to temperature. The results showed that the fruit set of the artificially pollinated plants inside the greenhouse was significantly higher than that outside the greenhouse. Furthermore, pollen germination and pollen tube growth gradually accelerated under the appropriate high-temperature range, resulting in reduced pollen tube travel time to the ovule. However, the percentage of the style traversed by the pollen tube did not increase at temperatures greater than 30 °C, and a high-temperature range could accelerate ovule degeneration. Therefore, impairment of pollen tube growth in the upper half of the style following pollen germination and ovule degeneration constituted important factors leading to reduced fruit setting under short periods of high temperature during the flowering phenophase in rabbiteye blueberry. This work advances our understanding of the effect of temperature on pollen germination, pollen tube growth, ovule longevity, and fruit setting in rabbiteye blueberry, and provides a foundation for continued cultivation and breeding enhancement. The findings propose that the tolerance of rabbiteye blueberry to a certain high-temperature range in the flowering phenophase should inform breeding strategies for temperature resistance and that temperature range is also an important indicator of suitable environments for cultivation to mitigate potential temperature stress.

Can elevated CO2 buffer the effects of heat waves on wheat in a dryland cropping system?

Increasing atmospheric CO2 concentration [CO2] drives the rise in global temperatures, with predictions of an increased frequency of heat waves (short periods of high temperatures). Both, CO2 and high temperature, have profound effects on wheat growth and productivity. We tested whether elevated [CO2] (eCO2) has a potential to ameliorate the effects of simulated heat waves (HT) on wheat in a dryland cropping system. Wheat was field-grown at the Australian Grains Free Air CO2 Enrichment (AGFACE) facility under ambient [CO2] (∼390 ppm) or eCO2 (∼550 ppm) for two growing seasons, one with ample water supply and one of severe drought. Using heated chambers, heat waves (3-day periods of high temperatures) were imposed at critical growth stages before anthesis (HT1) or post-anthesis (HT2, HT3). Gas exchange, chlorophyll content and concentration of nitrogen (N) in mainstem flag leaves, as well as concentrations of stem water-soluble carbohydrates (WSC) in mainstems were monitored throughout the season. Yield, biomass and thousand kernel weights (TKW) were measured at maturity. Elevated [CO2] moderated the effect on net CO2 assimilation rates of pre-anthesis (HT1), but not of post-anthesis heat waves (HT2, HT3). Growth under eCO2 increased stem WSC both, with and without experimental heat waves, but remobilisation decreased significantly under heat indicating that a greater WSC pool does not necessarily translate into greater remobilisation into the grain. Grain yield (g m−2) was greater under eCO2 and especially pre-anthesis heat stress decreased grain yield in the wetter season, and this decrease was stronger under eCO2 (up to 20%) than under aCO2 (up to 10%). Grain N decreased under eCO2, but less so under heat stress. We conclude that eCO2 may moderate some effects of heat stress in wheat but such effects strongly depend on seasonal conditions and timing of heat stress.

Short periods of high temperature during meiosis prevent normal meiotic progression and reduce grain number in hexaploid wheat (Triticum aestivum L.)

Key messageExposure of wheat to high temperatures during male meiosis prevents normal meiotic progression and reduces grain number. We define a temperature-sensitive period and link heat tolerance to chromosome 5D.This study assesses the effects of heat on meiotic progression and grain number in hexaploid wheat (Triticum aestivum L. var. Chinese Spring), defines a heat-sensitive stage and evaluates the role of chromosome 5D in heat tolerance. Plants were exposed to high temperatures (30 or 35 °C) in a controlled environment room for 20-h periods during meiosis and the premeiotic interphase just prior to meiosis. Examination of pollen mother cells (PMCs) from immature anthers immediately before and after heat treatment enabled precise identification of the developmental phases being exposed to heat. A temperature-sensitive period was defined, lasting from premeiotic interphase to late leptotene, during which heat can prevent PMCs from progressing through meiosis. PMCs exposed to 35 °C were less likely to progress than those exposed to 30 °C. Grain number per spike was reduced at 30 °C, and reduced even further at 35 °C. Chinese Spring nullisomic 5D-tetrasomic 5B (N5DT5B) plants, which lack chromosome 5D, were more susceptible to heat during premeiosis–leptotene than Chinese Spring plants with the normal (euploid) chromosome complement. The proportion of plants with PMCs progressing through meiosis after heat treatment was lower for N5DT5B plants than for euploids, but the difference was not significant. However, following exposure to 30 °C, in euploid plants grain number was reduced (though not significantly), whereas in N5DT5B plants the reduction was highly significant. After exposure to 35 °C, the reduction in grain number was highly significant for both genotypes. Implications of these findings for the breeding of thermotolerant wheat are discussed.

Transcriptome profiling of two Iberian freshwater fish exposed to thermal stress

The congeneric freshwater fish Squalius carolitertii and S. torgalensis inhabit different Iberian regions with distinct climates; Atlantic in the North and Mediterranean in the South, respectively. While northern regions present mild temperatures, fish in southern regions often experience harsh temperatures and droughts. Previous work with two hsp70 genes suggested that S. torgalensis is better adapted to harsher thermal conditions than S. carolitertii as a result of the different environmental conditions. We present a transcriptomic characterisation of these species' thermal stress responses. Through differential gene expression analysis of the recently available transcriptomes of these two endemic fish species, comprising 12 RNA-seq libraries from three tissues (skeletal muscle, liver and fins) of fish exposed to control (18°C) and test (30°C) conditions, we intend to lay the foundations for further studies on the effects of temperature given predicted climate changes.Results showed that S. carolitertii had more upregulated genes, many of which are involved in transcription regulation, whereas S. torgalensis had more downregulated genes, particularly those responsible for cell division and growth. However, both species displayed increased gene expression of many hsps genes, suggesting that they are able to deal with protein damage caused by heat, though with a greater response in S. torgalensis. Together, our results suggest that S. torgalensis may have an energy saving strategy during short periods of high temperatures, re-allocating resources from growth to stress response mechanisms. In contrast, S. carolitertii regulates its metabolism by increasing the expression of genes involved in transcription and promoting the stress response, probably to maintain homoeostasis. Additionally, we indicate a set of potential target genes for further studies that may be particularly suited to monitoring the responses of Cyprinidae to changing temperatures, particularly for species living in similar conditions in the Mediterranean Peninsulas.

Effects of plant growth regulators and high temperature on colour development in ‘Crimson Seedless’ grapes

Summary‘Crimson Seedless’ is a red, seedless table grape (Vitis vinifera) variety which may not develop adequate colour during hot weather. This problem can be alleviated, in part, by applying abscisic acid (ABA). We were interested to determine whether a gibberellin (GA3), which is used to improve the size of grape berries, might interfere with this response. We were also interested in examining whether short periods of high temperature would affect the response to ABA. In addition, we assessed the optimum time at which to apply ABA, and the role of ethylene in berry colour development. These experiments were conducted in commercial vineyards in Israel, and on detached berries held under controlled conditions. GA3 had no effect on the response to ABA. There was a mixed response to ethylene, and to the application of ethylene inhibitors in the vineyard or in controlled studies in growth chambers. The best response in terms of colour development (i.e., anthocyanin levels) on detached berries occurred when 200 mg l–1 ABA was applied in early August. A short period of high temperature did not affect the response to ABA, with similar accumulations of anthocyanins at 18ºC, 25ºC, or 38ºC for 24 h after ABA was applied. These results indicate that: (i) the practice of applying a GA3 spray to increase berry size did not affect the berry response to ABA; (ii) the responsiveness of grape berries to ABA was maintained for an extended time after the initiation of veraison; (iii) a period of high temperature after the application of ABA did not compromise the response of detached berries to ABA; and (iv) the response of berries to ethylene was not maintained after harvest. The best treatment to enhance colour in ‘Crimson Seedless’ grapes was therefore 200 – 400 mg l–1 ABA applied 2 – 3 weeks after veraison.

Grain yield in wheat as affected by short periods of high temperature, drought and their interaction during pre- and post-anthesis stages

Drought and high temperature are especially considered as key stress factors with high potential impact on crop yield. The aim of this study was to investigate the effect of short periods of high temperature, drought and their combination at different growth stages of plant under controlled conditions. The effects of high temperature and drought on grain yield were additive. However, high temperature increased the degree of water stress and the combined effects of drought and high temperature were much more severe than those of each individual treatment. High temperature and drought applied at post- anthesis shortened duration of maturation, grain filling duration and reduced grain yield, mean grain weight, grain number and thousand-grain weight.

Effects of soil temperature, moisture, and burial depths on carpogenic germination of Sclerotinia sclerotiorum and S. minor.

Extensive studies have been conducted on the carpogenic germination of Sclerotinia sclerotiorum, but carpogenic germination in S. minor has not been studied adequately. It remains unclear why apothecia of this pathogen have seldom been observed in nature. In this study, a new method was developed to produce apothecia in the absence of soil or sand, and carpogenic germination without preconditioning was recorded for 95 of the 96 S. sclerotiorum isolates tested. Carpogenic germination of the two species was compared under a variety of temperature, soil moisture, burial depths, and short periods of high temperature and low soil moisture. The optimal temperatures for rapid germination and for maximum germination rates were both lower for S. minor than for S. sclerotiorum. The temperature range for carpogenic germination was also narrower for S. minor than for S. sclerotiorum. A 5-day period at 30 degrees C, either starting on the 10th or 20th day of incubation, did not significantly affect carpogenic germination of S. sclerotiorum. For both S. minor and S. sclerotiorum, the percentage of carpogenically germinated sclerotia increased as soil water potential increased from -0.3 to -0.01 MPa. In the greenhouse, a 10- or 20-day dry period completely arrested carpogenic germination of S. sclerotiorum, and new apothecia appeared after an interval of 35 days following rewetting, similar to the initial carpogenic germination regardless of when the dry period was imposed. In naturally infested fields, the number of sclerotia in 100 cc of soil decreased as depth increased from 0 to 10 cm before tillage, but became uniform between 0 and 10 cm after conventional tillage for both species. Most apothecia of S. minor were, however, produced from sclerotia located at a depth shallower than 0.5 cm while some apothecia of S. sclerotiorum were produced from sclerotia located as deep as 4 to 5 cm. These results provide the much needed information to assess the epidemiological roles of inoculum from sexual reproduction in diseases caused by the two Sclerotinia species in different geographical regions. However, more studies on effects of shorter and incompletely dry periods are still needed to predict production of apothecia of S. sclerotiorum in commercial fields under fluctuating soil temperature and moisture.

Asymmetrical thermal constraints on the parapatric species boundaries of two widespread generalist butterflies

Abstract 1. The sibling species Papilio glaucus and Papilio canadensis meet in a narrow hybrid zone believed to be maintained by temperature thresholds acting independently on both species. The present study tests if this assertion is true for the cold‐adapted species, P. canadensis, which is presumed to be limited by the effect of high temperatures in late summer and/or autumn on pupal survival.2. Three experiments were conducted examining the effects of: (i) short periods of high temperature stress in autumn, (ii) prolonged warm temperatures in autumn, and (iii) temperatures simulating warmer winters/longer springs upon the survival of P. canadensis and P. glaucus.3. Results indicated that short periods of high temperatures did not induce the high mortality rates required to be the key factor limiting the range of P. canadensis. However, P. canadensis did exhibit a considerably lower tolerance to high temperature extremes, prolonged warm temperatures in autumn, and conditions simulating shorter/warmer winters than P. glaucus.4. Differences in temperature tolerance throughout the pupal stage are likely to be a significant factor in maintaining the southern range limit of P. canadensis. Further warming as may occur during climate change, particularly in winter and spring, will likely affect the dynamics of southerly populations of P. canadensis.

Integrated geophysical and geomorphological approach to investigate the snowmelt-triggered landslide of Bosco Piccolo village (Basilicata, southern Italy)

Most of Basilicata region in the southern Italian Apennines is characterized by landslides often developing in clayey–marly formations. Many events have been triggered by extreme rainfall or snowmelt. The most important happened (on February–March 2005) at Bosco Piccolo 5 km far from Potenza. This landslide developed subsequently to rapid snowmelt occurred during alternating short periods of high temperatures and intense and continuous snowfalls. This complex landslide affected 4 ha of surface and reached a maximum depth of 20 m inducing damage and collapse of about 80% of the buildings in the village. An integrated multidisciplinary approach has been adopted to study the landslide. A multitemporal aerial photo interpretation and detailed geomorphological surveys have been carried out. Combined Electrical Resistivity Tomography (ERT) and Self-Potential (SP) measurements have been performed and calibrated with boreholes stratigraphy. Such an integrated approach allowed us to partially reconstruct the geometry of the investigated body and to evaluate the effectiveness of drainage system planned for the area.

Canola and mustard response to short periods of temperature and water stress at different developmental stages

Seed yield of Brassica crops in semiarid environments can be increased by minimizing the crops’ exposure to high temperature and water stress that often occurs during the growing season. A growth chamber study was conducted to determine the effect of short periods of high temperature and water stress at different developmental stages on seed yield and yield components of Brassica crops. Two canola-quality Brassica juncea ‘PC98-44’ and ‘PC98-45’, a Brassica napus canola ‘Quantum’, and a B. juncea oriental mustard ‘Cutlass’ were grown under 20/18°C day/night temperatures with photoperiod of 16/8 h light/dark. High (35/18°C) and moderate (28/18°C) temperature stress was imposed for 10 d during bud formation, flowering, and pod development. Low (90% available water) and high (50% available water) water stress was imposed in combination with the temperature treatments. On average, the 35/18°C stress reduced main stem pods by 75%, seeds pod-1 25%, and seed weight 22% from the control. Seed yield per plant was reduced by 15% when plants were severely (35/18°C) stressed during bud formation, 58% when stressed during flowering, and 77% when stressed during pod development. Plants stressed at earlier growth stages exhibited recovery, whereas stress during pod development severely reduced most of the yield components. Effect of water stress on seed yield was minimal regardless of crop developmental stage. The four Brassica cultivars responded similarly to water stress. In response to temperature stress, B. juncea produced greater number of pods per plant but had a great rate of pod infertility than B. napus. Seed yield of B. juncea in semiarid environments can be increased by improving pod fertility, whereas the seed yield of B. napus can be increased by improving pod pro duction and retention. Key words: Oilseed, yield components, Brassica species, moisture.

A comparison of the 1998 and 2002 coral bleaching events on the Great Barrier Reef: spatial correlation, patterns, and predictions

Detailed mapping of coral bleaching events provides an opportunity to examine spatial patterns in bleaching over scales of 10 s to 1,000 s of km and the spatial correlation between sea surface temperature (SST) and bleaching. We present data for two large-scale (2,000 km) bleaching events on the Great Barrier Reef (GBR): one from 1998 and another from 2002, both mapped by aerial survey methods. We examined a wide range of satellite-derived SST variables to determine which one best correlated with the observed bleaching patterns. We found that the maximum SST occurring over any 3-day period (max3d) during the bleaching season predicted bleaching better than anomaly-based SST variables and that short averaging periods (3–6 days) predicted bleaching better than longer averaging periods. Short periods of high temperature are therefore highly stressful to corals and result in highly predictable bleaching patterns. Max3d SST predicted the presence/absence of bleaching with an accuracy of 73.2%. Large-scale (GBR-wide) spatial patterns of bleaching were similar between 1998 and 2002 with more inshore reefs bleached compared to offshore reefs. Spatial change in patterns of bleaching occurred at scales of ~10 s km, indicating that reefs bleach (or not) in spatial clusters, possibly due to local weather patterns, oceanographic conditions, or both. Approximately 42% of reefs bleached to some extent in 1998 with ~18% strongly bleached, while in 2002, ~54% of reefs bleached to some extent with ~18% strongly bleached. These statistics and the fact that nearly twice as many offshore reefs bleached in 2002 compared to 1998 (41 vs. 21%, respectively) makes the 2002 event the worst bleaching event on record for the GBR. Modeling of the relationship between bleaching and max3d SST indicates that a 1 °C increase would increase the bleaching occurrence of reefs from 50% (approximate occurrence in 1998 and 2002) to 82%, while a 2 °C increase would increase the occurrence to 97% and a 3 °C increase to 100%. These results suggest that coral reefs are profoundly sensitive to even modest increases in temperature and, in the absence of acclimatization/adaptation, are likely to suffer large declines under mid-range International Panel for Climate Change predictions by 2050.

Grain weight and malting quality in barley as affected by brief periods of increased spike temperature under field conditions

High temperature is usually one of the most important stresses during grain filling affecting both yield and quality in barley crops. In the present study, an attempt was made to assess in the field the effects of short periods of high temperature, using transparent boxes covering only the spikes, with thermostatically controlled electric resistance for increasing the temperature. Treatments consisted of 2 malting cultivars and 5 heat treatments of high temperatures (8�C above the environmental temperature for 6 h/day for 5 consecutive days) over different periods during grain filling. Final grain weight was reduced by 2–14%, depending on the timing of heat stress and the genotype. There was a significant increase in grain nitrogen percentage in both cultivars, and grain β-glucans decreased with high temperatures in Logan and were unchanged in Beka. The resulting malt extract was reduced with exposure to high temperatures, depending on the cultivar, implying that even mild heat stress may change malting performance.

Final grain weight in wheat as affected by short periods of high temperature during pre- and post-anthesis under field conditions

Individual grain weight is an important source of variation for grain yield in wheat. The aim of this study was to investigate the effect of short periods of high temperature immediately pre-anthesis, or during post-anthesis, on grain weight under field conditions. Thus, two wheat cultivars of different grain weight potential were sown on four different sowing dates to provide different temperature conditions during the pre- and post-anthesis periods. In addition, for two sowings, acrylic boxes were installed to increase spike temperature either immediately before anthesis, or during the lineal phase of the grain-filling period. Final grain weight was significantly affected by sowing date, genotype and grain position on the spike. Grain weight showed a clear relationship with the average temperature of the grain filling period, but this relationship was either linear or curvilinear, depending on the cultivar. Both high temperature treatments, i.e. at pre- or post-anthesis, significantly diminished final grain weight, and their effect was similar with the exception of heavier grains, which were unresponsive to the high temperature treatment at pre-anthesis. Finally, a better understanding of final grain weight was reached when temperatures from the pre-anthesis period were included in the analysis of grain weight response to temperature.

The regulation of isoprene emission responses to rapid leaf temperature fluctuations

Isoprene emission from leaves is temperature dependent and may protect leaves from damage at high temperatures. We measured the temperature of white oak (Quercus alba L.) leaves at the top of the canopy. The largest short‐term changes in leaf temperature were associated with changes in solar radiation. During these episodes, leaf temperature changed with a 1 min time constant, a measure of the rate of temperature change. We imposed rapid temperature fluctuations on leaves to study the effect of temperature change rate on isoprene emission. Leaf temperature changed with a 16 s time constant; isoprene responded more slowly with a 37 s time constant. This time constant was slow enough to cause a lag in isoprene emission when leaf temperature fluctuated rapidly but isoprene emission changed quickly enough to follow the large temperature changes observed in the oak canopy. This is consistent with the theory that isoprene functions to protect leaves from short periods of high temperature. Time constant analysis also revealed that there are two processes that cause isoprene emission to increase with leaf temperature. The fastest process likely reflects the influence of temperature on reaction kinetics, while the slower process may reflect the activation of an enzyme.

Effects of a period of high temperature during grain filling on the grain growth characteristics and malting quality of three Australian malting barleys

Short periods of high temperatures (up to 35°C) during mid grain filling appear to reduce yield and quality in barley. Plants of 3 malting barley varieties, Schooner, Arapiles, and Sloop (a new South Australian malting variety), were grown under constant environment conditions from germination to maturity and exposed to 5 days of high temperatures (up to 35°C) during mid grain filling. Schooner and Sloop showed similar patterns of accumulation of dry matter under control conditions (21°C/16°C, day/night temperature) and in response to high temperatures. In all varieties, the reduction in starch accumulation represented the most significant detrimental effect of high temperature and made the greatest contribution to the reduction in final grain weight. The reduction in absolute grain nitrogen (N) in heat-treated Arapiles grains represents a potentially important response under high temperature conditions. In this study, water loss did not have a decisive role in the termination of grain filling. Continued accumulation of endosperm dry matter at low moisture levels suggested that water distribution and/or components of water potential may be more important than overall water content in the cessation of grain filling. Final grain composition depended not only on the amount of endosperm storage component present in the grain but also on the contribution of the non-endosperm components (including the embryo and husk) to final grain dry weight. In some cases, changes in the contribution made by the non-endosperm components of the grain to final grain weight masked important high temperature effects on key endosperm storage components. Hot water extract (HWE) values were similar within treatments and ranged from 73% to 78%. High temperature exposure reduced HWE for all varieties. Malt b-glucan was lower in heat-treated grains than in control grains. Despite relatively high malt protein levels in all varieties, higher free amino N levels in heat-treated grains indicated a higher protein modification than in control grains.

Effects of Short Periods of Drought and High Temperature on Grain Growth and Starch Accumulation of Two Malting Barley Cultivars

Short periods (3-5 days) of high maximum temperature (>35�C), often accompanied by drought, commonly occur during grain filling of cereals. Short periods of high temperature have been shown to reduce grain weight and baking quality in wheat, but little is known about their effects on barley. Consequently, we examined the effects of high temperature and drought, alone or combined, on grain growth for two barley cultivars, Schooner and Franklin. Treatments started 15 days after anthesis and consisted of the factorial combination of three temperatures and three water regimes. The high temperature (maximum 40�C for 6 h day-1) and drought treatments were maintained for 5 or 10 days. Drought reduced individual grain weight much more (ca 20%) than high temperature (ca 5%) for both cultivars. Franklin appeared to be more sensitive to heat stress than Schooner. The reduction in individual grain weight was greatest when both stresses were combimed (ca 30%). The reduction in mature grain weight under high temperature was due to a reduction in duration of grain growth for Schooner and to a reduction in both rate and duration of grain growth for Franklin (8-12%). The reduction in duration of grain growth was the most important cause of reduced grain weight at maturity under drought alone (12-25%) or combined with high temperature (25-33%). Nitrogen content per grain was quite high and similar for all treatments, and nitrogen percentage increased when stress was severe enough to reduce starch accumulation, confirming that starch accumulation is more sensitive to post- anthesis stress than nitrogen accumulation. We conclude that drought, particularly when combined with high temperature, is more likely than heat stress to cause large reductions in grain weight of barley under field conditions.

Responses of grain growth and malting quality of barley to short periods of high temperature in field studies using portable chambers

Although environmental conditions during grain filling are often cited as the reason for decreases in malting quality of barley, little is actually known about the specific effects of different environmental conditions on grain yield and quality of barley. In the present study, an attempt was made to assess in the field the effects of short periods of high temperature (>35�C), using portable chambers with thermostatically controlled electric heaters, on grain yield and quality of barley. Two experiments were carried out in 2 consecutive years, involving the malting barley cultivars Schooner (first year) and Parwan (second year). The treatments were (i) control (no chamber, no heating) during the whole grain-filling period, (ii) plots with chambers heated to ca. 40�C for 6 h per day over 5 days starting 17 days after anthesis, and (iii) plots with non-heated chambers for 5 days from 17 days after anthesis. High temperature treatments reduced individual grain weight by 14% in Schooner and 25% in Parwan. There was a reduction in starch content and an increase in nitrogen content in the heat treatments compared to the control, but the G-glucan content was similar to the control. The resulting malt extract was reduced from 79 to 73% in Schooner and from 68.4 to 66.2% in Parwan in ,the heat stress treatment. The starch granule size distribution was also measured. Results indicate that decreases in grain dry matter were due to reductions in number rather than size of starch granules. It is concluded that high temperature reduced the amount of 'maltable' grain by reducing grain size and increasing the screening percentage, and also reduced malt extract by 3-7%, which represents a large decrease for the malting industry.