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High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (Anet) and stomatal conductance (gs). However, evidence for decoupling of gs from Anet under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum Tair > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced Anet and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well‐watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced—gs was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD. [ABSTRACT FROM AUTHOR]

Due to arid and semi-arid climates, identification of water deficit stress tolerant genotypes is really important in Iran. To select tolerant genotypes, it is necessary to use important and practical indicators. In this regard, this study was conducted to studying the effect of water deficit stress on 27 sugar beet genotypes in a split plots based on randomized complete block design in three replications at the University of Tabriz in 2019. The main factor included two irrigation levels (optimal and water deficit), and the sub-factor included 27 sugar beet genotypes. In this experiment, sugar beet seeds were planted in the PVC tubes. Studied traits included transpirational cooling before and after irrigation, relative water loss (RWL), relative water content (RWC), root weight (RW), electrolyte leakage (EL), initial fluorescence (F0), maximum fluorescence (Fm), and quantum yield of photosystem II. Based on the analysis of variance, irrigation, genotype, and irrigation × genotype interaction effects were significant for all studied traits at 1% of probability level. Also, the relative lost water, relative water content, maximum fluorescence, and quantum yield of photosystem II were significantly reduced due to water deficit stress. In terms of transpirational cooling indices after irrigation, relative water content, root weight, electrolyte leakage, initial fluorescence, and quantum yield of photosystem II, Full sib S1- 10- 8001 genotype was identified as the tolerant genotype. [ABSTRACT FROM AUTHOR]

Transpirational cooling is an effective thermal protection method in hypersonic vehicles. In order to properly manage the high heat load, an understanding of the convective flow regimes as well as the thermophysical properties of the working fluid are required. Often, the vehicle's fuel is re-purposed as the coolant or working fluid that is passed through the porous media. If the geometry is such that the coolant is heated from below, buoyancy-induced instability can ensue resulting in a mixed convection phenomena. Transpirational cooling applications require a unique analysis which combines a Darcy–Forchheimer relationship for the momentum relation, a flowing base state which introduces non-negligible convective terms for the energy equation, and a novel consideration of a cubic density dependence on temperature. This latter feature is justified by fitting thermodynamic data for typical transpirational cooling conditions. A base state solution is provided and the onset of instability is investigated using linear stability analysis. The governing equations are solved utilizing multiple methods, comparing results from a combination of analytical solutions, finite difference, power series, and Chebyshev methods. Results demonstrate excellent consistency in predictions across these methods and indicate that including non-linear density effects promote a stabilizing effect. Finally, the effect of varying the net through-flow in the porous media is investigated. [ABSTRACT FROM AUTHOR]

Transpirational cooling is an effective thermal protection method in hypersonic vehicles. In order to properly manage the high heat load, an understanding of the convective flow regimes as well as the thermophysical properties of the working fluid are required. Often, the vehicle’s fuel is re-purposed as the coolant or working fluid that is passed through the porous media. If the geometry is such that the coolant is heated from below, buoyancy-induced instability can ensue resulting in a mixed convection phenomena. Transpirational cooling applications require a unique analysis which combines a Darcy–Forchheimer relationship for the momentum relation, a flowing base state which introduces non-negligible convective terms for the energy equation, and a novel consideration of a cubic density dependence on temperature. This latter feature is justified by fitting thermodynamic data for typical transpirational cooling conditions. A base state solution is provided and the onset of instability is investigated using linear stability analysis. The governing equations are solved utilizing multiple methods, comparing results from a combination of analytical solutions, finite difference, power series, and Chebyshev methods. Results demonstrate excellent consistency in predictions across these methods and indicate that including non-linear density effects promote a stabilizing effect. Finally, the effect of varying the net through-flow in the porous media is investigated.

• Leaf-air temperature difference decreases with increasing environmental temperature. • Hot-dry habitat increases transpirational cooling, decreases leaf warming. • Plants minimize transpirational cooling, facilitate leaf warming in cold forest. • Leaf area and water content play significant roles in physical thermal regulation. The ecophysiological processes of leaves are more related to leaf temperature (Tl) than air temperature (Ta). Transpiration and leaf physical traits enable plants to maintain Tl within a thermal range. However, it is challenging to quantitatively study leaf thermal regulation strategies, due to the complex interaction between thermal effects of transpiration and leaf physical traits. We utilized a 3-T method that compares Tl, Ta, and Tn (the temperature of non-transpiring leaves) investigate thermal regulation strategies of dominant canopy species in four vegetation types, including a savanna woodland, a tropical rain forest, a subtropical evergreen broad-leaved forest, and a temperate mixed forest. Our results indicate that the difference between Tl and Ta decreased as the site mean temperature increased. Transpirational cooling was strongest in savanna woodland, and decreased from the hottest site to the coldest site. Without transpiration, sun-exposed leaves were consistently hotter under sunshine than air. This physical warming effect increased from the hottest site to the coldest site. We observed leaf area, water content and leaf angle played a significant role in physical thermal regulation. The present research quantitatively measured leaf thermal regulation strategies across a temperature and precipitation gradient, which advances our understanding of how plants adapt to their thermal environments. [Display omitted] [ABSTRACT FROM AUTHOR]

Rice florets are susceptible to high-temperature damage at anthesis, but rice production remains stable in the Riverina region of Australia even when the air temperature during flowering exceeds 40°C. To identify the mechanism that supports rice production under these conditions, we examined sterility and pollination in relation to microclimate and panicle temperature in an extremely hot paddy field in the Riverina region of New South Wales. In windy > 40°C weather, the panicle temperature was > 38°C at the windward edge of the crop but around 35°C inside the crop, probably because of strong transpirational cooling due to the extremely dry wind (15% RH). Pollen from the windward edge of the crop showed extremely poor germination, yet that from inside the crop showed sufficient germination for fertilization. Moreover, sterility inside the crop was significantly lower than that at windward edge. We concluded that the wind with large vapor pressure deficit enabled stable rice production under the extreme heat during flowering.

How desert shrubs respond to drought and heat remains poorly understood. We investigated how drought affected the morphology and physiology of two C 4 species with different root architecture, Calligonum arborescens and Haloxylon ammodendron , in the Taklamakan Desert. We measured leaf traits, chlorophyll fluorescence and temperature responses of photosynthesis after ∼3 years of experimental drought and asked: (1) Do both species adjust similarly to drought? (2) Does transpirational cooling increase at high temperature, and if so, how is this affected by drought? Drought plants of H. ammodendron had shorter and thinner leaves than controls, and lower chlorophyll content, photochemical efficiency, and electron transport rate. In contrast, C. arborescens leaves became shorter and thicker. Drought reduced maximum photosynthesis by 63 % and 21 % in H. ammodendron and C. arborescens , respectively. The optimum temperature did not change significantly in H. ammodendron and decreased by ∼2 °C in C. arborescens. Surprisingly, above 40–45 °C, stomatal conductance (g sw) and transpiration increased—even under drought stomata partially reopened. This consistent uncoupling of photosynthesis and g sw suggests that widely-implemented stomatal optimization models may poorly reflect high-temperature behaviors in dryland ecosystems. Our study established the basis for predicting the eco-physiological responses of C4 species in hyper-arid ecosystems to climate change. • Drought affected the morphology and physiology of two C4 desert species. • C4 woody shrubs exhibit decoupling of photosynthesis and stomatal conductance. • Drought weakened, but did not fully prevent this decoupling. • Different physiological adjustment mechanisms and morphological adjustment. [ABSTRACT FROM AUTHOR]

Evaporative cooling due to transpiration of urban trees in two contrasting climates is the subject of this study. Transpiration was studied experimentally on local tree species at 'tree lab' sites in Munich, Germany (temperate climate) and in Beer Sheva, Israel (hot arid climate), within various settings (park, street, square) with natural and sealed surface conditions. The comparison was based on linear relationship of midday canopy resistance to atmospheric vapor pressure deficit (VPD), where the slope is proportional to tree hydraulic conductance and midday stem water potential. Sap flux densities in all trees were similar but crown projected area (CPA) was larger in Beer Sheva, resulting in higher hydraulic resistance that limits transpirational cooling (regression slopes of 0.25–0.44 in Beer Sheva vs. 0.10–0.18 in Munich). The contribution of the transpirational cooling, studied as the proportion of latent heat to total available energy was about 40% less significant at noon hours in summer in Beer Sheva than in Munich. Results suggested that shading should be the main consideration in planting local trees in hot dry climates. The linear relationship of midday canopy resistance to VPD is proposed as an operative tool for comparing evaporative cooling between local trees in different climatic regions. • The canopy resistance to VPD relationship quantified tree hydraulic resistance • Due larger crown projected area, water use per m2 area was lower in the arid city • Latent heat dissipation was ∼40% less in hot arid climate than in temperate climate • Transpirational cooling from trees is prominent in temperate climates • Shading should be the main consideration in planting trees in hot dry climates [ABSTRACT FROM AUTHOR]

Even Under Extremely Hot (40ºC) Conditions During Anthesis, Heat-induced Floret Sterility Does Not Appear To Be A Serious Issue For Australian Rice Growers. This Contradicts Previously Reported Temperature Thresholds For Floret Sterility. To Determine The Factors Associated With Stable Rice Production Under Hot and Dry Conditions in The Riverina Region of New South Wales (Australia), We Examined Rice (Cv. ‘Langi’) Pollination At Different Distances From The Windward Edge of A Paddy Field and Its Association With Canopy Microclimate. With An Air Temperature of 34.5ºC and A Relative Humidity of 20.7% During Anthesis, Poor Pollination of Florets Occurred At The Windward Edge, But Pollination Remained Stable Farther From The Edge. The Temperature Difference Between The Air and The Panicles in The Canopy Reached As High As 6.8ºC Under These Conditions Because of Low Humidity and Strong Transpirational Cooling. Moreover, The Length of The Dehiscence At The Base of The Thecae During Anthesis Was Long; This Is A Desirable Trait For Heat Tolerance. The Long Basal Dehiscence of The Thecae of This Cultivar and The Lower Panicle Temperatures Relative To The Ambient Temperature Caused By High Transpirational Cooling Appear To Be The Key Factors Responsible For Stable Pollination Under The Extremely High Temperatures of The Riverina Region.

The purpose of this study was to investigate supposedly positive biological effects of coloured hailnets on microclimate, including photosynthetically active radiation (PAR), UV-B, air, soil, fruit and leaf temperature as well as humidity, which in turn may affect leaf anatomy, tree growth and fruit quality; apple was chosen as a model crop at Klein-Altendorf near Bonn, Germany; adjacent uncovered trees served as control. Red and green hailnets transmitted 3–6% more red or green light, without alteration of the red:far red (R–666 nm:FR–730 nm) ratio (0.99–1.01:1) and hence without affecting the phytochrome system. The microclimate was changed with a reduction by 12–23% in PAR and, to a larger extent, by 20–28% in UV, viz. shading. Light measurements at a 45° angle, to mimic the fruit or leaf position, showed that PAR was 90–210 µmol m−2 s−1 larger outside on a sunny summer day than under the white or red-white and 150–340 µmol m−2 s−1 larger than under red-black and green-black hailnets. Air temperature and relative humidity under coloured hailnets decreased by ca. 1.3°C and by ca 2% rh (cloudy) to 5% rh (sunny day), respectively, compared with outside; leaf temperature was decreased by up to 3°C and fruit temperature by up to 6°C. Soil temperatures at 5 cm depth were 0.5–1°C colder under red-black and green-black hailnets, but up to 0.9°C warmer under white and red-white hailnets compared with the uncovered control outside. Alternate bearing had a larger impact on vegetative growth in the affected year than the coloured hailnets; annual trunk diameter increments in cv. ‘Fuji’, i.e. the variety susceptible to alternate bearing, showed a larger variation than cv. ‘Pinova’ without alternate bearing. Reproductive growth, viz. return bloom and leaf anatomy were impaired by the coloured hailnets. Apple trees under dark hailnets developed thinner leaves with a thinner epidermis and fewer layers of palisade cells. These leaves were 3.5°C (dark hailnets) and 2.5°C (white hailnets) cooler than outside on a sunny day compared with ca. 1.5°C (dark hailnets) and 0.85°C (white hailnets) on a cloudy day. Transpirational cooling of cv. ‘Fuji’ leaves was 0.3–0.6°C outside and 1.4–1.6°C under the green-black hailnet on sunny days compared to <0.1°C on cloudy days. As a practical application, apple fruit colouration was dependent on light (PAR and UV-B) transmission of the respective hailnet colour. [ABSTRACT FROM AUTHOR]

Seed treatment of thermotolerant and sensitive cultivars of wheat (Triticum aestivum L. cv. Frederick and Glenlea, respectively) with uniconazole reduced shoot and increased root fresh and dry weights. When subjected to 48°C for 6 h, treated seedlings had lower leaf-temperatures and overall higher rates of evapotranspiration. Percent survival 11 days after high temperature incubation in untreated seedlings was 53% in Frederick and 30% in Glenlea, whereas in uniconazole-treated seedlings it was 94% and 80%, respectively. Transpiration resistance markedly increased in the control cultivars after 4 h of high-temperature incubation, but remained relatively unchanged in the treated cultivars. The increased root to shoot ratio exhibited in treated plants may have alleviated guard cell stress, even under high evaporative demand. Chlorophyll fluorescence measurements on leaves of heat-stressed seedlings indicated that increased photosynthetic metabolism in treated seedlings was correlated with lower leaf temperatures. High-temperature stress resulted in a dramatic decline in pigments and proteins in thylakoid extracts of control seedlings but not in extracts from treated seedlings. Integrity of thylakoid pigment protein complexes, as illustrated by reduced relative amounts of free pigment, was maintained after exposure to high temperatures in treated seedlings. Furthermore, the treated Glenlea seedlings displayed a reduction in the monomeric form of light-harvesting chlorophyll protein II (LHCP II) compared to control and Frederick seedlings. The heat-tolerant Frederick cultivar showed a greater protective effect from uniconazole treatment than the sensitive Glenlea cultivar. Uniconazole treatment did not affect heat-shock protein (HSP) synthesis in mesocotyl tissue. [ABSTRACT FROM AUTHOR]

We studied the effects of high temperature and drought on the survival, growth and water relations of seedlings of Pinus ponderosa (Dougl.) Lawson, one of few coniferous tree species that can successfully colonize drought-prone sites with high soil surface temperatures. Temperature profiles were measured with 0.07-mm thermocouples in a sparse ponderosa pine forest in northern Idaho. The soil surface and the adjacent 5 mm of air reached maximum temperatures exceeding 75 °C, well above the lethal temperature threshold for most plants. Air temperatures 50 mm above the soil surface (seedling needle height) rarely exceeded 45 °C. Pinus ponderosa seedlings that survived maintained basal stem temperatures as much as 15 °C lower than the surrounding air. The apparent threshold temperature at the seedling stem surface resulting in death was approximately 63 °C for less than 1 min. No correlation between seedling mortality and needle temperature was found, although some needles reached temperatures as high as 60 °C for periods of ≤ 1 min. Surviving seedlings had significantly higher stomatal conductance than seedlings that did not survive until fall. Transpiration rates, calculated from measured needle temperatures, stomatal conductance and evaporative demand, were high (up to 16.7 mmol m−2 s−1), indicating that water transport through seedling stems may have acted as a heat transfer mechanism, cooling the stem below the lethal threshold temperature. Heat exchange calculations showed that rapid water flow through seedling stems can absorb sufficient energy to reduce stem temperature by 30 °C during peak sunlight hours. [ABSTRACT FROM PUBLISHER]