The Temperatures of Leaves in Assimilation Chambers, and in the Open

The temperatures of apple leaves in assimilation chambers wereup to 17° C above the temperature of the outside air insunlight. Except when in deep shade, enclosed leaves had temperatureshigher than ambient. Leaves in the open in the sun were often2–3° C above ambient, but the greatest differencemeasured was 4.3° C. In shade, leaves in the open were asmuch as 0.8° C below the air temperature. Laboratory experimentswith an incandescent lamp showed that the temperature differencebetween an enclosed leaf and the outside air increased linearlywith increasing light intensity above a certain value. Belowthis value it is believed that changes in leaf permeabilitywere sufficiently large to affect the rate of transpirationand therefore the leaf excess temperature-light intensity relationship.Under field conditions leaves may not be in a steady state;this gives rise to more variable measurements which may indicatea non-linear relation between leaf excess temperature and lightintensity. Methods of cooling leaves in chambers were examined.Impractically high rates of flow of air at the ambient temperatureare necessary to reduce the temperature of enclosed leaves appreciably.Some reduction of the leaf excess temperature can be obtainedby filtering the infra-red from the incident light, or by usinga chamber made of material which transmits far infra-red, thoughcondensation reduces the effectiveness of the latter measure.Leaves exhibit rapid changes in temperature, so the heatingproblem cannot be circumvented by brief enclosure. The mosteffective of the techniques examined is to use a water-cooledchamber, though the temperatures of the leaf and water differby several degrees centigrade in bright light. A simple solutionto the heating problem for field assimilation measurements hasnot been found.     

The effect of root temperature on rose plants in relation to air temperature

Rose plants (Rosa hybrida ‘Sonia’=‘Sweet Promise’) were grown in heated (minimum night temperature 17°C), and unheated greenhouses with or without root heating to 21°C. These trials covered 6 growth cycles extending over two winter seasons. In the heated greenhouse, root heating did not increase yield, flower quality or plant development. In the unheated greenhouse, root-heated plants grew as well as those in the air-heated greenhouse as long as the air temperature did not fall below 6°C. When minimum night temperatures fell below 6°C, growth, yield and quality were reduced, irrespective of root temperature. Daytime plant water relations were studied in plants growing at 6 different root temperatures in the unheated greenhouse. Leaf resistance to water diffusion was lowest at optimal root temperature. Total leaf water potential was not significantly affected by root temperature.     

The cooling of convolvulaceous flowers in a tropical environment

The temperatures of flowers of Ipomoea pes‐caprae ssp. brasilensis, Ipomoea aquatica and Merremia borneensis in bright sunshine, were studied to determine the role of corollas and sepals in cooling the gynoecium. The corollas and sepals were prevented from transpiring by greasing, to investigate the extent of evaporative cooling. In the exposed natural habitats of these flowers the maximum temperatures of air and soil were high (32 and 42 °C, respectively) and corolla, sepal and gynoecium temperatures were often intermediate. Despite being almost astomatous, significant evaporative cooling was observed in the corolla. Between 20 and 80% of the energy absorbed by the corollas was dissipated as evaporation. The sepals were stomatous and their evaporative cooling was very important in reducing the temperature of the gynoecium. The temperatures of the non‐transpiring gynoecia and corollas were significantly higher than the temperatures of the normally transpiring corollas and gynoecia. Furthermore, the gynoecia temperatures were significantly higher with non‐transpiring corollas than with normally transpiring corollas, suggesting that the corollas alone play a role in maintaining the gynoecium within optimal temperatures levels. It was shown in an incubation experiment that temperatures exceeding 32 °C may damage the carpels, and temperatures exceeding 42 °C may damage sepals. Pollen grains were killed after 200 min of exposure to temperatures in the range 32 to 47 °C. It is concluded that the cooling mechanisms (evaporation and self‐shading) are critical for the reproductive success of these flowers in their natural environment.     

Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf, Hyptis emoryi

The temperature and water relations of sun versus shade leavesof Hyptis emoryi Torr. were evaluated from field measurementsmade in late summer. Throughout most of the day sun leaves hadhigher temperatures and higher resistances to water vapour diffusion,but lower transpiration rates and lower stem water potentials,than did shade leaves. Leaf absorptivity to solar irradiationwas less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shadeleaves (0.56). For both leaf types the stomatal resistance increasedas the water vapour concentration drop from the leaf to theair increased. Energy balance equations were used together with the measuredtemperature dependence of photosynthesis to predict the effectof variations in leaf absorptivity, length, and resistance onnet photosynthesis. The influence of leaf dimorphism on wholeplants was determined by calculating daily photosynthesis andtranspiration for plants with various percentages of sun andshade leaves. A hypothetical plant with all sun leaves in thesun had about twice the photosynthesis and half the transpirationratio as did plants with sun leaves in the shade or shade leavesin the sun or shade. Plants with both sun and shade leaves hadthe highest predicted photosynthesis per unit ground area. Thepossible adaptive significance of the seasonal variation insun and shade leaf percentages observed for individual H. emoryibushes is discussed in terms of water economy and photosynthesi     

The effects of high temperature on isoprene synthesis in oak leaves

Isoprene emission from plants is highly temperature sensitive and is common in forest canopy species that experience rapid leaf temperature fluctuations. Isoprene emission declines with temperature above 35 °C but the temperature at which the decline begins varies between 35 and 44 °C. This variability is caused by the rate at which leaf temperature is increased during measurement with lower temperatures associated with longer measurement cycles. To investigate this we exposed leaves of red oak (Quercus rubra L.) to temperature regimes of 35–45 °C for periods of 20–60 min. Isoprene emission increased during the first 10 min of high temperature exposure and then decreased over the next 10 min until it reached steady state. This phenomenon was common at temperatures above 35 °C but was not noticeable at temperatures below that. The response was reversible within 30 min by lowering leaf temperature to 30 °C. Because there is no storage of isoprene inside the leaf, this behaviour indicates regulation of isoprene synthesis in the leaf. We demonstrated that the variability in isoprene decline results from regulation and explains the variability in the temperature response. This is consistent with our theory that isoprene protects leaves from damage caused by rapid temperature fluctuations.     

Microsite characteristics of Muhlenbergia richardsonis (Trin.) Rydb., an alpine C4 grass from the White Mountains, California

C₄ plants are uncommon in cold environments and do not generally occur in the alpine tundra. In the White Mountains of California, however, the C₄ grass Muhlenbergia richardsonis is common in the alpine zone at 3,300-3,800 m, with the highest population observed at 3,960 m (13,000 feet) above sea level. This is the highest reported C₄ species in North America and is near the world altitude limit for C₄ plants (4,000-4,500 m). Above 3,800 m, M. richardsonis is largely restricted to southern slope aspects, with greatest frequency on southeast-facing slopes. In open tundra, M. richardsonis formed prostrate mats with a mean height of 2.5 cm. Neighboring C₃ grasses were two to three times taller. Because of its short stature, leaf temperature of M. richardsonis was greatly influenced by the boundary layer of the ground, rising over 20°C above air temperature in full sun and still air and over 10°C above air temperature in full sun and wind velocity of 1-4 m s^-1. Thus, although air temperatures did not exceed 15°C, midday leaf temperatures of M. richardsonis were routinely between 25°C and 35°C, a range favorable to C₄ photosynthesis. At night, leaf temperature of M. richardsonis was often 5-12°C below air temperature, resulting in regular exposure to subzero temperatures and frosting of the leaves. No visible injury was associated with exposure to freezing night temperatures. The presence of M. richardsonis in the alpine zone demonstrates that C₄ plants can tolerate extreme cold during the growing season. The localization to microsites where leaf temperatures can exceed 25°C during the day, however, indicates that even when cold tolerant, C₄ plants still require periods of high leaf temperature to remain competitive with C₃ species. In this regard, the prostrate growth form of M. richardsonis compensates for the alpine climate by allowing sufficient heating of the leaf canopy during the day.     

Phospholipid synthesis and exchange in castor bean endosperm homogenates

Leaves on a bush of Hyptis emoryi Torr. varied in length from less than 1 cm when development occurred in full sunlight (e.g. 40 Mjoules m⁻²) to over 7 cm when the total daily solar irradiance was less than 3 Mjoules m⁻². The 1-cm sun leaves were 3-fold higher than the 7-cm shade leaves in chlorophyll per unit area, mesophyll thickness, and the internal to external leaf area ratio (A^mes/A). The higher A^mes/A caused a 1.2-cm leaf to have a 3-fold lower CO₂ liquid phase resistance than did a 7.1-cm leaf. Large thin shade leaves captured photosynthetically active radiation effectively (less than 7% passed through), but were not adapted to full sunlight. Specifically, when a 6.9-cm leaf was placed at 910 w m⁻² for 30 min, its temperature exceeded that of the air by nearly 8 C. For the common daytime air temperatures above 30 C for this desert shrub, large shade leaves would have temperatures far in excess of that optimum for photosynthesis for H. emoryi, 29 to 32 C.     

Leaf temperatures and energy balance ofWelwitschia mirabilis in its natural habitat

Summary Welwitschia mirabilis is a perennial desert plant with extremely large leaves (0.5–1.0 m broad, 1–2 m long). Leaf temperatures were measured in the field and the energy budget was calculated. The portions of the leaf which were kept above the ground had leaf temperatures which were only 4–6°C above air temperature. In the leaf portions which were in contact with the ground leaf temperatures were 6–12°C above air temperature (absolute maximum 51°C). The important feature in the energy budget ofWelwitschia mirabilis is its high reflectivity (38% of the global radiation). Only about 56% of the global radiation is absorbed by the thick leathery leaves. The energy loss due to convection is of the same order of magnitude as the reflection and it is abouy the same in the portions of leaf on and above the ground. The difference in leaf temperatures found in these portions is due to the loss of thermal radiation from the section of leaf above the ground to the cooler ground which is shaded by the leaf. The provision of a heat sink due to the large area of shade cast by these large leaves is of significance to the existence ofWelwitschia mirabilis in its arid habitats.     

Do fruit sunburn control measures affect leaf photosynthetic rate and stomatal conductance in ‘Royal Gala’ apple?

A field study was conducted on a 5-year-old orchard of ‘Royal Gala’ apple (Malus domestica Borkh.) in Stellenbosch, South Africa, to investigate whether the measures employed to control sunburn in fruit, viz., evaporative cooling, Surround WP and 20% black shade net affect leaf photosynthetic gas exchange attributes in comparison to untreated control during the 2003/2004 season. Shade net significantly reduced midday leaf net photosynthetic rate (A) compared to evaporative cooling. Furthermore, shade net and Surround WP significantly reduced midday leaf stomatal conductance (g_s) compared to evaporative cooling and control. Evaporative cooling increased light saturated photosynthetic rate by 27 and 24% compared to shade net and Surround WP, respectively. Light compensation point and dark respiration of shaded leaves were about a third of the other treatments and about 50% less than the control leaves, respectively. Shade net down-regulated photosynthetic capacity of the leaves as evidenced by lower maximum rate of carboxylation and light saturated rate of electron transport compared to control leaves. Sunburn control treatments reduced day respiration by 60–70% compared to the control. Response of A and g_s to increasing temperature showed only slight increase in both A and g_s with increasing temperature from 20 to 30 °C. A declined at 35 °C in Surround WP and shade net leaves while it declined at 40 °C in evaporatively cooled and control leaves. Evaporative cooling and control had higher g_s than shade net and Surround WP at all leaf temperatures. In conclusion, shade net down-regulated photosynthetic reactions and Surround WP and shade net reduced leaf g_s and increased the vulnerability of leaf A and g_s to high temperature compared to evaporative cooling and control.     

Scaling carbon dioxide and water vapour exchange from leaf to canopy in a deciduous forest. I. Leaf model parametrization

In order to parametrize a leaf submodel of a canopy level gas-exchange model, a series of photosynthesis and stomatal conductance measurements were made on leaves of white oak (Quercus alba L.) and red maple (Acer rubrum L.) in a mature deciduous forest near Oak Ridge, TN. Gas-exchange characteristics of sun leaves growing at the top of a 30 m canopy and of shade leaves growing at a depth of 3–4 m from the top of the canopy were determined. Measured rates of net photosynthesis at a leaf temperature of 30°C and saturating photosynthetic photon flux density, expressed on a leaf area basis, were significantly lower (P = 0.01; n = 8) in shade leaves (7.9μmol m^?2 s^?1) than in sun leaves (11–5μmol m^?2 s^?1). Specific leaf area increased significantly with depth in the canopy, and when photosynthesis rates were expressed on a dry mass basis, they were not significantly different for shade and sun leaves. The percentage leaf nitrogen did not vary significantly with height in the canopy; thus, rates expressed on a per unit nitrogen basis were also not significantly different in shade and sun leaves. A widely used model integrating photosynthesis and stomatal conductance was parametrized independently for sun and shade leaves, enabling us to model successfully diurnal variations in photosynthesis and evapotranspiration of both classes of leaves. Key photosynthesis model parameters were found to scale with leaf nitrogen levels. The leaf model parametrizations were then incorporated into a canopy-scale gas-exchange model that is discussed and tested in a companion paper (Baldocchi & Harley 1995, Plant, Cell and Environment 18, 1157–1173).     

Physiological Responses of Apple Trees to Supraoptimal Root Temperature

Ungrafted apple rootstocks were grown in sand cultures at constant root temperatures between 20°C to 40°C. Temperatures of 30°C and above reduced root and shoot growth. Serious damage to the leaves occurred at 35°C and above. The O₂ consumption, CO₂ evolution and respiratory quotient (RQ) of the roots showed maximum values at 35°C. Different rootstock cultivars varied greatly in their susceptibility to damage by supraoptimal root temperatures apparently due to anaerobic respiration. The more susceptible ones differed from resistant types in the larger amount of ethanol they accumulated in their roots at supraoptimal root temperature, and the more severe reduction in the malic acid content of the roots at such temperature. Acetaldehyde was also found in roots and leaves at supraoptimal root temperatures, whereas the organic acid content of the leaves tended to decrease. Supraoptimal root temperature also caused a reduction of cytokinins in both roots and leaves accompanied by a reduction in the leaf chlorophyll content. This could be prevented by the application of kinetin or benzyladenine to the leaves. In a short experiment a rise in root temperature up to 40°C caused an increase in transpiration and a decrease in the resistance of the leaves to the passage of water vapor, whereas in prolonged experiments transpiration reached a maximum and leaf resistance a minimum at 30°C. The leaf water potential increased also with increasing root temperature. Leaf temperature increased with increasing root temperature, irrespective of increasing or decreasing transpiration rates.     

A Controlled-environment Leaf Chamber to Allow Measurement of Gas Exchange by Leaves Undergoing Rapid Fluctuations in Temperature

A leaf chamber is described which allows continuous measurementof transpiration from an attached leaf while leaf temperatureis controlled independently of air temperature. Leaf temperaturecan be varied from approximately 3 °C below air temperatureto 12 °C above air temperature while air temperature remainsrelatively constant (±2 °C). Leaf temperature canbe varied rapidly (by up to 12 °C in 30 s) in order to simulatethe rapid, short-term temperature fluctuations to which leavesare frequently exposed in the field. The chamber operates overa wide range of conditions of visible and total radiation, ofair and leaf temperatures, and of ambient carbon dioxide concentrationand water vapour density     

Leaf rosette closure in the alpine rock species Saxifraga paniculata Mill.: significance for survival of drought and heat under high irradiation

The significance of leaf rosette closure for survival of drought and heat under high irradiation on alpine rock sites was investigated in the cushion forming rosette plant, Saxifraga paniculata Mill. With decreasing water content the leaves fold over the rosette centre reducing reversibly the evaporative leaf surface area by 80%. Internal water redistribution driven by an osmotic gradient from older to younger leaves occurs. The oldest leaves dry out to promote the survival of the individual. Leaf temperatures above 45 °C (which match heat tolerance limits 45–57 °C; LT₅₀) co-occurred with low substrate water potentials (less than – 0·5 MPa) on 11·3% of summer days. Shading by leaves can be crucial to surviving high temperatures as it keeps the rosette centre up to 10 °C colder. Mutual shading prevented sustained drought-induced photoinhibition in upper leaf surfaces at relative water contents below 60%. In exposed lower leaf surfaces restoration of photosystem II took several days. Leaf temperatures above 40 °C (21·3% of summer days) induced photoinhibition in situ. Periods with sufficient water supply can be fully utilized as rehydration is fast ( < 12 h) and exposes the upper leaf surfaces that showed only minor photoinhibition. By reversible leaf rosette closure environmental extremes that otherwise could exceed tolerance are efficiently avoided.     

Differences in leaf thermoregulation and water use strategies between three co‐occurring Atlantic forest tree species

Given anticipated climate changes, it is crucial to understand controls on leaf temperatures including variation between species in diverse ecosystems. In the first study of leaf energy balance in tropical montane forests, we observed current leaf temperature patterns on 3 tree species in the Atlantic forest, Brazil, over a 10‐day period and assessed whether and why patterns may vary among species. We found large leaf‐to‐air temperature differences (maximum 18.3 °C) and high leaf temperatures (over 35 °C) despite much lower air temperatures (maximum 22 °C). Leaf‐to‐air temperature differences were influenced strongly by radiation, whereas leaf temperatures were also influenced by air temperature. Leaf energy balance modelling informed by our measurements showed that observed differences in leaf temperature between 2 species were due to variation in leaf width and stomatal conductance. The results suggest a trade‐off between water use and leaf thermoregulation; Miconia cabussu has more conservative water use compared with Alchornea triplinervia due to lower transpiration under high vapour pressure deficit, with the consequence of higher leaf temperatures under thermal stress conditions. We highlight the importance of leaf functional traits for leaf thermoregulation and also note that the high radiation levels that occur in montane forests may exacerbate the threat from increasing air temperatures.     

Moderating Night Radiative Cooling Reduces Frost Damage to Metrosideros polymorpha Seedlings Used for Forest Restoration in Hawaii

Winter frosts caused by radiative cooling were hypothesized to limit successful reintroduction of Hawaiian plants other than Acacia koa to alien‐dominated grasslands above 1700 m elevation. We determined, in the laboratory, the temperature at which irreversible tissue damage occurred to Metrosideros polymorpha leaves. We also conducted a field study of this species to determine if (1) leaf damage was correlated with sub‐zero leaf temperatures, (2) radiative cooling could be moderated by canopies of A. koa, and (3) low soil temperatures contributed to seedling damage. The last was evaluated by thermally buffering seedlings with water‐filled bladders placed at their base to keep roots warm, or by installing a radiation shield to reduce early morning transpiration when water uptake from cold soils would be least. Leaf temperatures were monitored between midnight and 7:00 a.m. using fine‐wire thermocouples, and leaf damage was recorded monthly. In the laboratory, supercooling protected leaves from mild sub‐zero temperatures; irreversible tissue damage occurred at about ?8°C. In the field, leaf damage was strongly correlated with degree‐hours below freezing. Unprotected seedlings suffered the greatest leaf damage. Those sheltered under A. koa trees rarely experienced temperatures below ?3°C, and damage was minimal. Shaded and thermally buffered seedlings suffered less damage than unprotected plants, probably due to elevated leaf temperatures rather than improved water relations. Using A. koa or artificial devices to reduce radiative cooling during winter nights should enhance establishment of M. polymorpha in high‐elevation rangeland.     

Effects of prescribed burning on leaves and flowering of Quercus garryana

Many woodland understories are managed with prescribed fire. While prescribed burns intended to manipulate understory vegetation and fuels usually do not cause excessive tree mortality, sublethal canopy damage may occur and can affect tree vigor and reproductive output. We monitored Quercus garryana trees in western Washington, USA with multiple canopy thermocouples during three prescribed burns. Peak temperatures recorded in tree canopies ranged from 36 to 649°C. We assessed leaf damage immediately after burning, and flower, leaf and acorn production in the following year in the vicinity of each thermocouple. Leaf scorch first occurred with peak thermocouple temperatures around 45°C, was variable up to 75°C, but above 75°C all leaves were killed. Buds, including their reproductive and leaf organs were more resistant to heat damage than leaves, but leaf scorch had predictive value in forecasting bud organ damage. Staminate and pistillate inflorescences and acorn production per bud decreased and bud mortality increased with maximum thermocouple temperature. In two burns where the highest peak temperatures reached 137°C, there was no difference in leaf production between burned and control plots in the spring following burning. However, no staminate or pistillate inflorescences were produced when thermocouple peak temperatures went above 55 or 68°C, respectively. While heat damage to bud organs was detected, production of reproductive organs was also curtailed at temperatures lower than could reasonably be attributed to heat damage. Thus, it is probable that some other fire-related factor, possibly smoke, was also involved.     

Leaf temperature of desert sand dune plants: perspectives on the adaptability of leaf morphology

The leaf temperature of six annual and six perennial plant species was monitored during spring and summer on a sand dune ecosystem in the delta Mediterranean coast of Egypt. During winter, leaves of all tested perennial species attained temperatures higher than the air temperature at night and shortly after sunrise, with maximum leaf–air temperature differences reaching up to 8°C. The lowest differences were less than 1°C. Around noon, the leaves of several species attained temperatures lower than that of the air whereas others showed higher temperatures. The opposite was true during summer, when leaf temperatures were lower than air temperature. The maximum leaf–air temperature differences occurred after midnight towards sunrise and reached up to 10°C. The lowest differences were found around noon and were of less than 5°C. The annual plant species have more pronounced variations than perennials in their leaf temperatures during the night and for most of the day. The leaves were heated or cooled a few degrees above or below the air temperature. The results are discussed in relation to the morphological characters of the leaves. The variation in leaf temperature at different times of the day was significantly related to leaf morphology, specific leaf area, thickness, volume, leaf area index and the surrounding environment.     

Temperature's influence on the activity budget,terrestriality, and sun exposure of chimpanzees in the Budongo Forest,Uganda

Chimpanzee (Pan troglodytes schweinfurthii) activity budget, terrestriality, and sun exposure were found to be influenced by the immediate environmental temperature. Thirty adult chimpanzees in the Budongo Forest, Uganda, were observed for 247 h. Temperatures in the shade and sun, sky cover, sun exposure, activity, and terrestriality were recorded at 5‐min intervals at <15 m from the center of the party. Terrestriality frequency was 26.5% for females and 41.5% for males. Terrestriality and resting both show a significant positive correlation with temperature in the sun. Controlling for seven potential confounding factors, temperature in the sun remained the strongest predictor of terrestriality. The difference between temperatures in the sun and shade had a significant effect on chimpanzee sun exposure frequency. Time spent continuously in the sun was negatively correlated with temperature, beginning to decrease around 30°C, and markedly decreasing around 40°C. A concurrent experiment determined that dark pelage (lacking physiological coping mechanisms) exposed to the same solar regime can easily reach 60°C within minutes. This study indicates that both temperature in the sun and sun exposure play a role in influencing chimpanzee activity behavior, and specifically suggests that chimpanzees thermoregulate behaviorally both by moving to the ground and by decreasing their activity level. These results, in the context of deforestation and increasing global temperatures, have physiological and conservation implications for wild chimpanzees. Am J Phys Anthropol, 2009. © 2008 Wiley‐Liss, Inc.     

Protective and injuring action of visible light on photosynthetic apparatus in wheat plants during hyperthermia treatment

Illumination of wheat (Triticum aestivum L.) leaves during heat treatment produced either additional injury or protection of photosynthetic apparatus depending on irradiance and the heating dose. Furthermore, illumination of leaves during hyperthermia exerted differential impacts on thermal tolerances of photosynthesis and photosystem II-driven electron transport assessed from the reduction of 2,6-dichlorophenolindophenol (DCPIP). Measurements with infrared gas analyzer showed that mild heating of leaves in darkness (10 min at 38–40°C) had stronger inhibitory effect on CO₂ uptake than heating of leaves exposed to low and moderate complex irradiances (3–30 klx), as well as excessive irradiance (75–100 klx). When the leaves were heated at higher temperatures (42–44°C), the low and moderate irradiances had a protective action, while high-intensity light aggravated the inhibition of photosynthesis. Illumination of leaves with weak light during heat treatment mitigated the impairment of chloroplast ultrastructure, whereas irradiation with high-intensity light (100 klx) destroyed the sensitive population of chloroplasts. The heat-stimulated photoinhibition was stronger for leaf photosynthesis than for DCPIP reduction in chloroplasts isolated from heat-treated leaves. No correlation was observed between the extent of violaxanthin deepoxidation, zeaxanthin accumulation, and the protective effect of light on photosynthetic apparatus during heat treatments.     

Passive heat treatment of sweet basil crops suppresses Peronospora belbahrii downy mildew

Sweet basil (Ocimum basilicum) is an annual herb crop grown in polyethylene‐covered structures in Israel. It is Israel's leading herb crop, grown in warm regions of the country. Downy mildew (caused by Peronospora belbahrii) is a severe disease in Israel and in many other crop‐growing regions worldwide. Experiments were carried out to identify potential climate‐management techniques for suppression of this disease on basil in non‐heated greenhouses. Disease severity was evaluated under commercial‐like conditions in three experiments, with 8–10 walk‐in tunnels at each location. Pathogen inoculum was introduced into all walk‐in tunnels. Regression analysis was performed between the disease values and air temperature, relative humidity (RH) and soil temperature. Downy mildew severity was negatively related to high (>25°C) air temperature, RH in the range of 65–85% and high (>21°C) soil temperature. The increase in air temperature did not result in a significant increase in leaf temperature; canopy surface median temperatures only reached 30°C. Symptomless plants from relatively warmer tunnels (peak temperatures of 45–48°C) that were transferred to conditions that promote downy mildew (22 ± 2°C, RH > 95%) became severely diseased, showing sporulation of P. belbahrii, suggesting that infection occurred but at the high temperatures symptom expression/tissue colonisation was suppressed. Pot experiments in which aerial and subterranean plant organs were differentially heated revealed that treating the roots with a high temperature (26–31°C), similar to the soil temperatures in the warmer greenhouses, while maintaining the upper plant parts at ambient temperature (20°C), suppresses canopy downy mildew. The effect lasted for 1–2 weeks after the plants were removed from the heated soil treatments and maintained under optimal conditions for pathogen development. Furthermore, oospores were found in the symptomatic leaves. Oospores are minimally affected by high temperature, and therefore the high temperature presumably did not affect pathogen survival. In conclusion, the effect of high greenhouse temperature on basil downy mildew may not result from a direct negative effect of high temperature on the pathogen but from an indirect high‐temperature effect on the host, rendering it less susceptible to pathogen development.