Elevated temperatures increase leaf senescence and root secondary metabolite concentrations in the understory herb Panax quinquefolius (Araliaceae)

The response of understory species to elevated temperatures is not well understood but is important because these plants are highly sensitive to their growth conditions. Three-year-old plants of Panax quinquefolius, an understory herb endemic to the eastern deciduous forests of North America, were grown in a greenhouse at 25/20°C (day/night) or 30/25°C for one growing season and analyzed each month. Plants grown at high temperatures had an early onset of leaf senescence and therefore accumulated less carbon. From May to July, P. quinquefolius grown at high temperatures had decreased photosynthesis (52%), stomatal conductance (60%), and root and total biomass (33% and 28%, respectively) compared to plants grown at low temperatures. As P. quinquefolius prepared to overwinter, plants grown at high temperatures had less root biomass (53%) than plants in low temperatures. The amount of storage-root ginsenosides was unaffected by temperature, and differences in storage root size may explain why plants grown at high temperatures had greater concentrations of storage root ginsenosides (49%) than plants grown at low temperatures. Panax quinquefolius is clearly sensitive to a 5°C increase in temperature, and therefore other understory species may be negatively impacted by future increases in global temperature.     

Seasonal and inter-annual variations of gas exchange in thirteen woody species along a climatic gradient in the Mediterranean island of Mallorca

We studied the influence of summer drought and winter temperatures on seasonal and spatial variations of light-saturated net photosynthesis and stomatal conductance in Mediterranean woody species. We measured variations in leaf gas exchange over 3 years in 13 Mediterranean trees and shrubs, located at four different sites along a climate gradient of temperature and precipitation in the island of Mallorca (West Mediterranean basin). Net photosynthesis and stomatal conductance were at a maximum during spring, autumn or winter and at a minimum during summer in most sites, species and years. Nevertheless, important spatial, temporal and species-specific variations were observed. Summer drought limitation to gas exchange was greatest in the dry part of the transect, where many species showed their maximum gas exchange rate during winter. In contrast, winter temperatures limited gas exchange of many species at the wet and cool end of the transect, while summer depression of gas exchange was shorter and less pronounced. These results suggest that the effect on carbon fixation and productivity by the predicted future increase of aridity in the Mediterranean basin will depend on whether gas exchange is mostly limited by summer drought or by low winter temperatures.     

Low soil temperatures induce water deficits in olive (Olea europaea) trees

Olive trees are often subjected to low temperatures during winter. To quantify the effects of low temperatures on the water relations of olive trees, we studied the responses to low soil temperatures on winter days of variable evaporative demand (ET₀) in 1-year-old potted olive (Oleo europaea L. cv. Picual) trees in 1996 and 1997. Low night (2.5 and 5.2°C) but ambient day soil temperatures (above 10°C) did not affect stomatal conductance (g_s), leaf (Ψ_leaf) and stem (Ψ_stem) water potentials. Soil temperature levels inducing water stress in olive trees were determined for winter days with ET₀ typical for southern Spain (ET₀= 1.5 ± 0.3 mm day^?1). Leaf and stem water potential decreased and root hydraulic resistance (r_root) increased when trees were exposed to night and day soil temperatures below 10°C. Stomatal conductance was not affected at soil temperatures between 6.4 and 10°C, but decreased at temperatures below 6.4°C. The soil temperature levels affecting the water uptake of olive trees remained relatively constant over the range of ET₀ of 1-2 mm day^?1 during winter and early spring months. However, the soil temperature influencing gs appeared to be more variable and was affected by ET₀. Olive tree recovery from low soil temperature stress depended on stress duration and severity and interacted with ET₀. Recovery of ψ started already during the stress period, probably induced by stomatal closure and high r_root, thus allowing tree rehydration overnight. Root hydraulic resistance contributed the major part of whole-tree hydraulic resistance in response to cold stress, accounting for 76 and 89% at 6.4 and 4.6°C, respectively; which indicates that r_root is the primary control of the water status in olive trees under low temperatures.     

Nocturnal and daytime stomatal conductance respond to root-zone temperature in ‘Shiraz’ grapevines

Background and Aims

Daytime root-zone temperature may be a significant factor regulating water flux through plants. Water flux can also occur during the night but nocturnal stomatal response to environmental drivers such as root-zone temperature remains largely unknown.

Methods

Here nocturnal and daytime leaf gas exchange was quantified in ‘Shiraz’ grapevines (Vitis vinifera) exposed to three root-zone temperatures from budburst to fruit-set, for a total of 8 weeks in spring.

Key Results

Despite lower stomatal density, night-time stomatal conductance and transpiration rates were greater for plants grown in warm root-zones. Elevated root-zone temperature resulted in higher daytime stomatal conductance, transpiration and net assimilation rates across a range of leaf-to-air vapour pressure deficits, air temperatures and light levels. Intrinsic water-use efficiency was, however, lowest in those plants with warm root-zones. CO₂ response curves of foliar gas exchange indicated that the maximum rate of electron transport and the maximum rate of Rubisco activity did not differ between the root-zone treatments, and therefore it was likely that the lower photosynthesis in cool root-zones was predominantly the result of a stomatal limitation. One week after discontinuation of the temperature treatments, gas exchange was similar between the plants, indicating a reversible physiological response to soil temperature.

Conclusions

In this anisohydric grapevine variety both night-time and daytime stomatal conductance were responsive to root-zone temperature. Because nocturnal transpiration has implications for overall plant water status, predictive climate change models using stomatal conductance will need to factor in this root-zone variable.     

植被恢复树种在不同实验光环境下叶片气体交换的生态适应特点

比较研究了华南丘陵地区植被恢复8种乔木树种在不同光条件下的光饱和点、日平均净光合速率、叶温、气孔导度日平均值、内在水分利用效率和光合辐射利用效率等生理生态特征。结果表明,火力楠和藜蒴具有演替后期种的特征,在荫蔽条件下生存和生长良好,它们不宜用作构建先锋群落的树种。大叶相思、马占相思和樟树是典型的阳生性树种,具有鲜明的演替后期种的特征;红椎和海南红豆显示除叶温以外的阳生性的特征。这些种类可被选作构建先锋群落的组成成分。桂西红荷则只能作为演替的过渡种类,它兼具有演替前期种和演替后期种的特征,尤其适用于对先锋群落进行林分改造。     

Water relations of crok-oak (Quercus suber L.) under natural conditions

Daily and annual courses of leaf transpiration, stomatal conductance and shoot water potential of four Quercus suber individuals were compared in a semi-natural stand in southwest Portugal, from spring 1989 to early summer 1990.The trees investigated showed annual patterns typical of evergreen sclerophyllous species but varied in their range of stomatal operation. This appeared to be related to differences in hydraulic conductivity in the root-to-leaf pathway.Maximum stomatal conductance and transpiration rates occurred from March to June.Water stress was found to be moderate and winter cold stress due to low air and soil temperatures appeared to have an influence on plant water balance through their effects on flow resistances.Abbreviations g_sw stomatal conductance - g_max maximum stomatal conductance - PAR photosynthetically active radiation - RH relative humidity of the air - T leaf transpiration - Ta air temperature - TL leaf temperature - T_max maximum leaf transpiration - W air-to-leaf vapor pressure difference - shoot water potential - PD predawn shoot water potential - MIN minimum shoot water potential     

Stomatal conductance in Cucumis sativus upon short-term and long-term exposures to low temperatures

We studied stomatal conductance (g_s) in leaves of cucumber plants (Cucumis sativus L., cv. Zozulya) subjected at early developmental stages to either short-term daily cooling (2 h at the end of night periods) or continuous chilling (12°C throughout the day and night). Irrespective of the irradiance during measurements, continuous chilling either lowered g_s or had no effect, as compared to g_s of control plants (23°C). In plants subjected to periodic short-term cooling, the g_s was found to increase at low temperatures both at moderate and high irradiance; it also increased at high temperature (33°C) but only at photosynthetically active irradiance of 800 ??mol/(m² s). It is supposed that heat-loving plants, subjected to different types of low-temperature treatment, mobilize different mechanisms of stomatal regulation and employ different strategies of adaptation in response to low- and high-temperature treatments. The unusual behavior of stomata, manifested in stomatal opening at both low and high temperatures, extends the adaptive potential of plants subjected daily to short-term low-temperature treatments. This leads to a high level of photosynthesis, biomass accumulation, and supports high physiological activity in plants.     

Selective oviposition by a leaf miner in response to temporal variation in abscission

Summary Responses to humidity of net photosynthesis and leaf conductance of single attached leaves were examined in populations of herbs from wet soil sites in Beltsville, Maryland and Davis, California, USA. Plants were grown in controlled environments under three conditions which differed in the magnitude of the day-night temperature difference and in daytime air saturation deficit. No population differences in response were found in Abutilon theophrasti. In Amaranthus hybridus stomatal conductance and net photosynthesis were more reduced by increasing leaf to air water vapor pressure difference (VPD) in the population from Beltsville, but only for the growth condition with a constant 25°C temperature. In Chenopodium album, stomatal conductance was more sensitive to VPD in the population from Davis, but only for the growth condition with 28/22°C day/night temperatures. Population differences in the sensitivity to VPD of leaf conductance were associated with differences in leaf area to root weight ratio. The relative reduction of net photosynthesis as VPD increased was greater than, equal to, or less than the relative decrease in substomatal carbon dioxide partial pressure. The pattern depended on species, and on growth condition. From these results one can not conclude that environmental humidity has been a strong selective force in determining sensitivity to humidity of stomatal conductance.     

Environmental and genetic control of phenotypic adaptation to drought in Potentilla glandulosa Lindl.

Summary Two populations of Potentilla glandulosa (Rosaceae) have strongly contrasting abilities to seasonally modify their phenotypic sensitivity to drought by low temperature-induced changes in leaf morphology. One population is native to an inland continental climate with unpredictable droughts occurring at any time during the year. The second population is native to a coastal mediterranean climate with a highly predictable annual cycle of winter rain and summer drought. In response to low temperatures in the autumn, the inland plants produce a compact rosette of small leaves and shed their large summer leaves, thereby reducing the total plant leaf area. The inland plants begin growth in the spring in the compact rosette phenotype and are much less sensitive to drought, maintaining higher values of stomatal conductance and leaf water potential, than when in the largeleaved summer phenotype. The coastal plants do not exhibit the low temperature-induced change in leaf morphology and are relatively sensitive to drought when grown at high or low temperatures. The F₁ hybrids of a coastal x inland cross exhibit an intermediate response to low temperature and drought. In the F₂ generation the inland parental class of individuals fully responding to low temperature segregates in a 1:63 ratio.     

Water relations under root chilling in a sensitive and tolerant tomato species

The shoots of cultivated tomato (Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling temperatures of around 5 °C. Under the same treatment, a chilling‐tolerant congener (Lycopersicon hirsutum LA 1778) maintains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure chambers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, however, in stomatal behaviour: in L. hirsutum, stomatal conductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 °C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differential stomatal behaviour during root chilling has distinct shoot and root components.     

SEASONAL PATTERNS OF CO2 AND WATER VAPOR EXCHANGE OF JUNCUS ROEMERIANUS SCHEELE IN A GEORGIA SALT MARSH

CO₂ and water vapor exchange studies of intact plants of black needle rush (Juncus roemerianus Scheele) were conducted in an undisturbed marsh community on Sapelo Island, Georgia. The seasonal patterns of the light and temperature responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration were determined five times over the year. Internal resistances to CO₂ uptake were also evaluated. Net photosynthesis was highest in early spring, but declined only slightly through the year. A distinct and moderate temperature optimum of net photosynthesis was observed with decreasing rates above 30 C. Leaf conductances to water vapor were similar at all seasons and were high at cooler temperatures and decreased with increasing temperature. Transpiration was relatively high and constant during all seasons. The water-use efficiency of photosynthesis was high below 25 C, but decreased sharply above that temperature. Dark respiration was relatively low. Seasonal changes reflected changes in leaf density. Decreasing stomatal conductances and increasing respiration rates reduced net photosynthesis at higher temperatures. The stomatal resistance increased and internal resistances to CO₂ uptake decreased over the year, but the total resistance remained constant. The internal resistance to CO₂ uptake was consistently higher than the stomatal resistance. These seasonal response patterns show that J. roemerianus is well adapted to the seasonal changes in ambient temperature and irradiance and other microenvironmental factors in the high marsh. These physiological characteristics permit this C₃ species to maintain a high productivity in a seasonally hot and stressful environment.     

The stomatal response to evaporative demand persists at night in Ricinus communis plants with high nocturnal conductance

Evidence is building that stomatal conductance to water vapour (g(s)) can be quite high in the dark in some species. However, it is unclear whether nocturnal opening reflects a mechanistic limitation (i.e. an inability to close at night) or an adaptive response (i.e. promoting water loss for reasons unrelated to carbon gain). Further, it is unclear if stomatal responses to leaf-air vapour pressure difference (D) persist in the dark. We investigated nocturnal stomatal behaviour in castor bean (Ricinus communis L.) by measuring gas exchange and stomatal responses to D in the light and in the dark. Results were compared among eight growth environments [two levels for each of three treatment variables: air saturation deficit (D(a)), light and water availability]. In most plants, stomata remained open and sensitive to D at night. g(s) was typically lower at night than in the day, whereas leaf osmotic pressure (Pi) was higher at night. In well-watered plants grown at low D(a), stomata were less sensitive to D in the dark than in the light, but the reverse was found for plants grown at high D(a). Stomata of droughted plants were less sensitive to D in the dark than in the light regardless of growth D(a). Drought also reduced g(s) and elevated Pi in both the light and the dark, but had variable effects on stomatal sensitivity to D. These results are interpreted with the aid of models of stomatal conductance.     

Chilling outweighs photoperiod in preventing precocious spring development

It is well known that increased spring temperatures cause earlier onset dates of leaf unfolding and flowering. However, a temperature increase in winter may be associated with delayed development when species' chilling requirements are not fulfilled. Furthermore, photosensitivity is supposed to interfere with temperature triggers. To date, neither the relative importance nor possible interactions of these three factors have been elucidated. In this study, we present a multispecies climate chamber experiment to test the effects of chilling and photoperiod on the spring phenology of 36 woody species. Several hypotheses regarding their variation with species traits (successional strategy, floristic status, climate of their native range) were tested. Long photoperiods advanced budburst for one‐third of the studied species, but magnitudes of these effects were generally minor. In contrast to prior hypotheses, photosensitive responses were not restricted to climax or oceanic species. Increased chilling length advanced budburst for almost all species; its effect greatly exceeding that of photoperiod. Moreover, we suggest that photosensitivity and chilling effects have to be rigorously disentangled, as the response to photoperiod was restricted to individuals that had not been fully chilled. The results indicate that temperature requirements and successional strategy are linked, with climax species having higher chilling and forcing requirements than pioneer species. Temperature requirements of invasive species closely matched those of native species, suggesting that high phenological concordance is a prerequisite for successful establishment. Lack of chilling not only led to a considerable delay in budburst but also caused substantial changes in the chronological order of species' budburst. The results reveal that increased winter temperatures might impact forest ecosystems more than formerly assumed. Species with lower chilling requirements, such as pioneer or invasive species, might profit from warming winters, if late spring frost events would in parallel occur earlier.     

In situ temperature relationships of biochemical and stomatal controls of photosynthesis in four lowland tropical tree species

Net photosynthetic carbon uptake of Panamanian lowland tropical forest species is typically optimal at 30–32 °C. The processes responsible for the decrease in photosynthesis at higher temperatures are not fully understood for tropical trees. We determined temperature responses of maximum rates of RuBP‐carboxylation (V_CMax) and RuBP‐regeneration (J_Max), stomatal conductance (G_s), and respiration in the light (R_Light) in situ for 4 lowland tropical tree species in Panama. G_s had the lowest temperature optimum (T_Opt), similar to that of net photosynthesis, and photosynthesis became increasingly limited by stomatal conductance as temperature increased. J_Max peaked at 34–37 °C and V_CMax ~2 °C above that, except in the late‐successional species Calophyllum longifolium, in which both peaked at ~33 °C. R_Light significantly increased with increasing temperature, but simulations with a photosynthesis model indicated that this had only a small effect on net photosynthesis. We found no evidence for Rubisco‐activase limitation of photosynthesis. T_Opt of V_CMax and J_Max fell within the observed in situ leaf temperature range, but our study nonetheless suggests that net photosynthesis of tropical trees is more strongly influenced by the indirect effects of high temperature—for example, through elevated vapour pressure deficit and resulting decreases in stomatal conductance—than by direct temperature effects on photosynthetic biochemistry and respiration.     

低温胁迫下红松与西伯利亚红松光合与气孔特性

红松与西伯利亚红松均为寒温带著名的成林树种,具有较强的耐寒性,与红松相比,西伯利亚红松具有更强的耐寒性。为探究低温胁迫下两树种的生理响应机制及抗寒生理机理,本研究以5年生的红松与西伯利亚红松幼苗为材料,对其进行低温处理,3个胁迫温度(0℃、-20℃和-40℃)和3个胁迫时间(6、24和48 h),20℃为对照,研究低温胁迫下红松与西伯利亚红松的光合特性和气孔特性。T检验和方差分析结果表明,各光合指标和气孔密度在红松与西伯利亚红松中的差异显著(P<0.05),低温及低温胁迫时间对红松与西伯利亚红松各光合指标具有极显著影响(P<0.01),低温对红松与西伯利亚红松的气孔开度与气孔面积具有极显著影响(P<0.01)。胁迫前(20℃)和0℃低温胁迫下,红松中的净光合速率、气孔导度和蒸腾速率均显著高于西伯利亚红松,但在-20℃条件下胁迫6 h,西伯利亚红松各光合测定指标显著高于红松。随着温度的降低与胁迫时间的延长,两树种的各光合指标均呈下降趋势。红松中的气孔密度显著高于西伯利亚红松,胁迫前(20℃),红松与西伯利亚红松的气孔均为椭圆形,随着温度的降低,两树种的气孔开度和气孔面积显著减小。     

Stomatal Behaviours of Aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) Plants in Response to Low Root Temperature in Hydroponics

Hydroponic-grown seedlings of aspen (Populus tremuloides Michx.) were used to investigate how low root temperatures (5°C) affect stomatal conductance and water relations. An isohydric manner of the stomatal behaviour was found with the seedlings when their roots were subjected to the low temperature. Stomatal conductance rapidly and dramatically reduced in response to the low root temperature, while the xylem water potential did not significantly alter. Under the low root temperature, pH value of the xylem sap increased from 6.15 to 6.72 within the initial 4 h, while abscisic acid (ABA) concentration increased by the eighth hour of treatment. K⁺ concentration of the xylem sap significantly decreased within the 8th h and then reversed by the 24th h. The ion change was accompanied by a decrease and then an increase in the electrical conductivity, and an increase and then a decrease in the osmotic potential. The tempo of physiological responses to the low root temperature suggests that the rapid pH change of the xylem sap was the initial factor which triggered stomatal closure in low temperature-treated seedlings, and that the role of the more slowly accumulating ABA was likely to reinforce the stomatal closure. Xylem sap from the seedlings subjected low root temperature affected stomatal aperture on leaf discs when they were floated on the sap solution. The stomatal aperture correlated (P = 0.006) with the changed pattern of [K⁺] in the sap while the range of pH or ABA found in the xylem sap did not influence stomatal aperture of leaf discs in solution. The effect of xylem sap on stomatal aperture on leaf discs was different from on stomatal conductance in the intact seedlings. Comparison was made with previous study with the soil-grown seedlings.     

High yields in advanced lines of Pima cotton are associated with higher stomatal conductance, reduced leaf area and lower leaf temperature

Advanced lines of Pima cotton ( Gossypium barbadense L.) bred for higher yield potential and heat resistance have higher stomata conductance and smaller leaf areas than those of obsolete lines. In controlled experiments, five commercial lines of Pima cotton having increasing lint yield and heat resistance showed a gradient of increasing stomatal conductance and decreasing leaf size. In field experiments, heat-sensitive, low yield Pima lines showed a lower stomatal conductance than high yielding, advanced lines. This indicates that selection for high yield potential and heat resistance has imposed a selection pressure for higher stomatal conductance and smaller leaf areas. The higher stomatal conductance and smaller leaf area in the advanced lines resulted in a lower leaf temperature in both controlled environments and in the field. The largest leaf temperature differences between obsolete and advanced lines were observed in the afternoon. These differences coincided with the largest differences in stomatal conductance and the highest air temperatures. Measurements of stomatal conductance and leaf temperature in field-grown progeny from a cross between the advanced line, Pima S-6. and the obsolete line, Pima 32, showed that genetically determined differences in stomatal conductance resulted in corresponding differences in leaf temperature. None of the altered physiological traits were selected for in the breeding program, indicating that selection for the desired agronomic traits imposed selection pressures on the altered physiological traits. The increases in stomatal conductance and decreases in leaf area could represent an integrated response to selection pressures on enhanced evaporative cooling, ensuing from selection for heat resistance.     

Seasonal and diurnal changes in photosynthetic limitation of young sweet orange trees

This study tests the hypothesis that potted sweet orange plants show a significant variation in photosynthesis over seasonal and diurnal cycles, even in well-hydrated conditions. This hypothesis was tested by measuring diurnal variations in leaf gas exchange, chlorophyll fluorescence, leaf water potential, and the responses of CO₂ assimilation to increasing air CO₂ concentrations in 1-year-old ‘Valência’ sweet orange scions grafted onto ‘Cleopatra’ mandarin rootstocks during the winter and summer seasons in a subtropical climate. In addition, diurnal leaf gas exchange was evaluated under controlled conditions, with constant environmental conditions during both winter and summer. In relation to our hypothesis, a greater rate of photosynthesis is found during the summer compared to the winter. Reduced photosynthesis during winter was induced by cool night conditions, as the diurnal fluctuation of environmental conditions was not limiting. Low air and soil temperatures caused decreases in the stomatal conductance and in the rates of the biochemical reactions underlying photosynthesis (ribulose-1,5-bisphosphate (RuBP) carboxylation and RuBP regeneration) during the winter compared to the values obtained for those markers in the summer. Citrus photosynthesis during the summer was not impaired by biochemical or photochemical reactions, as CO₂ assimilation was only limited by stomatal conductance due to high leaf-to-air vapor pressure difference (VPD) during the afternoon. During the winter, the reduction in photosynthesis during the afternoon was caused by decreases in RuBP regeneration and stomatal conductance, which are both precipitated by low night temperature.     

Photosynthesis affects following night leaf conductance in Vicia faba

Night-time stomatal opening in C₃ plants may result in significant water loss when no carbon gain is possible. The objective of this study was to determine if endogenous patterns of night-time stomatal opening, as reflected in leaf conductance, in Vicia faba are affected by photosynthetic conditions the previous day. Reducing photosynthesis with low light or low CO₂ resulted in reduced night-time stomatal opening the following night, irrespective of the effects on daytime stomatal conductance. Likewise, increasing photosynthesis with enriched CO₂ levels resulted in increased night-time stomatal opening the following night. Reduced night-time stomatal opening was not the result of an inability to regulate stomatal aperture as leaves with reduced night-time stomatal opening were capable of greater night-time opening when exposed to low CO₂. After acclimating plants to long or short days, it was found that night-time leaf conductance was greater in plants acclimated to short days, and associated with greater leaf starch and nitrate accumulation, both of which may affect night-time guard cell osmotic potential. Direct measurement of guard cell contents during endogenous night-time stomatal opening will help identify the mechanism of the effect of daytime photosynthesis on subsequent night-time stomatal regulation.     

Abscisic Acid may Mediate the Rapid Thermal Acclimatization of Photosynthesis in Wheat

Exposing cold-grown (6°C) plants of a winter cultivar (Avalon)and a spring cultivar (Highbury) of wheat to a single warm night(18 h and 25°C) significantly increased the photosyntheticrate and stomatal conductance of fully-expanded leaves withoutaffecting the calculated, substomatal CO₂ pressure. Prior to the warm-night treatment net photosynthesis and stomatalconductance in Avalon were very small at high leaf temperaturesduring measurement (25–30°C). Such high temperatureinhibitions were not observed in Highbury. Exposure to the warm-nighttreatment alleviated these inhibitions in Avalon. Consequently,the temperature response of photosynthesis was similar in theleaves of the treated cultivars and showed a relatively broadoptimum. The application of exogenous abscisic acid in the transpirationstream to the leaves of treated plants caused a reduction ofsimilar proportions in the steady-state rate of photosynthesisand in stomatal conductance and essentially reversed the effectsof the warm-night treatment in a manner which depended on thecultivar. The endogenous abscisic acid content of leaves declinedby 50% during the warm-night treatment. In conjunction, thesedata suggest that changes in the content of abscisic acid inthe leaf may regulate the observed temperature acclimatizationof apparent photosynthesis in the fully-expanded leaves of cold-grownwheat. Key words: Abscisic acid, photosynthesis, stomatal conductance, temperature, acclimatizatio