Leaf conductance and transpiration, and water relations of evergreen and deciduous perennials co-occurring in a moist chaparral site

Abstract. Diurnal courses of leaf conductance, water potential and environmental parameters were measured through the year. The seasonal decreases in plant water potentials were greatest in evergreen H. arbutifolia , intermediate in winter deciduous C. occidentalis and least in drought deciduous A. californica. The seasonal patterns of water use were very similar. Estimates of soil root-stem conductances to liquid water flux indicate that during the spring, conductances are high, that during the early summer there is a daily shift from high morning conductances to lower afternoon conductances, and that late summer conductances are low.     

Roles of leaf water potential and soil-to-leaf hydraulic conductance in water use by understorey woody plants

Diurnal changes of leaf water potential and stomatal conductance were measured for 12 deciduous shrubs and tree saplings in the understorey of a temperate forest. Sunflecks raised the leaf temperature by 4°C, and vapor pressure deficit to 2 kPa. Although the duration of the sunflecks was only 17% of daytime, the photon flux density (PFD) of sunflecks was 52% of total PFD on a sunny summer day. Leaf osmotic potential at full turgor decreased in summer, except in some species that have low osmotic potential in the spring. Plants that endured low leaf water potential had rigid cell walls and low osmotic potential at full turgor. These plants did not have lower relative water content and turgor potential than plants with higher leaf water potential. There were three different responses to an increase in transpiration rate: (i) plants had low leaf water potential and slightly increased soil-to-leaf hydraulic conductance; (ii) plants decreased leaf water potential and increased the hydraulic conductance; and (iii) plants had high leaf water potential and largely increased the hydraulic conductance.     

Physiological responses of beech and sessile oak in a natural mixed stand during a dry summer

Responses of CO2 assimilation and stomatal conductance to decreasing leaf water potential, and to environmental factors, were analysed in a mixed natural stand of sessile oak (Quercus petraea ssp. medwediewii) and beech (Fagus svlvatica L.) in Greece during the exceptionally dry summer of 1998. Seasonal courses of leaf water potential were similar for both species, whereas mean net photosynthesis and stomatal conductance were always higher in sessile oak than in beech. The relationship between net photosynthesis and stomatal conductance was strong for both species. Sessile oak had high rates of photosynthesis even under very low leaf water potentials and high air temperatures, whereas the photosynthetic rate of beech decreased at low water potentials. Diurnal patterns were similar in both species but sessile oak had higher rates of CO2 assimilation than beech. Our results indicate that sessile oak is more tolerant of drought than beech, due, in part, to its maintenance of photosynthesis at low water potential.     

Young daughter cladodes affect CO2 uptake by mother cladodes of Opuntia ficus-indica

BACKGROUND AND AIMS: Drought damages cultivated C3, C4 and CAM plants in the semi-arid lands of central Mexico. Drought damage to Opuntia is common when mother cladodes, planted during the dry spring season, develop young daughter cladodes that behave like C3 plants, with daytime stomatal opening and water loss. In contrast, wild Opuntia are less affected because daughter cladodes do not develop on them under extreme drought conditions. The main objective of this work is to evaluate the effects of the number of daughter cladodes on gas exchange parameters of mother cladodes of Opuntia ficus-indica exposed to varying soil water contents. METHODS: Rates of net CO2 uptake, stomatal conductance, intercellular CO2 concentration, chlorophyll content and relative water content were measured in mature mother cladodes with a variable number of daughter cladodes growing in spring under dry and wet conditions. KEY RESULTS: Daily carbon gain by mother cladodes was reduced as the number of daughter cladodes increased to eight, especially during drought. This was accompanied by decreased mother cladode relative water content, suggesting movement of water from mother to daughter cladodes. CO2 assimilation was most affected in phase IV of CAM (late afternoon net CO2 uptake) by the combined effects of daughter cladodes and drought. Rainfall raised the soil water content, decreasing the effects of daughter cladodes on net CO2 uptake by mother cladodes. CONCLUSIONS: Daughter cladodes significantly hasten the effects of drought on mother cladodes by competition for the water supply and thus decrease daily carbon gain by mother cladodes, mainly by inhibiting phase IV of CAM.     

Diurnal courses of leaf conductance and transpiration of mistletoes and their hosts in Central Australia

Summary In Australia, diurnal courses of leaf conductance and transpiration of hemiparasitic mistletoes (Loranthaceae) and their hosts were measured using steady-state porometers under conditions of partial drought and high evaporative demand. The sites spanned a diversity of climatic regions ranging from the subtropical arid zone with winter rainfall, through the subtropical arid zone with summer rainfall to the tropical summer rainfall zone. With one exception (Acacia farnesiana with deciduous leaves), the hosts were trees or shrubs with evergreen, sclerophyllous leaves or phyllodes.The measurements confirm previous observations that mistletoes transpire at higher rates than their hosts. For adult leaves from all of the 18 different host/mistletoe pairs investigated, the daily average leaf conductances were higher in the parasites than in their hosts. The ratios ranged from 1.5 to 7.9. In the most extreme case,Amyema maidenii had a daily rate of water loss 8.9 times higher than its hostAcacia cowleana. Hoever, the parasites did not exhibit unlimited transpiration. Despite high water loss rates, leaf conductance showed large and consistent changes during the course of the day, indicating definite stomatal regulation. The typical diurnal pattern of conductance in both mistletoes and hosts consisted of an early morning peak followed by a continuous decrease throughout the remainder of the day. There was no abrupt decrease in leaf conductance of the parasites that might be interpreted as a threshold response with respect to internal water potential. In most cases, the continuous stomatal closure occurred without substantial changes in leaf water potential over a time span of several hours. The decrease in leaf conductance was correlated with an increase in leaf-to-air water vapor difference, which was associated with increasing leaf temperatures. It seems probable that external humidity plays a major role in the stomatal response. Diurnal courses of leaf conductance of the host/parasite pairs usually showed similar general patterns, even when the absolute rates were quite different. Thus, mistletoes not only control their water loss by stomatal action but this regulation seems to occur in coordination with the stomatal response of their hosts.The integrated mistletoe/host system must also endure severe drought conditions. Controlled water use is necessary for long-term survival of the host. Assuming stomatal behavior in the host is well adapted to ensure its existence, then similar performance in the mistletoe would promote survival of both host and parasite.     

Diurnal and seasonal responses of stomatal conductance for cowpea plants subjected to different levels of environmental drought

Summary Previously we reported that leaf conductance of cowpea (Vigna unguiculata) decreased with small changes in soil water status without associated changes in leaf water status. In these studies a larger range of soil water deficits was imposed in a rain-free environment by prolonged soil drying, and by weekly irrigation with different amounts of water. With progressive soil water deficits, leaf conductance and xylem pressure potential both declined, but in a manner which indicated that they were not related. Diurnal courses of leaf conductance usually indicated that stomatal opening occurred in the morning, and partial or complete stomatal closure occurred during midday and afternoon. This stomatal closure was associated with increases in air vapor pressure deficit. Day-to-day increases in leaf conductance, at times when radiation was not limiting stomatal opening, were associated with decreases in air vapor pressure deficits. However, maximum leaf conductances and their responses to vapor pressure deficit were generally smaller for plants subjected to greater depletion of soil water.     

Leaf Gas Exchange in Canopy Species of a Venezuelan Cloud Forest

Tropical cloud forests are considered humid ecosystems with frequent cloud cover down to the ground surface. However, seasonal variation in precipitation may induce short-term water stress. For canopy leaves, this water stress may also be a consequence of large atmospheric vapor pressure deficits. The objective of this work was to study five canopy cloud forest species to determine if there are restrictions to leaf gas exchange as a consequence of seasonality in precipitation and to daily water deficit due to air evaporative demand mainly during maximum incoming radiation hours. Seasonal daily courses of microclimatic variables (air temperature, relative humidity, photosynthetic photon flux density) and plant responses (leaf water potential, stomatal conductance, CO₂ assimilation rates, leaf nitrogen concentration) were measured at 2400 m asl in Monterrey, an intermontane valley of the Venezuelan Andes. A gradient in terms of responses to water stress conditions was observed between the species, with Clusia multiflora (a 46% reduction in stomatal conductance between seasons) as the most affected and Miconia resimoides (increased stomatal conductance) responding more favorably to slight water stress conditions. If we consider the limitations of water stress and/or light conditions on CO₂ assimilation we may arrange the species into those in which water stress conditions have a greater impact on leaf carbon gain, those where light conditions are determinant and one in which both water stress and light conditions may affect leaf carbon assimilation.     

Limitations due to water stress on leaf net photosynthesis of Quercus coccifera in the Portuguese evergreen scrub

Summary Gas exchange characteristics in leaves of the sclerophyll shrub Quercus coccifera were studied in the natural habitat in Portugal during spring and during the summer dry period. Compared to other sclerophyll species growing at the same site, photosynthesis in leaves of Quercus coccifera was less affected by water stress. Moderate water stress after six weeks of drought led to large decreases in stomatal conductance but no change in mesophyll photosynthetic capacity as compared to late spring. Leaf internal CO₂ pressure remained near 220 bar during diurnal courses in the spring. On midsummer days, leaf internal CO₂ decreased from a late morning value of 200 bar to a late afternoon value of approximately 150 bar. In contrast to Quercus suber (Tenhunen et al. 1984), restriction of CO₂ supply due to stomatal closure reduced net CO₂ uptake at midday and in the afternoon during midsummer. A decrease in leaf carboxylation efficiency and an increase in CO₂ compensation point at midday also played an important role in determining the diurnal course of net photosynthesis. During the late stages of drought in September, severe water stress led to reduction in mesophyll photosynthetic capacity and further reduction in leaf conductance. The observed decrease in mesophyll photosynthetic capacity was correlated with decrease in the daily minimum leaf water potential to greater negative values than-30 bar. At this time, CO₂ saturated photosynthetic rates decreased as much as 50% over the course of a day when measured at constant saturating light, 32° C leaf temperature, and a water vapor mole fraction difference between leaf and air of 30 mbar bar^-1.     

Diurnal variations in leaf water potential and stomatal conductance of pigeon pea (Cajanus cajan (L.) Millsp.) cultivars as affected by irrigation levels

Two cultivars of pigeon pea (Cajanus cajan (L.)Millsp.) UPAS-120 and Parbhat were grown in the field under two irrigation treatments: no irrigation and irrigation when cumulative pan evaporation was equal to 50 per cent depletion of available water content in one metre root zone depth. Diurnal changes in leaf water potential and stomatal conductance were recorded on two daysi.e. October 1, 1979 and October 28, 1979 which corresponded to reproductive growth stage of the crop. Plant water potential decreased rapidly during the day up to about 15.00 and increased during evening hours. Higher leaf water potential was recorded in irrigated treatment on both dates. Adaxial and abaxial stomata differed in their response to water stress. Adaxial stomatal conductance started declining during early morning hours, however, abaxial conductance firstly increased up to 09.00 then decreased and increased again in the afternoon except in irrigated crop of cv. UPAS-120 on 28th October, where conductance never increased after 09.00. The reduced rate of stomatal conductance during day hours may be identified as one of the characteristics responsible for drought tolerance in pigeon pea.     

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.     

Loss of whole-tree hydraulic conductance during severe drought and multi-year forest die-off

Understanding the pathways through which drought stress kills woody vegetation can improve projections of the impacts of climate change on ecosystems and carbon-cycle feedbacks. Continuous in situ measurements of whole trees during drought and as trees die hold promise to illuminate physiological pathways but are relatively rare. We monitored leaf characteristics, water use efficiency, water potentials, branch hydraulic conductivity, soil moisture, meteorological variables, and sap flux on mature healthy and sudden aspen decline-affected (SAD) trembling aspen (Populus tremuloides) ramets over two growing seasons, including a severe summer drought. We calculated daily estimates of whole-ramet hydraulic conductance and modeled whole-ramet assimilation. Healthy ramets experienced rapid declines of whole-ramet conductance during the severe drought, providing an analog for what likely occurred during the previous drought that induced SAD. Even in wetter periods, SAD-affected ramets exhibited fivefold lower whole-ramet hydraulic conductance and sevenfold lower assimilation than counterpart healthy ramets, mediated by changes in leaf area, water use efficiency, and embolism. Extant differences between healthy and SAD ramets reveal that ongoing multi-year forest die-off is primarily driven by loss of whole-ramet hydraulic capability, which in turn limits assimilation capacity. Branch-level measurements largely captured whole-plant hydraulic limitations during drought and mortality, but whole-plant measurements revealed a potential role of other losses in the hydraulic continuum. Our results highlight the importance of a whole-tree perspective in assessing physiological pathways to tree mortality and indicate that the effects of mortality on these forests’ assimilation and productivity are larger than expected based on canopy leaf area differences.     

Effects of soil water deficit and nitrogen nutrition on water relations and photosynthesis of pot-grown Coffea canephora Pierre

Coffea canephora plants (clone INCAPER-99) were submitted to low N (LN) or high N (HN) applications and two watering regimes (daily irrigation and irrigation every 5 days for a month). Although water potential was not altered significantly by N, HN plants showed higher relative water content than did LN plants under water deficit. Only HN plants exhibited some ability for osmotic adjustment. Plants from both N treatments increased their cell wall rigidity under drought, with a more pronounced augmentation in HN plants. In well-watered plants, carbon assimilation rate increased with increasing N while stomatal conductance did not respond to N supply. Under drought conditions, carbon assimilation decreased by 68-80% compared to well-watered plants, whereas stomatal conductance and transpiration rate declined by 35% irrespective of the N applications. Stable carbon isotope analysis, combined with leaf gas exchange measurements, indicated that regardless of the watering treatments, N increased the long-term water use efficiency through changes in carbon assimilation with little or no effect on stomatal behaviour.     

RAPID LEAF MOVEMENT,MICROCLIMATE, AND WATER RELATIONS OF TWO TEMPERATE LEGUMES IN THREE CONTRASTING HABITATS

Diurnal heliotropic leaf movements, microclimate, stomatal conductance and leaf water potential of two leguminous species, Strophostyles helvola and Amphicarpa bracteata, were measured in three different habitats over two growing seasons. The habitats occurred along an environmental gradient from an open, sandy beach to a closed canopy deciduous forest understory. At the beach site, heliotropism in S. helvola resulted in higher irradiances in morning and afternoon hours and lower irradiances during midday periods compared to an horizontal leaf. In an exposed forest site A. bracteata responded within minutes to penetration of the direct solar beam by orienting its leaves to steep angles. In contrast, plants in closed canopy locations showed little diurnal leaf movement. The combined results of leaf energy budget calculations and plant physiological responses suggest that ecological ramifications of these movements vary with habitat. We hypothesize that heliotropism in open habitats increases water use efficiency and maximizes carbon returns on plant investment in photosynthesis. Conversely, we hypothesize that leaf movements in understory habitats represent a morphological mechanism to avoid thermal damage, photoinhibition of the photosynthetic apparatus, and water stress associated with high irradiances.     

Diurnal and Seasonal Changes in Water Balance of Acer saccharum and Betula papyrifera

Diurnal and seasonal changes in plant water potential, leaf diffusion resistance, and stem radial changes of Acer saccharum and Betula papyrifera trees were studied in northern Wisconsin during the 1974 and 1975 growing seasons. Water potential decreased during the day, following relatively high values in the morning, and increased in the late afternoon and evening. Diurnal patterns and actual values of water potential varied with species, soil water availability, and factors influencing transpiration (e.g., solar radiation, vapor pressure deficit, and transpiration flux density). When plant water deficits were not severe, leaf resistance of both species was rather stable during the day. During severe droughts, however, leaf resistance increased (stomata closed) during the day when light intensity was high. Leaf resistance at high light intensity was higher in Acer than in Betula. Stomatal closure with decreasing light intensity varied between species and among Acer trees. Tree stems of both species shrank during the day, as internal water deficits developed, and they expanded as trees rehydrated during the night. Stems of Acer shrank more than those of Betula. The amount of daily stem shrinkage increased as the season progressed if the trees were not under severe water deficits. During severe droughts the amount of diurnal stem shrinkage decreased. Shrinkage of stems lagged behind water potential changes by 1 to 2 h in Acer and less than 1 h in Betula. The relationship between stem radius and leaf water potential was not constant throughout the growing season.     

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.     

Effects of ultraviolet-B radiation on plants during mild water stress. IV. The insensitivity of soybean internal water relations to ultraviolet-B radiation

The combined effects of ultraviolet-B (UV-B, 280–320 nm) radiation and water stress were investigated on the water relations of greenhouse grown soybean [ Glycine max (L.) Merr. cv. Essex]. On a weighted (Caldwell 1971), total daily dose basis, plants received either 0 or 3 000 effective J m² UV-B_BE supplied by filtered FS-40 sunlamps. The latter dose simulated the solar UV-B radiation anticipated at College Park, Maryland, U.S.A. (39°N latitude) in the event that the global stratospheric ozone column is reduced by 25%. Plants were either well-watered or preconditioned by drought stress cycles. Diurnal measurements of water potential and stomatal conductance were made on the youngest fully expanded leaf. Various internal water relations parameters were determined for detached leaves. Plants were monitored before, during and after water stress. There were no significant differences in leaf water potential or stomatal conductance between treatments before plants were preconditioned to water stress. However, drought stress resulted in significantly lower midday and afternoon leaf water potentials and lower leaf conductances as compared to well-watered plants. UV-B radiation had no additional effect on leaf water potential; however, UV did result in lower leaf conductances in plants preconditioned to water stress. Turgid weight:dry weight ratio, elastic modulus, bound water and relative water content were unaffected by UV-B radiation. Osmotic potentials at full and zero turgor were significantly lower in the drought stressed treatments as compared to well-watered plants.     

Relationship between xylem ion concentration and bean growth responses to short-term salinisation in spring and summer

Bean plants, Phaseolus vulgaris L. cv. Contender, were grown in the spring and summer seasons to study the relationship between xylem Na+/Cl-, transpiration rate, and salt tolerance. Eight-day-old seedlings were transplanted to 50% modified Hoagland solution with 1 mM NaCl. Four days after transfer, one of two treatments was applied: a control of 1 mM NaCl or a treatment of 25 mM NaCl every two days to reach a final treatment concentration of 75 mM NaCl. Plants were sampled on the fourth day after the final salt concentration was reached, eight days after the salinisation treatment began. Relative growth rate was 2.6-fold greater in summer than in spring. However, while no differences were found between treatments in spring, summer salt-treated plants had growth rates that were 31% lower than those of controls. In summer, CO2 assimilation, stomatal conductance, and transpiration rate of salinised plants declined with respect to controls. Leaf Na+ and trifoliolate leaf Cl- were higher in salt-treated plants in summer, although root Na+ was significantly higher in spring. Moreover, in summer salinity inhibited Ca2+ and K+ uptake and changed its distribution. Summer salt-treated plants had an average of 17-fold higher xylem Na+ during the daily cycle, while xylem Cl-, only in the afternoon, showed higher values (1.5-fold) compared to spring-grown plants. Our results suggest that the faster growth response to salt in summer-grown bean was at least partly due to an increase in xylem Na+ independent of the transpiration rate and possibly related to an increase in xylem Na+ influx or/and Na+ recirculation.     

Photosynthesis in Encelia farinosa Gray in Response to Decreasing Leaf Water Potential

Photosynthetic responses of intact leaves of the desert shrub Encelia farinosa were measured during a long term drought cycle in order to understand the responses of stomatal and nonstomatal components to water stress. Photosynthetic rate at high irradiance and leaf conductance to water vapor both decreased linearly with declining leaf water potential. The intercellular CO₂ concentration (c_i) remained fairly constant as a function of leaf water potential in plants subjected to a slow drought cycle of 25 days, but decreased in plants exposed to a 12-day drought cycle. With increasing water stress, the slope of the dependence of photosynthesis on c_i (carboxylation efficiency) decreased, the maximum photosynthetic rates at high c_i became saturated at lower values, and water use efficiency increased. Both the carboxylation efficiency and photosynthetic rates were positively correlated with leaf nitrogen content. Associated with lower leaf conductances, the calculated stomatal limitation to photosynthesis increased with water stress. However, because of simultaneous changes in the dependence of photosynthesis on c_i with water stress, increased leaf conductance alone in water-stressed leaves would not result in an increase in photosynthetic rates to prestressed levels. Both active osmotic adjustment and changes in specific leaf mass occurred during the drought cycle. In response to increased water stress, leaf specific mass increased. However, the increases in specific leaf mass were associated with the production of a reflective pubescence and there were no changes in specific mass of the photosynthetic tissues. The significance of these responses for carbon gain and water loss under arid conditions are discussed.     

Coppicing alters ecophysiology of Quercus rubrasaplings in Wisconsin forest openings

In the spring of 1987, entire shoots were removed from Quercus rubra L. saplings in two southwestern Wisconsin forest openings. Shoots possessed newly expanding leaves at the time of coppicing. All coppiced individuals sprouted from dormant stem buds near the root collar. Leaf gas exchange and water potential were monitored on these sprouts and on untreated (control) Q. rubra saplings throughout several clear warm days during the 1987 growing season. Daily maxima and averages for sprout leaf photosynthesis and stomatal conductance generally exceeded those of controls. On average, treatment differences in daily maximum photosynthetic rate were modest (11–14%) and were attributed primarily to a 30–38% enhancement of sprout leaf stomatal conductance. Relative differences in daily average photosynthetic rate (29–39%) were substantially larger than those in daily maximum photosynthesis, owing to the fact that sprouts and controls exhibited distinct diurnal gas exchange patterns. Photosynthetic rate and stomatal conductance of control leaves typically declined during the day following a mid-morning maximum. Sprout leaves, on the other hand, tended to maintain gas exchange rates nearer to their morning maxima throughout the day. This difference in diurnal gas exchange pattern was associated with an apparent differential leaf sensitivity to leaf-to-air vapor pressure gradient (VPG). The relative decline in sprout leaf gas exchange rates with increasing VPG was less than that of controls. Treatment differences in gas exchange did not appear to be related to leaf water potential or tissue water relations, but sprouts had a higher soil-to-leaf hydraulic conductivity than controls.     

Photosynthesis and water relations of well-watered orange plants as affected by winter and summer conditions

The aim of this study was to evaluate how the summer and winter conditions affect the photosynthesis and water relations of well-watered orange trees, considering the diurnal changes in leaf gas exchange, chlorophyll (Chl) fluorescence, and leaf water potential (Ψ) of potted-plants growing in a subtropical climate. The diurnal pattern of photosynthesis in young citrus trees was not significantly affected by the environmental changes when compared the summer and winter seasons. However, citrus plants showed higher photosynthetic performance in summer, when plants fixed 2.9 times more CO₂ during the diurnal period than in the winter season. Curiously, the winter conditions were more favorable to photosynthesis of citrus plants, when considering the air temperature (< 29 °C), leaf-to-air vapor pressure difference (< 2.4 kPa) and photon flux density (maximum values near light saturation) during the diurnal period. Therefore, low night temperature was the main environmental element changing the photosynthetic performance and water relations of well-watered plants during winter. Lower whole-plant hydraulic conductance, lower shoot hydration and lower stomatal conductance were noticed during winter when compared to the summer season. In winter, higher ratio between the apparent electron transport rate and leaf CO₂ assimilation was verified in afternoon, indicating reduction in electron use efficiency by photosynthesis. The high radiation loading in the summer season did not impair the citrus photochemistry, being photoprotective mechanisms active. Such mechanisms were related to increases in the heat dissipation of excessive light energy at the PSII level and to other metabolic processes consuming electrons, which impede the citrus photoinhibition under high light conditions.