Acidic mist reduces foliar membrane-associated calcium and impairs stomatal responsiveness in red spruce

Acidic deposition can leach essential pools of calcium (Ca) directly from plant foliage. Because of the central role of Ca in environmental signal transduction, disruptions of labile foliar Ca pools could impair physiological responses to a variety of environmental stimuli and stressors. We investigated the possibility that acidic mist-induced depletion of membrane-associated Ca (mCa), which is one form of labile Ca, may alter stomatal responsiveness to water stress, a process known to include Ca in signal transduction cascades. Red spruce (Picea rubens Sarg.) seedlings were exposed to either pH 3.0 or pH 5.0 mist treatments for one growing season. Foliar nutrition was assessed following treatments, and declines in stomatal conductance and net photosynthesis were measured on current-year shoots following stem excision. Seedlings exposed to pH 3.0 acidic mist treatments had reduced mCa relative to the pH 5.0 treated seedlings. Seedlings subjected to the pH 3.0 acidic mist treatment exhibited impaired stomatal functions, including a smaller maximum aperture, slower closure and an increased lag time between stomatal closure and photosynthetic decline following experimental water stress. Delayed stomatal closure could undermine desiccation avoidance mechanisms. Previous work has demonstrated that acidic mist treatments deplete mCa in red spruce and impair cold tolerance, with similar effects in other species. The results we present provide further evidence that acidic mist-induced mCa depletion may cause disruption of a broad range of plant stress responses.     

Water relations of red spruce seedlings treated with acid mist

Pressure-volume curves, day and night transpiration rates, needle drying curves, and shoot water potentials were determined for 2-year-old red spruce trees that had been exposed for three months to a range of acid mists (pH 2.5 to pH 5.0) containing equimolar (NH(4))(2)SO(4) and HNO(3). No effect of acid mist was observed on cuticular resistance or on the rates of day and night transpiration, although trees exposed to acid mist exhibited symptoms of mild water stress. Significant decreases in maximum turgor, the relative water content (RWC) associated with zero turgor, and bulk volumetric elastic modulus occurred as the pH of the mist decreased from 5.0 to 2.5. At all RWC values, there was an increase in solute potential as mist pH decreased. Shoot water potential declined with a decrease in pH of the mist.     

Influence of temperature and leaf-to-air vapor pressure deficit on net photosynthesis and stomatal conductance in red spruce (Picea rubens)

The roles of temperature (T) and leaf-to-air vapor pressure deficit (VPD) in regulating net photosynthesis (A(net)) and stomatal conductance (G(s)) of red spruce (Picea rubens Sarg.) were investigated in a field study and in a controlled environment experiment. Both A(net) and G(s) exhibited a relatively flat response to temperatures between 16 and 32 degrees C. Temperatures between 32 and 36 degrees C markedly decreased both A(net) and G(s). Vapor pressure deficits above 2 kPa had significant effects on both A(net) and G(s). The influence of VPD on A(net) and G(s) fit a linear response model and did not interact significantly with T effects.     

Effects of mist acidity and ambient ozone removal on montane red spruce

Recent forest studies have established that high-elevation (> 900 m) populations of red spruce (Picea rubens Sarg.) in the northeastern USA are declining. Because it has been suggested that changes in air quality are responsible for the decline, we examined the effects of acidic mists and ozone on several biochemical and growth parameters in mature montane red spruce. We used branch-sized environmental chambers to introduce mists of controlled composition and exclude ambient clouds and ozone from individual branches within a tree. Mists consisting of distilled water increased the end-of-season pigment concentration and shoot length of enclosed branches relative to ambient or artificial mists. Needle and twig weights and starch concentrations were not significantly altered by the acidic mist treatments. Removal of ambient ozone had no apparent effect on the variables measured.     

Acid mist and soil Ca and Al alter the mineral nutrition and physiology of red spruce

We examined the effects and potential interactions of acid mist and soil solution Ca and Al treatments on foliar cation concentrations, membrane-associated Ca (mCa), ion leaching, growth, carbon exchange, and cold tolerance of red spruce (Picea rubens Sarg.) saplings. Soil solution Ca additions increased foliar Ca and Zn concentrations, and increased rates of respiration early in the growing season (July). Soil Al treatment had a broad impact, reducing foliar concentrations of Ca, Mg, Mn, P and Zn, and resulting in smaller stem diameters, sapling heights and shoot lengths compared with soil treatments with no added Al. Aluminum treatment also reduced respiration when shoots were elongating in July and decreased net photosynthesis at the end of the growing season (September). Three lines of evidence suggest that Al-induced alterations in growth and physiology were independent of foliar Ca status: (1) Ca concentrations in foliage of Al-treated saplings were within the range of sufficiency established for red spruce; (2) mCa concentrations were unaffected by Al treatment; and (3) no Al x Ca interactions were detected. Acid mist treatment increased foliar Fe and K concentrations and increased leaching of Ca, Mg, Mn, Zn, Fe, and Al from foliage. Leaching losses of Ca were more than twice those of the element with the next highest amount of leaching (Zn), and probably led to the reductions in mCa concentration and membrane stability of acid-treated saplings. Acidic mist resulted in enhanced shoot growth, and consistent reductions in foliar cold tolerance in the fall and winter. Of the few significant interactions among treatments, most involved the influence of mist pH and Al treatment on foliar nutrition. In general, reductions in cation concentration associated with Al addition were greater for pH 5.0-treated saplings than for pH 3.0-treated saplings. We propose that H(+)-induced leaching of mCa from mesophyll cells is the mechanism underlying acid-induced reductions in foliar cold tolerance of red spruce.     

Photosynthetic and transpirational responses of red spruce understory trees to light and temperature

Understory red spruce (Picea rubens Sarg.) trees, between 20 and 50 cm in height and 12 years or more in age, were collected from mid- and high-elevation stands in north-central Vermont and placed in a closed-cuvette system to measure photosynthetic and transpirational responses to photosynthetic photon flux density (PPFD) and temperature. Photosynthesis, dark respiration, transpiration and water-use efficiency of trees from both stands responded to changes in PPFD and temperature in similar ways. Trees from both stands exhibited maximum rates of net photosynthesis at temperatures between 15 and 20 degrees C, and exposure to higher temperatures resulted in reduced rates of photosynthesis and increased rates of respiration. Net photosynthetic rates generally increased with increasing light intensity but began to level off at 250 micro mol m(-2) s(-1). Water-use efficiency was maximal when temperature and PPFD were at 15 degrees C and above 400 micro mol m(-2) s(-1), respectively.     

Mean canopy stomatal conductance responses to water and nutrient availabilities in Picea abies and Pinus taeda

We compared sap-flux-scaled, mean, canopy stomatal conductance (GS) between Picea abies (L.) Karst. in Sweden and Pinus taeda (L.) in North Carolina, both growing on nutritionally poor soils. Stomatal conductance of Picea abies was approximately half that of Pinus taeda and the sensitivity of GS in Picea abies to vapor pressure deficit (D) was lower than in Pinus taeda. Optimal fertilization increased leaf area index (L) two- and threefold in Pinus taeda and Picea abies, respectively, regardless of whether irrigation was increased. Although it increased L, fertilization did not increase GS in Picea abies unless irrigation was also provided. In Pinus taeda growing on coarse, sandy soils, the doubling of L in response to fertilization reduced GS sharply unless irrigation was also provided. The reduction in GS with fertilization in the absence of irrigation resulted from the production of fine roots with low saturated hydraulic conductivity. When Pinus taeda received both fertilization and irrigation, the increase in L was accompanied by a large increase in GS. In Pinus taeda, a reference GS (defined as GS at D = 1 kPa; GSR) decreased in all treatments with decreasing volumetric soil water content (theta). In Picea abies, theta varied little within a treatment, but overall, GSR declined with theta, reaching lowest values when drought was imposed by the interception of precipitation. Despite the large difference in GS both between Picea abies and Pinus taeda and among treatments, stem growth was related to absorbed radiation, and stem growth response to treatment reflected mostly the changes in L.     

Abscisic acid in leaves and roots of willow: significance for stomatal conductance

Excised leaves and roots of willow (Salix dasyclados Wimm.) accumulated abscisic acid (ABA) in response to desiccation. The accumulation of ABA was greater in young leaves and roots than in old leaves and roots. In mature leaves, ABA accumulation was related to the severity and duration of the desiccation treatment. Water loss equal to 12% of initial fresh weight caused the ABA content of mature leaves to increase measurably within 30 min and to double in 2.5 h. The drying treatment caused significant (P = 0.05) reductions in leaf water potential and stomatal conductance. Recovery of leaf water potential to the control value occurred within 10 min of rewatering the dehydrated leaves, but recovery of stomatal conductance took an hour or longer, depending on the interval between dehydration and rewatering. The addition of ABA to the transpiration stream of well-watered excised leaves was sufficient to cause partial stomatal closure within 1 h and, depending on ABA concentration, more or less complete stomatal closure within 3 h. When the ABA solution was replaced with water, stomatal conductance increased at a rate inversely related to the concentration of the ABA solution with which the leaves had been supplied.     

Hydraulic and stomatal adjustment of Norway spruce trees to environmental stress

A study of how the water conducting systems of 30-50-year-old Norway spruce (Picea abies (L.) Karst.) trees growing at three sites adjust to shade and waterlogging indicated that water relations characteristics varied with the life histories of the trees. Xylem was more efficient at conducting water and stomata were more sensitive to atmospheric evaporative demand in trees subjected to favorable growth conditions (control trees) than in trees growing in shade or waterlogged conditions. At the same soil water availability, shade-grown trees suffered more severely from water deficit than control trees. Under conditions of high atmospheric vapor pressure deficit, foliage of shade-grown trees exhibited low water potentials, as a result of low hydraulic conductance of the vascular system and inefficient stomatal control. Because of the increased internal resistance to water flow, more negative leaf water potentials (Psi(x)) must be reached to provide an adequate water supply to the foliage. It is concluded that dynamic water stress is one of the main causes of the continuing growth retardation in suppressed Norway spruce trees after their release from the overstory. Trees growing in waterlogged soil (bog-grown trees) were characterized by weak stomatal control, resulting in large water losses from the foliage. Although bog-grown trees exhibited uneconomical water use, they possessed mechanisms (e.g., osmotic adjustment) that allowed leaves to tolerate low Psi(x) while stomata remained open. Under conditions of sufficient soil water availability and moderate atmospheric vapor pressure deficit, soil-to-leaf conductance was highest in bog-grown trees (1.45 +/- 0.06 mmol m(-2) s(-1) MPa(-1)), followed by control and shade-grown trees (1.04 +/- 0.04 and 0.77 +/- 0.05 mmol m(-2) s(-1) MPa(-1), respectively). The lowest soil-to-leaf conductance (0.45 +/- 0.04 mmol m(-2) s(-1) MPa(-1)) was recorded in control trees at high atmospheric evaporative demand, and was probably caused by tracheid cavitation.     

Fertilization effects on mean stomatal conductance are mediated through changes in the hydraulic attributes of mature Norway spruce trees

Stomatal conductance was quantified with sap flux sensors and whole-tree chambers in mature Norway spruce (Picea abies (L.) Karst.) trees after 3 years of exposure to elevated CO(2) concentration ([CO(2)]) in a 13-year nutrient optimization experiment. The long-term nutrient optimization treatment increased tree height by 3.7 m (67%) and basal diameter by 8 cm (68%); the short-term elevated [CO(2)] exposure had no effect on tree size or allometry. Nighttime transpiration was estimated as approximately 7% of daily transpiration in unchambered trees; accounting for the effect of nighttime flux on the processing of sap flux signals increased estimated daily water uptake by approximately 30%. Crown averaged stomatal conductance (g(s)) was described by a Jarvis-type model. The addition of a stomatal response time constant (tau) and total capacitance of stored water (C(tot)) improved the fit of the model. Model estimates for C(tot) scaled with sapwood volume of the bole in fertilized trees. Hydraulic support-defined as a lumped variable of leaf-specific hydraulic conductivity and water potential gradient (K(l)DeltaPsi) -was estimated from height, sapwood-to-leaf area ratio (A(s):A(l)) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in g(s) at reference conditions for trees across nutrient and [CO(2)] treatments. Removal of approximately 50% of A(l) from three trees yielded results suggesting that stomatal compensation (i.e., an increase in g(s)) after pruning scales inversely with K(l)DeltaPsi, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in A(s):A(l).     

干旱胁迫下云杉内源激素的响应及其气孔调节

为给抗旱性的深入研究提供理论依据,采用盆栽法比较了3年生红皮云杉和嫩江云杉苗木在不同水分条件下几种内源激素的变化.结果表明:随着土壤含水量的降低(40％～12％),2种云杉根、茎、叶中的ABA含量都有上升的趋势,可达到对照的2～5倍,且嫩江云杉的上升幅度大于红皮云杉.在轻度甚至中度干旱时,红皮云杉IAA的含量有所上升,...     

Responses of stomatal conductance to simultaneous changes in two environmental factors

To clarify interactions between stomatal responses to two simultaneous environmental changes, the rates of change in stomatal conductance were measured after simultaneously changing two environmental factors from the set of air humidity, leaf water potential (hydraulic environmental factors), air CO(2) concentration and light intensity (photosynthetic environmental factors). The stomatal responses to changes in leaf water potential were not significantly modified by any other simultaneous environmental change. A decrease in air humidity was followed by a decrease in stomatal conductance, and an increase in air humidity was followed by an increase in the conductance, irrespective of the character of the simultaneous change in the photosynthetic environmental factor. If the simultaneous change had an opposite effect on stomatal conductance, the rate of change in stomatal conductance was higher than the theoretical summed rate-the sum of the rate following one environmental change and the rate following another environmental change, measured separately. That is, the stomatal response to air humidity dominated over the responses to photosynthetic environmental factors. Yet, if the simultaneous change in photosynthetic factors had a codirectional effect on stomatal conductance, the rate of stomatal conductance change was lower than the theoretical summed rate. After a simultaneous change of two photosynthetic environmental factors, the rate of stomatal conductance change was very similar to the theoretical rate, if both the environmental changes had a codirectional effect on stomatal conductance. If the changes in the photosynthetic factors had opposite effects on stomatal conductance, the conductance increased, irrespective of the character of the increasing environmental factor. In drought-stressed trees, the rates of change in stomatal conductance tended to differ from the theoretical summed rates more than in well-watered trees. Stomatal closure following an increase in CO(2) concentration was the stomatal response that was most strongly suppressed by the response to another simultaneous environmental change. Six species of temperate deciduous trees were shown to be similar in their relations between the stomatal responses to two simultaneous environmental changes. The mechanism and ecological significance of the interactions between the two signal response pathways of stomata are discussed.     

Detection and quantification of changes in membrane-associated calcium in red spruce saplings exposed to acid fog

Five-year-old red spruce saplings (Picea rubens Sarg.) were exposed to either (1) acid fog consisting of a mixture of H(2)SO(4) and HNO(3) adjusted to pH 2.5, (2) distilled-water fog at pH 5.6, or (3) no fog (dry control) for 3.5 hours per day, five times a week during the 1996 and 1997 growing seasons. The effect of fog on cell membrane-associated calcium (mCa) of leaf mesophyll cells was investigated with the fluorescence probe chlortetracycline (CTC). In both years, mean mCa concentrations were significantly less in needles exposed to acid fog than in needles exposed to distilled-water fog or in untreated needles. In 1997, acid-fog treatment resulted in 25 and 12% reductions in mCa in current-year needles, and 18 and 15% reductions in 1-year-old needles, compared with untreated needles and needles exposed to distilled-water fog, respectively, indicating that acid deposition induced calcium leaching from the membranes of photosynthetic mesophyll cells. Exposure to distilled-water fog also led to reductions in mCa in young needles, suggesting that water films on needle surfaces can induce losses by diffusion between the needle interior and surface. Consistent with the chamber studies, field data obtained from red spruce trees at two sites in Maine showed that low mCa concentrations in needles were associated with exposure to acid fog.     

Effects of elevated CO(2) concentration and nutrition on net photosynthesis,stomatal conductance and needle respiration of field-grown Norway spruce trees

To study the effects of elevated CO(2) on gas exchange, nonstructural carbohydrate and nutrient concentrations in current-year foliage of 30-year-old Norway spruce (Picea abies (L.) Karst.) trees, branches were enclosed in ventilated, transparent plastic bags and flushed with ambient air (mean 370 &mgr;mol CO(2) mol(-1); control) or ambient air + 340 &mgr;mol CO(2) mol(-1) (elevated CO(2)) during two growing seasons. One branch bag was installed on each of 24 selected trees from control and fertilized plots. To reduce the effect of variation among trees, results from each treated branch were compared with those from a control branch on the same whorl of the same tree. Elevated CO(2) increased rates of light-saturated photosynthesis on average by 55% when measured at the treatment CO(2) concentration. The increase was larger in shoots with high needle nitrogen concentrations than in shoots with low needle nitrogen concentrations. However, shoots grown in elevated CO(2) showed a decrease in photosynthetic capacity compared with shoots grown in ambient CO(2). When measured at the internal CO(2) concentration of 200 &mgr;mol CO(2) mol(-1), photosynthetic rates of branches in the elevated CO(2) treatments were reduced by 8 to 32%. The elevated CO(2) treatment caused a 9 to 20% reduction in carboxylation efficiency and an 18% increase in respiration rates. In response to elevated CO(2), starch, fructose and glucose concentrations in the needles increased on average 33%, whereas concentrations of potassium, nitrogen, phosphorus, magnesium and boron decreased. Needle nitrogen concentrations explained 50-60% of the variation in photosynthesis and CO(2) acclimation was greater at low nitrogen concentrations than at high nitrogen concentrations. We conclude that the enhanced photosynthetic rates found in shoots exposed to elevated CO(2) increased carbohydrate concentrations, which may have a negative feedback on the photosynthetic apparatus and stimulate cyanide-resistant respiration. We also infer that the decrease in nutrient concentrations of needles exposed to elevated CO(2) was the result of retranslocation of nutrients to other parts of the branch or tree.     

Response of canopy stomatal conductance of Acacia mangium forest to environmental driving factors

Granier’s probes were applied to measure the sap flow of 14 sample trees in an Acacia mangium forest on the hilly lands in Heshan City, Guangdong, during the time period of October, 2003. The photosynthetically active radiation (PAR), air relative humidity (RH) and temperature of air (T) above the forest canopy were recorded. The sap flow measurement was used in combination with morphological characteristics of tree and forest structure to calculate the whole-tree transpiration (E), stand transpiration (E _t), and mean canopy stomatal conductance (g _c). Analyses on the relationships between tree morphological characters and whole-tree water use, and on the responses of g _c to PAR and vapor pressure deficit (D) were conducted. The results showed that whole-tree transpiration correlated significantly and positively with tree diameter at breast height (DBH) (p<0.0001), with sapwood area (p<0.0001), and with canopy size (p = 0.0007) logarithmically, but exponentially with tree height (p = 0.014). The analyses on the responses of canopy stomatal conductance showed that the maximum g _c (g _cmax) changed with PAR in a hyperbolic curve (p<0.0001) and with D in a logarithmic one (p<0.0001). The results obtained with sap flow technique indicate its reliability and accuracy of the methods of estimation of whole-tree and stand transpirations and canopy stomatal conductance. __________ Translated from Chinese Journal of Applied Ecology, 2006, 17(7): 1149–1156 [译自: 应用生态学报]     

The response of Pinus sylvestris to drought: stomatal control of transpiration and hydraulic conductance

We investigated the impact of drought on the physiology of 41-year-old Scots pine (Pinus sylvestris L.) in central Scotland. Measurements were made of the seasonal course of transpiration, canopy stomatal conductance, needle water potential, xylem water content, soil-to-needle hydraulic resistance, and growth. Comparison was made between drought-treated plots and those receiving average precipitation. In response to drought, transpiration rate declined once volumetric water content (VWC) over the top 20 cm of soil reached a threshold value of 12%. Thereafter, transpiration was a near linear function of soil water content. As the soil water deficit developed, the hydraulic resistance between soil and needles increased by a factor of three as predawn needle water potential declined from -0.54 to -0.71 MPa. A small but significant increase in xylem embolism was detected in 1-year-old shoots. Stomatal control of transpiration prevented needle water potential from declining below -1.5 MPa. Basal area, and shoot and needle growth were significantly reduced in the drought treatment. In the year following the drought, canopy stomatal conductance and soil-to-needle hydraulic resistance recovered. Current-year needle extension recovered, but a significant reduction in basal area increment was evident one year after the drought. The results suggest that, in response to soil water deficit, mature Scots pine closes its stomata sufficiently to prevent the development of substantial xylem embolism. Reduced growth in the year after a severe soil water deficit is most likely to be the result of reduced assimilation in the year of the drought, rather than to any residual embolism carried over from one year to the next.     

Sorption of metal ions from aqueous solution to spruce bark

The concentrations and protonation constants of the functional groups like carboxyl and phenolic hydroxyl groups in spruce bark were determined by a potentiometric acid–base titration method. The non-cellulosic carbohydrates in spruce bark were also characterized by acid methanolysis and GC, including determination of the uronic acid units, which are the key units involved in metal sorption. Sorption of metal ions to bark takes place by ion exchange, mainly by complexation to these functional groups. The sorption equilibrium time, studied by using a batch method, was approximately 5 min. The metal sorption capacity of spruce bark and the affinity order of metal ions were studied with four different metal ion mixtures using a column chromatographic method. Because a method of competition was used, concentration of metal ions adsorbed to bark depends on the metal ions present in the mixtures studied. In the sorption experiments with same metal ion mixtures, inner bark of spruce exhibited higher sorption capacity than outer bark. By combination of the results from several experiments, the following affinity order was obtained Fe³⁺ ? Pb²⁺ ? Cu²⁺ ? Cd²⁺ > Zn²⁺ > Ni²⁺ > Ba²⁺ > Ca²⁺ > Sr²⁺ > Mn²⁺ > Mg²⁺ ? K⁺ ~ Na⁺ ~ Li⁺. Bark has a great potential as an effective and inexpensive sorbent for removal of metal ions from, e.g., waste water.     

Rate of stomatal opening, shoot hydraulic conductance and photosynthetic characteristics in relation to leaf abscisic acid concentration in six temperate deciduous trees

Correlations between leaf abscisic acid concentration ([ABA]), stomatal conductance (gs), rate of stomatal opening in response to an increase in leaf water potential (si), shoot hydraulic conductance (L) and photosynthetic characteristics were examined in saplings of six temperate deciduous tree species: Acer platanoides L., Padus avium Mill., Populus tremula L., Quercus robur L., Salix caprea L. and Tilia cordata Mill. Species-specific values of foliar [ABA] were negatively related to the mean values of gs, si, L and light- and CO2- saturated net photosynthesis (P(max)), thus providing strong correlative evidence of a scaling of foliar gas exchange and hydraulic characteristics with leaf endogenous [ABA]. In addition, we suggest that mean gs, si, L and Pmax for mature leaves may partly be determined by the species-specific [ABA] during leaf growth. The most drought-intolerant species had the lowest [ABA] and the highest gs, suggesting that interspecific differences in [ABA] may be linked to differences in species-specific water-use efficiency. Application of high concentrations of exogenous ABA led to large decreases in gs, si and P(max), further underscoring the direct role of ABA in regulating stomatal opening and photosynthetic rate. Exogenous ABA also decreased L, but the decreases were considerably smaller than the decreases in gs, si and Pmax. Thus, exogenous ABA predominantly affected the stomata directly, but modification of L by ABA may also be an important mechanism of ABA action. We conclude that interspecific variability in endogenous [ABA] during foliage growth and in mature leaves provides an important factor explaining observed differences in L, gs, si and Pmax among temperate deciduous tree species.     

Winter desiccation and injury of subalpine red spruce

Montane red spruce (Picea rubens Sarg.) in the northeastern United States has undergone a decline during the past two decades. One symptom associated with the decline syndrome is the episodic browning of first-year foliage in early spring. To examine the potential role of winter desiccation in this browning, the water relations of red spruce foliage in a subalpine forest on Mt. Moosilauke, New Hampshire, USA, were monitored from January to May, 1989. All sampled trees lost water during the winter and the first-year foliage on some trees turned brown in early spring. The relative water content of first-year shoots during the winter was a significant predictor of spring browning; red spruce trees that showed browning had desiccated faster and reached lower relative water contents. Damaged trees also had more closely packed needles and lower cuticular resistances to water loss. The first-year shoots had a significantly lower average relative water content than older shoots before and after browning. Cuticular resistance to water loss decreased with elevation. Sun-exposed shoots lost more water than shaded shoots because of solar heating of needles. Winter desiccation can occur before the decline-related spring browning of red spruce foliage.     

Response of stomatal conductance to drought in ponderosa pine: implications for carbon and ozone uptake

To gain insight into the limitations imposed by a typical Mediterranean-climate summer drought on the uptake of carbon and ozone in the ponderosa pine (Pinus ponderosa Dougl. ex Laws.) ecosystem, we compared diurnal trends in leaf physiology of young trees in a watered and a control plot located in the Sierra Nevada Mountains, CA, USA (Blodgett Forest, 38 degrees 53' N, 120 degrees 37' W, 1315 m elevation). Predawn water potential of trees in the watered plot remained above -0.3 MPa throughout the growing season, whereas it dropped in the control plot from -0.24 to -0.52 MPa between late May and mid-August. Photosynthesis and stomatal conductance of trees in the watered plot were relatively insensitive to atmospheric vapor pressure deficit (VPD), whereas gas exchange of trees in the control plot varied with changes in soil water, VPD and temperature. Although the 1998 growing season was abnormally wet, we saw a pronounced drought effect at the control site. Over the 2 months following the onset of watering, carbon and ozone uptake were measured on three days at widely spaced intervals. Carbon uptake per unit leaf area by 1-year-old foliage of trees in the control plot was 39, 35 and 30% less, respectively, than in the watered plot, and estimated ozone deposition per unit leaf area (ozone concentration times stomatal conductance) was 36, 46 and 41% less.