Photosynthetic limitations in response to water stress and recovery in Mediterranean plants with different growth forms

* Whether photosynthesis is limited during water stress and recovery because of diffusive or biochemical factors is still open to debate, and apparent contradictions appear when various studies on species with different growth forms are compared. * Ten Mediterranean species, representing different growth forms, were subjected to different levels of water stress, the most severe followed by rewatering. A quantitative limitation analysis was applied to estimate the effects of water stress on stomatal (S(L)), mesophyll conductance (MC(L)) and biochemical limitations (B(L)). * Results confirmed a general pattern of photosynthetic response to water stress among C(3) plants when stomatal conductance (g(s)) is used as a reference parameter. As g(s) values decreased from a maximum to approx. 0.05 mol H(2)O m(-2) s(-1), the total photosynthetic limitation rose from 0 to approx. 70%, and this was caused by a progressive increase of both S(L) and MC(L) limitations, while B(L) remained negligible. When lower values of g(s) were achieved (total photosynthetic limitation increased from 70 to 100%), the contribution of S(L) declined, while MC(L) still increased and B(L) contributed significantly (20-50%) to the total limitation. * Photosynthetic recovery of severely stressed plants after rewatering showed a dominant role of MC(L), irrespective of the degree of photosynthesis recovery.     

Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na₂SO₄ were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (_w), leaf osmotic potential (_o) and leaf turgor potential (_p) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na₂SO₄-treated plants. The reduction in stomatal conductance was higher for NaCl-treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na⁺, Cl^– and SO₄ ^2–). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl^–, SO₄ ^2– and/or Na⁺. However, turgor of NaCl-treated plants was maintained at low EC (3 and 4 dS m^–1) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na⁺ in the plant tissues compared with the Na₂SO₄ treatment, could be the cause of the smaller decrease in growth.     

Seasonal patterns of gas exchange,water relations and growth of it Roupala montana,an evergreen savanna species

Roupala montana is an evergreen species widespread in the seasonal savannas of the central plains of Brazil. I examined the degree of coupling of photosynthetic gas-exchange characteristics, water relations and growth responses of R. montana with regard to seasonal changes in soil water availability. Despite a rainless period of over three months soil water potential at 60 cm depth reached values of only about -1.0 MPa, while pre-dawn leaf water potential (^l) reached about -0.4 MPa by the end of the three-month drought. Thus, R. montana had access to deep soil water in the dry period, but pre-dawn ^l did not reach the high wet season values of -0.2 MPa. Most of the shoot growth was concluded in the onset of the rainy season. Although some individual branches might have shown some extension thereafter, most of them remained inactive during the rest of the rainy season and the subsequent dry season. New leaf production was also restricted to the first part of the wet period. R. montana remained evergreen in the dry season, but there was a 27% decrease in the number of leaves and herbivory removed about 16% of the leaf area still present in the plant. CO₂-exchange rates of these leaves reached only ca. 55% of the maximum rainy season values of 14 µmol m^-2 s^-1. Thus, the estimated potential daily carbon gain was about 34% of the maximum by the end of the dry period. These values will be even lower, if we considered the decrease in photosynthetic rates that occurred around midday. These reductions in photosynthetic rates as a result of partial stomatal closure were measured both in the wet and dry season and they were related to increases in the evaporative demand of the atmosphere. In conclusion, the combined effect of herbivory, leaf loss and reductions in photosynthetic rates limited plant productivity in the dry season.     

Effects of plant size and water relations on gas exchange and growth of the desert shrub Larrea tridentata

Larrea tridentata is a xerophytic evergreen shrub, dominant in the arid regions of the southwestern United States. We examined relationships between gasexchange characteristics, plant and soil water relations, and growth responses of large versus small shrubs of L. tridentata over the course of a summer growing season in the Chihuahuan Desert of southern New Mexico, USA. The soil wetting front did not reach 0.6 m, and soils at depths of 0.6 and 0.9 m remained dry throughout the summer, suggesting that L. tridentata extracts water largely from soil near the surface. Surface soil layers (<0.3 m) were drier under large plants, but predawn xylem water potentials were similar for both plant sizes suggesting some access to deeper soil moisture reserves by large plants. Stem elongation rates were about 40% less in large, reproductively active shrubs than in small, reproductively inactive shrubs. Maximal net photosynthetic rates (P_max) occurred in early summer (21.3 mol m^-2 s^-1), when pre-dawn xylem water potential (XWP) reached ca. -1 MPa. Although both shrub sizes exhibited similar responses to environmental factors, small shrubs recovered faster from short-term drought, when pre-dawn XWP reached about -4.5 MPa and P_max decreased to only ca. 20% of unstressed levels. Gas exchange measurements yielded a strong relationship between stomatal conductance and photosynthesis, and the relationship between leaf-to-air vapor pressure deficit and stomatal conductance was found to be influenced by pre-dawn XWP. Our results indicate that stomatal responses to water stress and vapor pressure deficit are important in determining rates of carbon gain and water loss in L. tridentata.     

Modulation of relative growth rate and its components by water stress in Mediterranean species with different growth forms

Effects of water availability on seedling growth were analysed in eight Mediterranean species naturally occurring in the Balearic Islands. Seedlings were grown outdoors during summer under two irrigation treatments: field capacity and 35% of field capacity. The relative growth rate (RGR) strongly depended on the growth form, from highest values in herbs to lowest in woody perennials. The main component associated with interspecific variation in RGR was the specific leaf area (SLA), and a quantitative grouping of the different growth forms appeared along the regression line between both parameters. The slow-growing species, i.e. woody perennial shrubs, had the lowest SLA and the fast-growing perennial herbs, the highest, while woody semi-deciduous shrubs appeared intermediate. Decreases in RGR due to water stress were analysed in terms of the relative contribution of the leaf mass ratio (LMR), SLA and the net assimilation rate (NAR). Pooling all species, the decrease in RGR caused by water deficit was mainly explained by decreases in SLA. However, this general pattern was strongly dependent of growth form. Thus, in the woody perennial plants, the decrease in RGR was accompanied by a three-fold decrease in NAR which, however, increased in perennial herbs. SLA increased with decreasing water supply in woody perennial plants, and decreased in woody semi-deciduous shrubs and perennial herbs. Finally, decreases in LMR partly explained decreases in RGR in perennial herbs and woody perennial shrubs. This different response of the different growth forms may reflect differences in seedling adaptation and surviving strategies to drought periods.     

Effects of water stress and rewatering on leaf water relations of lemon plants

Potted two-year-old lemon plants (Citrus limon (L.) Burm. fil.) cv. Fino, growing under field conditions were subjected to drought by withholding irrigation for 13 d. After that, plants were re-irrigated and the recovery was studied for 5 d. Control plants were daily irrigated maintaining the soil matric potential at about -30 kPa. Young leaves of control plants presented higher leaf conductance (g1) and lower midday leaf water potential (Ψmd) than mature ones. Young leaves also showed higher leaf water potential at the turgor loss point (Ψtlp) than mature leaves. In both leaf types g1 decreased with increased vapour pressure deficit of the atmosphere. From day 1 of the withholding water, predawn and midday leaf water potentials (Ψpd and Ψmd) decreased, reaching in both cases minimum values of -5.5 MPa, with no significant differences between mature and young leaves. Water stress induced stomatal closure, leaf rolling and partial defoliation. No osmotic adjustment was found in response to water stress in either leaf type, but both were able to enhance the cell wall elasticity (elastic adjustment). After rewatering, leaf water potential recovered quickly (within 2 d) but g1 did not. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stomatal and photosynthetic responses during sun/shade transitions in subalpine plants: influence on water use efficiency

Summary Different response patterns in net photosynthesis (A) leaf conductance (g) and water use efficiency (WUE= a/transpiration) in three subalpine plants occurred during experimental sun/shade transitions that simulated natural cloudcover. In Frasera speciosa Dougl., a large-leaved herb characteristic of open sites, g was relatively insensitive to transitions in irradiance and variations in A. However, large decreases in leaf temperature resulted in reduced transpiration during shade intervals and relatively constant WUE throughout the experimental sun/shade regime. In the understory herb, Arnica cordifolia Hook., patterns of A and g were similar during sun/shade transitions, but WUE was substantially reduced compared to steady-state levels. A third, somewhat intermediate pattern of A, g, and WUE was found in Artemisia tridentata L., an open site shrub. Higher intercellular CO₂ values in A. tridentata suggested that internal, cellular limitations to A were high relative to stomatal limitations in this shrub when compared to the herbaceous species.     

Water relations and adaptations to increasing water deficit in three perennial legumes, Medicago sativa, Dorycnium hirsutum and Dorycnium rectum

Dorycnium hirsutum (L.) Ser. and Dorycnium rectum (L.) Ser. are Mediterranean perennial legumes that may have potential as alternative forage plants to Medicago sativa (lucerne, alfalfa) for low rainfall dryland agriculture. Strategies for surviving periods of water deficit are vital for perennial plants in water-limited environments. This experiment compared leaf physiological and morphological adaptations to increasing water deficit among D. hirsutum, D. rectum and M. sativa. Plants were grown in the glasshouse in large pots (7.8 L, 1 m deep, 10 cm diameter) containing a sandy clay loam (14% available water content) to limit differences between root foraging among the species. Watering was withheld for 21 days and predawn and midday leaf water and osmotic potential were determined. Mid-morning rates of gas exchange were measured at five times as soil water was depleted. After 35 days of withholding water, plant recovery was measured. D. hirsutum and M. sativa reduced stomatal conductance at leaf water potentials below −1.8 MPa and water-stressed D. hirsutum osmotically adjusted by up to 0.68 MPa. D. rectum differed from the other species; leaf water potential was maintained at high levels until soil water content had reached low levels, and reductions in stomatal conductance and photosynthesis were not associated with leaf water potential. D. hirsutum and M. sativa displayed leaf morphological adaptations that may contribute to greater resistance of water deficit. Only one of five D. rectum plants survived the water-stress treatment compared to five of five for D. hirsutum and four of five for M. sativa. The water relations and physiology of D. hirsutum observed in this study suggest that it possesses adaptations suitable for arid environments. On the other hand, the poor survival and water relations of D. rectum indicate that it is poorly adapted to situations where water deficit is common.     

Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring

Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO₂ concentrations in the proximity of a natural CO₂ spring. Trees at the spring site have been growing for generations in elevated atmospheric CO₂ concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid ‘runaway embolism’. Trees at the CO₂ spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO₂ was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO₂ spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO₂ spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO₂ spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO₂ on plant water use at the organ and whole-tree scale.     

Osmotic stress affects water relations, growth, and nitrogen fixation in Phaseolus vulgaris plants

The effects of mannitol-mediated osmotic stress on water relations, plant growth and symbiotic N₂-fixation in four common bean (Phaseolus vulgaris) lines (Coco Blanc, BAT 477, BRB 77 and Flamingo) were studied. After germination, seedlings were inoculated with a reference strain (Rhizobium tropici CIAT 899) and aerohydroponically grown in a glasshouse. Osmotic stress was applied by 50 mM mannitol. Plants were harvested 4 weeks after osmotic stress application. Measured parameters were plant water relations, growth, nodule development, and symbiotic N₂-fixation (SNF) as well as leghemoglobin contents. Osmotic stress induced significant changes in water relations, growth and symbiotic N₂-fixation in stressed plants compared to control ones in all lines studied. A noticeable different behaviour was observed in the end of the treatment: Flamingo was the most tolerant line, whereas Coco blanc was the most sensitive, the two other lines exhibited an intermediate behaviour. The four bean lines displayed significant differences in their responses to osmotic stress. This study indicated that the relative tolerance of Flamingo line seems to be due to its ability to maintain higher leaf water potential, adequate leaf area and abundant and efficient nodular system, which in turn determines an important rate of SNF.     

Water stress in Eucalyptus pauciflora: comparison of effects on stomatal conductance with effects on the mesophyll capacity for photosynthesis,and investigation of a possible involvement of photoinhibition

Seedlings of Eucalyptus pauciflora Sieb. ex Spreng., grown in 4-1 pots, were stressed by withholding water while relationships between net assimilation rate (A) and intercellular partial pressure of CO₂ (p_i) in selected leaves were obtained repeatedly throughout the stress cycle. Water stress at first caused stomatal closure without any decline in the A(p_i) relationship. As stress became more severe, the A(p_i) relationship was affected as well. This always affected assimilation rate at both high and low intercellular partial pressures of CO₂. It was then tested whether water-stressed leaves were more prone to photoinhibition than unstressed ones. Plants were water-stressed while at the same time subjected to strong photon flux area density (2000 mol quanta·m^-2·s^-1). A possible light-induced inhibition was assessed by comparing quantum yields of photosynthesis with light directed onto one or the other surface of the leaf. A decline in quantum yield was observed, and the decline on the previously irradiated side was more pronounced than on the previously shaded side, but the effect was small and disappeared entirely within 1 d of rewatering the plants. It is concluded that photoinhibition can play a role, but not an important one, in the effect of water stress on the A(p_i) relationship in leaves of E. pauciflora.Abbreviations and symbols RuBP ribulose-1,5-bisphosphate - A net assimilation rate - p_i intercellular partial pressure of CO₂ - quantum yield of photosynthesis (net assimilation or RuBP-regeneration rate) - w difference in water content between air saturated at leaf temperature and the actual vapor content of the air, expressed as mole fraction     

General relations of stomatal responses to xylem sap abscisic acid under stress in the rooting zone – A global perspective

A sensitivity factor that quantifies the responsiveness of stomata to xylem sap abscisic acid concentration ([ABA]_xyl) is described, using the relation between [ABA]_xyl and maximum leaf conductance (g_max). Plotting g_max against this factor results in a common linear relationship for woody and herbaceous species from boreal to (semi-) arid climates. The global distribution of the sensitivity factor reveals an unexpected pattern which is inverse to rainfall, i.e., plants in humid climates respond more sensitively to ABA than plants in arid areas. The implications for the response of natural vegetation and consequences for agriculture are discussed.     

Aquaporin expression in response to different water stress intensities and recovery in Richter-110 (<Emphasis Type="Italic">Vitis</Emphasis> sp.): relationship with ecophysiological status

Aquaporins seem essential for the regulation of plant water status and expenses. Richter-110 is a Vitis hybrid (Vitis berlandieri × rupestris) reputed to be strongly drought-tolerant. Three irrigation treatments were established in Richter-110 plants growing outdoors defined by the resulting maximum stomatal conductance (g _s), and ensuring water stress situations not severe enough as to stop photosynthesis and growth: well-watered plants (g _s about 250 mmol H₂O m⁻² s⁻¹), moderate water stress (g _s about 150 mmol H₂O m⁻² s⁻¹) and severe water stress (g _s about 50 mmol H₂O m⁻² s⁻¹). Plants under water stress were kept at constant water availability for 7 days to check for possible acclimation. Finally, plants were re-watered, and allowed to recover, for 3 days. Stomatal conductance, leaf water potential, xylem abscisic acid (ABA) content and root and stem hydraulic conductivity were determined. The relative amounts of expression of mRNA encoding seven putative aquaporins were determined in roots and leaves by RT-PCR. The decrease in stomatal conductance with moderate and severe water stress was associated with increasing ABA contents, but not with the leaf water potential and hydraulic conductivities, which remained unchanged during the entire experiment. Aquaporin gene expression varied depending on which aquaporin, water stress level and the plant organ. We suggest that aquaporin expression was responsive to water stress as part of the homeostasis, which resulted in constant leaf water potential and hydraulic conductivity.     

Mycorrhizal symbiosis and response of sorghum plants to combined drought and salinity stresses

Arbuscular mycorrhizal (AM) symbiosis can confer increased host resistance to drought stress, although the effect is unpredictable. Since AM symbiosis also frequently increases host resistance to salinity stress, and since drought and salinity stress are often linked in drying soils, we speculated that the AM influence on plant drought response may be partially the result of AM influence on salinity stress. We tested the hypothesis that AM-induced effects on drought responses would be more pronounced when plants of comparable size are exposed to drought in salinized soils. In two greenhouse experiments, several water relations characteristics were measured in sorghum plants colonized by Glomus intraradices (Gi), Gigaspora margarita (Gm) or a mixture of AM species, during a sustained drought following exposure to salinity treatments (NaCl stress, osmotic stress via concentrated macronutrients, or soil leaching). The presence of excess salt in soils widened the difference in drought responses between AM and nonAM plants in just two instances. Days required for plants to reach stomatal closure were similar for Gi and nonAM plants exposed to drought alone, but with exposure to combined NaCl and drought stress, stomates of Gi plants remained open 17-22% longer than in nonAM plants. Promotion of stomatal conductance by Gm occurred with exposure to NaCl/drought stress but not with drought alone or with soil leaching before drought. In other instances, however, the addition of salt tended to nullify an AM-induced change in drought response. Our findings confirm that AM fungi can alter host response to drought but do not lend much support to the idea that AM-induced salt resistance might help explain why AM plants can be more resilient to drought stress than their nonAM counterparts.     

Some physiological and morphological characteristics of citrus plants for drought resistance

Tolerance and avoidance mechanisms to drought stress were studied in 6-month-old plants of Newhall orange (Citrus sinensis (L.) Osbeck) and Ellendale tangor (orange × mandarin hybrid) (Citrus sinensis (L) Osbeck × Citrus reticulata Blanco) during a drought/rewatering cycle under controlled conditions. Drought stress did not promote osmotic adjustment, while elastic adjustment (tissue elasticity increase) was noted in stressed orange and tangor plants. Both citrus plants showed a parallel decrease in leaf conductance (g₁) and leaf water potential (Ψ₁) under water stress. Tangor plants had a more efficient water conservative strategy than orange, based on the characteristics of canopy architecture (lower canopy area and a more closed canopy with leaves nearly vertically oriented) together with a significant decrease in cuticular transpiration rates (TRc) under stress.     

Influence of leaf water status on stomatal response to humidity,hydraulic conductance,and soil drought in Betula occidentalis

Whole-canopy measurements of water flux were used to calculate stomatal conductance (g _s ) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (ΔN; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Ψ_s). As expected, g _s dropped in response to decreased k or Ψ_S, or increased ΔN(> 0.025). Field data showed a decrease in mid-day g _s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Ψ from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g _s were associated with approximately constant bulk leaf water potential (ψ_l), this does not logically exclude a feedback response between Ψ_L and g _s . To test the influence of leaf versus root water status on g _s , we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased ΔN, decreased k, and decreased Ψ_S was fully or partially reversed within 5 min of pressurizing the soil. Bulk Ψ_L remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When ΔN was low (i.e., < 0.025), pressurizing the soil either had no effect on g _s , or caused it to decline; and bulk Ψ_L increased. Increased Ψ_l may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high ΔN, low k, and low Ψ_S. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.     

Patterns of tree dieback in Queensland,Australia: the importance of drought stress and the role of resistance to cavitation

During the extreme 1992–1997 El Niño drought event, widespread stem mortality, or tree dieback, of both mature and juvenile eucalypts occurred within the tropical savannas of northeast Australia. Most of the dieback occurred in individuals of the ironbark species complex (Eucalyptus crebra – E. xanthoclada) while individuals of the bloodwood species Corymbia erythrophloia, exhibited significantly less stem mortality. Indicative of greater water stress, predawn and midday xylem water potentials of ironbark adults and saplings were significantly more negative than predawn values of bloodwoods. The very negative xylem water potentials in ironbarks suggest that stem mortality in both adult and juvenile ironbarks results from drought-induced embolism and that ironbarks perhaps have a shallower and less extensive root system than bloodwoods. Although predawn and midday water potentials for ironbark adults and saplings were similar, a census of mature and juvenile ironbark trees indicated that mortality was higher in adult trees. Cavitation vulnerability curves indicated that ironbark saplings may be better buffered against cavitation than adult trees. If they possess smaller root systems, saplings are more likely than adults to experience low xylem water potentials, even in non-drought years. Xylem conduits produced in adult trees during periods of normal rainfall, although perhaps more efficient in water conduction, may be more vulnerable to cavitation during infrequent severe droughts.     

Water relations and hydraulic control of stomatal behaviour in bell pepper plant in partial soil drying

Two experiments, a split-root experiment and a root pressurizing experiment, were performed to test whether hydraulic signalling of soil drying plays a dominant role in controlling stomatal closure in herbaceous bell pepper plants. In the split-root experiment, when both root parts were dried, synchronous decreases in stomatal conductance (g_s), leaf water potential (LWP) and stem sap flow (SF_stem) were observed. The value of g_s was found to be closely related to soil water potential (SWP) in both compartments. Tight relationships were observed between g_s and stem sap flow under all conditions of water stress, indicating a complete stomatal adjustment of transpiration. When the half-root system has been dried to the extent that its water uptake dropped to almost zero, declines in g_s of less than 20% were observed without obvious changes in LWP. The reduced plant hydraulic conductance resulting from decreased sap flow and unchanged LWP may be a hydraulic signal controlling stomatal closure; the results of root pressurizing supported this hypothesis. Both LWP and g_s in water-stressed plants recovered completely within 25 min of the application of root pressurizing, and decreased significantly within 40 min after pressure release, indicating the hydraulic control of stomatal closure. Our results are in contrast to those of other studies on other herbaceous species, which suggested that chemical messengers from the roots bring about stomatal closure when plants are in water stress.     

Plant water relations and the effects of elevated CO2: a review and suggestions for future research

Increased ambient carbon dioxide (CO₂) has been found to ameliorate water stress in the majority of species studied. The results of many studies indicate that lower evaporative flux density is associated with high CO₂-induced stomatal closure. As a result of decreases in evaporative flux density and increases in net photosynthesis, also found to occur in high CO₂ environments, plants have often been shown to maintain higher water use efficiencies when grown at high CO₂ than when grown in normal, ambient air. Plants grown at high CO₂ have also been found to maintain higher total water potentials, to increase biomass production, have larger root-to-shoot ratios, and to be generally more drought resistant (through avoidance mechanisms) than those grown at ambient CO₂ levels. High CO₂-induced changes in plant structure (i.e., vessel or tracheid anatomy, leaf specific conductivity) may be associated with changes in vulnerability to xylem cavitation or in environmental conditions in which runaway embolism is likely to occur. Further study is needed to resolve these important issues. Methodology and other CO₂ effects on plant water relations are discussed.Abbreviations A net photosynthesis - C_a ambient [CO₂] - C_i internal [CO₂] - E evaporative flux density - g₁ leaf conductance - g_s stomatal conductance - LSC leaf specific conductivity - IRGA infrared gas analyzer - LAI leaf area index - PAR photosynthetically active radiation - total plant water potential - _soil soil water potential - _s solute potential - _pt turgor pressure potential - _px xylem pressure potential - RH relative humidity - R : S root to shoot ratio - RWC relative water content - SLA specific leaf area - SLW specific leaf weight - SPAC soil-plant-atmosphere-continuum - SWC soil water content - VPD vapor pressure deficit - WUE water use efficiency