Low carbon dioxide concentrations can reverse stomatal closure during water stress

Leaf water potentials below threshold values result in reduced stomatal conductance (g_s). Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of g_s by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until g_s was reduced and then lowering the [CO₂] to determine whether g_s and transpiration rate could be increased and leaf water potential decreased and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low [CO₂] could reverse the stomatal closure because of drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. The relative sensitivity of g_s to internal [CO₂] in the leaves of dry plants of each species averaged three to four times higher than in leaves of wet plants. Two species in which g_s was reputed to be insensitive to [CO₂] were examined to determine whether high leaf to air water vapor pressure differences (D) resulted in increased stomatal sensitivity to [CO₂]. In both species, stomatal sensitivity to [CO₂] was indeed negligible at low D, but increased with D, and low [CO₂] partly or fully reversed closure caused by high D. In no case did low leaf water potential or low hydraulic conductance during drying of the air or the rooting medium prevent low [CO₂] from increasing g_s and transpiration rate.     

Alfalfa leaf senescence induced by drought stress: photosynthesis, hydrogen peroxide metabolism, lipid peroxidation and ethylene evolution

Exchange rates of CO₂ and H₂O and metabolism of hydrogen peroxide have been measured in leaves of alfalfa ev. Aragón) under drought stress. The inhibitory effect of drought upon photosynthesis depended on the severity of the stress treatment. Leaf water potential (Ψ_leaf) down to,-2.8 MPa reduced CO₂ availability due to stomatal closure and inhibited the rate of photosynthesis. Leaf water potential lower than,-2.8 MPa directly affected CO₂ fixation, although CO₂ was not limiting. Transpiration was more affected by stornatal closure than photosynthesis, which led to am apparent improvement in WUE (water use efficiency). Alfalfa leaves with Ψ_leaf lower than,-2.0 MPa had an increased quantum requirement, probably due to the severe stress effect on photoenergetic reactions.
Ethylene evolution from alfalfa leaves increased when they were subjected to Ψ_leaf of,- 1.6 MPa. Under more severe stress, the leaves showed low or almost no ethylene production. In parallel with the increase in ethyiene production, alfalfa leaves exhibited an increased membrane lipid peroxidation index (maloridialdehyde content) and an increased peroxide content. Superoxide disinutase activity (SOD; EC 1.15.1.1) was not affected by drought stress. Catalase (EC 1.11.1.6) was inhibited at slight stress, but significantly increased at a Ψ_leaf of -2.0 MPa. Peroxidase (EC 1.11.1.7) was progressively inhibited as drought stress developed. The possible implication of reactive O₂ intermediates in drought stress-induced senescence of alfalfa leaves is discussed in the light of the pattern of enzymatic scavenging systems.     

Salinity effects on water relations in Lycopersicon esculentum and its wild salt-tolerant relative species L. pennellii

Cultivated tomato Lycopersicon esculentum (L.) Mill. cv. P-73 and its wild salt tolerant relative L. pennellii (Correll) D'Arcy accession PE-47, were grown during spring-summer 1989 under unheated plastic greenhouse conditions. Plants were submitted to two different salt treatments using 0 and 140 mM NaCI irrigation water. In both tomato species, salinity caused a proportionally larger reduction in leaf area than in leaf weight and, in L. esculentum , a proportionally larger decrease in stem weight than in leaf weight. Daily variations in leaf water potential (Ψ₁) were fundamentally due to changes in the evaporative demand of the atmosphere. Reductions in Ψ₁ due to salinity were consistent only in L. esculentum . In all the conditions studied, leaf turgor was maintained. Leaf conductance (g₁)was higher in L. esculentum than in L. pennellii .Salinity induced a clear reduction in g₁ levels in L. esculentum whereas, in L. pennellii , this reduction was noted only in May. In both species the Ψ^o_s (leaf osmotic potential at full turgor) levels were reduced by salinity. The bulk modulus of elasticity (E) and relative water content at turgor loss point (RWC_tlp) were not affected by salinity. The RWC_tlp values in L. pennellii seem to be controlled by E values.     

Stomatal and photosynthetic responses to shade in sorghum, soybean and eastern gamagrass

We studied photosynthetic and stomatal responses of grain sorghum ( Sorghum bicolor [L.] Moench cv. Pioneer 8500), soybean ( Glycine max L. cv. Flyer) and eastern gamagrass ( Tripsacum dactyloides L.) during experimental sun and shade periods simulating summer cloud cover. Leaf gas exchange measurements of field plants showed that short-term (5 min) shading of leaves to 300–400 μmol m⁻² s⁻¹ photosynthetic photon flux density reduced photosynthesis, leaf temperature, stomatal conductance, transpiration and water use efficiency and increased intercellular CO₂ partial pressure. In all species, photosynthetic recovery was delayed when leaves were reilluminated, apparently by stomatal closure. The strongest stomatal response was in soybean. Photosynthetic recovery was studied further with soybeans grown indoors (maximum photosynthetic photon flux density 1 200 μmol m⁻² s⁻¹). Plants grown indoors had responses to shade similar to those of field plants, except for brief nonstomatal limitation immediately after reillumination. These responses indicated the importance of the light environment during leaf development on assimilation responses to variable light, and suggested different limitations on carbon assimilation in different parts of the soybean canopy. Photosynthetic oxygen evolution recovered immediately upon reillumination, indicating that the light reactions did not limit soybean photosynthetic recovery. While shade periods caused stomatal closure and reduced carbon gain and water loss in all species, the consequences for carbon gain/water loss were greatest in soybean. The occurrence of stomatal closure in all three species may arise from their shared phenologies and herbaceous growth forms.     

Changes in leaf hydraulics and stomatal conductance following drought stress and irrigation in Ceratonia siliqua (Carob tree)

Changes in leaf hydraulic conductance (K) were measured using the vacuum chamber technique during dehydration and rehydration of potted plants of Ceratonia siliqua . K of whole, compound leaves as well as that of rachides and leaflets decreased by 20–30% at leaf water potentials (Ψ_L) of −1.5 and −2.0 MPa, i.e. at Ψ_L values commonly recorded in field-growing plants of the species. Higher K losses (up to 50%) were measured for leaves at Ψ_L of −2.5 and −3.0 MPa, i.e. near or beyond the leaf turgor loss point. Leaves of plants rehydrated while in the dark for 30 min, 90 min and 12 h recovered from K loss with characteristic times and to extents inversely proportional to the initial water stress applied. Leaf conductance to water vapour of plants dehydrated to decreasing Ψ_L and rehydrated at low transpiration was inversely related to loss of K, thus suggesting that leaf vein embolism and refilling (and related changes in leaf hydraulics) may play a significant role in the stomatal response.     

Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

Water vapour fluxes and their impact under elevated CO2 in a C4-tallgrass prairie

We measured leaf-level stomatal conductance, xylem pressure potential, and stomate number and size as well as whole plant sap flow and canopy-level water vapour fluxes in a C4-tallgrass prairie in Kansas exposed to ambient and elevated CO₂. Stomatal conductance was reduced by as much as 50% under elevated CO₂ compared to ambient. In addition, there was a reduction in stomate number of the C4 grass, Andropogon gerardii Vitman, and the C3 dicot herb, Salvia pitcheri Torr., under elevated CO₂ compared to ambient. The result was an improved water status for plants exposed to elevated CO₂ which was reflected by a less negative xylem pressure potential compared to plants exposed to ambient CO₂. Sap flow rates were 20 to 30% lower for plants exposed to elevated CO₂ than for those exposed to ambient CO₂. At the canopy level, evapotranspiration was reduced by 22% under elevated CO₂. The reduced water use by the plant canopy under elevated CO₂ extended the photosynthetically-active period when water became limiting in the ecosystem. The result was an increased above- and belowground biomass production in years when water stress was frequent.     

Physiological response of Pinus halepensis needles under ozone and water stress conditions

The aim of this study was to evaluate how physiological processes of potted Pinus halepensis plants, grown under controlled conditions, were affected by ozone (O₃) and/or water stress, integrating the gas exchange and biochemical data with fluorescence OJIP polyphasic transient data. Plants submitted to only water stress (T₁) and with ozone (T₃) showed a strong decrease in stomatal conductance and gas exchange, coinciding with a reduction of maximum yield of photochemistry ( φ _po) and very negative values of leaf water potential. Simultaneously, a great increase of both PSII antenna size, indicated by absorption per reaction centre, and electron transport per reaction centre were found. The reduction of photosynthesis in the O₃-treated plants (T₂) by a slowing down of the Calvin cycle was supported by the increase of related fluorescence parameters such as relative variable fluorescence, heat de-excitation constant, energy de-excitation by spillover, and the decrease of φ _po. We suggest an antagonistic effect between the two stresses to explain the delayed ozone-induced decrease of stomatal conductance values for T₃ with respect to T₁ plants, by an alteration of the physiological mechanisms of stomatal opening, which involve the increase of intra-cellular free-calcium induced by ABA under co-occurring water shortage. We emphasise the importance of considering the intensity of the individual stress factor in studies concerning the interaction of stresses.     

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.     

Drought effects on photosynthesis and fluorescence in hard wheat cultivars grown in the field

Net photosynthesis of the flag leaf of hard wheat ( Triticum durum L. evs Valforte, Produra, Adamello, Karel, Appulo and El Amel from the collection of the Instituto di Cerealicultura. Foggia, Italy) of different water potential has been studied on three consecutive years. Net photosynthesis was measured in natural conditions with a LI-COR portable instrument and in saturating CO₂ with an oxygen electrode. Net photosynthesis and stomatal conductance were significantly lower in the unirrigated leaves. However, the ratio of intercellular CO₂, concentration (C₁) to ambient CO₃ concentration (C_a) around the stressed plants was similar to the irrigated control. The maximal rate of photosynthesis in saturating CO₂, (P_nmax). measured in the second year of the experiment, was quite close to photosynthesis under natural conditions, indicating that CO₂ supply was not limiting. These results suggest that altered mesophyll photosynthetic capacity, rather than stomatal closure, causes the observed reduction in photosynthesis in the unirrigated plants. The variable fluorescence yield (_v/F_m) in predarkened leaves measured for two consecutive years, did not show differences between treatments or between cultivars. However, the analysis of the slow transients, measured the last year of the experiment, showed a linear relation between the fluorescence decline from the maximum initial level (P) and maximum photosynthesis (P_nmax).     

Stomatal limitation of photosynthesis in winter production of greenhouse tomato plants

In May, greenhouse tomato ( Lycopersicon esculentum Mill.) plants near the end of their winter production cycle were shown to exhibit a diurnal photosynthetic decrease. In order to identify the physiological causes of this decline, we compared in May the photosynthetic characteristics of the fifth youngest leaves from tomato plants of different ages corresponding to a winter production (11-month-old plants) and to a spring production (5-month-old plants). Although the leaves were developed simultaneously under the same environmental conditions, only the ones from the winter production showed a diurnal decline of the in situ CO₂ assimilation rate (A _{CO 2}). This was accompanied by a decline of internal CO₂ and stomatal conductance and by large accumulations of hexoses. When stomatal closure was relieved under saturated CO₂ concentration (5%) using a leaf-disc electrode system, the fifth leaves of both tomato cultures had similar maximum quantum efficiency of O₂ evolution (Φ_max), light-saturated rate of O₂ evolution (P_max) and quantum efficiency of photosystem II (PSII) photochemistry (ΔF/F'_m, q _P and q _N ). We concluded that the diurnal decline of A _{CO 2} observed in winter tomato production during May originates from a stomatal limitation that is not dependent on environmental conditions but rather related to the developmental stage of the plants.     

Effects of leaf wetting and high humidity on stomatal function in leafy cuttings and intact plants of Corylus maxima

When rooted cuttings of Corylus maxima Mill. cv. Purpurea are moved from the wet and humid conditions of the rooting environment, the leaves frequently shrivel and die. Since the newly formed adventitious root system has been shown to be functional in supplying water to the shoot, stomatal behaviour in C. maxima was investigated in relation to the failure to prevent desiccation. Stomatal conductance (g_s) in expanding leaves (L3) of cuttings increased almost 10-fold over the first 14 days in the rooting environment (fog), from 70 to 650 mmol m⁻² s⁻¹. In contrast, g_s of expanded leaves (L1) changed little and was in the region of 300 mmol m⁻² s⁻¹. Midday leaf water potential was much higher in cuttings than in leaves on the mother stock-plant (−0.5 versus −1.2 MPa) even before any roots were visible. Despite this, leaf expansion of L3 was inhibited by >50% in cuttings and stomata showed a gradual reduction in their ability to close in response to abscisic acid (ABA). To determine whether the loss of stomatal function in cuttings was due to severance or to unnaturally low vapour pressure deficit and wetting in fog, intact plants were placed alongside cuttings in the rooting environment. The intact plants displayed reductions in leaf expansion and in the ability of stomata to close in response to dark, desiccation and ABA. However, in cuttings, the additional effect of severance resulted in smaller leaves than in intact plants and more severe reduction in stomatal closure, which was associated with a 2.5-fold increase in stomatal density and distinctively rounded stomatal pores. The similarities between stomatal dysfunction in C. maxima and that observed in many species propagated in vitro are discussed, as is the possible mechanism of dysfunction.     

Temporal changes in root growth and 15N uptake and water relations of two tussock grass species recovering from water stress

Physiological responses of Agropyron desertorum and Pseudoroegneria spicata , two common cold desert perennial tussock grass species of the North American Great Basin, were evaluated during and after a period of imposed drought in a pot study. The timing and the pattern of response of leaf water potential (Ψ₁), stomatal conductance (g_s), and root growth were strikingly similar in both species during and after drought. The severity of stress influenced the magnitude of Ψ₁ and g_s, but had little effect on the timing of these responses. Although drought inhibited total root length in prestressed plants, within 4 days after relief of drought both species showed similar increases in root growth which exceeded those of the control. Despite similarities in their root growth responses to increased soil water availability, the two grasses differed in their capacity to restore N uptake following drought. By 14 days after rewatering, N uptake in the prestressed Agropyron had recovered to levels of control plants, although both root biomass and root lenght were much less than those of the controls. This is attributed to elevated root uptake kinetics. Restoration of N uptake by prestressed Pseudoregneria was much less effective during the same period.     

CO2 enrichment, drought stress and growth of Alaska pea plants (Pisum sativum)

The interaction of CO₂ enrichment and drought on water status and growth of pea plants was investigated. Pisum sativum L. (cv. Alaska) plants were grown from seeds in growth chambers using 350 and 675 μl I1 CO₂, a photon flux density of 600 μmol M-2 S-1, a 16 h photoperiod and a temperature regime of 20/14°C. The drought treatment was started at the beginning of branch initiation and lasted for 9 or 11 days. The water status of the plants was monitored daily by measuring total leaf water potential and stomatal conductance. The total leaf water potential of well-watered plants was not affected by the CO₂ level. Under draughting conditions total leaf water potential decreased, with a slower decrease under the high CO₂ regime, due, at least in part, to reduced stomatal conductance. Upon rewatering, total leaf water potential and stomatal conductance recovered within one day. High CO₂ counteracted the reduction in height and, to some extent, leaf area that developed in low CO₂ unwatered plants. Additional CO₂ had no effect on branch number and did not prevent the complete inhibition of branch development that resulted from drought stress. Removing the drought conditions resulted in a rapid recovery of the internal water status and also a rapid recovery of most, but not all, plant growth parameters.     

Photosynthesis affects following night leaf conductance in Vicia faba

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

Leaf Gas Exchange Characteristics in Relation to Leaf Canopy Position of Myrica faya in its Native Environment (Tenerife, Canary Islands)

Abstract: Diurnal courses of gas exchange were measured throughout one year in fully expanded current-year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) of Myrica faya Ait., a dominant component of the Canarian laurel forest in Tenerife, Canary Islands, Spain.
M. faya showed large differences between sun and shade leaves in gas exchange characteristics (about 50 % of maximum carbon assimilation rate (A_max) reduction in shade leaves, but this reduction can be higher on specific days) that were modulated by strong light attenuation and high leaf area index (LAI) of the stand. This species presented low A_max, about 10 μmol m^-2 s^-1, high maximum transpiration (E, 8 mmol m^-2 s^-1) and stomatal conductance (g_s, 750 mmol m^-2 s^-1) and very low instantaneous water use efficiency (WUE, mean maximum 1.1 mmol mol^-1) and A/g_s (mean maximum 23.5 μmol mol^-1). M. faya responded to high air vapour pressure deficit (VPD), decreasing its g_s but maintaining relatively high values of A and E during the studied period. Stomatal response to VPD showed a higher sensitivity than its congeners, M. cerifera, and Laurus azorica, tree species co-occurring in the Canarian laurel forest. In general, all these gas exchange characteristics lead us to consider this species more similar to subtropical plants of humid regions than to species of the Mediterranean region.     

Phenology, Lignotubers, and Water Relations of Cochlospermum vitifolium, a Pioneer Tropical Dry Forest Tree in Costa Rica

We examined structural and physiological traits relevant to the phenology of the tropical dry forest (TDF) pioneer tree Cochlospermum vitifolium . Despite marked seasonality in rainfall, meristem activity occurred throughout the year. Leaves were produced almost continuously during the rainy season, while leaf shedding started early during drought, before changes in soil water content were observed. Phenological activity under drought included flowering and fruiting of leafless trees; bud break and shoot extension took place before the end of the dry season. Low wood density of C. vitifolium stems (0.17 g/cm³) and lignotubers (0.14 g/cm³) provided water and starch storage needed to support phenological events such as branch extension, leaf flushing, and reproduction during the dry season, and probably also contributed to survival following mechanical damage and fire, typical of early TDF successional stages. Lignotuber water and starch contents showed substantial seasonal variation, declining from the beginning of the dry season to their lowest levels at the time of reproduction and dry-season flushing. Stems progressively replaced lignotubers as main storage organs as tree size increased. Evidence for a role of water stores in buffering daily water deficits was weak. Leaf water potentials remained above −1.2 MPa and stomatal conductance below 350 mmol/m²/s, suggesting that gas exchange during the rainy season was limited to prevent xylem cavitation. Leaf shedding occurred when early-morning and mid-day Ψ_L converged at the rainy–dry season transition, without changes in lignotuber or soil water content, suggesting that leaves of C. vitifolium are closely tuned to atmospheric drought.
Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp .     

Water stress, carbon dioxide, and light effects on sucrosephosphate synthase activity in Phaseolus vulgaris

The characteristics of sucrose-phosphate synthase (SPS; EC 2.4.1.14) activity in leaves of Phaseolus vulgaris L. cv. Linden was studied in plants subjected to water stress and various CO₂ and light treatments. When water was withheld for 3 days causing mild water stress (–0.9 MPa), the activity of SPS measured in crude extracts was reduced ca 50%. The effect of water stress was most evident when the enzyme was assayed with saturating amounts of its substrates fructose 6-phosphate and UDP glucose. Placing a water-stressed plant in an atmosphere containing 1% CO₂ reversed the effect of water stress on SPS activity over 5 h even though the water stress was not relieved. Holding unstressed leaves in low CO₂ partial pressure reduced the extractable activity of SPS. After 1 h of low CO₂ treatment the effect of low CO₂ could be reversed by 20 min of 5% CO₂. However, after 24 h of low CO₂ treatment, less SPS activity was recovered by the 20 min treatment. The cytosolic protein synthesis inhibitor cycloheximide prevented the slow recovery of SPS activity, but did not affect the rapid recovery of SPS. We conclude that the effect of water stress on SPS activity was a consequence of the inhibition of photosynthesis caused by stomatal closure. Responses of Phaseolus vulgaris SPS to light were similar to the response to low CO₂ in that the effects were most pronounced under V_max assay conditions. This is the first report of this type of light response of SPS in a dicotyledonous species.     

Investigation of the limitations to photosynthesis induced by leaf water deficit in field-grown sunflower (Helianthus annuus L.)

Abstract. The diurnal cycling of leaf water potential (Ψ_leaf) in field-grown sunflower ( Helianthus annuus ) was used to investigate the cause of water deficitinduced limitation of net photosynthesis. Daily midafternoon decreases in Ψ_leaf of up to 1.5 MPa and in net photosynthesis of up to 50% were typical for irrigated sunflower during seed filling. These midafternoon values were lowered an additional 0.6 to 0.8 MPa by prolonged drought treatment. There was a nearly linear relationship between the decline in net photosynthesis and reductions in leaf conductance over the course of the day. Thus, it was unexpected to find that the low, midafternoon rates of photosynthesis were associated with the highest intercellular CO₂ concentrations. These and other observations suggest that the daily decline in photosynthesis represents a 'down regulation' of the biochemical demand for CO₂ that is coordinated with the diurnally developing need to conserve water, thus establishing a balanced limitation of photosynthesis involving both stomatal and non-stomatal factors. There were no indications that either short term (i.e. diurnal declines in Ψ_leaf) or long term (i.e. drought treatment) water deficits caused any damage or malfunctioning of photosynthesis. Rather, both the daily declines in photosynthesis and the nearly 25% decrease in leaf area induced by prolonged drought appeared to be well-controlled adaptive responses by field-grown sunflower plants to limited water availability.