Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO₂ concentrations (ambient and ambient + 250 μmol mol^?1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g_s) of well-watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO₂ by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO₂ concentration and nutrient level on A or g_s in beech or oak. Stomatal conductance was reduced in elevated CO₂ by an average of 34% in oak, but in beech there were no significant reductions in g_s except under cloudy conditions (–22% in elevated CO₂). During drought, there was no effect of CO₂ concentration on g_s in beech grown with high nutrients, but for beech grown with low nutrients, g_s was significantly higher in elevated CO₂, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO₂ due to increased leaf area. It appears that beech may substantially increase whole-plant water consumption in elevated CO₂, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.     

Diffusion limitations and metabolic factors associated with inhibition and recovery of photosynthesis from drought stress in a C3 perennial grass species

Stomatal closure and metabolic impairment under drought stress limits photosynthesis. The objective of this study was to determine major stomatal and metabolic factors involved in photosynthetic responses to drought and recovery upon re‐watering in a C₃ perennial grass species, Kentucky bluegrass (Poa pratensis L.). Two genotypes differing in drought resistance, ‘Midnight’ (tolerant) and ‘Brilliant’ (sensitive), were subjected to drought stress for 15 days and then re‐watered for 10 days in growth chambers. Single‐leaf net photosynthetic rate (A), stomatal conductance (g_s) and transpiration rate (Tr) decreased during drought, with a less rapid decline in ‘Midnight’ than in ‘Brilliant’. Photochemical efficiency, Rubisco activity and activation state declined during drought, but were significantly higher in ‘Midnight’ than in ‘Brilliant’. The relationship between A and internal leaf CO₂ concentration (A/Ci curve) during drought and re‐watering was analyzed to estimate the relative influence of stomatal and non‐stomatal components on photosynthesis. Stomatal limitation (Ls %), non‐stomatal limitation (Lns %), CO₂ compensation point (CP) and dark respiration (Rd) increased with stress duration in both genotypes, but to a lesser extent in ‘Midnight’. Maximum CO₂ assimilation rate (A_max), carboxylation efficiency (CE) and mesophyll conductance (g_m) declined, but ‘Midnight’ had significantly higher levels of A_max, CE and g_m than ‘Brilliant’. Maximum carboxylation rate of Rubisco (V_cmax) and ribulose‐1,5‐bisphospate (RuBP) regeneration capacity mediated by maximum electron transport rate (J_max) decreased from moderate to severe drought stress in both genotypes, but to a greater extent in ‘Brilliant’ than in ‘Midnight’. After re‐watering, RWC restored to about 90% of the control levels in both genotypes, whereas A, g_s, Tr and Fv/Fm was only partially recovered, with a higher recovery level in ‘Midnight’ than in ‘Brilliant’. Rubisco activity and activation state restored to the control level after re‐watering, with more rapid increase in ‘Midnight’ than in ‘Brilliant’. The values of Ls, Lns, CP and Rd declined, and A_max, CE, V_cmax, J_max and g_m increased after re‐watering, with more rapid change in all parameters in ‘Midnight’ than in ‘Brilliant’. These results indicated that the maintenance of higher A and A_max under drought stress in drought‐tolerant Kentucky bluegrass could be attributed to higher Rubico activation state, higher CE and less stomatal limitation. The ability to resume metabolic activity (A_max, CE, Fv/Fm and Rubisco) was observed in the drought‐tolerant genotype and is the most likely cause for the increased recuperative ability of photosynthesis. Incomplete recovery of photosynthesis upon re‐watering could be attributable to lasting stomatal limitations caused by severe drought damage in both genotypes. Promoting rapid stomatal recovery from drought stress may be critical for plants to resume full photosynthetic capacity in C₃ perennial grass species.     

Decoupling between stomatal conductance and photosynthesis occurs under extreme heat in broadleaf tree species regardless of water access

High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (A_net) and stomatal conductance (g_s). However, evidence for decoupling of g_s from A_net under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum T_air > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced A_net and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well-watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced—g_s was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD.     

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO₂ uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (k_s) and leaf hydraulic conductance (K_Leaf) across species. All species had similar maximum photosynthetic rates (A_max). The species with higher k_s and K_Leaf attained A_max in the morning, whereas the species with lower k_s and K_Leaf exhibited their A_max in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO₂ uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.     

The effects of elevated [CO2] and water availability on growth and physiology of peach (Prunus persica) plants

ABSTRACT

Peach (Prunus persica L.) seedlings were germinated and grown for two growing seasons either in open top chambers (OTC) with ambient (350 μmol mol^-1) or elevated (700 μmol mol^-1) [CO₂], or in an outside control plot, all located inside a glasshouse. The seedlings were grown in 10 dm³ pots and were fertilised once a week following Ingestad principles in order to supply mineral nutrients at free access rates. In the second growing season, rapid onset of water stress was imposed on rapidly growing peach seedlings by withholding water for a four-week drying cycle. In elevated [CO₂], seedlings had a total dry mass which was 33% higher than that in ambient [CO₂]. This increase was largely a consequence of increased height growth. [CO₂] and irrigation treatments had only small effects on allocation, and there was no increase in root allocation with low water availability possibly as consequence of the high-nutrient regime. Specific leaf area was significantly reduced in elevated [CO₂], and probably resulted from increases in starch concentrations. Stomatal conductance (g _s) was not affected by elevated [CO₂] both in well-watered and water-stressed seedlings. The combination of increased assimilation rate (A) and unchanged g _s led to large increases in intrinsic water use efficiency in response to elevated [CO₂]. The A/C _i curves were used to derive the parameters describing photosynthetic capacity, A_max, J_max and V_cmax . These parameters were similar among [CO₂] treatments; thus, there was no downward acclimation of photosynthesis in elevated [CO₂]. Moreover, A_max, J_max and V_cmax scaled linearly with leaf N content per unit leaf area. This indicates that the whole-plant source-sink balance of peach seedlings was not disrupted by growth in elevated [CO₂], because root volume and nutrient supply were non-restricting. These values may be used in scaling up models to improve their ability to predict the magnitude of tree responses to climate change in the Mediterranean area.     

Leaf gas exchange of trees in old-growth and young secondary forest stands in Sulawesi, Indonesia

In the tropics, old-growth forests are converted to other land cover types at a high rate and young secondary forest may gain in importance. Information on associated changes in leaf gas exchange and other leaf traits can be valuable for modelling biogeochemical fluxes under altered land-use patterns. We studied in situ photosynthetic parameters and stomatal conductance for water vapour in eight abundant tree species of young secondary forest and eight tree species of natural old-growth forest in Central Sulawesi, Indonesia. In sun leaves, the average maximal stomatal conductance (g _smax) in the secondary forest (SF) species was 2.1 times higher than in the old-growth forest (OGF) species. Species with a high g _smax reduced g _s sharply when vapour pressure deficit of the air increased, whereas species with a low g _smax were much less sensitive to air humidity. For area-based photosynthetic capacity (A _max-area), the SF species had a 2.3 times higher average than the OGF species. For both, g _smax and A _max-area the variation among species was higher in the OGF than in the SF. When all tree species (n=16) are considered, species means of specific leaf area (SLA), leaf N concentration and leaf P concentration were significantly correlated with g _smax and A _max-area. The strong correlation between A _max-area and foliar P (r ²=0.8) is remarkable as the alluvial soils in the study region are rich in nutrients. If the eight OGF species are analysed separately, the only significant correlation was observed between SLA and mass-based A _max; in the SF species strong correlations were found between leaf size and A _max-area and g _smax. These results show that the conversion of old-growth forest to young secondary forest in Sulawesi significantly alters tree leaf gas exchange characteristics and that chemical and structural leaf traits can be used for the prediction of these changes. The best correlations between leaf gas exchange parameters and leaf traits were obtained by different traits in the SF species, the OGF species and the entire pool of studied species.     

The influence of elevated temperature,elevated atmospheric CO2 concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern,central and southern sites in its native range

We investigated the effect of elevated [CO₂] (700 μmol mol^?1), elevated temperature (+2 °C above ambient) and decreased soil water availability on net photosynthesis (A_net) and water relations of one‐year old potted loblolly pine (Pinus taeda L.) seedlings grown in treatment chambers with high fertility at three sites along a north‐south transect covering a large portion of the species native range. At each location (Blairsville, Athens and Tifton, GA) we constructed four treatment chambers and randomly assigned each chamber one of four treatments: ambient [CO₂] and ambient temperature, elevated [CO₂] and ambient temperature, ambient [CO₂] and elevated temperature, or elevated [CO₂] and elevated temperature. Within each chamber half of the seedlings were well watered and half received much less water (1/4 that of the well watered). Measurements of net photosynthesis (A_net), stomatal conductance (g_s), leaf water potential and leaf fluorescence were made in June and September, 2008. We observed a significant increase in A_net in response to elevated [CO₂] regardless of site or temperature treatment in June and September. An increase in air temperature of over 2 °C had no significant effect on A_net at any of the sites in June or September despite over a 6 °C difference in mean annual temperature between the sites. Decreased water availability significantly reduced A_net in all treatments at each site in June. The effects of elevated [CO₂] and temperature on g_s followed a similar trend. The temperature, [CO₂] and water treatments did not significantly affect leaf water potential or chlorophyll fluorescence. Our findings suggest that predicted increases in [CO₂] will significantly increase A_net, while predicted increases in air temperature will have little effect on A_net across the native range of loblolly pine. Potential decreases in precipitation will likely cause a significant reduction in A_net, though this may be mitigated by increased [CO₂].     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

Stomatal, mesophyll conductance and biochemical limitations to photosynthesis as affected by drought and leaf ontogeny in ash and oak trees

Gas exchange measurements were carried out on ash and oak trees in a forest plantation during three whole growing seasons characterized by different water availability (2001, 2002 and 2003). A quantitative limitation analysis was applied to estimate the effects of drought and leaf ontogeny on stomatal (S_L) and non-stomatal limitations (NS_L) to light-saturated net photosynthesis (A_max), relative to the seasonal maximum rates obtained under conditions of optimal soil water content. Furthermore, based on combined gas exchange and chlorophyll fluorescence measurements, NS_L was partitioned into a diffusive (due to a decrease in mesophyll conductance, MC_L) and a biochemical component (due to a decrease in carboxylation capacity, B_L). During the wettest year (2002), the seasonal pattern of both A_max and stomatal conductance (g_sw) was characterized in both species by a rapid increase during spring and a slight decline over the summer. However, with a moderate (year 2001) or a severe (year 2003) water stress, the summer decline of A_max and g_sw was more pronounced and increased with drought intensity (30–40% in 2001, 60–75% in 2003). The limitation analysis showed that during the spring and the autumn periods S_L, MC_L and B_L were of similar magnitude. By contrast, from the summer data it emerged that all the limitations increased with drought intensity, but their relative contribution changed. At mild to moderate water stress (corresponding to values of g_sw > 100 mmol H₂O m⁻² s⁻¹) about two-thirds of the decline in A_max was attributable to S_L. However, with increasing drought intensity, NS_L increased more than S_L and nearly equalled it when the stress was very severe (i.e. with g_sw < 60 mmol H₂O m⁻² s⁻¹). Within NS_L, MC_L represented the main component, except at the most severe water stress levels when it was equalled by B_L. It is concluded that diffusional limitations (i.e. S_L + MC_L) largely affect net assimilation during most of the year, whereas biochemical limitations are quantitatively important only during leaf development and senescence or with severe droughts.     

Effects of Water-Deficit Stress on Photosynthesis, Its Components and Component Limitations, and on Water Use Efficiency in Wheat (Triticum aestivum L.)

It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO₂ and H₂O was determined in two growth-chamber-grown wheat cultivars (Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO₂-saturated A, and stomatal conductance (g_s) components. The limitations on A measured at ambient CO₂ concentration (A₃₅₀) were estimated. No cultivar difference was observed when A₃₅₀ was plotted versus leaf water potential (Ψ_w). Light- and CO₂-saturated A, c.e., and g_s decreased with decreasing leaf Ψ_w, but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and g_s increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A₃₅₀. WUE measured as A₃₅₀/g_s first increased with stomatal closure down to a g_s of about 0.25 mol H₂O m⁻² s⁻¹ (Ψ_w = −1.6 MPa). However, it was predicted that A₃₅₀/g_s would decrease with more severe stress due to inhibition of c.e.     

Most stomatal closure in woody species under moderate drought can be explained by stomatal responses to leaf turgor

Reduced stomatal conductance (g_s) during soil drought in angiosperms may result from effects of leaf turgor on stomata and/or factors that do not directly depend on leaf turgor, including root‐derived abscisic acid (ABA) signals. To quantify the roles of leaf turgor‐mediated and leaf turgor‐independent mechanisms in g_s decline during drought, we measured drought responses of g_s and water relations in three woody species (almond, grapevine and olive) under a range of conditions designed to generate independent variation in leaf and root turgor, including diurnal variation in evaporative demand and changes in plant hydraulic conductance and leaf osmotic pressure. We then applied these data to a process‐based g_s model and used a novel method to partition observed declines in g_s during drought into contributions from each parameter in the model. Soil drought reduced g_s by 63–84% across species, and the model reproduced these changes well (r² = 0.91, P < 0.0001, n = 44) despite having only a single fitted parameter. Our analysis concluded that responses mediated by leaf turgor could explain over 87% of the observed decline in g_s across species, adding to a growing body of evidence that challenges the root ABA‐centric model of stomatal responses to drought.     

Free-air CO2 enrichment in a wheat-weed assembly – effects on water relations

In a three-year free-air CO₂ enrichment study (Mini-FACE), spring wheat associated with typical arable weeds were grown under present and elevated atmospheric carbon dioxide concentrations [CO₂] (ambient air+150 μmol mol^?1). Analyses of plant stable carbon isotope ratios and in vivo measurements of leaf gas exchange were used to describe the CO₂ effects on water relations. For most species examined elevated [CO₂] significantly increased the intrinsic water-use efficiency (A/g_s) as derived from carbon isotope analyses. In some of the species, seasonal averages of the ratio between leaf internal to atmospheric CO₂ (c_i/c_a) were found to be significantly reduced by elevated [CO₂]. Periodic leaf gas exchange measurements confirmed the increased water-use efficiency, but significant CO₂ effects became evident only over the entire season by carbon isotope analysis. In both types of analysis conducted, spring wheat was found to react significantly different from all other species examined. The relation between A/g_s and biomass production was significantly influenced by elevated [CO₂] in all three years of the study. At the end of the drier growing seasons 2003 and 2004, the soil water content tended to be increased in the CO₂ enriched plots indicating a water saving effect. These observations demonstrate the impact of elevated [CO₂] on plant water relations with a likely positive feedback leading to higher soil water availability. Due to the differences in the CO₂ responses of spring wheat compared to the weeds we suggest that rising [CO₂] may cause shifts in the species composition of crop-weed communities.     

Leaf Photosynthetic Rate of Tropical Ferns Is Evolutionarily Linked to Water Transport Capacity

Ferns usually have relatively lower photosynthetic potential than angiosperms. However, it is unclear whether low photosynthetic potential of ferns is linked to leaf water supply. We hypothesized that there is an evolutionary association of leaf water transport capacity with photosynthesis and stomatal density in ferns. In the present study, a series of functional traits relating to leaf anatomy, hydraulics and physiology were assessed in 19 terrestrial and 11 epiphytic ferns in a common garden, and analyzed by a comparative phylogenetics method. Compared with epiphytic ferns, terrestrial ferns had higher vein density (D_vein), stomatal density (SD), stomatal conductance (g_s), and photosynthetic capacity (A_max), but lower values for lower epidermal thickness (LET) and leaf thickness (LT). Across species, all traits varied significantly, but only stomatal length (SL) showed strong phylogenetic conservatism. A_max was positively correlated with D_vein and g_s with and without phylogenetic corrections. SD correlated positively with A_max, D_vein and g_s, with the correlation between SD and D_vein being significant after phylogenetic correction. Leaf water content showed significant correlations with LET, LT, and mesophyll thickness. Our results provide evidence that A_max of the studied ferns is linked to leaf water transport capacity, and there was an evolutionary association between water supply and demand in ferns. These findings add new insights into the evolutionary correlations among traits involving carbon and water economy in ferns.     

Variability in water use efficiency at the leaf level among Mediterranean plants with different growth forms

Assessing natural variability of leaf water use efficiency in plants adapted to extreme conditions of the Mediterranean climate represents an important step in the evaluation of the usefulness of some plant ecophysiological traits under water stress. Eleven Mediterranean species naturally inhabiting the Balearic Islands and corresponding to different growth forms (herbs, semi-deciduous shrubs, woody evergreen shrubs and woody evergreen semi-shrubs) were subject to progressive soil water depletion. Leaf intrinsic water use efficiency was measured by gas exchange at four different degrees of water stress. Under well watered conditions, differences in leaf intrinsic water use efficiency (A _N/g _s) among growth forms were limited to woody evergreen semi-shrubs, which presented the highest values. Under water stress conditions, differences became more evident, with a trend for an increase in A _N/g _s from woody evergreen shrubs, through semi-deciduous shrubs and herbaceous to woody evergreen semi-shrubs. The observed variation in A _N/g _s correlated with several physiological (leaf water potential, soil to leaf hydraulic conductance and stomatal conductance) and morphological (stomatal density) parameters, displaying a general relationship for all growth forms. This suggests that the capacity for withstanding water limitation is adaptive for all Mediterranean species. However, when A _N/g _s was related to leaf mass area, this relationship was not generally applicable, and depended on growth forms, suggesting that different growth forms display specific morphological adjustments in response to water shortage.     

Soil drying and its effect on leaf conductance and CO2 assimilation of Vigna unguiculata (L.) Walp

Summary Well watered plants of Vigna unguiculata (L.) Walp cv. California Blackeye No. 5 had maximum photosynthetic rates of 16 mol m^-2 s^-1 (at ambient CO₂ concentration and environmental parameters optimal for high CO₂ uptake). Leaf conductance declined with increasing water vapour concentration difference between leaf and air (w), but it increased with increasing leaf temperature at a constant small w. When light was varied, CO₂ assimilation and leaf conductance were correlated linearly. We tested the hypothesis that g was controlled by photosynthesis via intercellular CO₂ concentration (c _i). No unique relationship between (1) c _i, (2) the difference between ambient CO₂ concentration (c _a) and c _i, namely c _a-c _i, or (3) the c _i/c _a ratio and g was found. g and A appeared to respond to environmental factors fairly independently of each other. The effects of different rates of soil drying on leaf gas exchange were studied. At unchanged air humidity, different rates of soil drying were produced by using (a) different soils, (b) different irrigation schemes and (c) different soil volumes per plant. Although the soil dried to wilting point the relative leaf water content was little affected. Different soil drying rates always resulted in the same response of photosynthetic capacity (A _max) and corresponding leaf conductance (g(_Amax)) when plotted against percent relative plant-extractable soil water content (W _e %) but the relationship with relative soil water content (W _e ) was less clear. Above a range of W _e of 15%–25%, A _max and g(_Amax) were both high and responded little to decreasing W _e . As soon as W _e fell below this range, A _max and g(_Amax) declined. The data suggest root-to-leaf communication not mediated via relative leaf water content. However, g(_Amax) was initially more affected than A _max.List of abbreviations A CO₂ assimilation - A _max photosynthetic capacity at favourable ambient conditions - c _a CO₂ concentration of the air in the leaf chamber - c _i intercellular - CO₂ concentration - E transpiration - g leaf conductance - g(_Amax) leaf conductance corresponding to photosynthetic capacity - I photon flux rate - T _l leaf temperature - W _e relative plant-extractable soil water content - W _e absolute plant-extractable soil water content - W _l relative leaf water content - W _s relative soil water content - w difference in water vapour mole fraction between leaf and air - leaf water potential     

Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO2 enrichment

Light‐saturated photosynthetic and stomatal responses to elevated CO₂ were measured in upper and mid‐canopy foliage of a sweetgum (Liquidambar styraciflua L) plantation exposed to free‐air CO₂ enrichment (FACE) for 3 years, to characterize environmental interactions with the sustained CO₂ effects in an intact deciduous forest stand. Responses were evaluated in relation to one another, and to seasonal patterns and natural environmental stresses, including high  temperatures, vapour pressure deficits (VPD), and drought. Photosynthetic CO₂ assimilation (A) averaged 46% higher in the +200 µmol mol^?1 CO₂ treatment, in mid‐ and upper canopy foliage. Stomatal conductance (g_s) averaged 14% (mid‐canopy) and 24% (upper canopy) lower under CO₂ enrichment. Variations in the relative responses of A and g_s were linked, such that greater relative stimulation of A was observed on dates when relative reductions in g_s were slight. Dry soils and high VPD reduced g_s and A in both treatments, and tended to diminish treatment differences. The absolute effects of CO₂ on A and g_s were minimized whenever g_s was low (<0·15 mol m^?2 s^?1), but relative effects, as the ratio of elevated to ambient rates, varied greatly under those conditions. Both stomatal and non‐stomatal limitations of A were involved during late season droughts. Leaf temperature had a limited influence on A and g_s, and there was no detectable relationship between prevailing temperature and CO₂ effects on A or g_s. The responsiveness of A and g_s to elevated CO₂, both absolute and relative, was maintained through time and within the canopy of this forest stand, subject to seasonal constraints and variability associated with limiting air and soil moisture.     

Effects of elevated CO2, drought and temperature on the water relations and gas exchange of groundnut (Arachis hypogaea) stands grown in controlled environment glasshouses

Stands of groundnut (Arachis hypogaea L. cv. Kadiri‐3) were grown in controlled environment glasshouses at mean atmospheric CO₂ concentrations of 375 or 700 μmol mol^?1 and daily mean air temperatures of 28 or 32°C on irrigated or drying soil profiles. Leaf water (Ψ_l) and solute potential (Ψ_s), relative water content (RWC), stomatal conductance (g_l) and net photosynthesis (P_n) were measured at midday for the youngest mature leaf throughout the growing season. Elevated CO₂ and temperature had no detectable effect on the water relations of irrigated plants, but higher values of RWC, Ψ_l and Ψ_s were maintained for longer under elevated CO₂ during progressive drought. Turgor potential (Ψ_p) reached zero when Ψ_l declined to ?1.6 to ?1.8 MPa in all treatments; turgor was lost sooner when droughted plants were grown under ambient CO₂. A 4°C increase in mean air temperature had no effect on Ψ_s in droughted plants, but elicited a small increase in Ψ_l; midday g_l values were lower under elevated than under ambient CO₂, and Ψ_l and g_l declined below ?1.5 MPa and 0.25 cm s^?1, respectively, as the soil dried. Despite the low g_l values recorded for droughted plants late in the season, P_n was maintained under elevated CO₂, but declined to zero 3 weeks before final harvest under ambient CO₂. Concurrent reductions in g_l and increases in water use efficiency under elevated CO₂ prolonged photosynthetic activity during drought and increased pod yields relative to plants grown under ambient CO₂. The implications of future increases in atmospheric CO₂ for the productivity of indeterminate C₃ crops grown in rainfed subsistence agricultural systems in the semi‐arid tropics are discussed.     

Shade does not ameliorate drought effects on the tree fern species Dicksonia antarctica and Cyathea australis

We examined the responses of two tree fern species (Dicksonia antarctica and Cyathea australis) growing under moderate and high light regimes to short-term water deficit followed by rewatering. Under adequate water supply, morphological and photosynthetic characteristics differed between species. D. antarctica, although putatively the more shade and less drought adapted species, had greater chlorophyll a/b ratio, and greater water use efficiency and less negative δ¹³C. Both species were susceptible to water deficit regardless of the light regime showing significant decreases in photosynthetic parameters (A _max, V _cmax, J _max) and stomatal conductance (g _s ) in conjunction with decreased relative frond water content (RWC) and predawn frond water potential (Ψ_predawn). During the water deficit period, decreases in g _s in both species started one day later, and were at lower soil water content, under moderate light compared with high light. D. antarctica under moderate light was more vulnerable to drought than all other plants as was indicated by greater decreases in Ψ_predawn, lowest stomatal conductance, and photosynthetic rates. Both tree fern species were able to recover after a short but severe water stress.     

Do photosynthetic limitations of evergreen Quercus ilex leaves change with long‐term increased drought severity?

Seasonal drought can severely impact leaf photosynthetic capacity. This is particularly important for Mediterranean forests, where precipitation is expected to decrease as a consequence of climate change. Impacts of increased drought on the photosynthetic capacity of the evergreen Quercus ilex were studied for two years in a mature forest submitted to long‐term throughfall exclusion. Gas exchange and chlorophyll fluorescence were measured on two successive leaf cohorts in a control and a dry plot. Exclusion significantly reduced leaf water potential in the dry treatment. In both treatments, light‐saturated net assimilation rate (A_max), stomatal conductance (g_s), maximum carboxylation rate (V_cmax), maximum rate of electron transport (J_max), mesophyll conductance to CO₂ (g_m) and nitrogen investment in photosynthesis decreased markedly with soil water limitation during summer. The relationships between leaf photosynthetic parameters and leaf water potential remained identical in the two treatments. Leaf and canopy acclimation to progressive, long‐term drought occurred through changes in leaf area index, leaf mass per area and leaf chemical composition, but not through modifications of physiological parameters.     

Soybean leaf hydraulic conductance does not acclimate to growth at elevated [CO2] or temperature in growth chambers or in the field

Background and Aims

Leaf hydraulic properties are strongly linked with transpiration and photosynthesis in many species. However, it is not known if gas exchange and hydraulics will have co-ordinated responses to climate change. The objective of this study was to investigate the responses of leaf hydraulic conductance (K_leaf) in Glycine max (soybean) to growth at elevated [CO₂] and increased temperature compared with the responses of leaf gas exchange and leaf water status.

Methods

Two controlled-environment growth chamber experiments were conducted with soybean to measure K_leaf, stomatal conductance (g_s) and photosynthesis (A) during growth at elevated [CO₂] and temperature relative to ambient levels. These results were validated with field experiments on soybean grown under free-air elevated [CO₂] (FACE) and canopy warming.

Key results

In chamber studies, K_leaf did not acclimate to growth at elevated [CO₂], even though stomatal conductance decreased and photosynthesis increased. Growth at elevated temperature also did not affect K_leaf, although g_s and A showed significant but inconsistent decreases. The lack of response of K_leaf to growth at increased [CO₂] and temperature in chamber-grown plants was confirmed with field-grown soybean at a FACE facility.

Conclusions

Leaf hydraulic and leaf gas exchange responses to these two climate change factors were not strongly linked in soybean, although g_s responded to [CO₂] and increased temperature as previously reported. This differential behaviour could lead to an imbalance between hydraulic supply and transpiration demand under extreme environmental conditions likely to become more common as global climate continues to change.