A study on the relationship between leaf conductance,CO2 concentration and carboxylation rate in various species

Leaf conductance g_L is strongly influenced by environmental factors like CO₂, irradiance and air humidity. According to Ball et al. (1987), g_L is correlated with an index calculated as the product of net CO₂ exchange rate A and ambient water vapour concentration W_a, divided by ambient CO₂ concentration c_a. However, this empirical model does not apply to high values of g_L observed at c_a below CO₂ compensation concentration . Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P₁ and P₂, were determined by adding to A the quotient of and the sum of gas phase resistance r_g and intracellular resistance for CO₂ exchange r_i, P₁ = A+/(r_g + r_i), or the quotient of and r_i, P₂ = A + /r_i. If P₂ is chosen, c_a in the Ball index has to be replaced by the intercellular CO₂ concentration c_i. By using the modified indices P₁·W_a/c_a and P₂·W_a/c_i, we analysed data from the C₃ species Nicotiana tabacum and Nicotiana plumbaginifolia, the C₃–C₄ intermediate species Diplotaxis tenuifolia, and the C₄ species Zea mays. The data were collected at widely varying levels of irradiance and CO₂ concentration. For all species uniform relationships between g_L and the new indices were found for the whole range of CO₂ concentrations below and above . Correlations between g_L and P₁·W_a/c_a were closer than those between g_L and P₂·W_a/c_i because P₁/c_a implicitly contains g_L. Highly significant correlations were also obtained for the relationships between g_L and the ratios P₁/c_a and P₂/c_i.     

Analysis of inhibition of photosynthesis due to water stress in the C3 species Hordeum vulgare and Vicia faba: Electron transport, CO2 fixation and carboxylation capacity

A C₃ monocot, Hordeum vulgare and C₃ dicot, Vicia faba, were studied to evaluate the mechanism of inhibition of photosynthesis due to water stress. The net rate of CO₂ fixation (A) and transpiration (E) were measured by gas exchange, while the true rate of O₂ evolution (J _O2) was calculated from chlorophyll fluorescence analysis through the stress cycle (10 to 11 days). With the development of water stress, the decrease in A was more pronounced than the decrease in J _O2 resulting in an increased ratio of Photosystem II activity per CO₂ fixed which is indicative of an increase in photorespiration due to a decrease in supply of CO₂ to Rubisco. Analyses of changes in the J _O2 A ratios versus that of CO₂ limited photosynthesis in well watered plants, and RuBP pool/RuBP binding sites on Rubisco and RuBP activity, indicate a decreased supply of CO₂ to Rubisco under both mild and severe stress is primarily responsible for the decrease in CO₂ fixation. In the early stages of stress, the decrease in C _i (intercellular CO₂) due to stomatal closure can account for the decrease in photosynthesis. Under more severe stress, CO₂ supply to Rubisco, calculated from analysis of electron flow and CO₂ exchange, continued to decrease. However, C _i, calculated from analysis of transpiration and CO₂ exchange, either remained constant or increased which may be due to either a decrease in mesophyll conductance or an overestimation of C _i by this method due to patchiness in conductance of CO₂ to the intercellular space. When plants were rewatered after photosynthesis had dropped to 10–30% of the original rate, both species showed near full recovery within two to four days.Abbreviations A- net CO₂ assimilation rate - A ^*- net CO₂ assimilation rate plus dark respiration - ATP- adenosine triphosphate - CABP- carboxyarabinitol 1,5-bisphosphate - C _a- ambient CO₂ concentration - C _c- CO₂ concentration in the chloroplast - C _i- intercellular CO₂ concentration - E- transpiration rate - g _m- mesophyll conductance - g _s- stomatal conductance - J _O2 true rate of O₂ evolution - LSD- least significant difference - PPFD- photosynthetic photon flux density - PS II- Photosystem II - R _n- dark respiration rate - Rubisco- ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP- ribulose 1,5-bisphosphate - RWC- relative water content - _c- rate of carboxylation - _o- rate of oxygenation - _PSII- quantum yield of Photosystem II - - CO₂ compensation point in the absence of R _n - - water potential     

Photosynthesis of cotton plants exposed to elevated levels of carbon dioxide in the field

The cotton (Gossypium hirsutum L.) plant responds to a doubling of atmospheric CO₂ with almost doubled yield. Gas exchange of leaves was monitored to discover the photosynthetic basis of this large response. Plants were grown in the field in open-top chambers with ambient (nominally 350 l/l) or enriched (nominally either 500 or 650 l/l) concentrations of atmospheric CO₂. During most of the season, in fully-irrigated plants the relationship between assimilation (A) and intercellular CO₂ concentration (c_i) was almost linear over an extremely wide range of c_i. CO₂ enrichment did not alter this relationship or diminish photosynthetic capacity (despite accumulation of starch to very high levels) until very late in the season, when temperature was somewhat lower than at midseason. Stomatal conductance at midseason was very high and insensitive to CO₂, leading to estimates of c_i above 85% of atmospheric CO₂ concentration in both ambient and enriched chambers. Water stress caused A to show a saturation response with respect to c_i, and it increased stomatal closure in response to CO₂ enrichment. In fully-irrigated plants CO₂ enrichment to 650 l/l increased A more than 70%, but in water-stressed plants enrichment increased A only about 52%. The non-saturating response of A to c_i, the failure of CO₂ enrichment to decrease photosynthetic capacity for most of the season, and the ability of the leaves to maintain very high c_i, form in part the basis for the very large response to CO₂ enrichment.Abbreviations c_a- atmospheric CO₂ concentration - c_i- intercellular CO₂ concentration - A- rate of assimilation of CO₂ - g_s- stomatal conductance to water vapor - g_b- boundary layer conductance to water vapor - g_m- mesophyll conductance to CO₂ - VPD- vapor pressure deficit - _w leaf water potential - L- stomatal limitation to CO₂ uptake     

Overcoming drought-induced decreases in soybean leaf photosynthesis by measuring with CO2-enriched air

Soybean [Glycine max (L.) Merr. cv. Williams 82 and A3127] plants were grown in the field under long-term soil moisture deficit and irrigation to determine the effects of severe drought stress on the photosynthetic capacity of soybean leaves. Afternoon leaf water potentials, stomatal conductances, intercellular CO₂ concentrations and CO₂-assimilation rates for the two soil moisture treatments were compared during the pod elongation and seed enlargement stages of crop development. Leaf CO₂-assimilation rates were measured with either ambient (340 l CO₂ l^–1) or CO₂-enriched (1800 l CO₂ l^–1) air. Although seed yield and leaf area per plant were decreased an average of 48 and 31%, respectively, as a result of drought stress, leaf water potentials were reduced only an average of 0.27 MPa during the sampling period. Afternoon leaf CO₂-assimilation rates measured with ambient air were decreased an average of 56 and 49% by soil moisture deficit for Williams 82 and A3127, respectively. The reductions in leaf photosynthesis of both cultivars were associated with similar decreases in leaf stomatal conductance and with small increases in leaf intercellular CO₂ concentration. When the CO₂-enriched air was used, similar afternoon leaf CO₂-assimilation rates were found between the soil moisture treatments at each stage of crop development. These results suggest that photosynthetic capacity of soybean leaves is not reduced by severe soil moisture deficit when a stress develops gradually under field conditions.Abbreviations C_i intercellular CO₂ concentrations - A_a rates of CO₂ assimilation measured with ambient air - A_e rates of CO₂ assimilation measured with CO₂-enriched air - g_s stomatal conductances - RuBPCase ribulose-1,5-bisphosphate carboxylase     

Separating the contribution of the upper and lower mesophyll to photosynthesis in Zea mays L. leaves

The appearance of transverse sections of maize leaves indicates the existence of two airspace systems serving the mesophyll, one connected to the stomata of the upper epidermis and the other to the stomata of the lower surface, with few or no connections between the two. This study tests the hypothesis that the air-space systems of the upper and lower mesophyll are separated by a defined barrier of measurable conductance. A mathematical procedure, based on this hypothesis, is developed for the quantitative separation of the contributions made by the upper and lower halves of the mesophyll to carbon assimilation using gasexchange data. Serial paradermal sections and three-dimensional scanning-electron-microscope images confirmed the hypothesis that there were few connections between the two air-systems. Simultaneous measurements of nitrous-oxide diffusion across the leaf and of transpiration from the two surfaces showed that the internal conductance was about 15% of the maximum observed stomatal conductance. This demonstrates that the poor air-space connections, indicated by microscopy, represent a substantial barrier to gas diffusion. By measuring the CO₂ and water-vapour fluxes from each surface independently, the intercellular CO₂ concentration (c _i) of each internal air-space system was determined and the flux between them calculated. This allowed correction of the apparent CO₂ uptake at each surface to derive the true CO₂ uptake by the mesophyll cells of the upper and lower halves of the leaf. This approach was used to analyse the contribution of the upper and lower mesophyll to CO₂ uptake by the leaf as a whole in response to varying light levels incident on the upper leaf surface. This showed that the upper mesophyll was light-saturated by a photon flux of approx. 1000 mol·m^-2·s^-1 (i.e. about one-half of full sunlight). The lower mesophyll was not fully saturated by photon fluxes of nearly double full sunlight. At low photon fluxes the c _i of the upper mesophyll was significantly less than that of the lower mesophyll, generating a significant upward flux of CO₂. At light levels equivalent to full sunlight, and above, c _i did not differ significantly between the two air space systems. The physiological importance of the separation of the air-space systems of the upper and lower mesophyll to gas exchange is discussed.Abbreviations and symbols A net leaf CO₂ uptake rate - A _upper ^app. and A _lower ^app. net rates of CO₂ uptake across the upper and lower surfaces - A _upper and A _lower derived net rates of CO₂ uptake by the upper and lower mesophyll - A _upward net flux of CO₂ from the lower to upper mesophyll - c _a, c _{a, upper} and c _{a, lower} the CO₂ concentrations in the air around the leaf above the upper surface and below the lower surface - c _N2O the concentration of N₂O in the air around the leaf - c _i, c _{i, upper} and c _{i, lower} the mesophyll intercellular CO₂ concentration of the whole leaf, the upper mesophyll and the lower mesophyll - g _i leaf internal conductance to CO₂ - g _s, g _{s, lower} and g _{s, upper} the stomatal conductance of the whole leaf, the lower surface and the upper surface - g the total conductance across the leaf - Q the photosynthetically active photon flux density     

Net CO2 assimilation of cacao seedlings during periods of plant water deficit

The effect of leaf water potential () on net CO₂ assimilation rate (A), stomatal conductance (g), transpiration (E) and water-use efficiency (WUE) was measured for three cultivars of cacao (Theobroma cacao L.) seedlings during three recurrent drought cycles. Net assimilation varied greatly at high water potentials, but as dropped below approximately -0.8 and -1.0 MPa, A was reduced to less than 1.5 mol CO₂ m^-2 s^-1. The relation between g and A was highly significant and conformed to an asymptotic exponential model, with A approaching maximal values at stomatal conductances of 55–65 mmol H₂O m^-2 s^-1. Net assimilation varied linearly (r=0.95) with transpiration, and the slope of the A-E relation (WUE) was approximately 3.0 mol CO₂ mmol^-1 H₂O throughout the range of stomatal conductances observed. C _i was insensitive to water stress, even though both g and A were strongly affected. Under the experimental conditions used here, mesophyll photosynthesis did not appear to control g through changes in C _i. As stress intensified within each drying cycle, WUE of nonirrigated seedlings did not decline relative to that of controls even though CO₂ and water vapor exchange rates underwent large displacements. The effect of seed source was highly significant for WUE, and the basis for observed differences among genotypes is discussed.Abbreviations ABA Abscisic Acid     

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     

Carbon fixation in eucalypts in the field

Summary The rate of CO₂ assimilation at light saturation and an intercellular CO₂ concentration of 350 l l^-1 (photosynthetic capacity), measured in leaves of Eucalyptus pauciflora, E. behriana, E. delegatensis and Acacia melanoxylon, declined over the course of cloudless days under naturally varying environmental conditions as well as under constant optimal conditions for high CO₂ uptake. Since the capacity did not recover during the light period, it was different from the midday depression of gas exchange. The change appeared to be caused neither by the diurnal variation of total leaf water potential, by photoinhibition of redox-reaction centres in photosystems nor by changes in the intrinsic properties of Ribulose-bisphosphate carboxylase-oxygenase. The decline was more pronounced in winter than in summer. It was related to the duration of illumination or the cumulative carbon gain. It was reversible in the following dark phase, and it did not occur on changeable days with short peaks of high light.Despite the decline in photosynthetic capacity, the initial slope of the CO₂ response of net photosynthesis, as obtained at low intercellular CO₂ concentrations, remained constant during the day, but declined at night when photosynthetic capacity recovered. In all cases stomatal conductance varied in parallel with photosynthetic capacity. The relevance of changes in photosynthetic capacity for the intercellular CO₂ concentration is discussed.Abbreviations and symbols A CO₂ assimilation - ABA abscisic acid - A_c350 photosynthetic capacity at c_i=350l l^-1 - c_i intercellular CO₂ concentration - g leaf conductance to water vapour - I photon flux density (irradiance) - P air pressure - P_i inorganic phosphate - R_d net CO₂ release at _* - Rubisco Ribulose-bisphosphate carboxylase-oxygenase - RuBP Ribulose-bisphosphate - T leaf temperature - w leaf-to-air water vapour concentration difference - A/c_i carboxylation efficiency at low c_i - _* light-independent CO₂ compensation point - total leaf water potential     

Disentangling the contributions of ontogeny and water stress to photosynthetic limitations in almond trees

Very few studies have attempted to disentangle the respective role of ontogeny and water stress on leaf photosynthetic attributes. The relative significance of both effects on photosynthetic attributes has been investigated in leaves of field‐grown almond trees [Prunus dulcis (Mill.) D. A. Webb] during four growth cycles. Leaf ontogeny resulted in enhanced leaf dry weight per unit area (W_a), greater leaf dry‐to‐fresh weight ratio and lower N content per unit of leaf dry weight (N_w). Concomitantly, area‐based maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max), mesophyll conductance to CO₂ diffusion (gm)′ and light‐saturated net photosynthesis (A_max) declined in both well‐watered and water‐stressed almond leaves. Although g_m and stomatal conductance (g_s) seemed to be co‐ordinated, a much stronger coordination in response to ontogeny and prolonged water stress was observed between g_m and the leaf photosynthetic capacity. Under unrestricted water supply, the leaf age‐related decline of A_max was equally driven by diffusional and biochemical limitations. Under restricted soil water availability, A_max was mainly limited by g_s and, to a lesser extent, by photosynthetic capacity and g_m. When both ontogeny and water stress effects were combined, diffusional limitations was the main determinant of photosynthesis limitation, while stomatal and biochemical limitations contributed similarly.     

Gas exchange and resource-use efficiency of Leymus cinereus (Poaceae): diurnal and seasonal responses to naturally declining soil moisture

We examined factors that limit diurnal and seasonal photosynthesis in Leymus cinereus, a robust tussock grass from shrub-steppes of western North America. Data from plants in a natural stand and in experimental field plots indicate that this bunchgrass has 1) a high photosynthetic capacity, 2) high leaf nitrogen content and high nitrogen-use efficiency, 3) a steep leaf-to-air diffusion gradient for carbon dioxide, which enhances intrinsic water-use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO₂ assimilation (A) and stomatal conductance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderately stressed plants reduced intercellular carbon dioxide concentration (c_i) by ≈40 μl liter^-1. The maximum rate of A achieved during the day for severely stressed plants (predawn water potential = -4 MPa) was one-third and daily carbon gain per unit leaf area was about one-fourth that of well-watered plants. For plants in the natural stand, CO₂-saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas there was little effect on A at CO₂ ambient levels or on carboxylation efficiency until predawn water potentials reached -1.8 MPa. Nitrogen-use efficiency declined with diminishing soil moisture, but there was no seasonal change in stomatal limitation or instantaneous water-use efficiency as estimated from A vs. c_i curves at optimal leaf temperature and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diumally, but over the growing season, stomatal limitation estimated from A vs. c_i curves is relatively constant because maximum stomatal conductance is closely tuned to the CO₂ assimilatory capacity of the mesophyll.     

Interactions of SO2 with other environmental stresses in influencing leaf gas exchange

Summary Leaves of two field growing co-occuring perennial shrubs (drought-deciduous Diplacus aurantiacus and the evergreen Heteromeles arbutifolia) from the Californian chaparral were exposed to small doses of SO₂. During this exposure the leaf environment was manipulated to determine how the presence of SO₂ alters the response of gas exchange to other environmental stresses. The data show that no direct changes in stomatal conductance (g) or net assimilation rate (A) could be attributed to short-term (7 h) SO₂ (4.2 mol m^-3, 0.1 l l^-1) exposure. D. aurantiacus leaves possessed features which demonstrate that they were sensitive to changes in environment e.g. light flux and atmospheric relative humidity. The interspecific differences in stomatal sensitivity to water vapour were extremely important, as relative humidity is a major factor influencing carbon fixation and the rate of pollutant absorption. Conditions of high relative humidity and high xylem water potentials are suggested to pre-dispose leaves of D. aurantiacus to greater pollutant doses than the more stomatally-conservative evergreen, H. arbutifolia. In the presence of SO₂ there was some indication of increased g for both D. aurantiacus and H. arbutifolia as W became smaller. This SO₂-effect was only obvious as increasing atmospheric humidity induced further stomatal opening. The important consequences of an SO₂ enhanced g, were a reduction in WUE, which may cause earlier leaf abscission and a concomitant decline in productivity.Abbreviations A net photosynthesis - A _max maximum rate light saturated photosynthesis - E transpiration; g stomatal conductance to water vapour - QY apparent incident quantum yield - W water vapour mole fraction difference between the leaf and the air - SO₂ Sulphur dioxide - WUE water use efficiency (mol CO₂ fixed per mol H₂O^-1 transpired)     

Negative effects of hydroxyl radical-generating mists (simulated dew water) on the photosynthesis and growth of Japanese apricot seedlings (Prunus mume)

The hydroxyl (OH) radical, which is generated in polluted dew water on leaf surfaces of the Japanese apricot (Prunus mume), is known to be a potent oxidant. In order to investigate the effects of the OH radical formed in polluted dew water on the photosynthesis and growth of 3-year-old seedlings of P. mume, OH radical-generating solutions simulating polluted dew water were sprayed in the early morning as a mist throughout a growing season onto the leaf surfaces of seedlings growing in experimental greenhouses. Four OH radical-generating solutions (0, 6, 18 and 54 M H₂O₂ with Fe(III) and an oxalate ion) were used in the mist treatment. Five months after the beginning of treatment, the leaves exposed to the mist containing 54 M H₂O₂ showed a significantly smaller maximum CO₂ assimilation rate (A_max) and stomatal conductance (g_s) as compared to the leaves exposed to the mist containing 0 M H₂O₂. Exposure of P. mume seedlings to the OH radical-generating mist had caused a reduction in the dry weight and relative growth rate (RGR) of the above-ground parts (stem + branch) at the end of the growing season. A significant positive correlation was shown between RGR and A_max. Thus, the effects of oxidants generated in polluted dew water on leaf surfaces can be considered to be a cause of the decrease in leaf photosynthesis and growth of P. mume.     

An in vivo analysis of photosynthesis during short-term O3 exposure in three contrasting species

The depressions of photosynthetic CO₂ uptake following O₃ exposures of 200 and 400 nmol mol^-1 for between 4 and 16 h were compared between Pisum sativum, Quercus robur and Triticum aestivum, and the potential causes of change identified in vivo. Photosynthetic change was examined by analysis of CO₂, O₂, O₃ and water vapour exchanges together with chlorophyll fluorescence in controlled environments. Under identical fumigation conditions, each species showed very similar rates of O₃ consumption. The light-saturated rate of CO₂ uptake showed a statistically significant decrease in each species with increasing O₃ dose. Although stomatal conductance declined in parallel with CO₂ uptake this did not account for the observed decrease in photosynthesis. The decrease in mesophyll conductance resulted primarily from a decrease in the apparent carboxylation capacity, implying in decreased activity of ribulose 1,5-bisphosphate carboxylase/oxygenase. The maximum capacity of carboxylation was consequently reduced by over 30% and 50% after 16 h fumigation with 200 and 400 nmol mol^-1 O₃ respectively. Additionally, in Q. robur, a statistically significant inhibition of the CO₂ saturated rate of photosynthesis occurred after 16 h with 400 nmol mol^-1 O₃, suggesting that the ability to regenerate ribulose 1,5-bisphosphate was also impaired. None of the species showed any significant decrease in the efficiency of light-limited photosynthesis following fumigation at 200 nmol mol^-1 O₃, but effects were apparent at 400 nmol mol^-1 O₃. The common feature in all three species was a decline in carboxylation capacity which preceded any other change in the photosynthetic apparatus.Abbreviations A_sat net CO₂ uptake rate per unit leaf area at light saturation - A net CO₂ uptake rate per unit leaf area - A_max net CO₂ uptake rate per unit leaf area at CO₂ and light saturation - c_i mole fraction of CO₂ in the intercellular air space - g_s stomatal conductance to CO₂ - F_m maximum chlorophyll fluorescence - F_v variable chlorophyll fluorescence - _c quantum yield of CO₂ uptake for absorbed light - ₀ quantum yield of oxygen evolution for incident light - PPFD photosynthetically active radiation - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate - Vc_max maximum rate of carboxylation     

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.     

Maximizing leaf carbon gain in varying saline conditions: An optimization model with dynamic mesophyll conductance

While the adverse effects of elevated salinity levels on leaf gas exchange in many crops are not in dispute, representing such effects on leaf photosynthetic rates (A) continues to draw research attention. Here, an optimization model for stomatal conductance (g_c) that maximizes A while accounting for mesophyll conductance (g_m) was used to interpret new leaf gas exchange measurements collected for five irrigation water salinity levels. A function between chloroplastic CO₂ concentration (c_c) and intercellular CO₂ concentration (c_i) modified by salinity stress to estimate g_m was proposed. Results showed that with increased salinity, the estimated g_m and maximum photosynthetic capacity were both reduced, whereas the marginal water use efficiency λ increased linearly. Adjustments of g_m, λ and photosynthetic capacity were shown to be consistent with a large corpus of drought‐stress experiments. The inferred model parameters were then used to evaluate the combined effects of elevated salinity and atmospheric CO₂ concentration (c_a) on leaf gas exchange. For a given salinity level, increasing c_a increased A linearly, but these increases were accompanied by mild reductions in g_c and transpiration. The c_a level needed to ameliorate A reductions due to increased salinity is also discussed using the aforementioned model calculations.     

Leaf and canopy responses of Lolium perenne to long-term elevated atmospheric carbon-dioxide concentration

The relationship between leaf photosynthetic capacity (p _{n, max}), net canopy CO₂- and H₂O-exchange rate (NCER and E _t, respectively) and canopy dry-matter production was examined in Lollium perenne L. cv. Vigor in ambient (363±30 l· l^-1) and elevated (631±43 l·l^-1) CO₂ concentrations. An open system for continuous and simultaneous regulation of atmospheric CO₂ concentration and NCER and E _t measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCER_max of full-grown canopies was 49% higher at elevated CO₂ level, stimulation of p _n, max was only 46% (in spite of a 50% rise in one-sided stomatal resistance for water-vapour diffusion), clearly indicating the effect of a higher leaf-area index under high CO₂ (approx. 10% in one growing period examined). A larger amount of CO₂-deficient leaves resulted in higher canopy dark-respiration rates and higher canopy light compensation points. The structural component of the high-CO₂ effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy darkrespiration rates under elevated CO₂ level and low irradiance during the growing period are the primary causes for the increase in dry-matter production (19%) being much lower than expected merely based on the NCER_max difference. While total water use was the same under high and low CO₂ levels, water-use efficiency increased 25% on the canopy level and 87% on a leaf basis. In the course of canopy development, allocation towards the root system became greater, while stimulation of shoot dry-matter accumulation was inversely affected. Over an entire growing season the root/shoot production ratio was 22% higher under high CO₂ concentration.Abbreviations and symbols C350 ambient CO₂, 363±30 l·l^-1 - C600 high CO₂, 631±43 l·l^-1 - c _a atmospheric CO₂ level - c _i CO₂ concentration in the intracellular spaces of the leaf - E_t canopy evapotranspiration - I _o canopy light compensation point - NCER canopy CO₂-exchange rate - p _n leaf photosynthetic rate - PPFD photosynthetic photon flux density - r _a leaf boundary-layer resistance - RD canopy dark-respiration rate - r _s stomatal resistance - WUE water use efficiency     

Water relations of crok-oak (Quercus suber L.) under natural conditions

Daily and annual courses of leaf transpiration, stomatal conductance and shoot water potential of four Quercus suber individuals were compared in a semi-natural stand in southwest Portugal, from spring 1989 to early summer 1990.The trees investigated showed annual patterns typical of evergreen sclerophyllous species but varied in their range of stomatal operation. This appeared to be related to differences in hydraulic conductivity in the root-to-leaf pathway.Maximum stomatal conductance and transpiration rates occurred from March to June.Water stress was found to be moderate and winter cold stress due to low air and soil temperatures appeared to have an influence on plant water balance through their effects on flow resistances.Abbreviations g_sw stomatal conductance - g_max maximum stomatal conductance - PAR photosynthetically active radiation - RH relative humidity of the air - T leaf transpiration - Ta air temperature - TL leaf temperature - T_max maximum leaf transpiration - W air-to-leaf vapor pressure difference - shoot water potential - PD predawn shoot water potential - MIN minimum shoot water potential     

On the complementary relationship between marginal nitrogen and water-use efficiencies among Pinus taeda leaves grown under ambient and CO2-enriched environments

Background and Aims

Water and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration (E) can be decreased by reducing leaf stomatal conductance (g_s) at the expense of lowering CO₂ uptake (A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when g_s declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration (N) varies on time scales much longer than g_s, an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO₂ concentration (c_a).

Methods

Optimization theory is employed to quantify interactions between resource-use efficiencies under elevated c_a and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, λ = (∂A/∂g_s)/(∂E/∂g_s) and η = ∂A/∂N, respectively. The relationship between the two efficiencies and related variation in intercellular CO₂ concentration (c_i) were examined using A/c_i curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO₂ Enrichment experiment (North Carolina, USA).

Key Results

Optimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where η is complementary to the square-root of λ. The data support the model predictions that elevated c_a increased η and λ, and at given c_a and needle age-class, the two quantities varied among needles in an approximately complementary manner.

Conclusions

The derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of η, and assist in predicting how increasing c_a influences ecosystem water use.     

Biomass Partitioning and Gas Exchange in Dalbergia sissoo seedlings under water stress

Biomass, leaf water potential (_l), net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), leaf to air temperature difference (T _diff), and instantaneous water use efficiency (WUE) were measured in the seedlings of Dalbergia sissoo Roxb. grown under irrigation of 20 (W₁), 14 (W₂), 10 (W₃), and 8 (W₄) mm. Treatments were maintained by re-irrigation when water content of the soil reached 7.4% in W₁, 5.6% in W₂, 4.3% in W₃, and 3.2% in W₄. Seedlings in a control (W₅) were left without irrigation after maintaining the soil field capacity (10.7%). Seedlings of W₁ had highest biomass that was one tenth in W₅. Biomass allocation was highest in leaf in W₂ and in root in W₄ and W₅ treatments. Difference between predawn leaf water potential (_Pd) and midday (_mid) increased with soil water stress and with vapour pressure deficit (VPD) in April and May slowing down the recovery in plant leaf water status after transpiration loss. P _N, E, and g _s declined and T _diff increased from W₁ to W₅. Their values were highly significant in April and May for the severely stressed seedlings of W₄ and W₅. P _N increased from 08:00 to 10:00 and E increased until 13:00 within the day for most of the seedlings whereas g _s decreased throughout the day from 08:00 to 17:00. P _N and E were highest in March but their values were low in January, February, April, and May. Large variations in physiological variables to air temperature, photosynthetically active radiation, and vapour pressure deficit (VPD) indicated greater sensitivity of the species to environmental factors. WUE increased from W₁ to W₂ but decreased drastically at high water stress particularly during hot summer showing a kind of adaptation in D. sissoo to water stress. However, low biomass and reduced physiological functions at <50% of soil field capacity suggest that this species does not produce significant biomass at severe soil water stress or drought of a prolonged period.     

Gas exchange and SO2 fumigation studies with irrigated and unirrigated field grown Diplacus aurantiacus and Heteromeles arbutifolia

Summary Experiments were performed on an evergreen (Heteromeles arbutifolia) and a drought deciduous shrub (Diplacus aurantiacus) to determine, 1) whether approaches for evaluating SO₂ absorption by leaves in laboratory studies could be extended to field studies, 2) the effects of irrigation on metabolism and SO₂ responses of the study species during a season when water was limiting, 3) to interpret SO₂ responses on the basis of SO₂ flux rates. Laboratory-developed approaches for evaluating SO₂ absorption by leaves were found to be suitable for use with field plants, despite field plants having lower gas exchange rates. Supplementing water during times of deficit did not override all the biological and environmental factors that limited photosynthesis (A). Irrigation increased leaf longevity of D. aurantiacus, and stomatal conductance to water vapour (g); g was also shown to increase with H. arbutifolia on irrigation. Irrigation profoundly influenced plant response to SO₂. Unwatered D. aurantiacus had only a small g and therefore a reduced capacity to absorb SO₂ and respond to SO₂; which resulted in apparent SO₂ avoidance. Water availability and SO₂ both affect g and therefore, SO₂ flux rates into the mesophyll. Different ambient SO₂ concentrations of 8.3 and 26.2 mol m^-3 (0.2 and 0.6 ppm) were both found to result in similar SO₂ flux rates into the leaf, due to variations in g in response to water availability. Changes in g did not always result in changes in A, implying that carbon fixation may be little affected by some SO₂ exposures, although still potentially affecting such processes as maintenance of leaf water potential, transpirational cooling and nutrient uptake.Abbreviations SO₂ sulphur dioxide - A net photosynthesis - E transpiration - g stomatal conductance to water vapour - W Water vapour mole fraction difference between the leaf and air - WUE water use efficiency (mol CO₂ uptake per mol H₂O transpired)