Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves

A series of experiments is presented investigating short term and long term changes of the nature of the response of rate of CO₂ assimilation to intercellular p(CO₂). The relationships between CO₂ assimilation rate and biochemical components of leaf photosynthesis, such as ribulose-bisphosphate (RuP₂) carboxylase-oxygenase activity and electron transport capacity are examined and related to current theory of CO₂ assimilation in leaves of C₃ species. It was found that the response of the rate of CO₂ assimilation to irradiance, partial pressure of O₂, p(O₂), and temperature was different at low and high intercellular p(CO₂), suggesting that CO₂ assimilation rate is governed by different processes at low and high intercellular p(CO₂). In longer term changes in CO₂ assimilation rate, induced by different growth conditions, the initial slope of the response of CO₂ assimilation rate to intercellular p(CO₂) could be correlated to in vitro measurements of RuP₂ carboxylase activity. Also, CO₂ assimilation rate at high p(CO₂) could be correlated to in vitro measurements of electron transport rate. These results are consistent with the hypothesis that CO₂ assimilation rate is limited by the RuP₂ saturated rate of the RuP₂ carboxylase-oxygenase at low intercellular p(CO₂) and by the rate allowed by RuP₂ regeneration capacity at high intercellular p(CO₂).     

Effects of Nitrate Application on Amaranthus powellii Wats. : III. Optimal Allocation of Leaf Nitrogen for Photosynthesis and Stomatal Conductance

Optimal allocation of leaf nitrogen maximizes daily CO₂ assimilation for a given leaf nitrogen concentration. According to the hypothesis of optimization, this condition occurs when the partial derivative of assimilation rate with respect to leaf nitrogen concentration is constant. This hypothesis predicts a linear increase of assimilation rate with leaf nitrogen concentration under constant conditions. Plants of Amaranthus powellii Wats. were grown at 1, 5, 10, or 45 millimolar nitrate to obtain leaves with different nitrogen concentrations. Assimilation rate at 340 microbar CO₂/bar, stomatal conductance, CO₂- and light-saturated net photosynthetic rate, the initial slope of the CO₂ response of photosynthesis, ribulose-1,5′-bisphosphate carboxylase activity, and phosphoenolpyruvate carboxylase activity were linearly related to estimated or actual leaf nitrogen concentration. The data are consistent with the optimal use of leaf nitrogen. This hypothesis and the hypothesis of optimal stomatal conductance were combined to determine the relationship between conductance and leaf nitrogen concentration. The slope of conductance versus leaf nitrogen concentration was not significantly different than the slope predicted by the combination of the two hypotheses. Stomatal conductance was linearly related to leaf nitrogen in the field and the slope decreased with lower xylem pressure potentials in a manner consistent with the hypotheses. Finally, apparent maximum stomatal aperture of isolated abaxial epidermal strips was linearly related to leaf nitrogen suggesting stomatal conductance and assimilation rate are controlled in parallel by leaf nitrogen concentration or some factor correlated with leaf nitrogen.     

Leaf Conductance in Relation to Rate of CO(2) Assimilation: III. Influences of Water Stress and Photoinhibition

Rates of CO₂ assimilation and leaf conductances to CO₂ transfer were measured in plants of Zea mays during a period of 14 days in which the plants were not rewatered, and leaf water potential decreased from −0.5 to −8.0 bar. At any given ambient partial pressure of CO₂, water stress reduced rate of assimilation and leaf conductance similarly, so that intercellular partial pressure of CO₂ remained almost constant. At normal ambient partial pressure of CO₂, the intercellular partial pressure of CO₂ was estimated to be 95 microbars. This is the same as had been estimated in plants of Zea mays grown with various levels of nitrogen supply, phosphate supply and irradiance, and in plants of Zea mays examined at different irradiances.

After leaves of Phaseolus vulgaris L. and Eucalyptus pauciflora Sieb. ex Spreng had been exposed to high irradiance in an atmosphere of CO₂-free N₂ with 10 millibars O₂, rates of assimilation and leaf conductances measured in standard conditions had decreased in similar proportions, so that intercellular partial pressure of CO₂ remained almost unchanged. As the conductance of each epidermis that had not been directly irradiated had declined as much as that in the opposite, irradiated surface it was hypothesized that conductance may have been influenced by photoinhibition within the mesophyll tissue.

     

Effects of phosphorus nutrition on the response of photosynthesis to CO2 and O2, activation of ribulose bisphosphate carboxylase and amounts of ribulose bisphosphate and 3-phosphoglycerate in spinach leaves

Phosphorus-deficient spinach plants were grown by transferring them to nutrient solutions without PO₄. Photosynthetic rates were measured at a range of intercellular CO₂ partial pressures from 50–500 bar and then the leaves were freeze-clamped in situ to measure ribulose bisphosphate carboxylase (Rubisco) activity and metabolite concentrations. Compared with control leaves, deficient leaves had significantly lower photosynthetic rates, percentage activation of Rubisco, and amounts of ribulose bisphosphate and 3-phosphoglycerate at all CO₂ partial pressures. After feeding 10 mM PO₄ to the petioles of detached deficient leaves, all these measurements increased within 2 hours. At atmospheric CO₂ partial pressure the photosynthetic rate was stimulated in 19 mbar O₂ compared with 200 mbar. At higher CO₂ partial pressures this stimulation was less but the percentage stimulation in deficient leaves was no different from controls in either CO₂ partial pressure. It was concluded that phosphorus deficiency affects both Rubisco activity and the capacity for ribulose bisphosphate regeneration, and possible causes are discussed.Abbreviations A CO₂ assimilation rate - C_i intercellular CO₂ partial pressure - PGA 3-phosphoglycerate - RuP₂ ribulose 1,5-bisphosphate - Rubisco RuP₂ carboxylase/oxygenase     

Leaf Conductance in Relation to Assimilation in Eucalyptus pauciflora Sieb. ex Spreng: Influence of Irradiance and Partial Pressure of Carbon Dioxide

Rates of assimilation and transpiration in Eucalyptus pauciflora Sieb. ex Spreng were measured at various ambient partial pressures of CO₂ and various irradiances and were used to estimate leaf conductance and intercellular partial pressure of CO₂. The responses of leaf conductance and rate of assimilation to change in intercellular partial pressure of CO₂ were expressed in terms of feedback. They are small in the sense that their combined effect was to reduce disturbances in intercellular partial pressure of CO₂ by 30% only. The magnitude of the feedback had no influence on the system as affected by irradiance, because the direct responses of conductance and rate of assimilation to changes in irradiance in the range 0.25 to 2 millieinsteins per meter per second were such that intercellular partial pressure was maintained almost constant.     

Salinity Induced Limitations on Photosynthesis in Prunus salicina, a Deciduous Tree Species

The response of photosynthetic CO₂ assimilation to salinization in 19 year old Prunus salicina was evaluated under field conditions for a 3 year period. The observed decline in CO₂ assimilation capacity was apparently related to increasing leaf chloride (Cl⁻) content, and independent of changes in leaf carbohydrate status. The response of net CO₂ assimilation (A) to leaf intercellular CO₂ partial pressure (C_i) indicated that the reduction in the capacity for A with Cl⁻ was not the result of decreased stomatal conductance but a consequence of nonstomatal inhibition. The nonstomatal limitations to CO₂ assimilation capacity, as determined by the response of A to C_i and biochemical assay, were related to a decline in the activity of ribulose 1,5-bisphosphate carboxylase (Rubpcase) and the pool size of triose phosphate, ribulose 1,5-bisphosphate (Rubp) and phosphoglycerate with increasing salinity. Lack of agreement between the initial slope of the A to C_i response curve and Rubpcase activity suggests the occurrence of heterogeneous stomatal apertures with the high salinity treatment (28 millimolar). Prolonged exposure to chloride salts appeared to increase the Rubp or Pi regeneration limitation, decrease Rubpcase activity and reduce leaf chlorophyll content. Observed changes in the biochemical components of CO₂ fixation may, in turn, affect total leaf carbohydrates, which also declined with time and salinity. The reduction in Rubpcase activity was apparently a consequence of a reduced Rubpcase protein level rather than either a regulatory or inhibitory effect.     

Differences between Wheat Genotypes in Specific Activity of Ribulose-1,5-bisphosphate Carboxylase and the Relationship to Photosynthesis

The in vitro ribulose-1,5-bisphosphate (RuBP) carboxylase activity per unit of leaf nitrogen was found to be 30% greater in Triticum aestivum than in T. monococcum. This was due to a higher specific activity of the enzyme from T. aestivum, as the amount of RuBP carboxylase protein per unit of total leaf nitrogen did not differ between the genotypes. The occurrence of higher specific activity of RuBP carboxylase is shown to correlate with possession of the large subunit derived from the B genome of wheat.

Despite the greater RuBP carboxylase activity per unit of leaf nitrogen in T. aestivum, the initial slopes of curves relating rate of CO₂ assimilation to intercellular p(CO₂) are similar in T. aestivum and T. monococcum for the same nitrogen content per unit leaf area. The similarity of the initial slopes is the result of a greater resistance to CO₂ transfer between the intercellular spaces and the site of carboxylation in T. aestivum than in T. monococcum.

     

Effects of doubled atmospheric carbon dioxide concentration on the responses of assimilation and conductance to humidity

Experiments were performed to determine if growth at elevated partial pressure of CO₂ altered the sensitivity of leaf water vapour conductance and rate of CO₂ assimilation to the leaf-to-air difference in the partial pressure of water vapour (Δw). Comparisons were made between plants grown and measured at 350 and 700 μPa Pa^?1 partial pressures of CO₂ for amaranth, soybean and sunflower grown in controlled environment chambers, soybean grown outdoors in pots, and orchard grass grown in field plots. In amaranth, soybean and orchard grass, both the absolute and the relative sensitivity of conductance to Δw at the leaf surface were less in plants grown and measured at the elevated CO₂. In sunflower, there was no change in the sensitivity of conductance to Δw for the two CO₂ partial pressures. Tests in soybeans and amaranth showed that the change in sensitivity resulted from elevated CO₂ during the measurement of the Δw response. Assimilation rate of CO₂ was not altered by Δw in amaranth, which has C₄ metabolism. In sunflower, the assimilation rate of plants grown and measured at elevated CO₂ was insensitive to Δw, consistent with the response of assimilation rate to intercellular CO₂ partial pressure in the prevailing range. In soybean, the sensitivity of assimilation rate to Δw was not different between CO₂ treatments, in contrast to what would be expected from the response of assimilation rate to intercellular CO₂ partial pressure.     

Simultaneous and independent effects of abscisic acid on stomata and the photosynthetic apparatus in whole leaves

(±)-Abscisic acid (ABA) at 10^-5 M was added to the transpiration stream of leaves of 16 species (C₃ and C₄, monocotyledons and dicotyledons). Stomatal responses followed one of three patterns: i) stomata that were wide and insensitive to CO₂ initially, closed partially and became sensitive to CO₂; ii) for stomata that were sensitive to CO₂ before the application of ABA, the range of highest sensitivity to CO₂ shifted from high to low intercellular partial pressures of CO₂, for instance in leaves of Zea mays from 170–350 to 70–140 bar; iii) when stomata responded strongly to ABA, their conductance was reduced to a small fraction of the initial conductance, and sensitivity to CO₂ was lost. The photosynthetic apparatus was affected by applications of ABA to various degrees, from no response at all (in agreement with several previous reports on the absence of effects of ABA on photosynthesis) through a temporary decrease of its activity to a lasting reduction. Saturation curves of photosynthesis with respect to the partial pressure of CO₂ in the intercellular spaces indicated that application of ABA could produce three phenomena: i) a reduction of the initial slope of the saturation curve (which indicates a diminished carboxylation efficiency); ii) a reduction of the level of the CO₂-saturated rate of assimilation (which indicates a reduction of the ribulose-1,5-bisphosphate regeneration capacity); and iii) an increase of the CO₂ compensation point. Photosynthesis of isolated mesophyll cells was not affected by ABA treatments. Responses of the stomatal and photosynthetic apparatus were usually synchronous and often proportional to each other, with the result that the partial pressure of CO₂ in the intercellular spaces frequently remained constant in spite of large changes in conductance and assimilation rate. Guard cells and the photosynthetic apparatus were able to recover from effects of ABA applications while the ABA supply continued. Recovery was usually partial, in the case of the photosynthetic apparatus occasionally complete. Abscisic acid did not cause stomatal closure or decreases in the rate of photosynthesis when it was applied during a phase of stomatal opening and induction of photosynthesis that followed a transition from darkness to light.Abbreviations and symbols A rate of CO₂ assimilation - ABA (±)-abscisic acid - c _a partial pressure of CO₂ in the ambient air or in the gas supplied to the leaf chambers - c _i partial pressure of CO₂ in the intercellular spaces of a leaf - e _a partial pressure of H₂O in the air - g conductance for water vapor - J quantum flux - T ₁ leaf temperature     

Physiological Site of Ethylene Effects on Carbon Dioxide Assimilation in Glycine max L. Merr

The physiological site of ethylene action on CO₂ assimilation was investigated in intact plants of Glycine max L., using a whole-plant, open exposure system equipped witha remotely operated single-leaf cuvette. The objective of the study was met by investigating in control and ethylene-treated plants the (a) synchrony in response of CO₂ assimilation, stomatal conductance to water vapor, and substomatal CO₂ partial pressure; (b) response of CO₂ assimilation as a function of a range of substomatal CO₂ partial pressures; and (c) response of CO₂ assimilation as a function of a range of photon flux densities. After exposure to 410 micromoles per cubic meter of ethylene for 2.0 hours, CO₂ assimilation and stomatal conductance declined in synchrony, while substomatal CO₂ partial pressure remained unchanged until exposure times equaled and exceeded 3.0 hours. Because incipient changes in CO₂ assimilation occurred without a change in the CO₂ partial pressure in the leaf interior, it is concluded that both stomatal physiology and the chloroplast's CO₂ assimilatory capacity were initial sites of ethylene action. After 3.5 hours the effect of ethylene on stomatal conductance and CO₂ assimilation exhibited saturation kinetics, and the effect was substantially more pronounced for stomatal conductance than for CO₂ assimilation. Based on the response of CO₂ assimilation to a range of substomatal CO₂ partial pressures, ethylene did not affect either the CO₂ compensation point or carboxylation efficiency at subsaturating CO₂ partial pressures. Above-ambient supplies of CO₂ did not alleviate the diminished rates of CO₂ assimilation. In partitioning the limitations imposed on CO₂ assimilation in control and ethylene-treated plants, the stomatal component accounted for only 16 and 4%, respectively. The response of CO₂ assimilation to a range of photon flux densities suggests that ethylene reduced apparent quantum yield by nearly 50%. Thus, the pronounced decline in net photosynthetic CO₂ assimilation in the presence of ethylene was due more to a loss in the mesophyll tissue's intrinsic capacity to assimilate CO₂ than to a reduction in stomatal conductance.     

Inhibition of photosynthesis by carbohydrates in wheat leaves

The rate of net CO₂ assimilation of mature wheat (Triticum aestivum L.) leaves in ambient air (21% O₂, 340 microbars CO₂) declined with time of illumination at temperatures lower than 25°C, but not at higher temperatures, and the rate of decline increased when maintained in air with higher CO₂ concentration (700-825 microbars). In this latter case, the decline in the rate of net CO₂ assimilation also occurred at high temperatures. Stomatal conductance also declined with time in some cases and stomata became more sensitive to CO₂, but this was not the primary cause of the decrease in CO₂ assimilation because internal partial pressure of CO₂ remained constant. Treatments which reduced the rate of translocation (e.g. lower temperatures, chilling the base of the leaf) produced a marked decline in CO₂ assimilation of leaves in atmospheric and high CO₂ concentrations. The decreased net CO₂ assimilation was correlated with carbohydrate accumulation in each case, suggesting end product inhibition of photosynthesis. Analysis of CO₂ assimilation in high carbohydrate leaves as a function of intercellular CO₂ partial pressure showed reduction in the upper part of the curve. The initial slope of this curve, however, was not affected. Photosynthetic rates in the upper part of this curve generally recovered after a short period in darkness in which carbohydrates were removed from the leaf. The stimulation of net CO₂ assimilation by 2% O₂ (Warburg effect), and the apparent quantum yield, decreased after several hours of light.     

Identification of two binding sites of the D-ribulose 1,5-bisphosphate carboxylase/oxygenase from spinach for D-ribulose 1,5-bisphosphate and effectors of the carboxylation reaction.

Fluorimetric studies of the binding of d-ribulose 1,5-bisphosphate (RuP₂) and the effectors 6-phosphogluconate and fructose 1,6-bisphosphate to the d-ribulose 1,5-bisphosphate carboxylase/oxygenase from spinach were correlated with the functions of these sugar phosphates in the carboxylation reaction. These agents compete for two binding sites of the enzyme. At relatively low concentrations they bind to an allosteric site, where 6-phosphogluconate and fructose 1,6-bisphosphate display their stimulating effect on the fixation of CO₂. At higher concentrations these compounds inhibit the carboxylation reaction and compete with RuP₂ for the reaction center of the carboxylase. Preincubation of the enzyme with low concentrations of RuP₂ (0.1–5 μm) inhibits the activity of these effectors as well as the effector-induced fluorescence changes of the enzyme-2-p-toluidinonapthalene-6-sulfonate (TNS) complex by competition for the regulatory center which could be identified as the high affinity binding site of the enzyme for RuP₂ with a K_D = 0.6 μm. The deactivation of the carboxylase which is observed on preincubation of the enzyme with RuP₂ in the absence of bicarbonate and Mg²⁺ cannot be correlated to the binding of RuP₂ to the effector site. The deactivation process occurs in an RuP₂ concentration range similar to that for CO₂ fixation.     

The kinetics of ribulose-1,5-bisphosphate carboxylase/oxygenase in vivo inferred from measurements of photosynthesis in leaves of transgenic tobacco

Transgenic tobacco (Nicotiana tabacum L. cv. W38) with an antisense gene directed against the mRNA of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit was used to determine the kinetic properties of Rubisco in vivo. The leaves of these plants contained only 34% as much Rubisco as those of the wild type, but other photosynthetic components were not significantly affected. Consequently, the rate of CO₂ assimilation by the antisense plants was limited by Rubisco activity over a wide range of CO₂ partial pressures. Unlike in the wild-type leaves, where the rate of regeneration of ribulose bisphosphate limited CO₂ assimilation at intercellular partial pressures above 400 ubar, photosynthesis in the leaves of the antisense plants responded hyperbolically to CO₂, allowing the kinetic parameters of Rubisco in vivo to be inferred. We calculated a maximal catalytic turnover rate, k_cat, of 3.5+0.2 mol CO₂·(mol sites)^–1·s^–1 at 25° C in vivo. By comparison, we measured a value of 2.9 mol CO₂·(mol sites)^–1·^–1 in vitro with leaf extracts. To estimate the Michaelis-Menten constants for CO₂ and O₂, the rate of CO₂ assimilation was measured at 25° C at different intercellular partial pressures of CO₂ and O₂. These measurements were combined with carbon-isotope analysis (¹³C/¹²C) of CO₂ in the air passing over the leaf to estimate the conductance for transfer of CO₂ from the substomatal cavities to the sites of carboxylation (0.3 mol·m^–2·s^–1·bar^–1) and thus the partial pressure of CO₂ at the sites of carboxylation. The calculated Michaelis-Menten constants for CO₂ and O₂ were 259 ±57 bar (8.6±1.9M) and 179 mbar (226 M), respectively, and the effective Michaelis-Menten constant for CO₂ in 200 mbar O₂ was 549 bar (18.3 M). From measurements of the photocompensation point (_* = 38.6 ubar) we estimated Rubisco's relative specificity for CO₂, as opposed to O₂ to be 97.5 in vivo. These values were dependent on the size of the estimated CO₂-transfer conductance.Abbreviations and Symbols A CO₂-assimilation rate - g_w conductance for CO₂ transfer from the substomatal cavities to the sites of carboxylation - K_c, K_o Michaelis-Menten constants for carboxylation, oxygenation of Rubisco - k_cat V_cmax/[active site] - O partial pressure of O₂ at the site of carboxylation - p_c partial pressure of CO₂ at the site of carboxylation - p_i intercellular CO₂ partial pressure - R_d day respiration (non-photorespiratory CO₂ evolution) - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - S_c/o relative specificity factor for Rubisco - SSu small subunit of Rubisco - V_cmax, V_omax maximum rates of Rubisco carboxylation, oxygenation - _* partial pressure of CO₂ in the chloroplast at which photorespiratory CO₂ evolution equals the rate of carboxylation     

Nitrogen fertilization enhances water‐use efficiency in a saline environment

Effects of salinity and nutrients on carbon gain in relation to water use were studied in the grey mangrove, Avicennia marina, growing along a natural salinity gradient in south‐eastern Australia. Tall trees characterized areas of seawater salinities (fringe zone) and stunted trees dominated landward hypersaline areas (scrub zone). Trees were fertilized with nitrogen (+N) or phosphorus (+P) or unfertilized. There was no significant effect of +P on shoot growth, whereas +N enhanced canopy development, particularly in scrub trees. Scrub trees maintained greater CO₂ assimilation per unit water transpired (water‐use efficiency, WUE) and had lower nitrogen‐use efficiency (NUE; CO₂ assimilation rate per unit leaf nitrogen) than fringe trees. The CO₂ assimilation rates of +N trees were similar to those in other treatments, but were achieved at lower transpiration rates, stomatal conductance and intercellular CO₂ concentrations. Maintaining comparable assimilation rates at lower stomatal conductance requires greater ribulose 1·5‐bisphosphate carboxylase/oxygenase activity, consistent with greater N content per unit leaf area in +N trees. Hence, +N enhanced WUE at the expense of NUE. Instantaneous WUE estimates were supported by less negative foliar δ¹³C values for +N trees and scrub control trees. Thus, nutrient enrichment may alter the structure and function of mangrove forests along salinity gradients.     

Changes in Photosynthetic Capacity in Rice Leaves from Emergence through Senescence. Analysis from Ribulose-1,5-bisphosphate Carboxylase and Leaf Conductance

Changes in the rates of gas exchange and the amount of ribulose1,5-bisphosphate (RuBP) carboxylase protein were determinedin the 12th leaf blades of rice during the reproductive stages.RuBP carboxylase exhibited a large change similar to that inthe assimilation rate at 2% O₂ throughout the leaf's life, butits decrease during senescence was barely faster than the decreasein the assimilation rate. Consequently, the overall relationshipwas slightly curvilinear. By contrast, leaf conductance decreasedmore slowly than the assimilation rate which resulted in theintercellular CO₂ concentration increasing during senescence. In order to determine the maximum activity of RuBP carboxylaseat the intercellular CO₂ concentration, the kinetic parametersand their pH response were determined using purified, and completelyactivated, rice RuBP carboxylase. The maximum carboxylase activityat the intercellular CO₂ concentration was linearly correlatedwith the assimilation rate at 2% O₂ (r=0.989), and was veryclose to that needed to account for the assimilation rate. We conclude that changes in both the amount of RuBP carboxylaseprotein and leaf conductance reflect the change in photosynthesisduring the life span of the leaf. (Received November 26, 1983; Accepted February 20, 1984)     

Differences between the flag leaf and the ear of a spring wheat cultivar (Triticum aestivum cv. Arkas) with respect to the CO2 response of assimilation, respiration and stomatal conductance

The CO₂- and H₂O-exchanges in the flag leaf and the ear of a spring wheat cultivar (Triticum aestivum L. cv. Arkas) were measured at CO₂ partial pressures, p_i(CO₂), between 8 and 400 Pa under high photosynthetic photon flux densities (2000 μmol m^?2 s^?1). The experiments were carried out on each organ separately while attached to the intact plant, from the time of ear emergence through senescence. To study the contribution of the kernels to the gas exchange of ears, experiments were also carried out on sterilized ears (treatment A), and on ears from which the kernels were removed (treatment B). Flag leaves and ears differed considerably with regard to CO₂-dependence of assimilation, response of stomata to varying p_a(CO₂), CO₂ compensation point (and its temperature dependence), dark respiration, and dissimilation in the light (i.e. CO₂ production which is not due to oxygenation of ribulose 1,5-bisphosphate). The higher dark respiration of the ear originated mainly from the kernels and continued to some extent in the light. Thus, the CO₂ compensation point was attained at higher CO₂ partial pressures for the ear than for the flag leaf. The CO₂ uptake of the ear was not saturated at intercellular CO₂ partial pressures below 180 Pa CO₂, while that of the flag leaf reached saturation at about 80 Pa CO₂. CO₂-saturated rates of CO₂ uptake were 2.5 and 1.5 times the rates at natural CO₂ partial pressure for ear and flag leaf, respectively. The stomatal conductance decreased with rising CO₂ partial pressure above 35 Pa, in a more pronounced manner for the flag leaf than for the ear.     

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m^?2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO₂ concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO₂-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO₂ assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO₂ assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO₂ assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO₂ assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO₂ assimilation.     

Effect of soil moisture on leaf ecophysiology of <Emphasis Type="Italic">Parasenecio yatabei</Emphasis>, a summer-green herb in a cool–temperate forest understory in Japan

Leaf physiological and gas-exchange traits of a summer-green herbaceous perennial, Parasenecio yatabei, growing along a stream were examined in relation to leaf age. In its vegetative phase, the aerial part of this plant consists of only one leaf and provides an ideal system for the study of leaf longevity. Volumetric soil water content (SWC) decreased with increasing distance from the stream, whereas relative light intensity was nearly constant. The light-saturated net CO₂ assimilation rate (A _sat) and leaf stomatal conductance (gs) were approximately 1.5-fold and 1.4-fold higher, respectively, in the lower slope near the mountain stream than in the upper slope far from the mountain stream. The lifespan of aerial parts of vegetative plants significantly increased with decreasing SWC. The leaf mass-based nitrogen content of the leaves (N _mass) was almost constant (ca. 2.2%); however, the maximum carboxylation rate by ribulose-1,5-biphosphate carboxylase/oxygenase (rubisco) (V _cmax) and photosynthetic nitrogen use efficiency (PNUE, A _sat/N _area) decreased more slowly in the upper slope than in the lower slope. The higher leaf photosynthetic activity of P. yatabei plants growing lower on the slope leads to a decrease in V _cmax and PNUE in the early growing season, and to a shorter leaf lifespan.     

Acclimation of CO(2) Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO₂ assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO₂ assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO₂ partial pressure. The slopes of net CO₂ assimilation versus intercellular CO₂ partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of −0.3 and −0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at −0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately −0.5 megapascal. The accumulation of Cl⁻ and Na⁺ in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO₂ partial pressure, the declined inhibition in CO₂ assimilation of stress-acclimated plants was found for both salinity and water stress.     

Temperature response of carbon isotope discrimination and mesophyll conductance in tobacco

The partial pressure of CO₂ at the sites of carboxylation within chloroplasts depends on the conductance to CO₂ diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (g_m). We investigated the temperature response of g_m in tobacco (Nicotiana tabacum) by combining gas exchange in high light, ambient CO₂ in either 2 or 21% O₂ with carbon isotope measurements using tuneable diode laser spectroscopy. The g_m increased linearly with temperature in 2 or 21% O₂. In 21% O₂, isotope discrimination associated with g_m decreased from 5.0 ± 0.2 to 1.8 ± 0.2‰ as temperature increased from 15 to 40 °C, but the photorespiratory contribution to the isotopic signal is significant. While the fractionation factor for photorespiration (f = 16.2 ± 0.7‰) was independent of temperature between 20 and 35 °C, discrimination associated with photorespiration increased from 1.1 ± 0.01 to 2.7 ± 0.02‰ from 15 to 40 °C. Other mitochondrial respiration contributed around 0.2 ± 0.03‰. The drawdown in CO₂ partial pressure from ambient air to intercellular airspaces was nearly independent of leaf temperature. By contrast, the increase in g_m with increasing leaf temperature resulted in the drawdown in CO₂ partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.