Relationship between stomatal conductance and light intensity in leaves of Zea mays L., derived from experiments using the mesophyll as shade

Attached leaves of Zea mays were illuminated with monochromatic light, with either the upper or the lower epidermis facing the light source. The mesophyll absorbed between 99.5 and 99.6% of the red or blue light used. An inversion of the light direction therefore caused a 200- to 250-fold change in the quantum flux into each epidermis. This variation in quantum flux did not affect stomatal conductance. Stomatal conductance was however correlated with intercellular CO₂ concentration, c_i, and the relationship between stomatal conductance and c_i appeared also to remain the same if changes in c_i were brought about by changes in atmospheric CO₂ concentration instead of light. A close inspection of the data showed that stomata of the upper (adaxial) epidermis exhibited a small increase in conductance (<0.1 cm s^-1) in response to blue light that was superimposed on the dominating response to c_i.     

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     

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     

Light effects on leaf development and photosynthetic capacity of Hydrocotyle bonariensis Lam.

Rates of net CO₂ uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m^-2 day^-1 PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m^-2 day^-1). Maximum net CO₂ uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO₂ diffusion conductances and the mesophyll surface available for CO₂ absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m^-2 day^-1) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m^-2 day^-1).     

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.     

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     

On the relationship between leaf anatomy and CO2 diffusion through the mesophyll of hypostomatous leaves

Internal conductances to CO₂ transfer from the stomatal cavity to sites of carboxylation (g_i) in hypostomatous sun-and shade-grown leaves of citrus, peach and Macadamia trees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m^?2) for all leaves, including sclerophyllous Macadamia, whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high g_i, the chloro-plast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a), the lowest tissue density (T_d) and the highest chloro-plast surface area (S_c) exposed to ias. There were negative correlations between g_i and leaf thickness or D/a, but positive correlations between g_i and S_c or S_c/T_d. We developed a one-dimensional diffusion model which partitioned g_i into a gaseous diffusion conductance through the ias (g_ias) plus a liquid-phase conductance through mesophyll cell walls (g_cw). The model accounted for a significant amount of variation (r₂=0.80) in measured g_i by incorporating both components. The g_ias component was related to the one-dimensional path-length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The g_cw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between g_i and leaf thickness or D/a related to g_ias whereas positive correlations between g_i and S_c or S_c/T_d, related to g_cw. The g_cw was consistently lower than g_ias, and thus represented a greater constraint on CO₂ diffusion in the mesophylls of these hypostomatous species.     

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

Leaves of twelve C₃ species and six C₄ species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO₂ diffusion conductance expressed per unit mesophyll cell surface area (g_CO2^cell) cell was determined using measurements of the net rate of CO₂ uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A^mes/A). A^mes/A averaged 31 for the C₃ species and 16 for the C₄ species. For the C₃ species g_CO2^cell ranged from 0.12 to 0.32 mm s^-1, and for the C₄ species it ranged from 0.55 to 1.5 mm s^-1, exceeding a previously predicted maximum of 0.5 mm s^-1. Although the C₃ species Cammissonia claviformis did not have the highest g_CO2^cell, the combination of the highest A^mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO₂ uptake in low oxygen, 93 μmol m^-2 s^-1 (147 mg dm^-2 h^-1). The high g_CO2^cell of the C₄ species Amaranthus retroflexus (1.5 mm s^-1) was in part attributable to its thin cell wall (72 nm thick).     

Effects of salt stress on the growth,ion content,stomatal behaviour and photosynthetic capacity of a salt-sensitive species,Phaseolus vulgaris L.

Phaseolus vulgaris (cv. Hawkesbury Wonder) was grown over a range of NaCl concentrations (0–150 mM), and the effects on growth, ion relations and photosynthetic performance were examined. Dry and fresh weight decreased with increasing external NaCl concentration while the root/shoot ratio increased. The Cl^- concentration of leaf tissue increased linearly with increasing external NaCl concentration, as did K⁺ concentration, although to a lesser degree. Increases in leaf Na⁺ concentration occurred only at the higher external NaCl concentrations (100 mM). Increases in leaf Cl^- were primarily balanced by increases in K⁺ and Na⁺. X-ray microanalysis of leaf cells from salinized plants showed that Cl^- concentration was high in both the cell vacuole and chloroplast-cytoplasm (250–300 mM in both compartments for the most stressed plants), indicating a lack of effective intracellular ion compartmentation in this species. Salinity had little effect on the total nitrogen and ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) content per unit leaf area. Chlorophyll per unit leaf area was reduced considerably by salt stress, however. Stomatal conductance declined substantially with salt stress such that the intercellular CO₂ concentration (C _i) was reduced by up to 30%. Salinization of plants was found to alter the ¹³C value of leaves of Phaseolus by up to 5 and this change agreed quantitatively with that predicted by the theory relating carbon-isotope fractionation to the corresponding measured intercellular CO₂ concentration. Salt stress also brought about a reduction in photosynthetic CO₂ fixation independent of altered diffusional limitations. The initial slope of the photosynthesis versus C _i response declined with salinity stress, indicating that the apparent in-vivo activity of RuBP carboxylase was decreased by up to 40% at high leaf Cl^- concentrations. The quantum yield for net CO₂ uptake was also reduced by salt stress.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - RuBP ribulose-1,5-bisphosphate - ¹³C ratio of ¹³C to ¹²C relative to standard limestone     

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     

Gas exchange and canopy structure of 9-year-old loblolly pine,pitch pine and pitch x loblolly hybrids

Summary Seasonal gas exchange and canopy structure were compared among 9-year-old loblolly pine (Pinus taeda L.), pitch pine (Pinus rigida Mill.), and pitch x loblolly hybrids (Pinus rigida x taeda) growing in an F2 plantation located in Critz, Va., USA. Leaf net photosynthesis, conductance, internal CO₂ concentration (c_i), water use efficiency (WUE; photosynthesis/conductance), dark respiration and the ratio of net photosynthesis/respiration did not vary among or within the three taxa. Significant differences in volume production, crown length, total crown leaf surface area and the silhouette area of shade shoots among the taxa were observed. The loblolly-South Carolina source had greater volume and crown surface area than the pitch pine, and the hybrid taxa were intermediate between the two. Although the silhouette area ratio of shade foliage varied among taxa, it was not related to volume. A strong relationship between total leaf surface area and volume was observed. Leaf conductance, c_i, WUE and leaf water potential were the physiological parameters significantly and positively correlated with volume. This study suggests that the amount of needle surface in the canopy is more important in early stand volume growth than the leaf carbon exchange rate and the degree of needle self-shading in the lower canopy.     

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     

Leaf expansion and carbon assimilation in cotton leaves grown at two photosynthetic photon flux densities

Increasing photosynthetic photon flux density (PPFD) received during development from 5.5 to 31.2 mol m^-2 d^-1 resulted in greater leaf and mesophyll cell surface areas in cotton (Gossypium hirsutum L.). The relationships between the amounts of these surface areas and potential CO₂ assimilation by these leaves were evaluated. Leaf area (epidermal surface area of one side of a leaf), mesophyll cell surface area, and net rate of CO₂ uptake (P_n) were measured from the time leaves first unfolded until P., was substantially reduced. At the higher PPFD, leaf and mesophyll surface areas increased more rapidly during expansion, and P_n per unit leaf area was greater than at the lower PPFD. Although leaves at the higher PPFD reached the maximum P., per unit mesophyll cell surface area 4 to 5 days earlier than leaves at the lower PPFD, the maxima for these P., were similar. Leaves grown at the higher PPFD had the potential to assimilate 2.2, 3.5, or 5.8 times the amount of CO₂ as leaves from the lower PPFD when P., was expressed per unit mesophyll surface, per unit leaf surface, or per whole leaf, respectively. Greater and earlier development of both P., and mesophyll cell surface area at higher PPFD apparently had a compounding effect on the potential for carbon assimilation by a leaf.     

Diffusional conductance to CO2 is the key limitation to photosynthesis in salt‐stressed leaves of rice (Oryza sativa)

Salinity significantly limits leaf photosynthesis but the factors causing the limitation in salt‐stressed leaves remain unclear. In the present work, photosynthetic and biochemical traits were investigated in four rice genotypes under two NaCl concentration (0 and 150 mM) to assess the stomatal, mesophyll and biochemical contributions to reduced photosynthetic rate (A) in salt‐stressed leaves. Our results indicated that salinity led to a decrease in A, leaf osmotic potential, electron transport rate and CO₂ concentrations in the chloroplasts (C_c) of rice leaves. Decreased A in salt‐stressed leaves was mainly attributable to low C_c, which was determined by stomatal and mesophyll conductance. The increased stomatal limitation was mainly related to the low leaf osmotic potential caused by soil salinity. However, the increased mesophyll limitation in salt‐stressed leaves was related to both osmotic stress and ion stress. These findings highlight the importance of considering mesophyll conductance when developing salinity‐tolerant rice cultivars.     

Gas exchange of ears of cereals in response to carbon dioxide and light

One cultivar each of spring wheat (Triticum aestivum L. cv. Arkas), oat (Avena sativa L. cv. Lorenz), and barley (Hordeum vulgare L. cv. Aramir) was chosen in order to study the relative contributions of individual bracts to the gas exchange of whole ears. The distribution and frequency of the stomata on the bracts were examined. Gas exchange was measured at normal atmospheric CO₂ (330 bar) and at high CO₂ (2000 bar) on intact ears and on ears from which glumes or lemmas and pleae (wheat and oat) or awns (barley) had been removed.The relative contribution to the gas exchange of the whole organ is highest for the awns of barley ears. In wheat, the contribution of the glumes is slightly higher than that of the inner bracts before anthesis. Two weeks after anthesis the inner bracts contribute more than the glumes. This tendency of increasing importance of the inner bracts is also found in oat ears, but the relative amount of CO₂ uptake by the glumes is higher than in wheat. These changes during ontogeny result from the better supply of light to the inner bracts caused by opening of the ears' structures during grain filling, which in part compensates for the decreasing photosynthetic capacity.The ratio of the photosynthesis rate at high CO₂ to that at normal CO₂ is lower for the glumes of oat and for the awns of barley than for the other bracts.Abbreviations A³³⁰, A²⁰⁰⁰ net photosynthesis rate, A³³⁰ at normal atmospheric CO₂ (330 bar), A²⁰⁰⁰ at high CO₂ (2000 bar) - PPFD photosynthetic photon flux density - p^c intercellular partial pressure of CO₂     

Leaf and canopy photosynthetic CO2 uptake of a stand of Echinochloa polystachya on the Central Amazon floodplain

The C₄ grass Echinochloa polystachya, which forms dense and extensive monotypic stands on the Varzea floodplains of the Amazon region, provides the most productive natural higher plant communities known. The seasonal cycle of growth of this plant is closely linked to the annual rise and fall of water level over the floodplain surface. Diurnal cycles of leaf photosynthesis and transpiration were measured at monthly intervals, in parallel with measurements of leaf area index, canopy light interception and biomass. By artificial manipulation of the light flux incident on leaves in the field light-response curves of photosynthesis at the top and near to the base of the canopy were generated. Fitted light-response curves of CO₂ uptake were combined with information of leaf area index, incident light and light penetration of the canopy to estimate canopy rates of photosynthesis. Throughout the period in which the floodplains were submerged photosynthetic rates of CO₂ uptake (A) for the emergent leaves were high with a mean of c. 30 mol m^-2 s^-1 at mid-day and occasional values of 40 mol m^-2 s^-1. During the brief dry phase, when the floodplain surface is uncovered, there was a significant depression of A, with mid-day mean values of c. 17 mol m^-2 s^-1. This corresponded with a c. 50% decrease in stomatal conductance, and a c. 35% depression in the ratio of the leaf inter-cellular to external CO₂ concentration (c _i/c _a). During the dry phase, a midday depression of rates of CO₂ assimilation was observed. The lowest leaf area index (F) was c. 2 in November–December, when the flood plain was dry, and again in May, when the rising floodwaters were submerging leaves faster than they were replaced. The maximum F of c. 5 was in August when the floodwaters were receding rapidly. Canopy light interception efficiency varied from 0.90 to 0.98. Calculated rates of canopy photosynthesis exceeded 18 mol C m^-2 mo^-1 throughout the period of flooding, with a peak of 37 mol C m^-2 mo^-1 in August, but declined to 13 mol C m^-2 mo^-1 in November during the dry phase. Estimated uptake of carbon by the canopy from the atmosphere, over 12 months, was 3.57 kg C m^-2. This was insufficient to account for the 3.99 kg C m^-2 of net primary production, measured simultaneously by destructive harvesting. It is postulated that this discrepancy might be accounted for by internal diffusion of CO₂ from the CO₂-rich waters and sediments via the roots and stems to the sites of assimilation in the leaves.     

Growth of cotton under continuous salinity stress: influence on allocation pattern,stomatal and non-stomatal components of photosynthesis and dissipation of excess light energy

Cotton (Gossypium hirsutum L.) plants were grown in flowing-culture solutions containing 0%, 26% and 55% natural seawater under controlled and otherwise identical conditions. Leaf Na⁺ content rose to 360 mM in 55% seawater, yet the K⁺ content was maintained above 100 mM. The K⁺/Na⁺ selectivity ratio was much greater in the saline plants than in the control plants. All plants were healthy and able to complete the life cycle but relative growth rate fell by 46% in 26% seawater and by 83% in 55% seawater. Much of this reduction in growth was caused by a decreased allocation of carbon to leaf growth versus root growth. The ratio of leaf area/plant dry weight fell by 32% in 26% seawater and by 50% in 55 % seawater while the rate of carbon gain per unit leaf area fell by only 20% in 26% seawater and by as much as 66% in 55% seawater. Partial stomatal closure accounted for nearly all of the fall in the photosynthesis rate in 26% seawater but in 55% seawater much of the fall also can be attributed to non-stomatal factors. As a result of the greater effect of salinity on stomatal conductance than on CO₂-uptake rate, photosynthetic water-use efficiency was markedly improved by salinity. This was also confirmed by stablecarbon-isotope analyses of leaf sugar and of leaf cellulose and starch. — Although non-stomatal photosynthetic capacity at the growth light was reduced by as much as 42% in 55% seawater, no effects were detected on the intrinsic photon yield of photosynthesis nor on the efficiency of photosystem II photochemistry, chlorophyll a/b ratio, carotenoid composition or the operation of the xanthophyll cycle. Whereas salinity caused in increase in mesophyll thickness and content of chloroplast pigments it caused a decrease in total leaf nitrogen content. The results indicate that the salinity-induced reduction in non-stomatal photosynthetic capacity was not caused by any detrimental effect on the photosynthetic apparatus but reflects a decreased allocation to enzymes of carbon fixation. — Rates of energy dissipation via CO₂ fixation and photorespiration, calculated from gas-exchange measurements, were insufficient to balance the rate of light-energy absorption at the growth light. Salinity therefore would have been expected to cause the excess excitation energy to rise, leading to an increased nonradiative dissipation in the pigment bed and resulting increases in non-photochemical fluorescence quenching and zeaxanthin formation. However, no such changes could be detected, implying that salinity may have increased energy dissipation via a yet unidentified energy-consuming process. This lack of a response to salinity stress is in contrast to the responses elicited by short-term water stress which caused strong non-photochemical quenching and massive zeaxanthin formation.Abbreviations and Symbols A net rate of CO₂ uptake - A_c calculated rate of CO₂ uptake at constant p_i - Chl chlorophyll - E rate of transpiration - EPS epoxidation state of xanthophyll cycle components - F, F_m fluorescence emission at the actual, full reduction of PSII reaction centers - F_v variable fluorescence - g_s stomatal conductance to water vapor - g_w conductance to CO₂transfer from intercellular spaces to chloroplasts - NPQ non-photochemical fluorescence quenching - p_a, p_i, p_c atmospheric, intercellular and chloroplastic partial pressures of CO₂ - PCRO photosynthetic carbon reduction and oxygenation cycle sum of the rate of carboxylation and oxygenation - PFD photon flux density - PSII photosystem II - V+A+Z pool size of xanthophyll cycle components - ¹³C carbon-isotope composition - (_PSII) photon yield of PSII photochemistry at the actual reduction state in the light * C.I.W.-D.P.B. Publication No. 1115, CNR-RAISA paper No. XXXWe thank Connie Shih for skilful assistance in growing plants and for conducting HPLC analyses and Barbara Mortimer for conducting the nitrogen analyses. Thanks are also due to C. Barry Osmond (now, Australian National University, Research School of Biological Sciences, Canberra) and Larry Giles of the Department of Botany, Duke University, Durham, N.C., for conducting carbonisotope analysis. E.B. was partially supported by the National Research Council of Italy, Special Project RAISA, Sub-Project No. 2. A Carnegie Institution Fellowship to E.B. is also gratefully acknowledged. This work was supported by grant No. 89-37-280-4902 of the Competitive Grant Program of the U.S. Department of Agriculture to O.B.     

Correlation of Stomatal Conductance with Photosynthetic Capacity of Cotton Only in a CO2-Enriched Atmosphere: Mediation by Abscisic Acid?

Some evidence indicates that photosynthetic rate (A) and stomatal conductance (g) of leaves are correlated across diverse environments. The correlation between A and g has led to the postulation of a “messenger” from the mesophyll that directs stomatal behavior. Because A is a function of intercellular CO₂ concentration (c_i), which is in turn a function of g, such a correlation may be partially mediated by c_i if g is to some degree an independent variable. Among individual sunlit leaves in a cotton (Gossypium hirsutum L.) canopy in the field, A was significantly correlated with g (r² = 0.41, n = 63). The relative photosynthetic capacity of each leaf was calculated as a measure of mesophyll properties independent of c_i. This approach revealed that, in the absence of c_i effects, mesophyll photosynthetic capacity was unrelated to g (r² = 0.06). When plants were grown in an atmosphere enriched to about 650 microliters per liter of CO₂, however, photosynthetic capacity remained strongly correlated with g even though the procedure discounted any effect of variable c_i. This “residual” correlation implies the existence of a messenger in CO₂-enriched plants. Enriched CO₂ also greatly increased stomatal response to abscisic acid (ABA) injected into intact leaves. The data provide no evidence for a messenger to coordinate g with A at ambient levels of CO₂. In a CO₂-enriched atmosphere, though, ABA may function as such a messenger because the sensitivity of the system to ABA is enhanced.     

CO2 transfer conductance, leaf structure and carbon isotope composition of Polygonum cuspidatum leaves from low and high altitudes

Anatomy and some physiological characteristics of the leaves in Polygonum cuspidatum Sieb. et Zucc., a dioecious clonal herb, were compared between two populations, one from a lowland in Shizuoka City (10 m above sea level), and another from a highland on Mt. Fuji (2500 m above sea level). Leaf mass per area (LMA) of the highland plants was about twice that of the lowland plants. The greater leaf thickness, thicker mesophyll cell walls and higher mesophyll cell density in the highland leaves contributed to the larger LMA. Although mesophyll area exposed to intercellular airspaces was greater in the highland leaves than in the lowland leaves by 30%, the surface area of chloroplasts facing intercellular airspaces was similar between these leaves. CO₂ transfer conductance inside the leaf (g_i) of the highland leaves (0·75 μmol m^?2 s^?1 Pa^?1) is the lowest recorded for herbaceous plants and was only 40% of that in the lowland leaves. On the other hand, the difference in stomatal conductance was small. δ¹³C values in the leaf dry matter were greater in the highland leaves by 4‰. These data and the estimation of CO₂ partial pressures in the intercellular air spaces and in the chloroplast suggested that the greater dry matter δ¹³C in the highland leaves, indicative of lower long‐term ratio of the chloroplast stroma to the ambient CO₂ partial pressures, would be mainly attributed to their lower g_i.     

Effect of increased salinity on CO2 assimilation,O2 evolution and the δ13C values of leaves of Plantago maritima L. developed at low and high NaCl levels

The effect of increased salinity on photosynthesis was studied in leaves of Plantago maritima L. that developed while plants were at low and high NaCl levels. In leaves that developed while plants were grown at 50 mol·m^-3, exposure to 200 and 350 mol·m^-3 NaCl resulted in reductions in net CO₂ assimilation and stomatal conductance. The decline in CO₂ assimilation in plants at 200 and 350 mol·m^-3 NaCl occurred almost exclusively at high intercellular CO₂ concentrations. The initial slope of the CO₂ assimilation-intercellular CO₂ (A-C _i) curve, determined after salinity was increased, was identical or very similar to that measured initially. In contrast to the reductions observed in CO₂ assimilation, there were no significant differences in O₂ evolution rates measured at 5% CO₂ among leaves from plants exposed to higher salinity and plants remaining at low salinity.Leaves that developed while plants were at increased salinity levels also had significantly lower net CO₂ assimilation rates than plants remaining at 50 mol·m^-3 NaCl. The lower CO₂ assimilation rates in plants grown at 200 and 350 mol·m^-3 NaCl were a result of reduced stomatal conductance and low intercellular CO₂ concentration. There were no significant differences among treatments for O₂ evolution rates measured at high CO₂ levels. The increased stomatal limitation of photosynthesis was confirmed by measurements of the ¹³C/¹²C composition of leaf tissue. Water-use efficiency was increased in the plants grown at high salinity.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - ¹³C isotopic ratio (¹³C/¹²C) expressed relative to a standard - RuBP ribulose-1,5-bisphosphate