Photosynthetic and Stomatal Responses of the Grey Mangrove, Avicennia marina, to Transient Salinity Conditions

Measurements of gas exchange characteristics were made on intact, attached leaves of hydroponically grown seedlings of Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke as the NaCl concentration of the culture solution was varied by step changes of 50 millimolar NaCl every 2nd day from 50 to 500 to 50 millimolar NaCl. The CO₂ assimilation rate, stomatal conductance, intercellular CO₂ concentration, and evaporation rate decreased at salinities above 250 millimolar NaCl and recovered substantially upon return to the original salinity.

The assimilation rate was measured as a function of the intercellular CO₂ concentration [A(c_i) curve]. The lower linear portion of this curve was insensitive to variation in salinity, whereas the upper nonlinear portion declined with increasing salinity, indicating a reduction in the capacity for CO₂ assimilation which recovered upon return to the original salinity. Stomatal conductance changed such that the intercellular CO₂ concentration measured under normal atmospheric conditions occurred in the transition between the lower, linear and upper nonlinear portions of the A(c_i) curve. Thus, stomatal conductance and photosynthetic capacity together co-limited the assimilation rate. The changes in gas exchange characteristics were such that water loss was minimal relative to carbon gain.

     

Regulation of mesophyll photosynthesis in intact wheat leaves by cytoplasmic phosphate concentrations

The effect of D-(+)-mannose, inorganic phosphate (Pi) and mannose-6-phosphate on net mesophyll CO₂ assimilation rate (A) and stomatal conductance (g_s) of wheat (Triticum aestivum L.) leaves was studied. The compounds were supplied through the transpiration stream of detached leaves from plants grown in sand in growth cabinets or glasshouses, with different concentrations of Pi (0.25, 1.0 and 4.0 mM) supplied during growth. In all cases, 10 mM D-(+)mannose caused 40–60% reduction of A within 30 min, though the time courses differed for flag leaves and the sixth leaf on the mainstem of glasshouse- and cabinet-grown plants. D-(+)Mannose had a similar effect on A in leaves having a fourfold range in total phosphate content. Effects of D-(+)mannose in reducing g_s were always slower than on A. When the CO₂ concentration in the leaf chamber was adjusted to maintain intercellular CO₂ concentration (C_i) constant as A declined after mannose supply, g_s still declined indicating that stomatal closure was not caused by changing C_i. Supplying mannose-6-phosphate at 10 and 1 mM and Pi at 5 and 10 mM concentrations caused rapid reductions in g_s and also direct reductions in A. The observed effects of mannose and Pi on assimilation are consistent with the proposed regulatory role of cytoplasmic Pi in determining mesophyll carbon assimilation that has been derived previously using leaf discs, protoplasts and chloroplasts.Abbreviations and symbols A net mesophyll CO₂-assimilation rate - C_a, C_i external (assimilation-chamber) and intercellular CO₂ concentration, respectively - g_s stomatal conductance - Man6P mannose-6-phosphate - Pi orthophosphate     

Internal conductance to CO2 transfer of adult Fagus sylvatica: Variation between sun and shade leaves and due to free-air ozone fumigation

Two separate objectives were considered in this study. We examined (1) internal conductance to CO₂ (g_i) and photosynthetic limitations in sun and shade leaves of 60-year-old Fagus sylvatica, and (2) whether free-air ozone fumigation affects g_i and photosynthetic limitations. g_i and photosynthetic limitations were estimated in situ from simultaneous measurements of gas exchange and chlorophyll fluorescence on attached sun and shade leaves of F. sylvatica. Trees were exposed to ambient air (1× O₃) and air with twice the ambient ozone concentration (2× O₃) in a free-air ozone canopy fumigation system in southern Germany (Kranzberg Forest). g_i varied between 0.12 and 0.24 mol m⁻² s⁻¹ and decreased CO₂ concentrations from intercellular spaces (C_i) to chloroplastic (C_c) by approximately 55 μmol mol⁻¹. The maximum rate of carboxylation (V_cmax) was 22–39% lower when calculated on a C_i basis compared with a C_c basis. g_i was approximately twice as large in sun leaves compared to shade leaves. Relationships among net photosynthesis, stomatal conductance and g_i were very similar in sun and shade leaves. This proportional scaling meant that neither C_i nor C_c varied between sun and shade leaves. Rates of net photosynthesis and stomatal conductance were about 25% lower in the 2× O₃ treatment compared with 1× O₃, while V_cmax was unaffected. There was no evidence that g_i was affected by ozone.     

Mapping Intercellular CO2 Mole Fraction (Ci) in Rosa rubiginosa Leaves Fed with Abscisic Acid by Using Chlorophyll Fluorescence Imaging : Significance of Ci Estimated from Leaf Gas Exchange

Imaging of photochemical yield of photosystem II (PSII) computed from leaf chlorophyll fluorescence images and gas-exchange measurements were performed on Rosa rubiginosa leaflets during abscisic acid (ABA) addition. In air ABA induced a decrease of both the net CO₂ assimilation (A_n) and the stomatal water vapor conductance (g_s). After ABA treatment, imaging in transient nonphotorespiratory conditions (0.1% O₂) revealed a heterogeneous decrease of PSII photochemical yield. This decline was fully reversed by a transient high CO₂ concentration (7400 μmol mol⁻¹) in the leaf atmosphere. It was concluded that ABA primarily affected A_n by decreasing the CO₂ supply at ribulose-1,5-bisphosphate carboxylase/oxygenase. Therefore, the A_n versus intercellular mole fraction (C_i) relationship was assumed not to be affected by ABA, and images of C_i and g_s were constructed from images of PSII photochemical yield under nonphotorespiratory conditions. The distribution of g_s remained unimodal following ABA treatment. A comparison of calculations of C_i from images and gas exchange in ABA-treated leaves showed that the overestimation of C_i estimated from gas exchange was only partly due to heterogeneity. This overestimation was also attributed to the cuticular transpiration, which largely affects the calculation of the leaf conductance to CO₂, when leaf conductance to water is low.     

Limitation factors for photosynthesis in ‘Bluecrop’ highbush blueberry (Vaccinium corymbosum) leaves in response to moderate water stress

The levels of stomatal, mesophyll and biochemical limitations in CO₂ assimilation of ‘Bluecrop’ highbush blueberry leaves were compared at two different levels of leaf water potential. The leaf water potentials were ?1.49 and ?1.94 MPa in daily-irrigated (DI) and non-irrigated (NI) shrubs, respectively. The NI shrubs represented plants under moderate water stress. Mesophyll conductance (g _m) and chloroplastic CO₂ concentration (C _c) were estimated by combined measurements of gas exchange and chlorophyll fluorescence under various intercellular CO₂ concentrations (C _i). Net CO₂ assimilation rates (A _n) as a function of C _c were used for calculating maximum carboxylation efficiency (α _cmax) at the real sites of CO₂ assimilation. Maximum A _n (A _nmax) from the light response curves at 400 μmol mol^?1 air of ambient CO₂ concentration (C _a) were lower in the leaves of NI shrubs than in those of DI ones. However, electron transport rates were higher in the leaves of NI shrubs than in those of DI ones. The decrease in CO₂ assimilation following water stress may be caused by a decrease in g _m rather than a decrease in stomatal conductance (g _s) according to limitation analysis. Limitation rates by g _s, calculated at 400 μmol mol^?1 air of C _a in A _n-C _i curves, were not significantly different between the leaves of DI and NI shrubs. However, limitation rates by g _m from A _n-C _c curves were significantly higher in the leaves of NI shrubs than in those of DI ones. Maximum carboxylation efficiency (α _cmax) values calculated from the A _n-C _c curve, contrary to those calculated from the A _n-C _i curve, were higher in the leaves of NI shrubs than in those of DI ones. Consequently, mesophyll limitation than stomatal and biochemical limitations mainly down-regulated the photosynthesis in the leaves of ‘Bluecrop’ blueberry shrubs during moderate water stress.     

Photosynthetic Plasticity in Flaveria brownii: Growth Irradiance and the Expression of C(4) Photosynthesis

Photosynthesis was examined in leaves of Flaveria brownii A. M. Powell, grown under either 14% or 100% full sunlight. In leaves of high light grown plants, the CO₂ compensation point and the inhibition of photosynthesis by 21% O₂ were significantly lower, while activities of ribulose 1,5-bisphosphate carboxylase/oxygenase and various C₄ cycle enzymes were considerably higher than those in leaves grown in low light. Both the CO₂ compensation point and the degree of O₂ inhibition of apparent photosynthesis were relatively insensitive to the light intensity used during measurements with plants from either growth conditions. Partitioning of atmospheric CO₂ between Rubisco of the C₃ pathway and phosphoenolpyruvate carboxylase of the C₄ cycle was determined by exposing leaves to ¹⁴CO₂ for 3 to 16 seconds, and extrapolating the labeling curves of initial products to zero time. Results indicated that ~94% of the CO₂ was fixed by the C₄ cycle in high light grown plants, versus ~78% in low light grown plants. Thus, growth of F. brownii in high light increased the expressed level of C₄ photosynthesis. Consistent with the carbon partitioning patterns, photosynthetic enzyme activities (on a chlorophyll basis) in protoplasts from leaves of high light grown plants showed a more C₄-like pattern of compartmentation. Pyruvate, Pi dikinase and phosphoenolpyruvate carboxylase were more enriched in the mesophyll cells, while NADP-malic enzyme and ribulose 1,5-bisphosphate carboxylase/oxygenase were relatively more abundant in the bundle sheath cells of high light than of low light grown plants. Thus, these results indicate that F. brownii has plasticity in its utilization of different pathways of carbon assimilation, depending on the light conditions during growth.     

Intraspecific variation in photosynthetic responses of trebouxioid lichens with reference to the activity of a carbon-concentrating mechanism

The photosynthetic responses of a range of trebouxioid lichens were investigated to determine whether variations in net assimilation rates shown by populations of the same species collected from different habitats could be correlated with adjustments in carbon-concentrating mechanism (CCM) activity. The activity of a CCM was inferred from the high affinity for CO₂ [i.e. low CO₂ compensation point (Γ); low external CO₂ concentration at which half-maximal assimilation rates are reached (K _{0.5 CO2})], the release of a pool of accumulated dissolved inorganic carbon (C_i) during light/dark transient measurements of CO₂ exchange and values for carbon isotope discrimination intermediate between those characteristic of C₃ and C₄ terrestrial plants. Higher net and gross assimilation rates were expressed by lichens collected from shaded woodland habitats. The higher rates were not accounted for by variations in chlorophyll content. Lichens with high assimilation rates also showed an increased affinity for CO₂ as demonstrated by low CO₂ compensation points and K _0.5 values and the magnitude of the C_i pool accumulated upon illumination and released after darkening of the thalli. However, there was no correlation between assimilation rates and organic matter or instantaneous carbon isotope discrimination measurements, with the latter remaining roughly consistent whatever the provenance or species of the lichen material. The data are discussed with reference to significant environmental factors which are likely to control photosynthesis in the habitats studied. Received: 5 April 1997 / Accepted: 9 September 1997     

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     

Photosynthesis and photorespiration in a mutant of the cyanobacterium Synechocystis PCC 6803 lacking carboxysomes

A mutant of the cyanobacterium Synechocystis PCC 6803 was obtained by replacing the gene of the carboxylation enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) with that of the photosynthetic bacterium Rhodospirillum rubrum. This mutant consequently lacks carboxysomes — the protein complexes in which the original enzyme is packed. It is incapable of growing at atmospheric CO₂ levels and has an apparent photosynthetic affinity for inorganic carbon (C_i) which is 1000 times lower than that of the wild type, yet it accumulates more C_i than the wild type. The mutant appears to be defective in its ability to utilize the intracellular C_i pool for photosynthesis. Unlike the carboxysomal carboxylase activity of Rubisco, which is almost insensitive to inhibition by O₂ in vitro, the soluble enzyme is competitively inhibited by O₂. The photosynthetic rate and C_i compensation point of the wild type were hardly affected by low O₂ levels. Above 100 μM O₂, however, both parameters became inhibited. The CO₂ compensation point of the mutant was linearly dependent on O₂ concentration. The higher sensitivity of the mutant to O₂ inhibition than that expected from in-vitro kinetics parameters of Rubisco, indicates a low capacity to recycle photorespiratory metabolites to Calvin-cycle intermediates.     

Reduction of ribulose-1,5-bisphosphate carboxylase/oxygenase content by antisense RNA reduces photosynthesis in transgenic tobacco plants

A complementary DNA for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was cloned from tobacco (Nicotiana tabacum) and fused in the antisense orientation to the cauliflower mosaic virus 35S promoter. This antisense gene was introduced into the tobacco genome, and the resulting transgenic plants were analyzed to assess the effect of the antisense RNA on Rubisco activity and photosynthesis. The mean content of extractable Rubisco activity from the leaves of 10 antisense plants was 18% of the mean level of activity of control plants. The soluble protein content of the leaves of anti-small subunit plants was reduced by the amount equivalent to the reduction in Rubisco. There was little change in phosphoribulokinase activity, electron transport, and chlorophyll content, indicating that the loss of Rubisco did not affect these other components of photosynthesis. However, there was a significant reduction in carbonic anhydrase activity. The rate of CO₂ assimilation measured at 1000 micromoles quanta per square meter per second, 350 microbars CO₂, and 25°C was reduced by 63% (mean value) in the antisense plants and was limited by Rubisco activity over a wide range of intercellular CO₂ partial pressures (p_i). In control leaves, Rubisco activity only limited the rate of CO₂ assimilation below a p_i of 400 microbars. Despite the decrease in photosynthesis, there was no reduction in stomatal conductance in the antisense plants, and the stomata still responded to changes in p_i. The unchanged conductance and lower CO₂ assimilation resulted in a higher p_i, which was reflected in greater carbon isotope discrimination in the leaves of the antisense plants. These results suggest that stomatal function is independent of total leaf Rubisco activity.     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

Limitation of net CO2 assimilation rate by internal resistances to CO2 transfer in the leaves of two tree species (Fagus sylvatica L. and Castanea sativa Mill.)

Using a combination of gas-exchange and chlorophyll fluorescence measurements, low apparent CO₂/O₂ specificity factors (1300 mol mol_?1) were estimated for the leaves of two deciduous tree species (Fagus sylvatica and Castanea sativa). These low values contrasted with those estimated for two herbaceous species and were ascribed to a drop in the CO₂ mole fraction between the intercellular airspace (C_i) and the catalytic site of Rubisco (C_c) due to internal resistances to CO₂ transfer. C_c. was calculated assuming a specificity of Rubisco value of 2560 mol mol^?1. The drop between C_i and C_c was used to calculate the internal conductance for CO₂ (g_i). A good correlation between mean values of net CO₂ assimilation rate (A) and g_i was observed within a set of data obtained using 13 woody plant species, including our own data. We report that the relative limitation of A, which can be ascribed to internal resistances to CO₂ transfer, was 24–30%. High internal resistances to CO₂ transfer may explain the low apparent maximal rates of carboxylation and electron transport of some woody plant species calculated from A/C_i curves.     

Regulation of Photosynthesis in Triazine-Resistant and -Susceptible Brassica napus

The response of photosynthetic carbon assimilation and chlorophyll fluorescence quenching to changes in intercellular CO₂ partial pressure (C_i), O₂ partial pressure, and leaf temperature (15-35°C) in triazine-resistant and -susceptible biotypes of Brassica napus were examined to determine the effects of the changes in the resistant biotype on the overall process of photosynthesis in intact leaves. Three categories of photosynthetic regulation were observed. The first category of photosynthetic response, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis, was observed at 15, 25, and 35°C leaf temperatures with low C_i. When the carbon assimilation rate was Rubisco-limited, there was little difference between the resistant and susceptible biotypes, and Rubisco activity parameters were similar between the two biotypes. A second category, called feedback-limited photosynthesis, was evident at 15 and 25°C above 300 microbars C_i. The third category, photosynthetic electron transport-limited photosynthesis, was evident at 25 and 35°C at moderate to high CO₂. At low temperature, when the response curves of carbon assimilation to C_i indicated little or no electron transport limitation, the carbon assimilation rate was similar in the resistant and susceptible biotypes. With increasing temperature, more electron transport-limited carbon assimilation was observed, and a greater difference between resistant and susceptible biotypes was observed. These observations reveal the increasing importance of photosynthetic electron transport in controlling the overall rate of photosynthesis in the resistant biotype as temperature increases. Photochemical quenching of chlorophyll fluorescence (q_P) in the resistant biotype never exceeded 60%, and triazine resistance effects were more evident when the susceptible biotype had greater than 60% q_P, but not when it had less than 60% q_P.     

Mechanism of Photosynthesis Decrease by Verticillium dahliae in Potato

Young, visually symptomless leaves from potato (Solanum tuberosum) plants infected with Verticillium dahliae exhibited reduced carbon assimilation rate, stomatal conductance, and intercellular CO₂, but no increase in dark respiration, no change in the relationship between carbon assimilation rate versus intercellular CO₂, and no change in light use efficiency when intercellular CO₂ was held constant. Therefore, the initial decrease in photosynthesis caused by V. dahliae was caused by stomatal closure. Errors in the intercellular CO₂ calculation caused by uneven distribution of carbon assimilation rate across the leaf were tested by ¹⁴CO₂ autoradiography. Patchiness was found at a low frequency. Low stomatal conductance was correlated with low leaf water potentials. Infection did not affect leaf osmotic potentials.     

Mechanism of c(4) photosynthesis: a model describing the inorganic carbon pool in bundle sheath cells

A theoretical model of the composition of the inorganic carbon pool generated in C₄ leaves during steady-state photosynthesis was derived. This model gives the concentrations of CO₂ and O₂ in the bundle sheath cells for any given net photosynthesis rate and inorganic carbon pool size. The model predicts a bundle sheath CO₂ concentration of 70 micromolar during steady state photosynthesis in a typical C₄ plant, and that about 13% of the inorganic carbon generated in bundle sheath cells would leak back to the mesophyll cells, predominantly as CO₂. Under these circumstances the flux of carbon through the C₄ acid cycle would have to exceed the net rate of CO₂ assimilation by 15.5%. With the calculated O₂ concentration of 0.44 millimolar, the potential photorespiratory CO₂ loss in bundle sheath cells would be about 3% of CO₂ assimilation. Among the factors having a critical influence on the above values are the permeability of bundle sheath chloroplasts to HCO₃⁻, the activity of carbonic anhydrase within these chloroplasts, the assumed stromal volume, and the permeability coefficients for CO₂ and O₂ diffusion across the interface between bundle sheath and mesophyll cells. The model suggests that as the net photosynthesis rate changes in C₄ plants, the level and distribution of the components of the inorganic carbon pool change in such a way that C₄ acid overcycling is maintained in an approximately constant ratio with respect to the net photosynthesis rate.     

The photosynthesis of young Panicum C4 leaves is not C3-like

Evidence is presented contrary to the suggestion that C₄ plants grow larger at elevated CO₂ because the C₄ pathway of young C₄ leaves has C₃-like characteristics, making their photosynthesis O₂ sensitive and responsive to high CO₂. We combined PAM fluorescence with gas exchange measurements to examine the O₂ dependence of photosynthesis in young and mature leaves of Panicum antidotale (C₄, NADP-ME) and P. coloratum (C₄, NAD-ME), at an intercellular CO₂ concentration of 5 Pa. P. laxum (C₃) was used for comparison. The young C₄ leaves had CO₂ and light response curves typical of C₄ photosynthesis. When the O₂ concentration was gradually increased between 2 and 40%, CO₂ assimilation rates (A) of both mature and young C₄ leaves were little affected, while the ratio of the quantum yield of photosystem II to that of CO₂ assimilation (Φ_PSII/Φ_CO2) increased more in young (up to 31%) than mature (up to 10%) C₄ leaves. A of C₃ leaves decreased by 1·3 and Φ_PSII/Φ_CO2 increased by 9-fold, over the same range of O₂ concentrations. Larger increases in electron transport requirements in young, relative to mature, C₄ leaves at low CO₂ are indicative of greater O₂ sensitivity of photorespiration. Photosynthesis modelling showed that young C₄ leaves have lower bundle sheath CO₂ concentration, brought about by higher bundle sheath conductance relative to the activity of the C₄ and C₃ cycles and/or lower ratio of activities of the C₄ to C₃ cycles.     

Leaf photosynthesis and conductance of selected triticum species at different water potentials

Leaf gas exchange characteristics of a desert annual (Triticum kotschyi [Boiss.] Bowden) and the wheat cultivar TAM W-101 (Triticum aestivum L. em Thell) were compared over a range of leaf water potentials from −0.50 to −2.9 megapascals. At an ambient [CO₂] of 330 microliters per liter, T. kotschyi had higher conductance and CO₂ assimilation (A) at a given water potential than T. aestivum. Under well watered conditions, A versus internal CO₂ concentration (C_i) response curves for both species were similar in shape and magnitude, and the higher A of T. kotschyi at an ambient [CO₂] of 330 microliters per liter was mostly related to the higher stomatal conductance of T. kotschyi. The higher conductance of T. kotschyi than T. aestivum under well watered conditions was associated with higher C_i and lower water use efficiency. Under water deficits, however, C_i at 330 microliters per liter ambient [CO₂] did not differ significantly between species. T. kotschyi had higher A under water deficits than T. aestivum primarily because its A versus C_i response curves had higher A at C_i values above about 150 microliters per liter. The results show that conductance played an important role in the high A of T. kotschyi under well watered conditions, but under water deficits the high A of T. kotschyi was related more to the maintenance of a higher capacity for mesophyll photosynthesis.     

Stomatal sensitivity to carbon dioxide and humidity: a comparison of two c(3) and two c(4) grass species

The sensitivity of stomatal conductance to changes of CO₂ concentration and leaf-air vapor pressure difference (VPD) was compared between two C₃ and two C₄ grass species. There was no evidence that stomata of the C₄ species were more sensitive to CO₂ than stomata of the C₃ species. The sensitivity of stomatal conductance to CO₂ change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO₂ concentration, C_i/C_a, was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, C_i/C_a was 0.7 and 0.5 (approximately) in C₃ and C₄ species, respectively. Transpiration efficiency (net CO₂ assimilation rate/transpiration rate) was larger in the C₄ species than in the C₃ species at current atmospheric CO₂ concentrations, but the relative increase due to high CO₂ was larger in the C₃ than in the C₄ species.     

Inhibition of inorganic carbon transport by oxygen in a high CO2-requiring mutant (E1) of Anacystis nidulans R2

The effect of O₂ on inorganic carbon (C_i) transport was studied with a high CO₂-requiring mutant (E₁) of Anacystis nidulans R₂. Oxygen (above 2%) inhibited C_i transport by 15–35|X% at CO₂ concentrations above 200 μl/l, but had no apparent effect at low, limiting CO₂ concentration. The action spectra for C_i transport measured in the presence or absence of 20% O₂ showed two peaks around 684 and 625 nm, corresponding to chlorophyll a and phycocyanin absorption, respectively. The difference between these two spectra (anaerobic minus aerobic) showed one peak around 625 nm, which indicates that a linear electron transport from water to O₂ is involved in the O₂ inhibition of C_i transport. Dithiothreitol could overcome the inhibition by O₂. The results suggested that the O₂ inhibition is a result of inactivation of the C_i-transporting system.     

Partitioning of CO2 Fixation in the Colonial Cyanobacterium Microcystis aeruginosa: Mechanism Promoting Formation of Surface Scums

Constraints on inorganic carbon (C_i) availability stimulated buoyancy in natural, photosynthetically active populations of the colonial blue-green alga (cyanobacterium) Microcystis aeruginosa. In nonmixed eutrophic river water and cultures, O₂ evolution determinations indicated C_i limitation of photosynthesis, which was overcome either by CO₂ additions to the aqueous phase or by exposure of buoyant colonies to atmospheric CO₂. Microautoradiographs of M. aeruginosa colonies revealed partitioning of ¹⁴CO₂ fixation and photosynthate accumulation between peripheral and internal cells, particularly in large colonies. When illuminated colonies were suspended in the aqueous phase, peripheral cells accounted for at least 90% of the ¹⁴CO₂ assimilation, whereas internal cells remained unlabeled. However, when ¹⁴CO₂ was allowed to diffuse into colonies 15 min before illumination, a more uniform distribution of labeling was observed. Resultant differences in labeling patterns were most likely due to peripheral cells more exclusively utilizing CO₂ when ambient C_i concentrations were low. Among colonies located at the air-water interface, internal cells showed an increased share of photosynthate production when atmospheric ¹⁴CO₂ was supplied. This indicated that C_i transport was restricted in large colonies below the water surface, forcing internal cells to maintain a high degree of buoyancy, thus promoting the formation of surface scums. At the surface, C_i restrictions were alleviated. Accordingly, scum formation appears to have an ecological function, allowing cyanobacteria access to atmospheric CO₂ when the C_i concentration is growth limiting in the water column.