Theoretical Considerations when Estimating the Mesophyll Conductance to CO(2) Flux by Analysis of the Response of Photosynthesis to CO(2)

The conductance for CO₂ diffusion in the mesophyll of leaves can limit photosynthesis. We have studied two methods for determining the mesophyll conductance to CO₂ diffusion in leaves. We generated an ideal set of photosynthesis rates over a range of partial pressures of CO₂ in the stroma and studied the effect of altering the mesophyll diffusion conductance on the measured response of photosynthesis to intercellular CO₂ partial pressure. We used the ideal data set to test the sensitivity of the two methods to small errors in the parameters used to determine mesophyll conductance. The two methods were also used to determine mesophyll conductance of several leaves using measured rather than ideal data sets. It is concluded that both methods can be used to determine mesophyll conductance and each method has particular strengths. We believe both methods will prove useful in the future.     

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.

     

A Microscale Model for Combined CO2 Diffusion and Photosynthesis in Leaves

Transport of CO₂ in leaves was investigated by combining a 2-D, microscale CO₂ transport model with photosynthesis kinetics in wheat (Triticum aestivum L.) leaves. The biophysical microscale model for gas exchange featured an accurate geometric representation of the actual 2-D leaf tissue microstructure and accounted for diffusive mass exchange of CO_2. The resulting gas transport equations were coupled to the biochemical Farquhar-von Caemmerer-Berry model for photosynthesis. The combined model was evaluated using gas exchange and chlorophyll fluorescence measurements on wheat leaves. In general a good agreement between model predictions and measurements was obtained, but a discrepancy was observed for the mesophyll conductance at high CO₂ levels and low irradiance levels. This may indicate that some physiological processes related to photosynthesis are not incorporated in the model. The model provided detailed insight into the mechanisms of gas exchange and the effects of changes in ambient CO₂ concentration or photon flux density on stomatal and mesophyll conductance. It represents an important step forward to study CO₂ diffusion coupled to photosynthesis at the leaf tissue level, taking into account the leaf''s actual microstructure.     

Photosynthetic and stomatal responses of spinach leaves to salt stress

The gas exchange of spinach plants, salt-stressed by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar, was studied 3 weeks after starting NaCl treatment. Photosynthesis became light saturated at 1100 to 1400 micromoles per square meter per second in salt-treated plants and at approximately 2000 micromoles per square meter per second in control plants. Photosynthetic capacity of the mesophyll measured as a function of intercellular partial pressure of CO₂ at the light intensity prevailing during growth and at light saturation were both decreased in the salttreated plants. The CO₂ compensation points and relative enhancements of photosynthesis at low O₂ were not affected by salinity. The lower photosynthetic rates in salt-treated leaves at 450 micromoles per square meter per second were associated with a 70% reduction in stomatal conductance and low intercellular CO₂ (219 microbars; cf. 285 microbars for controls). Increasing photon flux density to light saturation extended the linear portions of the CO₂ response curves, increased stomatal conductances, increased intercellular CO₂ in the salt-treated plants, but lowered it in controls, and accentuated differences in photosynthetic rate (area basis) between the treatments.

Leaves from salt-treated plants were thicker but contained about 73% of the chlorophyll per unit area of control plants. When photosynthetic rates were expressed on a chlorophyll basis there was no difference in initial slope of assimilation versus intercellular CO₂ between treatments. Photosynthetic rates (chlorophyll basis) at light saturation differed only by 20% which was also observed earlier with isolated, intact chloroplasts (Robinson et al. 1983 Plant Physiol 73: 238-242).

Measurement of carbon isotope ratio revealed less discrimination against ¹³C with salt treatment and confirmed the persistence of low intercellular partial pressures of CO₂ during plant growth. The development of a thicker leaf with less chlorophyll per unit area during salt treatment permitted stomatal conductance and intercellular partial pressure of CO₂ to decline without restricting photosynthesis and had the benefit of greatly increasing water use efficiency.

     

Carbon dioxide diffusion across stomata and mesophyll and photo-biochemical processes as affected by growth CO2 and phosphorus nutrition in cotton

Nutrients such as phosphorus may exert a major control over plant response to rising atmospheric carbon dioxide concentration (CO₂), which is projected to double by the end of the 21st century. Elevated CO₂ may overcome the diffusional limitations to photosynthesis posed by stomata and mesophyll and alter the photo-biochemical limitations resulting from phosphorus deficiency. To evaluate these ideas, cotton (Gossypium hirsutum) was grown in controlled environment growth chambers with three levels of phosphate (Pi) supply (0.2, 0.05 and 0.01 mM) and two levels of CO₂ concentration (ambient 400 and elevated 800 μmol mol⁻¹) under optimum temperature and irrigation. Phosphate deficiency drastically inhibited photosynthetic characteristics and decreased cotton growth for both CO₂ treatments. Under Pi stress, an apparent limitation to the photosynthetic potential was evident by CO₂ diffusion through stomata and mesophyll, impairment of photosystem functioning and inhibition of biochemical process including the carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxyganase and the rate of ribulose-1,5-bisphosphate regeneration. The diffusional limitation posed by mesophyll was up to 58% greater than the limitation due to stomatal conductance (g_s) under Pi stress. As expected, elevated CO₂ reduced these diffusional limitations to photosynthesis across Pi levels; however, it failed to reduce the photo-biochemical limitations to photosynthesis in phosphorus deficient plants. Acclimation/down regulation of photosynthetic capacity was evident under elevated CO₂ across Pi treatments. Despite a decrease in phosphorus, nitrogen and chlorophyll concentrations in leaf tissue and reduced stomatal conductance at elevated CO₂, the rate of photosynthesis per unit leaf area when measured at the growth CO₂ concentration tended to be higher for all except the lowest Pi treatment. Nevertheless, plant biomass increased at elevated CO₂ across Pi nutrition with taller plants, increased leaf number and larger leaf area.     

Evaluation of the potential to measure photosynthetic rates in C3 plants (Flaveria pringlei and Oryza sativa) by combining chlorophyll fluorescence analysis and a stomatal conductance model

The response curves of leaf photosynthesis to varying light, temperature and leaf-to-air vapour pressure deficit were measured in the C₃ plants Flaveria pringlei and Oryza sativa in normal air with a computerized open infrared gas analysis (IRGA) system, and the photochemical efficiency of photosystem II, described as (1–F,/F′_m) after Genty. Briantais & Baker (1989, Biochimica et Biophysica Acta 990, 87–92), was simultaneously measured with a modulated fluorometer. A model was written for rates of CO₂ fixation as a function of the true rate of O₂ evolution measured by fluorescene analysis (Jo₂), mesophyll conductance and intercellular CO₂ partial pressure. A second model was developed for rates of CO₂ fixation as a function of Jo₂, mesophyll conductance and stomatal conductance. In the latter case, leaf stomatal conductance was simulated using the stomatal model proposed by Leuning (1995, Plant, Cell and Environment 18 , 339–355). The rates of CO₂ fixation predicted from the models were similar to rates measured by IRGA. The results indicate that there is potential to measure CO₂ fixation in C₃ plants by combining the non-invasive measurement of Jo₂ by chlorophyll fluorescence analysis with the stomatal conductance model.     

Stomatal and Mesophyll Limitations of Photosynthesis in Phosphate Deficient Sunflower, Maize and Wheat Plants

The effects of phosphate deficiency on the composition and photosyntheticCO₂ assimilation rates of fully expanded leaves of sunflower,maize and wheat plants are described. The regulation of photosynthesisby stomatal and mesophyll characteristics of leaves of differentphosphate status is analysed and related to structure. Phosphatedeficient leaves had small concentrations of inorganic phosphate,Pi, in the tissue water. Rate of photosynthesis in leaves andstomatal conductance were smaller in plants grown with inadequatephosphate when measured under any given light intensity or CO₂partial pressure. Despite the decrease in stomatal conductance(and without evidence of patchy stomatal closure), the relativestomatal limitation of photosynthesis was similar in the plantsgrown with deficient or abundant phosphate. However, the mesophyllcapacity for photosynthesis was greatly limited by phosphatedeficiency. Leaves deficient in phosphate had larger numbersof small size cells per unit leaf area than leaves with adequatephosphate. The total soluble protein content of leaves decreasedwith phosphate deficiency in all three species; however, theleaf chlorophyll content was decreased only in sunflower andmaize and not in wheat. These results suggest that stomatalconductance did not restrict the CO₂ diffusion rate, ratherthe metabolism of the mesophyll was the limiting factor. Thisis shown by poor carboxylation efficiency and decreased apparentquantum yield for CO₂ assimilation, both of which contributedto the increase in relative mesophyll limitation of photosynthesisin phosphate deficient plants. Key words: Apparent quantum yield, carboxylation efficiency, phosphate nutrition, photosynthesis, stomatal and mesophyll limitation     

Photosynthetic light-response curves

The shapes of photosynthetic light-response curves for leaves of Eucalyptus maculata (Hook) and E. pauciflora (Sieber ex Sprengel) were examined. Three different methods were used to measure photosynthesis: CO₂ and H₂O-vapour exchange, O₂ evolution at a 5-kPa CO₂ partial pressure, and chlorophyll fluorescence. The three methods were compared and gave good agreement when measured under equivalent conditions. However, O₂ evolution was inhibited by high CO₂ partial pressures. A non-rectangular hyperbolic curve has been used widely to describe photosynthetic light-response curves. It has three variables which define the maximum quantum yield (photosynthetic rate divided by absorbed irradiance at very low irradiances), the maximum capacity and the curvature (Θ). We found that Θ was affected by the CO₂ partial pressure, declining to a minimum of about 0.6 as CO₂ partial pressure increased to 100 Pa. Further increases in the CO₂ partial pressure began to inhibit the rate of O₂ evolution at 2000 μmol quanta · m^?2·^?1 and Θ increased back to 0.95 by 5 kPa CO₂ partial pressure. At low irradiances, photosynthesis is limited by the rate of electron transport while at high irradiances, photosynthesis is frequently limited by the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). The dependence of Θ on CO₂ partial pressure arises because the transition between limitations changes as a function of the CO₂ partial pressure. The light-response curve is truncated by the transition to a Rubisco limitation and the lower the irradiance at the transition, the higher the value of Θ. There is a gradient in light absorption through the leaf which influences the photosynthetic capacity of different layers within the leaf. The gradient in photosynthetic capacity can be demonstrated by the fact that the shape of the light-response curve changes when the leaf is illuminated unilaterally onto either the adaxial or abaxial surface. We compared two Eucalyptus species which had either isolateral or dorsiventral leaf anatomy. Leaves were able to reverse completely the gradients in photosynthetic capacity following inversion of the leaves for a week, irrespective of their anatomy.     

A model of photosynthetic CO2 assimilation and carbon-isotope discrimination in leaves of certain C3−C4 intermediates

A model of leaf, photosynthesis has been developed for C₃–C₄ intermediate species found in the generaPanicum, Moricandia, Parthenium andMollugo where no functional C₄ pathway has been identified. Model assumptions are a functional C₃ cycle in both mesophyll and bundle-sheath cells and that glycine formed in the mesophyll, as a consequence of the oxygenase activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39), diffuses to the bundle sheath, where most of the photorespiratory CO₂ is released. The model describes the observed gas-exchange characteristics of these C₃–C₄ intermediates, such as low CO₂-compensation points () at an O₂ pressure of 200 mbar, a curvilinear response of to changing O₂ pressures, and typical responses of CO₂-assimilation rate to intercellular CO₂ pressure. The model predicts that bundle-sheath CO₂ concentration is highest at low mesophyll CO₂ pressures and decreases as mesophyll CO₂ pressure increases. A partitioning of 5–15% of the total leaf Rubisco into the bundle-sheath cells and a bundlesheath conductance similar to that proposed for C₄ species best mimics the gas-exchange results. The model predicts C₃-like carbon-isotope discrimination for photosynthesis at atmospheric levels of CO₂, but at low CO₂ pressures it predicts a higher discrimination than is typically found during C₃ photosynthesis at lower CO₂ pressures.Abbreviations and symbols PEP phosphoenolpyruvate - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) - RuBP ribulose-1,5-bisphosphate - p(CO₂) partial pressure of CO₂ - p(O₂) partial pressure of O₂. See also p. 471     

Stomatal Behavior and CO(2) Exchange Characteristics in Amphistomatous Leaves

The possibility that differences in stomatal conductance between upper and lower surfaces of amphistomatous leaves are adaptations to differences in CO₂ exchange characteristics for the two surfaces was investigated. The ratio of upper to lower stomatal conductance was found to change little in response to light and humidity for well-watered sunflower (Helianthus annuus L.) plants. Stressing the plants (ψ = −17 bars) and rewatering 1 day before gas exchange measurements reduced upper conductance more severely than lower in both indoor- and outdoor-grown plants, and caused small changes in conductance ratio with light and humidity. A similar pattern was found using outdoor grown sunflower and cocklebur (Xanthium strumarium L.) plants. Calculated intercellular CO₂ concentrations for upper and lower surfaces were always close to identical for a particular set of environmental conditions for both sunflower and cocklebur, indicating that no differences in CO₂ exchange characteristics exist between the two surfaces. By artificially creating a CO₂ gradient across the leaf, the resistance to CO₂ diffusion through the mesophyll was estimated and found to be so low that despite possible nonhomogeneity of the mesophyll, differences in CO₂ exchange characteristics for the two surfaces are unlikely. It is concluded that differences in conductance between upper and lower stomates are not adaptations to differences in CO₂ exchange characteristics.     

The F<Subscript>O</Subscript>F<Subscript>1</Subscript> ATP synthase: from atomistic three-dimensional structure to the rotary-chemical function

There are numerous studies describing how growth conditions influence the efficiency of C₄ photosynthesis. However, it remains unclear how changes in the biochemical capacity versus leaf anatomy drives this acclimation. Therefore, the aim of this study was to determine how growth light and nitrogen availability influence leaf anatomy, biochemistry and the efficiency of the CO₂ concentrating mechanism in Miscanthus × giganteus. There was an increase in the mesophyll cell wall surface area but not cell well thickness in the high-light (HL) compared to the low-light (LL) grown plants suggesting a higher mesophyll conductance in the HL plants, which also had greater photosynthetic capacity. Additionally, the HL plants had greater surface area and thickness of bundle-sheath cell walls compared to LL plants, suggesting limited differences in bundle-sheath CO₂ conductance because the increased area was offset by thicker cell walls. The gas exchange estimates of phosphoenolpyruvate carboxylase (PEPc) activity were significantly less than the in vitro PEPc activity, suggesting limited substrate availability in the leaf due to low mesophyll CO₂ conductance. Finally, leakiness was similar across all growth conditions and generally did not change under the different measurement light conditions. However, differences in the stable isotope composition of leaf material did not correlate with leakiness indicating that dry matter isotope measurements are not a good proxy for leakiness. Taken together, these data suggest that the CO₂ concentrating mechanism in Miscanthus is robust under low-light and limited nitrogen growth conditions, and that the observed changes in leaf anatomy and biochemistry likely help to maintain this efficiency.     

Gas-Exchange Properties of Salt-Stressed Olive (Olea europea L.) Leaves

The effects of two levels of salinity on photosynthetic properties of olive (Olea europea L.) leaves were observed either in low or in high H₂O vapor pressure deficit (vpd). Under moderate salt stress, stomata were found to be less open and responsive both to light and vpd, but the predominant limitation of photosynthesis was due to the mesophyll capacity of CO₂ fixation. We elaborate a procedure to correlate mesophyll capacity and liquid phase diffusive conductance. The estimated liquid phase diffusive conductance was reduced by salt and especially by high vpd; morphological and physiological changes could be responsible for this reduction. As a result, the chloroplast CO₂ partial pressure was found to decrease both under salt and vpd stress, thus resulting in a ribulose-1,5-bisphosphate carboxylase limitation of assimilation. However, under combined salt and vpd stress, O₂ sensitivity of assimilation increased, as would be expected under conditions of limiting ribulose 1,5-bisphosphate regeneration. Fluorescence induction measurements indicated that, under these conditions, energy supply may become limiting. When Cl⁻ concentration exceeded 80 millimolar in tissue water, zero growth and 50% leaf drop was observed. Fluorescence induction showed irreversible damage at Cl⁻ levels higher than 200 millimolar and basal leaves reached this concentration earlier than the apical ones.     

The After-effect of Wind on the Photosynthesis and Transpiration of Festuca arundinacea

The after-effect of wind on photosynthesis and transpiration of Festuca arundinacea Schreb, was determined. Following a period of exposure In a controlled environment wind tunnel the wind-treated plants showed reduced rates of photosynthesis when compared with the controls under standard conditions. Evaporation from paper model tillers was measured and the boundary layer resistance was shown to be low in all but very low wind speeds. Analysis of CO₂ and H₂O diffusion pathways indicated that mesophyll resistance in wind-treated plants was higher whilst leaf surface resistance was lower than in the controls. The high mesophyll resistance in the wind-treated plants was attributed to reduced water content.     

Nonstomatal Inhibition of Net CO(2) Uptake by (+/-) Abscisic Acid in Pharbitis nil

(±) Abscisic acid (ABA) injected into petioles of attached transpiring leaves of Pharbitis nil Chois. cv violet reduced the photosynthetic capacity of the mesophyll of these leaves as well as the stomatal conductance to CO₂ diffusion. Greater than 75% of the injected ABA was recovered as ABA, suggesting that ABA rather than some metabolite thereof was the active compound. The nonstomatal effect of ABA increased from 30% reduction in photosynthesis at 0.25 micromolar ABA in the leaf blade to 90% reduction at 18 micromolar. Despite the effect of ABA on the nonstomatal component of leaf net CO₂ uptake, it was calculated that a substantial part of the reduction in leaf net CO₂ uptake (50-80%) could be accounted for by the effect of ABA on stomatal conductance.     

Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis

In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO₂ assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO₂ loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO₂ assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36°C and 50 millibars bar⁻¹, respectively). Studies on the relationship between CO₂ assimilation rate and intercellular CO₂ partial pressure at various temperatures and humidities showed that this decrease in CO₂ assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO₂ diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher ¹³C content of Welwitschia compared to the majority of C₃ species.     

Isoprene emission from aspen leaves : influence of environment and relation to photosynthesis and photorespiration

Isoprene emission rates from quaking aspen (Populus tremuloides Michx.) leaves were measured simultaneously with photosynthesis rate, stomatal conductance, and intercellular CO₂ partial pressure. Isoprene emission required the presence of CO₂ or O₂, but not both. The light response of isoprene emission rate paralleled that of photosynthesis. Isoprene emission was inhibited by decreasing ambient O₂ from 21% to 2%, only when there was oxygen insensitive photosynthesis. Mannose (10 millimolar) fed through cut stems resulted in strong inhibition of isoprene emission rate and is interpreted as evidence that isoprene biosynthesis requires either the export of triose phosphates from the chloroplast, or the continued synthesis of ATP. Light response experiments suggest that photosynthetically generated reductant or ATP is required for isoprene biosynthesis. Isoprene biosynthesis and emission are not directly linked to glycolate production through photorespiration, contrary to previous reports. Isoprene emission rate was inhibited by above-ambient CO₂ partial pressures (640 microbar outside and 425 microbar inside the leaf). The inhibition was not due to stomatal closure. This was established by varying ambient humidity at normal and elevated CO₂ partial pressures to measure isoprene emission rates over a range of stomatal conductances. Isoprene emission rates were inhibited at elevated CO₂ despite no change in stomatal conductance. Addition of abscisic acid to the transpiration stream dramatically inhibited stomatal conductance and photosynthesis rate, with a slight increase in isoprene emission rate. Thus, isoprene emission is independent of stomatal conductance, and may occur through the cuticle. Temperature had an influence on isoprene emission rate, with the Q₁₀ being 1.8 to 2.4 between 35 and 45°C. At these high temperatures the amount of carbon lost through isoprene emission was between 2.5 and 8% of that assimilated through photosynthesis. This represents a significant carbon cost that should be taken into account in determining midsummer carbon budgets for plants that are isoprene emitters.     

Very high CO2 partially restores photosynthesis in sunflower at low water potentials

We re-examined the question of whether the stomata limit photosynthesis in dehydrated sunflower (Helianthus annuus L.) plants having low leaf water potentials. A gas-exchange apparatus was modified to operate at external CO₂ partial pressures as high as 3000 Pa (3%), which were much higher than previously achieved. This allowed photosynthesis and stomatal behavior to be monitored simultaneously at very high CO₂ in the same leaf. The data were compared with those from leaves treated with abscisic acid (ABA) where effects on photosynthesis are entirely stomatal. Photosynthesis was inhibited at low water potential and was only slightly enhanced by increasing the external CO₂ partial pressure from 34 Pa (normal air) to 300 Pa. Photosynthesis in ABA-treated leaves was similarly inhibited but recovered fully at 300 Pa. In both cases, the stomata closed to the same extent as judged from the average conductance of the leaves. Because the ABA effect resulted from diffusion limitation for CO₂ caused by stomatal closure, the contrasting data show that most of the dehydration effect was nonstomatal at low water potentials. When CO₂ partial pressures were raised further to 3000 Pa, photosynthesis increased somewhat at low water potentials but not in ABA-treated leaves. This indicates that some nonstomatal component of photosynthesis responded differently in leaves at low water potential and leaves treated with ABA. Because this component was only partially restored by very high CO₂, it was likely to be metabolic and was an important source of photosynthetic inhibition.Abbreviations and Symbol ABA abscisic acid - Chl chlorophyll - p_a external partial pressure of CO₂ - P_i intercellular partial pressure of CO₂ - _w water potential This work was supported by grant DE-FG02-87ER13776 from the Department of Energy and a grant from E.I. DuPont de Nemours and Company.     

Light-harvesting in photosystem I

Improving Rubisco catalysis is considered a promising way to enhance C₃-photosynthesis and photosynthetic water use efficiency (WUE) provided the introduced changes have little or no impact on other processes affecting photosynthesis such as leaf photochemistry or leaf CO₂ diffusion conductances. However, the extent to which the factors affecting photosynthetic capacity are co-regulated is unclear. The aim of the present study was to characterize the photochemistry and CO₂ transport processes in the leaves of three transplantomic tobacco genotypes expressing hybrid Rubisco isoforms comprising different Flaveria L-subunits that show variations in catalysis and differing trade-offs between the amount of Rubisco and its activation state. Stomatal conductance (g _s) in each transplantomic tobacco line matched wild-type, while their photochemistry showed co-regulation with the variations in Rubisco catalysis. A tight co-regulation was observed between Rubisco activity and mesophyll conductance (g _m) that was independent of g _s thus producing plants with varying g _m/g _s ratios. Since the g _m/g _s ratio has been shown to positively correlate with intrinsic WUE, the present results suggest that altering photosynthesis by modifying Rubisco catalysis may also be useful for targeting WUE.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: IV. ANALYSIS OF REDUCED OXYGEN RESPONSE IN PANICUM MILIOIDES AND PANICUM SCHENCKII

Reduced photorespiration has been reported in Panicum milioides on the basis of lower CO₂ compensation concentrations than in C₃ species, lower CO₂ evolution in the light, and less response of apparent photosynthesis to O₂ concentration. The lower response to O₂ in P. milioides could be due to reduced O₂ competition with CO₂ for reaction with ribulose 1,5-bisphosphate, to a reduced loss of CO₂, or to an initial fixation of CO₂ by phosphoenolpyruvate carboxylase. Experiments were carried out with Panicum maximum Jacq., a C₄ species having no apparent photorespiration; tall fescue (Festuca arundinacea Schreb.), a C₃ species; P. milioides Nees ex Trin.; and Panicum schenckii Hack. The latter two species are closely related and have low photorespiration rates. CO₂ exchange was measured at five CO₂ concentrations ranging from 0 to 260 microliters per liter at both 2 and 21% O₂. Mesophyll conductance or carboxylation efficiency was estimated by plotting substomatal CO₂ concentrations against apparent photosynthesis. In the C₄ species P. maximum, mesophyll conductance was 0.96 centimeters per second and was unaffected by O₂ concentration. At 21% O₂ mesophyll conductance of tall fescue was decreased 32% below the value at 2% O₂. Decreases in mesophyll conductance at 21% O₂ for P. milioides and P. schenckii were similar to that for tall fescue. On the other hand, loss of CO₂ in CO₂-free air, estimated by extrapolating the CO₂ response curve to zero CO₂, was increased from 1.8 to 6.5 milligrams per square decimeter per hour in tall fescue as O₂ was raised from 2-21%. Loss of CO₂ was less than 1 milligram per square decimeter per hour for P. milioides and P. schenckii and was unaffected by O₂. The results suggest that the reduced O₂ response in P. milioides and P. schenckii is due to a lower loss of CO₂ in the light rather than less inhibition of carboxylation by O₂, since the decrease in carboxylation efficiency at 21% O₂ was similar for P. milioides, P. schenckii, and tall fescue. The inhibition of apparent photosynthesis by 21% O₂ in these three species at low light intensities was similar at 31 to 36% which also indicates similar O₂ effects on carboxylation. Apparent photosynthesis at high light intensity was inhibited less by 21% O₂ in P. milioides (16.8%) and P. schenckii (23.8%) than in tall fescue (28.4%). This lower inhibition in the Panicum species may have been due to a higher degree of recycling of photorespired CO₂ in these species than in tall fescue.     

Intercellular Diffusion Limits to CO(2) Uptake in Leaves : Studies in Air and Helox

We studied plants of five species with hypostomatous leaves, and six with amphistomatous leaves, to determine the extent to which gaseous diffusion of CO₂ among the mesophyll cells limits photosynthetic carbon assimilation. In helox (air with nitrogen replaced by helium), the diffusivities of CO₂ and water vapor are 2.3 times higher than in air. For fixed estimated CO₂ pressure at the evaporating surfaces of the leaf (pⁱ), assimilation rates in helox ranged up to 27% higher than in air for the hypostomatous leaves, and up to 7% higher in the amphistomatous ones. Thus, intercellular diffusion must be considered as one of the processes limiting photosynthesis, especially for hypostomatous leaves. A corollary is that CO₂ pressure should not be treated as uniform through the mesophyll in many leaves. To analyze our helox data, we had to reformulate the usual gas-exchange equation used to estimate CO₂ pressure at the evaporating surfaces of the leaf; the new equation is applicable to any gas mixture for which the diffusivities of CO₂ and H₂O are known. Finally, we describe a diffusion-biochemistry model for CO₂ assimilation that demonstrates the plausibility of our experimental results.