Photosynthesis can be enhanced by lateral CO2 diffusion inside leaves over distances of several millimeters

This study examines the extent to which lateral gas diffusion can influence intercellular CO(2) concentrations (c(i)) and thus photosynthesis in leaf areas with closed stomata. Leaves were partly greased to close stomata artificially, and effects of laterally diffusing CO(2) into the greased areas were studied by gas-exchange measurement and chlorophyll fluorescence imaging. Effective quantum yields (Delta F/F(m)') across the greased areas were analysed with an image-processing tool and transposed into c(i) profiles, and lateral CO(2) diffusion coefficients (D(C'lat)), directly proportional to lateral conductivities (), were estimated using a one-dimensional (1D) diffusion model. Effective CO(2) diffusion distances in Vicia faba (homobaric), Commelina vulgaris (homobaric) and Phaseolus vulgaris (heterobaric) leaves clearly differed, and were dependent on D(C'lat), light intensity, [CO(2)], and [O(2)]: largest distances were approx. 7.0 mm for homobaric leaves (with high D(C'lat)) and approx. 1.9 mm for heterobaric leaves (low D(C'lat)). Modeled lateral CO(2) fluxes indicate large support of photosynthesis over submillimeter distances for leaves with low D(C'lat), whereas in leaves with large D(C'lat), photosynthesis can be stimulated over distances of several millimeters. For the plant species investigated, the surplus CO(2) assimilation rates of the greased leaf areas (A(gr)) differed clearly, depending on lateral conductivities of the respective leaves.     

Lateral gas diffusion inside leaves

Diffusion of CO2 inside leaves is generally regarded to be from the substomatal cavities to the assimilating tissues, i.e. in the vertical direction of the leaf blades. However, lateral gas diffusion within intercellular air spaces may be much more effective than hitherto considered. In a previous work it was demonstrated that, when 'clamp-on' leaf chambers are used, leaf internal 'CO2 leakage' beyond the leaf chamber gaskets may seriously affect gas exchange measurement. This effect has been used in the present paper to quantify gas conductance (g(leaf,l), mmol m(-2) s(-1)) in the lateral directions within leaves and significant differences between homo- and heterobaric leaves were observed. For the homobaric leaves, lateral gas conductance measured over a distance of 6 or 8 mm (the widths of the chamber gaskets) was 2-20% of vertical conductance taken from published data measured over much smaller distances of 108-280 microm (the thickness of the leaves). The specific internal gas diffusion properties of the leaves have been characterized by gas conductivities (g*(leaf), micromol m(-1) s(-1)). Gas conductivities in the lateral directions of heterobaric leaves were found to be small but not zero. In homobaric leaves, they were between 67 and 209 micromol m(-1) s(-1) and thus even larger than those in the vertical direction of the leaf blades (between 15 and 78 micromol m(-1) s(-1)). The potential implications for experimentalists performing gas exchange measurements are discussed.     

Lateral diffusion of CO2 from shaded to illuminated leaf parts affects photosynthesis inside homobaric leaves

Gas exchange is generally regarded to occur between the leaf interior and ambient air, i.e. in vertical (anticlinal) directions of leaf blades. However, inside homobaric leaves, gas movement occurs also in lateral directions. The aim of the present study was to ascertain whether lateral CO2 diffusion affects leaf photosynthesis when illuminated leaves are partially shaded. Measurements using gas exchange and chlorophyll fluorescence imaging techniques were performed on homobaric leaves of Vicia faba and Nicotiana tabacum or on heterobaric leaves of Glycine max and Phaseolus vulgaris. For homobaric leaves, gas exchange inside a clamp-on leaf chamber was affected by shading the leaf outside the chamber. The quantum yield of photosystem II (Phi(PSII)) was highest directly adjacent to a light/shade border (LSB). Phi(PSII) decreased in the illuminated leaf parts with distance from the LSB, while the opposite was observed for nonphotochemical quenching. These effects became most pronounced at low stomatal conductance. They were not observed in heterobaric leaves. The results suggest that plants with homobaric leaves can benefit from lateral CO2 flux, in particular when stomata are closed (e.g. under drought stress). This may enhance photosynthetic, instead of nonphotochemical, processes near LSBs in such leaves and reduce the photoinhibitory effects of excess light.     

Lateral diffusion of CO2 in leaves is not sufficient to support photosynthesis

Lateral diffusion of CO(2) was investigated in photosynthesizing leaves with different anatomy by gas exchange and chlorophyll a fluorescence imaging using grease to block stomata. When one-half of the leaf surface of the heterobaric species Helianthus annuus was covered by 4-mm-diameter patches of grease, the response of net CO(2) assimilation rate (A) to intercellular CO(2) concentration (C(i)) indicated that higher ambient CO(2) concentrations (C(a)) caused only limited lateral diffusion into the greased areas. When single 4-mm patches were applied to leaves of heterobaric Phaseolus vulgaris and homobaric Commelina communis, chlorophyll a fluorescence images showed dramatic declines in the quantum efficiency of photosystem II electron transport (measured as F(q)'/F(m)') across the patch, demonstrating that lateral CO(2) diffusion could not support A. The F(q)'/F(m)' values were used to compute images of C(i) across patches, and their dependence on C(a) was assessed. At high C(a), the patch effect was less in C. communis than P. vulgaris. A finite-volume porous-medium model for assimilation rate and lateral CO(2) diffusion was developed to analyze the patch images. The model estimated that the effective lateral CO(2) diffusion coefficients inside C. communis and P. vulgaris leaves were 22% and 12% of that for free air, respectively. We conclude that, in the light, lateral CO(2) diffusion cannot support appreciable photosynthesis over distances of more than approximately 0.3 mm in normal leaves, irrespective of the presence or absence of bundle sheath extensions, because of the CO(2) assimilation by cells along the diffusion pathway.     

Mechanisms underlying CO2 diffusion in leaves

Plants provide an excellent system to study CO(2) diffusion because, under light saturated conditions, photosynthesis is limited by CO(2) availability. Recent findings indicate that CO(2) diffusion in leaves can be variable in a short time range. Mesophyll CO(2) conductance could change independently from stomata movement or CO(2) fixing reactions and it was suggested that, beside others, the membranes are mesophyll CO(2) conductance limiting components. Specific aquaporins as membrane intrinsic pore proteins are considered to have a function in the modification of membrane CO(2) conductivity. Because of conflicting data, the mechanism of membrane CO(2) diffusion in plants and animals is a matter of a controversy vivid debate in the scientific community. On one hand, data from biophysics are in favor of CO(2) diffusion limiting mechanisms completely independent from membrane structure and membrane components. On the other, there is increasing evidence from physiology that a change in membrane composition has an effect on CO(2) diffusion.     

Carbon dioxide biofixation by Chlorella vulgaris at different CO2 concentrations and light intensities

In order to develop an effective CO₂ mitigation process using microalgae for potential industrial application, the growth and physiological activity of Chlorella vulgaris in photobioreactor cultures were studied. C. vulgaris was grown at two CO₂ concentrations (2 and 13% of CO₂ v/v) and at three incident light intensities (50, 120 and 180 μmol m^?2 s^?1) for 9 days. The measured specific growth rate was similar under all conditions tested but an increase in light intensity and CO₂ concentration affected the biomass and cell concentrations. Although carbon limitation was observed at 2% CO₂, similar cellular composition was measured in both conditions. Light limitation induced a net change in the growth behavior of C. vulgaris. Nitrogen limitation seemed to decrease the nitrogen quota of the cells and rise the intracellular carbon:nitrogen ratio. Exopolysaccharide production per cell appeared to be affected by light intensity. In order to avoid underestimation of the CO₂ biofixation rate of the microalgae, exopolysaccharide production was taken into account. The maximum CO₂ removal rate (0.98 g CO₂ L^?1 d^?1) and the highest biomass concentration (4.14 g DW L^?1) were determined at 13% (v/v) CO₂ and 180 μmol m^?2 s^?1. Our results show that C. vulgaris has a real potential for industrial CO₂ remediation.     

Effects of growth and measurement light intensities on temperature dependence of CO2 assimilation rate in tobacco leaves

Effects of growth light intensity on the temperature dependence of CO₂ assimilation rate were studied in tobacco (Nicotiana tabacum) because growth light intensity alters nitrogen allocation between photosynthetic components. Leaf nitrogen, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) contents increased with increasing growth light intensity, but the cyt f/Rubisco ratio was unaltered. Mesophyll conductance to CO₂ diffusion (g_m) measured with carbon isotope discrimination increased with growth light intensity but not with measuring light intensity. The responses of CO₂ assimilation rate to chloroplast CO₂ concentration (C_c) at different light intensities and temperatures were used to estimate the maximum carboxylation rate of Rubisco (V_cmax) and the chloroplast electron transport rate (J). Maximum electron transport rates were linearly related to cyt f content at any given temperature (e.g. 115 and 179 µmol electrons mol^?1 cyt f s^?1 at 25 and 40 °C, respectively). The chloroplast CO₂ concentration (C_trans) at which the transition from RuBP carboxylation to RuBP regeneration limitation occurred increased with leaf temperature and was independent of growth light intensity, consistent with the constant ratio of cyt f/Rubisco. In tobacco, CO₂ assimilation rate at 380 µmol mol^?1 CO₂ concentration and high light was limited by RuBP carboxylation above 32 °C and by RuBP regeneration below 32 °C.     

Carotenoid contents in leaves grown under various light intensities

The catotenoid content in the leaves of mosses, pteridophyt, gymnosperms and angiosperms (dicotyledons and monocotyledons0 growing in the sunlight and in the shade was studied. In the leaves of the plants growing in the shade the total carotenoid content and β-carotene increases as does chlorophyll. On the other hand, in the leaves of all plants a marked increase in lutein content was observed only in the leaves exposed to sunlight. The data indicates that carotenoids play a significant role, equal to that of chlorophyll, as ‘antennae’ absorbing light in the plants.     

Lateral diffusion on tubular membranes: quantification of measurements bias

Single Particle Tracking (SPT) is a powerful technique for the analysis of the lateral diffusion of the lipid and protein components of biological membranes. In neurons, SPT allows the study of the real-time dynamics of receptors for neurotransmitters that diffuse continuously in and out synapses. In the simplest case where the membrane is flat and is parallel to the focal plane of the microscope the analysis of diffusion from SPT data is relatively straightforward. However, in most biological samples the membranes are curved, which complicates analysis and may lead to erroneous conclusions as for the mode of lateral diffusion. Here we considered the case of lateral diffusion in tubular membranes, such as axons, dendrites or the neck of dendritic spines. Monte Carlo simulations allowed us to evaluate the error in diffusion coefficient (D) calculation if the curvature is not taken into account. The underestimation is determined by the diameter of the tubular surface, the frequency of image acquisition and the degree of mobility itself. We found that projected trajectories give estimates that are 25 to 50% lower than the real D in case of 2D-SPT over the tubular surface. The use of 3D-SPT improved the measurements if the frequency of image acquisition was fast enough in relation to the mobility of the molecules and the diameter of the tube. Nevertheless, the calculation of D from the components of displacements in the axis of the tubular structure gave accurate estimate of D, free of geometrical artefacts. We show the application of this approach to analyze the diffusion of a lipid on model tubular membranes and of a membrane-bound GFP on neurites from cultured rat hippocampal neurons.     

Stomatal responses to light and CO2 in greening wheat leaves

The development of two types of stomatal transpiration, oneinduced by light (light-induced stomatal transpiration) andthe other induced by CO₂-free air in the dark (CO₂-sensitivestomatal transpiration), in greening leaves of wheat (Triticumaestivum L.) was studied in respect to the development of CO₂uptake and chlorophyll formation. Light-induced stomatal transpirationwas not observed at all in etiolated leaves and was generatedafter 3 hr of illumination for greening, when the activity ofCO₂ uptake was generated. CO₂-sensitive stomatal transpirationwas low in etiolated leaves and started to increase at the sametime during greening as the start of CO₂ uptake. The activitiesof both light-induced and CO₂-sensitive stomatal transpirationincreased as the activity of CO₂ uptake and the chlorophyllcontent increased. Pre-illumination of etiolated leaves for1 min followed by 4 hr of dark incubation eliminated the lagfor the development of the two types of stomatal transpirationand CO₂ uptake. (Received September 4, 1978; )     

Regulation of CO2-fixation in Chlorella by light of varied wavelengths and intensities

1) The wavelength effects on ¹⁴CO₂-fixation by Chlorella cellswere studied, using monochromatic light of different light intensities. 2) Blue light (453 mµ) stimulated the incorporation of¹⁴C into aspartate, glutamate and malate. Red light (679 mµ),on the other hand, stimulated its incorporation into P-esters,free sugars and insoluble material. 3) The blue light effect was observed in the presence of CMUat concentrations completely suppressing ordinary photosyntheticCO₂-fixation. 4) The blue light effect in the presence of CMU was inducedat very low intensities. At 453 mµ, 300 erg cm^–2sec^–1 was sufficient for complete saturation. 5) Time courses of ¹⁴C-incorporation into individual compoundswere investigated. Irrespective of the wavelength of the illuminatinglight, the first stable CO₂-fixation product formed under weaklight (400–500 erg cm^–2 sec^–1) was citrulline.At higher light intensities (4,000–7,000 erg cm^–2sec^–1), PGA was the first stable CO₂-fixation product.The incorporation of ¹⁴C into citrulline was not inhibited byCMU. 6) Experimental results indicate that both blue light-inducedincorporation of ¹⁴C into amino and organic acids and the incorporationof ¹⁴C into citrulline induced by low intensity light are operatedby a mechanism(s) independent of ordinary photosynthetic CO₂-fixation.Possible effects of light regulating the carbon metabolism inalgal cells are discussed. (Received July 24, 1969; )     

Photosynthetic characteristics of coffee (Coffea arabusta) plantlets in vitro in response to different CO2 concentrations and light intensities

The photosynthetic characteristics of coffee ( Coffea arabusta) plantlets cultured in vitro in response to different CO₂ concentrations inside the culture vessel and photosynthetic photon flux (PPF) were investigated preliminarily. The estimation of net photosynthetic rate (P_n) of coffee plantlets involved three methods: (1) estimating time courses of actual P_n in situ based on measuring CO₂ concentrations inside and outside the vessel during a 45-day period, (2) estimating P_n in situ at different CO₂ concentrations and PPFs using the above measuring approach for 10-day and 30-day old in vitro plantlets, and (3) estimating P_n of a single leaf at different CO₂ concentrations and PPFs by using a portable photosynthesis measurement system for 45-day old in vitro coffee plantlets. The results showed that coffee plantlets in vitro had relatively high photosynthetic ability and that the P_n increased with the increase in CO₂ concentration inside the vessel. The CO₂ saturation point of in vitro coffee plantlets was high (4500–5000 μmol mol^-1); on the other hand, the PPF saturation point was not so high as compared to some other species, though it increased with increasing CO₂ concentration inside the vessel. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chlorophyll fluorescence in the leaves of Tradescantia species of different ecological groups: Induction events at different intensities of actinic light

Chlorophyll fluorescence analysis is one of the most convenient and widespread techniques used to monitor photosynthesis performance in plants. In this work, after a brief overview of the mechanisms of regulation of photosynthetic electron transport and protection of photosynthetic apparatus against photodamage, we describe results of our study of the effects of actinic light intensity on photosynthetic performance in Tradescantia species of different ecological groups. Using the chlorophyll fluorescence as a probe of photosynthetic activity, we have found that the shade-tolerant species Tradescantia fluminensis shows a higher sensitivity to short-term illumination (≤20 min) with low and moderate light (≤200 μE m⁻² s⁻¹) as compared with the light-resistant species Tradescantia sillamontana. In T. fluminensis, non-photochemical quenching of chlorophyll fluorescence (NPQ) and photosystem II operational efficiency (parameter Φ_PSII) saturate as soon as actinic light reaches ≈200 μE m⁻² s⁻¹. Otherwise, T. sillamontana revealed a higher capacity for NPQ at strong light (≥800 μE m⁻² s⁻¹). The post-illumination adaptation of shade-tolerant plants occurs slower than in the light-resistant species. The data obtained are discussed in terms of reactivity of photosynthetic apparatus to short-term variations of the environment light.     

七种热带雨林树苗叶片气孔特征及其可塑性对不同光照强度的响应

对生长在荫棚3种不同光照条件下和全自然光下的热带雨林4个冠层种(望天树、绒毛番龙眼、团花、红厚壳)和3个中层种(玉蕊、藤黄、滇南风吹楠)树苗叶片气孔特征以及它们的可塑性进行了研究、结果表明,这些植物的气孔全部着生在远轴面．7种植物中,玉蕊和绒毛番龙眼的气孔密度较大,滇南红厚壳和团花的保卫细胞最长．随光强的增大,气孔密度和气孔指数增大,单位叶气孔数在低光强下较大．除团花外,其它植物叶片气孔导度在50％光强处最大,而光强对保卫细胞的长度影响不显著．相关分析表明,气孔密度与植物单位叶的面积呈负相关。而与气孔导度的相关性不显著、尽管两种不同生活型植物气孔指数和单位叶气孔数在不同光强下的可塑性差异较小,但冠层树种气孔密度和气孔导度的可塑性显著高于中层树种．     

CO2 exchange under varying light intensities in some under- and overtemperature subtropical tree species

CO₂ exchange, transpiration and stomatal conductance of 39 subtropical tree species were studied under five light intensities at around atmospheric air temperatures found in subtropics during the active growth period of these species. Photosynthesis rates under different light intensities were strongly dependent on leaf to air temperature differences (T). Based on T, 39 species fell in two distinct categories namely, undertemperature and overtemperature. Majority of the species in the former group were found to have relatively higher rate of photosynthesis, stomatal conductance, transpiration as well as water use efficiency. These species also showed higher light saturation for photosynthesis. The significance of the results is discussed in terms of adaptive potential in the two types of species.