Estimation of canopy average mesophyll conductance using δ(13) C of phloem contents

Conductance to CO(2) inside leaves, known as mesophyll conductance (g(m)), imposes large limitations on photosynthesis. Because g(m) is difficult to quantify, it is often neglected in calculations of (13)C photosynthetic discrimination. The 'soluble sugar method' estimates g(m) via differences between observed photosynthetic discrimination, calculated from the δ(13)C of soluble sugars, and discrimination when g(m) is infinite. We expand upon this approach and calculate a photosynthesis-weighted average for canopy mesophyll conductance ((c) g(m)) using δ(13)C of stem phloem contents. We measured gas exchange at three canopy positions and collected stem phloem contents in mature trees of three conifer species (Pseudotsuga menziesii, Thuja plicata and Larix occidentalis). We generated species-specific and seasonally variable estimates of (c)g(m) . We found that (c)g(m) was significantly different among species (0.41, 0.22 and 0.09 mol m(-2) s(-1) for Larix, Pseudotsuga and Thuja, respectively), but was similar throughout the season. Ignoring respiratory and photorespiratory fractionations ((c)Δ(ef)) resulted in ≈30% underestimation of (c)g(m) in Larix and Pseudotsuga, but was innocuous in Thuja. Substantial errors (~1-4‰) in photosynthetic discrimination calculations were introduced by neglecting (c)g(m) and (c)Δ(ef) . Our method is easy to apply and cost-effective, captures species variation and would have captured seasonal variation had it existed. The method provides an average canopy value, which makes it suitable for parameterization of canopy-scale models of photosynthesis, even in tall trees.     

Developmental changes in mesophyll diffusion conductance and photosynthetic capacity under different light and water availabilities in Populus tremula: how structure constrains function

Finite mesophyll diffusion conductance (g(m) ) significantly constrains net assimilation rate (A(n) ), but g(m) variations and variation sources in response to environmental stresses during leaf development are imperfectly known. The combined effects of light and water limitations on g(m) and diffusion limitations of photosynthesis were studied in saplings of Populus tremula L. An one-dimensional diffusion model was used to gain insight into the importance of key anatomical traits in determining g(m) . Leaf development was associated with increases in dry mass per unit area, thickness, density, exposed mesophyll (S(mes) /S) and chloroplast (S(c) /S) to leaf area ratio, internal air space (f(ias) ), cell wall thickness and chloroplast dimensions. Development of S(mes) /S and S(c) /S was delayed under low light. Reduction in light availability was associated with lower S(c) /S, but with larger f(ias) and chloroplast thickness. Water stress reduced S(c) /S and increased cell wall thickness under high light. In all treatments, g(m) and A(n) increased and CO(2) drawdown because of g(m) , C(i) -C(c) , decreased with increasing leaf age. Low light and drought resulted in reduced g(m) and A(n) and increased C(i) -C(c) . These results emphasize the importance of g(m) and its components in determining A(n) variations during leaf development and in response to stress.     

Fitting photosynthetic carbon dioxide response curves for C(3) leaves

Photosynthetic responses to carbon dioxide concentration can provide data on a number of important parameters related to leaf physiology. Methods for fitting a model to such data are briefly described. The method will fit the following parameters: V(cmax), J, TPU, R(d) and g(m)[maximum carboxylation rate allowed by ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), rate of photosynthetic electron transport (based on NADPH requirement), triose phosphate use, day respiration and mesophyll conductance, respectively]. The method requires at least five data pairs of net CO(2) assimilation (A) and [CO(2)] in the intercellular airspaces of the leaf (C(i)) and requires users to indicate the presumed limiting factor. The output is (1) calculated CO(2) partial pressure at the sites of carboxylation, C(c), (2) values for the five parameters at the measurement temperature and (3) values adjusted to 25 degrees C to facilitate comparisons. Fitting this model is a way of exploring leaf level photosynthesis. However, interpreting leaf level photosynthesis in terms of underlying biochemistry and biophysics is subject to assumptions that hold to a greater or lesser degree, a major assumption being that all parts of the leaf are behaving in the same way at each instant.     

Temperature response of mesophyll conductance in cultivated and wild Oryza species with contrasting mesophyll cell wall thickness

A critical component of photosynthetic capacity is the conductance of CO(2) from intercellular airspaces to the sites of CO(2) fixation in the stroma of chloroplasts, termed mesophyll conductance (g(m)). Leaf anatomy has been identified as an important determinant of g(m). There are few studies of the temperature response of g(m) and none has examined the implications of leaf anatomy. Hence, we compared a cultivar of Oryza sativa with two wild Oryza relatives endemic to the hot northern savannah of Australia, namely Oryza meridionalis and Oryza australiensis. All three species had similar leaf anatomical properties, except that the wild relatives had significantly thicker mesophyll cell walls than O. sativa. Thicker mesophyll cell walls in the wild rice species are likely to have contributed to the reduction in g(m) , which was associated with a greater drawdown of CO(2) into chloroplasts (C(i) -C(c) ) compared with O. sativa. Mesophyll conductance increased at higher temperatures, whereas the rate of CO(2) assimilation was relatively stable between 20 and 40 °C. Consequently, C(i) -C(c) decreased for all three species as temperature increased.     

叶肉细胞导度研究进展

叶肉细胞导度指叶片叶肉细胞内部的CO2扩散能力,在植物生理生态及全球气候变化和陆地生态系统相互关系的研究中具有重要作用。系统介绍了叶肉细胞导度的发现、发展过程及其研究进展、几种目前国际上常用的叶肉细胞导度测度方法的原理、计算过程;强调了叶肉细胞导度作为光合作用扩散过程一部分的重要意义,明确了叶肉细胞导度的定义及分布范围。并探讨了不同方法的优缺点及注意事项。总结分析了叶肉细胞导度对不同环境因子(温度、水分及环境中CO2和O3浓度等)的响应,从不同角度对叶肉细胞导度的生态学意义进行了简单的概括。对叶肉细胞导度的未来研究进行了展望。     

Tobacco aquaporin NtAQP1 is involved in mesophyll conductance to CO2 in vivo

Leaf mesophyll conductance to CO(2) (g(m)) has been recognized to be finite and variable, rapidly adapting to environmental conditions. The physiological basis for fast changes in g(m) is poorly understood, but current reports suggest the involvement of protein-facilitated CO(2) diffusion across cell membranes. A good candidate for this could be the Nicotiana tabacum L. aquaporin NtAQP1, which was shown to increase membrane permeability to CO(2) in Xenopus oocytes. The objective of the present work was to evaluate its effect on the in vivo mesophyll conductance to CO(2), using plants either deficient in or overexpressing NtAQP1. Antisense plants deficient in NtAQP1 (AS) and NtAQP1 overexpressing tobacco plants (O) were compared with their respective wild-type (WT) genotypes (CAS and CO). Plants grown under optimum conditions showed different photosynthetic rates at saturating light, with a decrease of 13% in AS and an increase of 20% in O, compared with their respective controls. CO(2) response curves of photosynthesis also showed significant differences among genotypes. However, in vitro analysis demonstrated that these differences could not be attributed to alterations in Rubisco activity or ribulose-1,5-bisphosphate content. Analyses of chlorophyll fluorescence and on-line (13)C discrimination indicated that the observed differences in net photosynthesis (A(N)) among genotypes were due to different leaf mesophyll conductances to CO(2), which was estimated to be 30% lower in AS and 20% higher in O compared with their respective WT. These results provide evidence for the in vivo involvement of aquaporin NtAQP1 in mesophyll conductance to CO(2).     

Temperature response of mesophyll conductance. Implications for the determination of Rubisco enzyme kinetics and for limitations to photosynthesis in vivo

CO(2) transfer conductance from the intercellular airspaces of the leaf into the chloroplast, defined as mesophyll conductance (g(m)), is finite. Therefore, it will limit photosynthesis when CO(2) is not saturating, as in C3 leaves in the present atmosphere. Little is known about the processes that determine the magnitude of g(m). The process dominating g(m) is uncertain, though carbonic anhydrase, aquaporins, and the diffusivity of CO(2) in water have all been suggested. The response of g(m) to temperature (10 degrees C-40 degrees C) in mature leaves of tobacco (Nicotiana tabacum L. cv W38) was determined using measurements of leaf carbon dioxide and water vapor exchange, coupled with modulated chlorophyll fluorescence. These measurements revealed a temperature coefficient (Q(10)) of approximately 2.2 for g(m), suggesting control by a protein-facilitated process because the Q(10) for diffusion of CO(2) in water is about 1.25. Further, g(m) values are maximal at 35 degrees C to 37.5 degrees C, again suggesting a protein-facilitated process, but with a lower energy of deactivation than Rubisco. Using the temperature response of g(m) to calculate CO(2) at Rubisco, the kinetic parameters of Rubisco were calculated in vivo from 10 degrees C to 40 degrees C. Using these parameters, we determined the limitation imposed on photosynthesis by g(m). Despite an exponential rise with temperature, g(m) does not keep pace with increased capacity for CO(2) uptake at the site of Rubisco. The fraction of the total limitations to CO(2) uptake within the leaf attributable to g(m) rose from 0.10 at 10 degrees C to 0.22 at 40 degrees C. This shows that transfer of CO(2) from the intercellular air space to Rubisco is a very substantial limitation on photosynthesis, especially at high temperature.     

Photosynthetic performance along a light gradient as related to leaf characteristics of a naturally occurring <Emphasis Type="Italic">Cypripedium flavum</Emphasis>

Photosynthesis, leaf structure, nitrogen content and nitrogen allocation in photosynthetic functions of Cypripedium flavum were studied in a naturally varying light regime. Light-saturated leaf net photosynthetic rate (A (max)) was strongly correlated with leaf dry mass per area (LMA), mesophyll conductance (g (m)) and area-based leaf nitrogen content (N(area)), with all variables increasing with increasing irradiance. Such coordinate variation of all these parameters illustrates the plastic response of leaf structure to high light (HL). Leaf N(area) was greater under HL than in low light (LL). The fractions of leaf nitrogen partitioning in carboxylation (P (R)) and bioenergetics (P (B)) were positively related to LMA. In contrast, P (R) and P (B) decreased with increasing mass-based leaf nitrogen content (N(mass)). However, no correlation was found between leaf nitrogen investment in light harvesting (P (L)) and either LMA or N(mass). Like maximum rate of carboxylation (V (cmax)) and electron transport (J (max)), the J (max)/V (cmax) ratio, which was strongly correlated to LMA, also increased significantly with irradiance. Under HL, leaf maximum photosynthetic nitrogen efficiency (ANUE) and intrinsic water use efficiency (WUE) were greater than in LL conditions, despite a small difference in WUE. This suggests that a functional balance in the photosynthetic machinery favors leaf photosynthetic plasticity of C. flavum in response to different light conditions. Given an ample soil nitrogen supply, C. flavum may offset its susceptibility to HL by efficient nitrogen use and higher stomatal and mesophyll conductance against photoinhibition so as to keep leaf photosynthesis positive.     

The effect of temperature on C(4)-type leaf photosynthesis parameters

C(4)-type photosynthesis is known to vary with growth and measurement temperatures. In an attempt to quantify its variability with measurement temperature, the photosynthetic parameters - the maximum catalytic rate of the enzyme ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) (V(cmax)), the maximum catalytic rate of the enzyme phosphoenolpyruvate carboxylase (PEPC) (V(pmax)) and the maximum electron transport rate (J(max)) - were examined. Maize plants were grown in climatic-controlled phytotrons, and the curves of net photosynthesis (A(n)) versus intercellular air space CO(2) concentrations (C(i)), and A(n) versus photosynthetic photon flux density (PPFD) were determined over a temperature range of 15-40 degrees C. Values of V(cmax), V(pmax) and J(max) were computed by inversion of the von Caemmerer & Furbank photosynthesis model. Values of V(pmax) and J(max) obtained at 25 degrees C conform to values found in the literature. Parameters for an Arrhenius equation that best fits the calculated values of V(cmax), V(pmax) and J(max) are then proposed. These parameters should be further tested with C(4) plants for validation. Other model key parameters such as the mesophyll cell conductance to CO(2) (g(i)), the bundle sheath cells conductance to CO(2) (g(bs)) and Michaelis-Menten constants for CO(2) and O(2) (K(c), K(p) and K(o)) also vary with temperature and should be better parameterized.     

Internal conductance to CO(2) diffusion and C(18)OO discrimination in C(3) leaves

(18)O discrimination in CO(2) stems from the oxygen exchange between (18)O-enriched water and CO(2) in the chloroplast, a process catalyzed by carbonic anhydrase (CA). A proportion of this (18)O-labeled CO(2) escapes back to the atmosphere, resulting in an effective discrimination against C(18)OO during photosynthesis (Delta(18)O). By constraining the delta(18)O of chloroplast water (delta(e)) by analysis of transpired water and the extent of CO(2)-H(2)O isotopic equilibrium (theta(eq)) by measurements of CA activity (theta(eq) = 0.75-1.0 for tobacco, soybean, and oak), we could apply measured Delta(18)O in a leaf cuvette attached to a mass spectrometer to derive the CO(2) concentration at the physical limit of CA activity, i.e. the chloroplast surface (c(cs)). From the CO(2) drawdown sequence between stomatal cavities from gas exchange (c(i)), from Delta(18)O (c(cs)), and at Rubisco sites from Delta(13)C (c(c)), the internal CO(2) conductance (g(i)) was partitioned into cell wall (g(w)) and chloroplast (g(ch)) components. The results indicated that g(ch) is variable (0.42-1.13 mol m(-2) s(-1)) and proportional to CA activity. We suggest that the influence of CA activity on the CO(2) assimilation rate should be important mainly in plants with low internal conductances.     

The efficiency of C(4) photosynthesis under low light conditions: assumptions and calculations with CO(2) isotope discrimination

Leakiness (Φ), the proportion of carbon fixed by phosphoenolpyruvate carboxylation that leaks out of the bundle-sheath cells, determines C(4) photosynthetic efficiency. Large increases in Φ have been described at low irradiance. The underlying mechanisms for this increase remain uncertain, but changes in photorespiration or the energy partitioning between the C(4) and C(3) cycles have been suggested. Additionally, values of Φ at low light could be magnified from assumptions made when comparing measured photosynthetic discrimination against (13)C (Δ) with the theoretical formulation for Δ. For example, several simplifications are often made when modelling Δ to predict Φ including: (i) negligible fractionation during photorespiration and dark respiration; (ii) infinite mesophyll conductance; and (iii) CO(2) inside bundle-sheath cells (C(s)) is much larger than values in mesophyll cells (C(m)). Theoretical models for C(4) photosynthesis and C(4) Δ were combined to evaluate how these simplifications affect calculations of Δ and Φ at different light intensities. It was demonstrated that the effects of photorespiratory fractionations and mesophyll conductance were negligible at low light. Respiratory fractionation was relevant only when the magnitude of the fractionation factor was artificially increased during measurements. The largest error in estimating Φ occurred when assuming C(s) was much larger than C(m) at low light levels, when bundle-sheath conductance was large (g(s)), or at low O(2) concentrations. Under these conditions, the simplified equation for Δ overestimated Φ, and compromised comparisons between species with different g(s), and comparisons across O(2) concentrations.     

Mesophyll conductance to CO2 in Arabidopsis thaliana

The close rosette growth form, short petioles and small leaves of Arabidopsis thaliana make measurements with commercial gas exchange cuvettes difficult. This difficulty can be overcome by growing A. thaliana plants in 'ice-cream cone-like' soil pots. This design permitted simultaneous gas exchange and chlorophyll fluorescence measurements from which the first estimates of mesophyll conductance to CO(2) (g(m)) in Arabidopsis were obtained and used to determine photosynthetic limitations during plant ageing from c. 30-45 d. Estimations of g(m) showed maximum values of 0.2 mol CO(2) m(-2) s(-1) bar(-1), lower than expected for a thin-leaved annual species. The parameterization of the response of net photosynthesis (A(N)) to chloroplast CO(2) concentrations (C(c)) yielded estimations of the maximum velocity of carboxylation (V(c,max_Cc)) which were also lower than those reported for other annual species. As A. thaliana plants aged from 30 to 45 d, there was a 40% decline of A(N) that was entirely the result of increased diffusional limitations to CO(2) transfer, with g(m) being the largest. The results suggest that in A. thaliana A(N) is limited by low g(m) and low capacity for carboxylation. Decreased g(m) is the main factor involved in early age-induced photosynthetic decline.     

Chloroplast downsizing under nitrate nutrition restrained mesophyll conductance and photosynthesis in rice (Oryza sativa L.) under drought conditions

The phenomenon whereby ammonium enhances the tolerance of rice seedlings (Oryza sativa L., cv. 'Shanyou 63' hybrid indica China) to water stress has been reported in previous studies. To study the intrinsic mechanism of biomass synthesis related to photosynthesis, hydroponic experiments supplying different nitrogen (N) forms were conducted; water stress was simulated by the addition of polyethylene glycol. Water stress decreased leaf water potential (Ψ(leaf)) under nitrate nutrition, while it had no negative effect under ammonium nutrition. The decreased Ψ(leaf) under nitrate nutrition resulted in chloroplast downsizing and subsequently decreased mesophyll conductance to CO(2) (g(m)). The decreased g(m) and stomatal conductance (g(s)) under nitrate nutrition with water stress restrained the CO(2) supply to the chloroplast and Rubisco. The relatively higher distribution of leaf N to Rubisco under ammonium nutrition might also be of benefit for photosynthesis under water stress. In conclusion, chloroplast downsizing induced a decline in g(m), a relatively higher decrease in g(s) under nitrate nutrition with water stress, restrained the CO(2) supply to Rubisco and finally decreased the photosynthetic rate.     

Physiological effects of kaolin applications in well-irrigated and water-stressed walnut and almond trees

BACKGROUND AND AIMS: Kaolin applications have been used to mitigate the negative effects of water and heat stress on plant physiology and productivity with variable results, ranging from increased to decreased yields and photosynthetic rates. The mechanisms of action of kaolin applications are not clear: although the increased albedo reduces leaf temperature and the consequent heat stress, it also reduces the light available for photosynthesis, possibly offsetting benefits of lower temperature. The objective of this study was to investigate which of these effects are prevalent and under which conditions. METHODS: A 6% kaolin suspension was applied on well-irrigated and water-stressed walnut (Juglans regia) and almond (Prunus dulcis) trees. Water status (i.e. stem water potential, psi(s)), gas exchange (i.e. light-saturated CO2 assimilation rate, Amax; stomatal conductance, g(s)), leaf temperature (T(l)) and physiological relationships in treated and control trees were then measured and compared. KEY RESULTS: In both species, kaolin did not affect the daily course of psi(s) whereas it reduced Amax by 1-4 micromol CO2 m(-2) s(-1) throughout the day in all combinations of species and irrigation treatments. Kaolin did not reduce g(s) in any situation. Consequently, intercellular CO2 concentration (C(i)) was always greater in treated trees than in controls, suggesting that the reduction of Amax with kaolin was not due to stomatal limitations. Kaolin reduced leaf temperature (T(l)) by about 1-3 degrees C and leaf-to-air vapour pressure difference (VPD(l)) by about 0.1-0.7 kPa. Amax was lower at all values of g(s), T(l) and VPD(l) in kaolin-treated trees. Kaolin affected the photosynthetic response to the photosynthetically active radiation (PAR) in almond leaves: kaolin-coated leaves had similar dark respiration rates and light-saturated photosynthesis, but a higher light compensation point and lower apparent quantum yield, while the photosynthetic light-response curve saturated at higher PAR. When these parameters were used to model the photosynthetic response curve to PAR, it was estimated that the kaolin film allowed 63% of the incident PAR to reach the leaf. CONCLUSIONS: The main effect of kaolin application was the reduction, albeit minor, of photosynthesis, which appeared to be related to the shading of the leaves. The reduction in T(l) and VPD(l) with kaolin did not suffice to mitigate the adverse effects of heat and water stress on Amax.     

A/C(i) curve analysis across a range of woody plant species: influence of regression analysis parameters and mesophyll conductance

The analysis and interpretation of A/C(i) curves (net CO(2) assimilation rate, A, versus calculated substomatal CO(2) concentration, C(i)) is dependent upon a number of underlying assumptions. The influence of the C(i) value at which the A/C(i) curve switches between the Rubisco- and electron transport-limited portions of the curve was examined on A/C(i) curve parameter estimates, as well as the effect of mesophyll CO(2) conductance (g(m)) values on estimates of the maximum rate of Rubisco-mediated carboxylation (V(cmax)). Based on an analysis using 19 woody species from the Pacific Northwest, significant variation occurred in the C(i) value where the Rubisco- and electron transport-limited portions of the curve intersect (C(i_t)), ranging from 20 Pa to 152 Pa and averaging c. 71 Pa and 37 Pa for conifer and broadleaf species, respectively. Significant effects on estimated A/C(i) parameters (e.g. V(cmax)) may arise when preliminary estimates of C(i_t), necessary for the multiple regression analyses, are set either too high or too low. However, when the appropriate threshold is used, a significant relationship between A/C(i) and chlorophyll fluorescence estimates of carboxylation is achieved. The use of the V(cmax) parameter to describe accurately the Rubisco activity from the A/C(i) curve analysis is also dependent upon the assumption that C(i) is approximately equal to chloroplast CO(2) concentrations (C(c)). If leaf mesophyll conductance is low, C(c) will be much lower than C(i) and will result in an underestimation of V(cmax) from A/C(i) curves. A large range of mesophyll conductance (g(m)) values was observed across the 19 species (0.005+/-0.002 to 0.189+/-0.011 mol m(-2) s(-1) for Tsuga heterophylla and Quercus garryana, respectively) and, on average, g(m) was 1.9 times lower for the conifer species (0.058+/-0.017 mol m(-2) s(-1) for conifers versus 0.112+/-0.020 mol m(-2) s(-1) for broadleaves). When this mesophyll limitation was accounted for in V(cmax) estimates, considerable variation still existed between species, but the difference in V(cmax) between conifer and broadleaf species was reduced from c. 11 micromol m(-2) s(-1) to 4 micromol m(-2) s(-1). For example, A/C(i) curve estimates of V(cmax) were 31.2+/-6.2 and 42.2+/-4.4 micromol m(-2) s(-1), and A/C(c) curve estimates were 41.2+/-7.1 micromol m(-2) s(-1) and 45.0+/-4.8 micromol m(-2) s(-1), for the conifer and broadleaf species, respectively.     

Salicylic acid treatment via the rooting medium interferes with stomatal response, CO2 fixation rate and carbohydrate metabolism in tomato, and decreases harmful effects of subsequent salt stress

Salicylic acid (SA) applied at 10(-3) m in hydroponic culture decreased stomatal conductance (g(s)), maximal CO(2) fixation rate (A(max) ) and initial slopes of the CO(2) (A/C(i)) and light response (A/PPFD) curves, carboxylation efficiency of Rubisco (CE) and photosynthetic quantum efficiency (Q), resulting in the death of tomato plants. However, plants could acclimate to lower concentrations of SA (10(-7) -10(-4) m) and, after 3 weeks, returned to control levels of g(s), photosynthetic performance and soluble sugar content. In response to high salinity (100 mm NaCl), the pre-treated plants exhibited higher A(max) as a function of internal CO(2) concentration (C(i) ) or photosynthetic photon flux density (PPFD), and higher CE and Q values than salt-treated controls, suggesting more effective photosynthesis after SA treatment. Growth in 10(-7) or 10(-4) m SA-containing solution led to accumulation of soluble sugars in both leaf and root tissues, which remained higher in both plant parts during salt stress at 10(-4) m SA. The activity of hexokinase (HXK) with glucose, but not fructose, as substrate was reduced by SA treatment in leaf and root samples, leading to accumulation of glucose and fructose in leaf tissues. HXK activity decreased further under high salinity in both plant organs. The accumulation of soluble sugars and sucrose in roots of plants growing in the presence of 10(-4) m SA contributed to osmotic adjustment and improved tolerance to subsequent salt stress. Apart from its putative role in delaying senescence, decreased HXK activity may divert hexoses from catabolic reactions to osmotic adaptation.     

Physiological basis of genetic variation in leaf photosynthesis among rice (Oryza sativa L.) introgression lines under drought and well-watered conditions

To understand the physiological basis of genetic variation and resulting quantitative trait loci (QTLs) for photosynthesis in a rice (Oryza sativa L.) introgression line population, 13 lines were studied under drought and well-watered conditions, at flowering and grain filling. Simultaneous gas exchange and chlorophyll fluorescence measurements were conducted at various levels of incident irradiance and ambient CO(2) to estimate parameters of a model that dissects photosynthesis into stomatal conductance (g (s)), mesophyll conductance (g (m)), electron transport capacity (J (max)), and Rubisco carboxylation capacity (V (cmax)). Significant genetic variation in these parameters was found, although drought and leaf age accounted for larger proportions of the total variation. Genetic variation in light-saturated photosynthesis and transpiration efficiency (TE) were mainly associated with variation in g (s) and g (m). One previously mapped major QTL of photosynthesis was associated with variation in g (s) and g (m), but also in J (max) and V (cmax) at flowering. Thus, g (s) and g (m), which were demonstrated in the literature to be responsible for environmental variation in photosynthesis, were found also to be associated with genetic variation in photosynthesis. Furthermore, relationships between these parameters and leaf nitrogen or dry matter per unit area, which were previously found across environmental treatments, were shown to be valid for variation across genotypes. Finally, the extent to which photosynthesis rate and TE can be improved was evaluated. Virtual ideotypes were estimated to have 17.0% higher photosynthesis and 25.1% higher TE compared with the best genotype investigated. This analysis using introgression lines highlights possibilities of improving both photosynthesis and TE within the same genetic background.     

Effects of HgCl(2) on CO(2) dependence of leaf photosynthesis: evidence indicating involvement of aquaporins in CO(2) diffusion across the plasma membrane

Experiments were conducted to examine whether mercury-sensitive aquaporins facilitate photosynthetic CO(2) diffusion across the plasma membrane of leaf mesophyll cells. Discs without abaxial epidermes from Vicia faba leaflets were treated with HgCl(2), an inhibitor of aquaporins. Hydraulic conductivity of the plasma membrane of these discs, measured as the weight loss of the discs in the 1 M sorbitol solution, was inhibited by sub-mM concentrations of HgCl(2) by 70 to 80%. Photosynthetic CO(2) fixation was also inhibited by the HgCl(2) treatment in a similar concentration range. When 0.3 mM HgCl(2) solution was fed to the V. faba leaflets with intact epidermes via the transpiration stream, the rate of photosynthesis on leaf area basis (A) measured at photosynthetically active photon flux density of 700 micromol m(-2) s(-1) and at leaf temperature of 25 degrees C, decreased by about 20 to 30% at any CO(2) concentration in the intercellular spaces (C(i)). However, when CO(2) concentration in the chloroplast stroma (C(c)) was calculated from fluorescence and gas exchange data and A was plotted against C(c), A at low C(c) concentrations did not differ before and after the treatment. The conductance for CO(2) diffusion from the intercellular spaces to the chloroplast stroma (g(i)) decreased to 40 and 30% of the control value, when the leaflets were fed with 0.3 mM and 1.2 mM HgCl(2), respectively. Similar results were obtained with leaves of Phaseolus vulgaris. Although effects of HgCl(2) were not specific, the present results showed that HgCl(2) consistently lowered g(i). It is, thus, probable that the photosynthetic CO(2) uptake across the plasma membrane of the mesophyll cells is facilitated by mercury-sensitive aquaporins.     

A Mathematical Model for Leaf Photosynthesis of Mulberry

A mathematical model of leaf photosynthesis has been established. In this model, the processes of photosynthesis are divided into two parts, ie., the carboxylation process driven by light which is dependent on temperature and CO2 concentration, and the diffusion of CO2 from atmosphere to the carboxylation site. Finatly, CO2 uptake by the leaf is understood as dependent on 1), the CO2 response curve of the leaf mesophyll and 2). the CO2 partial pressure in the intercellular space in leaf. The COs response curve of the leaf photosynthesis is described mathematically in terms of carboxylation efficiency (Ca) or its initial slope and the photosynthetic capacity (Pm) or the CO2-saturated uptake rate of CO2 uptake, and dark respiration (Rd). The dependency of photosynthesis on leaf temperature and incident light intensity is incorporated into variations of those parameters which establish an appropriate response to internal CO2 pressure for particular light and temperature conditions prevailing at any time. Secondly the interactiion of stomata with photosynthesis is represented as an empirical relation between stomatal conductance and a combined environmental physiological index, APn·Hx/CaThe parameters used in the modelwere estimated with Marquardt-Newton method for non-linear function. Field measurements of mulberry leaf photosynthesis provided a data set for model testing. The resuks show that the simulated values of the model agree well with observed data. The model was used to analyse the response surface of leaf conductance and photosynthesis to environmental factors—Applications and limitations of the model are discussed