SPECIALIZATION OF MESOPHYLL MORPHOLOGY IN RELATION TO C4 PHOTOSYNTHESIS IN THE POACEAE

Our current understanding of the photosynthetic process in species utilizing the C₄ photosynthetic pathway suggests that photosynthetic efficiency should be enhanced by: 1) maximizing the conductance of the gas phase transport pathway from the leaf exterior to the mesophyll cell surfaces; 2) maximizing cytoplasmic connections and metabolite transport between bundle sheath and mesophyll parenchyma cells; and 3) minimizing the conductance of the gas phase transport pathway from the bundle sheath cells to the leaf exterior. In this study we have examined several species in the Poaceae with C₄ photosynthesis to determine if there is any evidence for anatomical specialization which would lead to enhanced photosynthetic efficiency by these processes. Observations with light and scanning electron microscopes revealed specializations in mesophyll cell morphology and arrangement which include branched cells forming intercellular channels. These specializations are hypothesized to contribute to photosynthetic efficiency through its influence on the above transport processes.     

Variation in mesophyll anatomy and photosynthetic capacity during leaf development in a deciduous mesophyte fruit tree (<Emphasis Type="Italic">Prunus persica</Emphasis>) and an evergreen sclerophyllous Mediterranean shrub (<Emphasis Type="Italic">Olea europaea</Emphasis>)

The relative importance that biomechanical and biochemical leaf traits have on photosynthetic capacity would depend on a complex interaction of internal architecture and physiological differences. Changes in photosynthetic capacity on a leaf area basis and anatomical properties during leaf development were studied in a deciduous tree, Prunus persica, and an evergreen shrub, Olea europaea. Photosynthetic capacity increased as leaves approached full expansion. Internal CO₂ transfer conductance (g _i) correlated with photosynthetic capacity, although, differences between species were only partially explained through structural and anatomical traits of leaves. Expanding leaves preserved a close functional balance in the allocation of resources of photosynthetic component processes. Stomata developed more rapidly in olive than in peach. Mesophyll thickness doubled from initial through final stages of development when it was twice as thick in olive as in peach. The surface area of mesophyll cells exposed to intercellular air spaces per unit leaf area tended to decrease with increasing leaf expansion, whereas, the fraction of mesophyll volume occupied by the intercellular air spaces increased strongly. In the sclerophyllous olive, structural protection of mesophyll cells had priority over efficiency of photochemical mechanisms with respect to the broad-leaved peach. The photosynthetic capacity of these woody plants during leaf development relied greatly on mesophyll properties, more than on leaf mass per area ratio (LMA) or nitrogen (N) allocation. Age-dependent changes in diffusion conductance and photosynthetic capacity affected photosynthetic relationships of peach versus olive foliage, evergreen leaves maturing functionally and structurally a bit earlier than deciduous leaves in the course of adaptation for xeromorphy.     

Leaf Factors Affecting Light-Saturated Photosynthesis in Ecotypes of Solidago virgaurea from Exposed and Shaded Habitats

Plants of Solidago virgaurea L. from exposed and shaded habitats differ with respect to the response of the photosynthetic apparatus to the level of irradiance during growth. An analysis was carried out on leaf characteristies which might be responsible for the differences established in the rates of Hght-saturated CO₂ uptake. The clones were grown in controlled environment chambers at high and low levels of irradiance. Light-saturated rates of photosynthesis and transpiration were measured at natural and lower ambient CO₂ concentrations. A low temperature dependence of light-saturated CO₂ uptake at natural CO₂ concentrations, and a strong response to changes in stomatal width, suggested that the rate of CO₂ transfer from ambient air towards reaetion sites in chloroplasts was mainly limiting the pholosynthetic rate. Resistances to transfer of CO₂ for different parts of the pathway were calculated. There was a weak but significant correlation between stomatal conductance and the product stomatal frequency ± pore length. Mesopbyll conductance and dry weight per unit area were highly correlated in leaves not damaged by high irradiance. This suggests that mesophyll conductance increases with increasing cross sectional area (per unit leaf area) of the pathways of CO₂ transfer in the mesophyll from cell surfaces to reaction sites. The higher light-saturated photosynthesis in clones from exposed habitats when grown at high irradiance than when grown at low irradiance was attributable mainly to a lower mesophyll resistance. In shade clones the effect upon CO₂ uptake of the increase in leaf thickness when grown at high irradiance was counteracted by the associated inactivation of the photosynthetic apparatus. The difference in CO₂ uptake present between clones from exposed and shaded habitats when preconditioned to high irradiance resulted from differences in both mesophyll and stomatal resistances. A few hybrid clones of an F₁-population from a cross between a clone from an exposed habitat and a clone from a shaded habitat reacted, on the whole, in the same way as the exposed habitat parent. When grown at high irradiance, the hybrid clones showed higher photosynthetic rates than either parent; this was largely attributable to the unusually low stomatal resistance of the hybrid leaves.     

Variation of photosynthetic capacity with leaf age in an alpine orchid, Cypripedium flavum

Photosynthetic rate, chlorophyll fluorescence, leaf nitrogen and chlorophyll content of Cypripedium flavum were studied at different leaf ages. The photosynthetic capacity changed significantly with leaf age. Net photosynthesis and chlorophyll content peaked when leaf age was 60 days, decreasing at 30, 90 and 120 days. Stomatal conductance showed the highest value at 60 days, while mesophyll conductance decreased with increasing leaf age. Both leaf nitrogen content per unit area and leaf nitrogen content per unit mass decreased with increasing leaf age. The age-dependent variation in photosynthetic capacity could be linked to the changes in biochemical efficiency, leaf nitrogen content and CO₂ diffusion limitation.     

Stomatal function,density and pattern,and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1‐1 mutants

• Stomata modulate the exchange of water and CO₂ between plant and atmosphere. Although stomatal density is known to affect CO₂ diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO₂ assimilation is not fully understood.
• We used wild types Col‐0 and C24 and stomatal mutants sdd1‐1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO₂ assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed.
• Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1‐1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis.
• Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO₂ transport.

     

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.     

Drought response of mesophyll conductance in forest understory species – impacts on water‐use efficiency and interactions with leaf water movement

Regulation of stomatal (g_s) and mesophyll conductance (g_m) is an efficient means for optimizing the relationship between water loss and carbon uptake in plants. We assessed water‐use efficiency (WUE)‐based drought adaptation strategies with respect to mesophyll conductance of different functional plant groups of the forest understory. Moreover we aimed at assessing the mechanisms of and interactions between water and CO₂ conductance in the mesophyll. The facts that an increase in WUE was observed only in the two species that increased g_m in response to moderate drought, and that over all five species examined, changes in mesophyll conductance were significantly correlated with the drought‐induced change in WUE, proves the importance of g_m in optimizing resource use under water restriction. There was no clear correlation of mesophyll CO₂ conductance and the tortuosity of water movement in the leaf across the five species in the control and drought treatments. This points either to different main pathways for CO₂ and water in the mesophyll either to different regulation of a common pathway.     

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.     

Estimation of Mesophyll Conductance to CO(2) Flux by Three Different Methods

The resistance to diffusion of CO₂ from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, ¹³C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO₂ diffusion determined in unstressed plants at high light. The relatively low CO₂ partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O₂ sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO₂ partial pressure upon switching to low O₂. We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO₂ and O₂ partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO₂-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO₂ level.     

Structural Adaptation of the Leaf Mesophyll to Shading

Structural characteristics of the mesophyll were studied in five boreal grass species experiencing a wide range of light and water supply conditions. Quantitative indices of the palisade and spongy mesophyll tissues (cell and chloroplast sizes, the number of chloroplasts per cell, the total cell and chloroplast surface area per unit leaf surface area) were determined in leaves of each of the species. The cell surface area and the cell volume in spongy mesophyll were determined with a novel method based on stereological analysis of cell projections. An important role of spongy parenchyma in the photosynthetic apparatus was demonstrated. In leaves of the species studied, the spongy parenchyma constituted about 50% of the total volume and 40% of the total surface area of mesophyll cells. The proportion of the palisade to spongy mesophyll tissues varied with plant species and growth conditions. In a xerophyte Genista tinctoria, the total cell volume, cell abundance, and the total surface area of cells and chloroplasts were 30–40% larger in the palisade than in the spongy mesophyll. In contrast, in a shade-loving species Veronica chamaedris, the spongy mesophyll was 1.5–2 times more developed than the palisade mesophyll. In mesophyte species grown under high light conditions, the cell abundance and the total cell surface area were 10–20% greater in the palisade mesophyll than in the spongy parenchyma. In shaded habitats, these indices were similar in the palisade and spongy mesophyll or were 10–20% lower in the palisade mesophyll. In mesophytes, CO₂ conductance of the spongy mesophyll accounted for about 50% of the total mesophyll conductance, as calculated from the structural characteristics, with the mesophyll CO₂ conductance increasing with leaf shading.     

Genome-Dependent Factors in the Development of Leaf Phototrophic Tissues in Diploid and Alloploid Wheat Species

Growth and mesostructure of the photosynthetic apparatus were studied in leaves of ten Triticum L. species. Plants with the A^u genome were shown to develop larger leaf assimilation areas due to expanding areas of individual leaves and an increase in the absolute growth rate. Leaf and mesophyll thickness and mesophyll cell size decreased in the G-genome species. Leaf compactness, which depended on cell size and number per unit leaf area and leaf folding, determined the specific patterns of internal leaf organization in wheat species with diverse genotypes. These patterns did not affect cell plastid-to-cytoplasm ratio as shown by the stable indices of cell surface area/cell volume, cell surface area per chloroplast, and cell volume per chloroplast. The structural indices of leaf phototrophic tissues, mesophyll density, and mesophyll CO₂ conductance in alloploids, as compared to diploid species, depended on both ploidy and genome constitution.     

Decrease of mesophyll conductance to CO<Subscript>2</Subscript> is a possible mechanism of abscisic acid influence on photosynthesis in seedlings of pea and wheat

Treatment of plants by phytohormones is a perspective method of regulation of the plant stress resistance and productivity. However, the mechanisms of phytohormone effects on physiological processes require investigations. The aim of this work was the analysis of the influence of exogenous abscisic acid (ABA) on photosynthesis in seedlings of pea and wheat, and in particular, an estimation of the involvement of the mesophyll conductance to CO₂ in the realization of the ABA effects. A standard system for recording of photosynthetic parameters and a system for intracellular measurements of electrical activity were used in the experiments. It was shown that the effect of exogenous ABA on photosynthesis was most prominent 1 day after spraying of a plant. A detailed analysis of photosynthetic processes showed that ABA decreased photosynthetic assimilation of CO₂ and increased cyclic electron flow in plants under study; these processes were interconnected. A decrease of the mesophyll conductance to CO₂ was probably a mechanism of the decrease in the photosynthetic assimilation of CO₂, because ABA did not significantly influence water conductance of a leaf and parameters of the CO₂ fixation in Calvin–Benson cycle. It is likely that the decrease of the mesophyll conductance to CO₂ was related with a decrease of the plasma membrane conductance to carbon dioxide. Inactivation of H⁺-ATP-ase and changes in extracellular pH can be a mechanism of this decrease. A decrease of the metabolic component of the electrical resting potential after the ABA treatment testifies in favor of this possibility.     

Limitations due to water stress on leaf net photosynthesis of Quercus coccifera in the Portuguese evergreen scrub

Summary Gas exchange characteristics in leaves of the sclerophyll shrub Quercus coccifera were studied in the natural habitat in Portugal during spring and during the summer dry period. Compared to other sclerophyll species growing at the same site, photosynthesis in leaves of Quercus coccifera was less affected by water stress. Moderate water stress after six weeks of drought led to large decreases in stomatal conductance but no change in mesophyll photosynthetic capacity as compared to late spring. Leaf internal CO₂ pressure remained near 220 bar during diurnal courses in the spring. On midsummer days, leaf internal CO₂ decreased from a late morning value of 200 bar to a late afternoon value of approximately 150 bar. In contrast to Quercus suber (Tenhunen et al. 1984), restriction of CO₂ supply due to stomatal closure reduced net CO₂ uptake at midday and in the afternoon during midsummer. A decrease in leaf carboxylation efficiency and an increase in CO₂ compensation point at midday also played an important role in determining the diurnal course of net photosynthesis. During the late stages of drought in September, severe water stress led to reduction in mesophyll photosynthetic capacity and further reduction in leaf conductance. The observed decrease in mesophyll photosynthetic capacity was correlated with decrease in the daily minimum leaf water potential to greater negative values than-30 bar. At this time, CO₂ saturated photosynthetic rates decreased as much as 50% over the course of a day when measured at constant saturating light, 32° C leaf temperature, and a water vapor mole fraction difference between leaf and air of 30 mbar bar^-1.     

Seasonal changes in photosynthetic characteristics of Anemone raddeana,a spring-active geophyte,in the temperate region of Japan

Summary Seasonal changes in the photosynthetic characteristics of intact involucral leaves of Anemone raddeana were investigated under laboratory conditions. Net photosynthesis and constant water vapor pressure deficit showed almost the same seasonal trend. They increased rapidly from mid-April immediately after unfolding of the leaves and reached the maximum in late-April, before the maximum expansion of the leaves. They retained the maximum values until early-May and then decreased toward late-May with a progress of leaf senescence. The calculated values of intercellular CO₂ concentration and relative stomatal limitation of photosynthesis showed no significant change throughout the season. The carboxylation efficiency as assessed by the initial slope of Ci-photosynthesis curve and the net photosynthesis under a high Ci regime varied seasonally in parallel with the change of the light-saturated photosynthesis. The results indicate that the seasonal changes in light-saturated net photosynthesis are not due to a change of stomatal conductance, but to a change in the photosynthetic capacity of mesophyll. Nevertheless, leaf conductance changed concomitantly with photosynthetic capacity, indicating that the seasonal change in stomatal conductance is modulated by the mesophyll photosynthetic capacity such that the intercellular CO₂ concentrations is maintained constant. The shape of light-photosynthesis curve was similar to that of sun-leaf type. The quantum yield also changed simultaneously with the photosynthetic capacity throughout the season.Contribution No. 2965 from the Institute of Low Temperature Science     

EVOLUTION OF C3 AND C4 PLANTS ALONG AN ENVIRONMENTAL MOISTURE GRADIENT: PATTERNS OF PHOTOSYNTHETIC DIFFERENTIATION IN HAWAIIAN SCAEVOLA AND EUPHORBIA SPECIES

The endemic Hawaiian species of Scaevola and Euphorbia grow in a wide variety of native habitats and exhibit a wide range of variation in photosynthetic responses. Light-saturated photosynthetic capacities range from 12.0 to 24.7 μmol CO₂ m^−-2 s^−-1 in the Scaevola species and from 18.2 to 51.4 μmol CO₂ m^−-2 s^−-1 in the Euphorbia species. Within each genus, differences in light-saturated photosynthetic capacity are paralleled by differences in mesophyll and leaf conductances to CO₂. Within each habitat, the C₄ Euphorbia species exhibits a significantly higher photosynthetic capacity and a significantly higher mesophyll conductance than the corresponding C₃ Scaevola species. These differences are greatest in the dry scrub habitat and least in the wet forest habitat. One photosynthetic characteristic that exhibits little variation among the species within each genus, yet that exhibits a consistently large difference between the species within each habitat, is photosynthetic water-use efficiency. The C₄ Euphorbia species possess water-use efficiencies that are 2–3½ times as high as those of the C₃ Scaevola species, regardless of whether these species are native to very dry or very wet habitats. At present, the ecological significance of this large inherent difference in photosynthetic water-use efficiency is unknown. Indeed, it appears that neither photosynthetic pathway has imposed any major inherent constraints on the ability of the Scaevola and Euphorbia species to diversify into a wide variety of habitats.     

Separating the contribution of the upper and lower mesophyll to photosynthesis in Zea mays L. leaves

The appearance of transverse sections of maize leaves indicates the existence of two airspace systems serving the mesophyll, one connected to the stomata of the upper epidermis and the other to the stomata of the lower surface, with few or no connections between the two. This study tests the hypothesis that the air-space systems of the upper and lower mesophyll are separated by a defined barrier of measurable conductance. A mathematical procedure, based on this hypothesis, is developed for the quantitative separation of the contributions made by the upper and lower halves of the mesophyll to carbon assimilation using gasexchange data. Serial paradermal sections and three-dimensional scanning-electron-microscope images confirmed the hypothesis that there were few connections between the two air-systems. Simultaneous measurements of nitrous-oxide diffusion across the leaf and of transpiration from the two surfaces showed that the internal conductance was about 15% of the maximum observed stomatal conductance. This demonstrates that the poor air-space connections, indicated by microscopy, represent a substantial barrier to gas diffusion. By measuring the CO₂ and water-vapour fluxes from each surface independently, the intercellular CO₂ concentration (c _i) of each internal air-space system was determined and the flux between them calculated. This allowed correction of the apparent CO₂ uptake at each surface to derive the true CO₂ uptake by the mesophyll cells of the upper and lower halves of the leaf. This approach was used to analyse the contribution of the upper and lower mesophyll to CO₂ uptake by the leaf as a whole in response to varying light levels incident on the upper leaf surface. This showed that the upper mesophyll was light-saturated by a photon flux of approx. 1000 mol·m^-2·s^-1 (i.e. about one-half of full sunlight). The lower mesophyll was not fully saturated by photon fluxes of nearly double full sunlight. At low photon fluxes the c _i of the upper mesophyll was significantly less than that of the lower mesophyll, generating a significant upward flux of CO₂. At light levels equivalent to full sunlight, and above, c _i did not differ significantly between the two air space systems. The physiological importance of the separation of the air-space systems of the upper and lower mesophyll to gas exchange is discussed.Abbreviations and symbols A net leaf CO₂ uptake rate - A _upper ^app. and A _lower ^app. net rates of CO₂ uptake across the upper and lower surfaces - A _upper and A _lower derived net rates of CO₂ uptake by the upper and lower mesophyll - A _upward net flux of CO₂ from the lower to upper mesophyll - c _a, c _{a, upper} and c _{a, lower} the CO₂ concentrations in the air around the leaf above the upper surface and below the lower surface - c _N2O the concentration of N₂O in the air around the leaf - c _i, c _{i, upper} and c _{i, lower} the mesophyll intercellular CO₂ concentration of the whole leaf, the upper mesophyll and the lower mesophyll - g _i leaf internal conductance to CO₂ - g _s, g _{s, lower} and g _{s, upper} the stomatal conductance of the whole leaf, the lower surface and the upper surface - g the total conductance across the leaf - Q the photosynthetically active photon flux density     

Interspecific difference in the photosynthesis–nitrogen relationship: patterns,physiological causes,and ecological importance

The photosynthesis–nitrogen relationship is significantly different among species. Photosynthetic capacity per unit leaf nitrogen, termed as photosynthetic nitrogen-use efficiency (PNUE), has been considered an important leaf trait to characterise species in relation to their leaf economics, physiology, and strategy. In this review, I discuss (1) relations between PNUE and species ecology, (2) physiological causes and (3) ecological implications of the interspecific difference in PNUE. Species with a high PNUE tend to have high growth rates and occur in disturbed or high productivity habitats, while those with a low PNUE occur in stressful or low productivity habitats. PNUE is an important leaf trait that correlates with other leaf traits, such as leaf mass per area (LMA) and leaf life span, irrespective of life form, phylogeny, and biomes. Various factors are involved in the interspecific difference. In particular, nitrogen allocation within leaves and the mesophyll conductance for CO₂ diffusion are important. To produce tough leaves, plants need to allocate more biomass and nitrogen to make thick cell walls, leading to a reduction in the mesophyll conductance and in nitrogen allocation to the photosynthetic apparatus. Allocation of biomass and nitrogen to cell walls may cause the negative relationship between PNUE and LMA. Since plants cannot maximise both PNUE and leaf toughness, there is a trade-off between photosynthesis and persistence, which enables the existence of species with various leaf characteristics on the earth.     

Seasonal changes in photosynthetic characteristics of Pachysandra terminalis (Buxaceae), an evergreen woodland chamaephyte,in the cool temperate regions of Japan

Summary Seasonal changes in photosynthetic capacity, and photosynthetic responses to intercellular CO₂ concentration and irradiance were investigated under laboratory conditions on intact leaves of Pachysandra terminalis. Photosynthetic capacity and stomatal conductance under saturating light intensity and constant water vapor pressure deficit showed almost the same seasonal trend. They increased from early June just after the expansion of leaves, reached the maximum in late-Septemer, and then decreased to winter. In over-wintering leaves they recovered and increased immediately after snow-melting, reached a first maximum in late April, and then decreased to early July in response to the reduction of light intensity on the forest floor. There-after, they increased from mid August, reached a second maximum in late September, and then decreased to winter. The parallel changes of photosynthesis and stomatal conductane indicate a more or less constant intercellular CO₂ concentration throughout the year. The calculated values of relative stomatal limitation of photosynthesis were nearly constant throughout the year, irrespective of leaf age. The results indicate that the seasonal changes in light-saturated photosynthetic capacity are not due to a change of stomatal conductance, but to a change in the photosynthetic capacity of mesophyll. Indeed, carboxylation efficiency assessed by the inital slope of the Ci-photosynthesis curve changed in proportion to seasonal changes of the photosynthetic capacity in both current-year and over-wintered leaves. High photosynthetic capacity in current-year leaves as compared with one-year-old leaves was also due to the high photosynthetic capacity of mesophyll. Nevertheless, stomatal conductance changed in proportion to photosynthetic capacity, indicating that stomatal conductance is regulated by the mesophyll photosynthetic capacity such that the intercellular CO₂ concentrations are maintained constant. The quantum yield also changed seasonally parallel with that in the photosynthetic capacity.Contribution No. 2893 from the Institute of Low Temperature Science     

喀斯特石漠化地区土壤养分对泡核桃功能性状的影响

为了探究喀斯特石漠化地区植物叶片功能性状及影响因素,以及揭示其对石漠化环境的适应机理,该文以中国南方喀斯特高原峡谷地区的泡核桃(Juglans sigillata)为对象,揭示土壤养分对叶片结构和光合性状的影响效应。结果表明：(1)泡核桃叶功能性状随石漠化等级增加,叶面积减小,比叶面积增大,叶干物质含量和叶组织密度先降后升,蒸腾速率、胞间CO₂浓度、气孔导度和光能利用率先下降后升高,其他性状变化趋势不显著。(2)冗余分析表明土壤养分能够解释37.4%的光合性状变异与53.4%的结构性状变异,其中全磷和溶解性有机碳对光合性状影响最大,而对结构性状影响最显著的是碱解氮和速效磷。(3)比叶面积分别与叶干物质含量极显著负相关,与净光合速率极显著正相关,叶厚度与叶组织密度极显著负相关,蒸腾速率与胞间CO₂浓度、气孔导度极显著正相关,水分利用速率与蒸腾速率、胞间CO₂浓度、气孔导度极显著负相关,光能利用率与净光合速率显著正相关。研究结果表明,泡核桃为适应喀斯特石漠化的特殊生境采取增强生长功能性状,同时提高资源获取能力的开拓型生长策略...     

Gas-exchange analysis of chloroplastic fructose-1,6-bisphosphatase antisense potatoes at different air humidities and at elevated CO2

Gas-exchange measurements were performed to analyze the leaf conductances and assimilation rates of potato (Solanum tuberosum L. cv. Desireé) plants expressing an antisense construct against chloroplastic fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) in response to increasing photon flux densities, different relative air humidities and elevated CO₂ concentrations. Assimilation rates (A) and transpiration rates (E) were observed during a stepwise increase of photon flux density. These experiments were carried out under atmospheric conditions and in air containing 500 μmol mol⁻¹ CO₂. In both gas atmospheres, two levels of relative air humidity (60–70% and 70–80%) were applied in different sets of measurements. Intercellular CO₂ concentration, leaf conductance, air-to-leaf vapour pressure deficit, and instantaneous water-use efficiency (A/E) were determined. As expected, assimilation rates of the FBPase antisense plants were significantly reduced as compared to the wild type. Saturation of assimilation rates in transgenic plants occurred at a photon flux density of 200 μmol m⁻² s⁻¹, whereas saturation in wild type plants was observed at 600 μmol m⁻² s⁻¹. Elevated ambient CO₂ levels did not effect assimilation rates of transgenic plants. At 70–80% relative humidity and atmospheric CO₂ concentration the FBPase antisense plants had significantly higher leaf conductances than wild-type plants while no difference emerged at 60–70%. These differences in leaf conductance vanished at elevated levels of ambient CO₂. Stomatal response to different relative air humidities was not affected by mesophyll photosynthetic activity. It is suggested that the regulation of stomatal opening upon changes in photon flux density is merely mediated by a signal transmitted from mesophyll cells, whereas the intercellular CO₂ concentration plays a minor role in this kind of stomatal response. The results are discussed with respect to stomatal control by environmental parameters and mesophyll photosynthesis. Received: 24 September 1998 / Accepted: 9 February 1999