The role of the epidermis in the generation of the circadian rhythm of carbon dioxide fixation in leaves of Bryophyllum fedtschenkoi

The role of the epidermis in the generation of the endogenous circadian rhythm of CO₂ exchange in leaves of Bryophyllum fedtschenkoi has been examined. At 25° C the rhythm of CO₂ output exhibited by whole leaves kept in continuous darkness and an initially CO₂-free air stream also occurs in isolated pieces of mesophyll. The sensitivity to light of the rhythms in whole leaves and in isolated mesophyll appears to be identical. At 15° C, however, no rhythm is observed in isolated mesophyll tissue, despite there being a conspicuous rhythm in intact leaves. The rhythm of net CO₂ assimilation in whole leaves kept in continuous light and a stream of normal air at either 25° C or at 15° C is abolished by removal of the epidermis, although at 15° C and under the higher of the two light levels used, there is an indication that rhythmicity may begin to reappear after the third day of the experiment. Thus, only under certain environmental conditions is the rhythm of CO₂ exchange in Bryophyllum leaves independent of the epidermis. The results indicate that the rhythm of carbon dioxide fixation in continuous darkness and CO₂-free air is generated primarily in the mesophyll cells, whereas the rhythm in continuous light and normal air is generated in the stomatal guard cells or in an interaction of these cells with the mesophyll cells.Abbreviation PEPCase phosphoenolpyruvate carboxylase     

Effects of salt on growth,ion accumulation,photosynthesis and leaf anatomy of the mangrove,<Emphasis Type="Italic"> Bruguiera parviflora</Emphasis>

The effects of a range of salinity (0, 100, 200 and 400 mM NaCl) on growth, ion accumulation, photosynthesis and anatomical changes of leaves were studied in the mangrove, Bruguiera parviflora of the family Rhizophoraceae under hydroponically cultured conditions. The growth rates measured in terms of plant height, fresh and dry weight and leaf area were maximal in culture treated with 100 mM NaCl and decreased at higher concentrations. A significant increase of Na⁺ content of leaves from 46.01 mmol m^-2 in the absence of NaCl to 140.55 mmol m^-2 in plants treated with 400 mM NaCl was recorded. The corresponding Cl^- contents were 26.92 mmol m^-2 and 97.89 mmol m^-2. There was no significant alteration of the endogenous level of K⁺ and Fe²⁺ in leaves. A drop of Ca²⁺ and Mg²⁺ content of leaves upon salt accumulation suggests increasing membrane stability and decreased chlorophyll content respectively. Total chlorophyll content decreased from 83.44 g cm^-2 in untreated plants to 46.56 g cm^-2 in plants treated with 400 mM NaCl, suggesting that NaCl has a limiting effect on photochemistry that ultimately affects photosynthesis by inhibiting chlorophyll synthesis (ca. 50% loss in chlorophyll). Light-saturated rates of photosynthesis decreased by 22% in plants treated with 400 mM NaCl compared with untreated plants. Both mesophyll and stomatal conductance by CO₂ diffusion decreased linearly in leaves with increasing salt concentration. Stomatal and mesophyll conductance decreased by 49% and 52% respectively after 45 days in 400 mM NaCl compared with conductance in the absence of NaCl. Scanning electron microscope study revealed a decreased stomatal pore area (63%) in plants treated with 400 mM NaCl compared with untreated plants, which might be responsible for decreased stomatal conductance. Epidermal and mesophyll thickness and intercellular spaces decreased significantly in leaves after treatment with 400 mM NaCl compared with untreated leaves. These changes in mesophyll anatomy might have accounted for the decreased mesophyll conductance. We conclude that high salinity reduces photosynthesis in leaves of B. parviflora, primarily by reducing diffusion of CO₂ to the chloroplast, both by stomatal closure and by changes in mesophyll structure, which decreased the conductance to CO₂ within the leaf, as well as by affecting the photochemistry of the leaves.     

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     

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

Some effects of air temperature and humidity on crop and leaf photosynthesis, transpiration and resistance to gas transfer

Measurements of carbon dioxide exchange and transpiration were made, at various air temperatures, on wheat and barley using a field enclosure system. From these were derived the stomatal and mesophyll resistances to carbon dioxide transfer. Optimum temperatures for net CO₂ uptake were about 24°C for wheat and barley. Above these optima, as temperature increased so net CO₂ uptake rates decreased, because of increasing stomatal and mesophyll resistances; transpiration rates decreased in wheat but were constant in barley. In laboratory growth cabinets, wheat plants were subjected to different regimes of temperature and humidity. Optimum temperature for net CO₂ uptake of individual leaves was 25°C. At constant humidity, a decline in net uptake rates above 25°C was associated with large increases in mesophyll resistance. At a constant 25°C, as the vapour pressure deficit (v.p.d. was increased above 1 k Pa (10 mb) v.p.d. the net uptake declined, with an increase in mesophyll resistance and a small increase in stomatal resistance. When the v.p.d. exceeded 1 k Pa at a temperature of 30°C, conditions that are experienced by field plants, then there were large increases in both mesophyll and stomatal resistances and the net uptake rates declined. Photo-respiration, as measured by CO₂ uptake in oxygen-free air, was independent of temperature, but both dark respiration and CO₂ compensation concentration increased with temperature.     

Low CO(2) Prevents Nitrate Reduction in Leaves

The correlation between CO₂ assimilation and nitrate reduction in detached spinach (Spinacia oleracea L.) leaves was examined by measuring light-dependent changes in leaf nitrate levels in response to mild water stress and to artificially imposed CO₂ deficiency. The level of extractable nitrate reductase (NR) activity was also measured. The results are: (a) In the light, detached turgid spinach leaves reduced nitrate stored in the vacuoles of mesophyll cells at rates between 3 and 10 micromoles per milligram of chlorophyll per hour. Nitrate fed through the petiole was reduced at similar rates as storage nitrate. Nitrate reduction was accompanied by malate accumulation. (b) Under mild water stress which caused stomatal closure, nitrate reduction was prevented. The inhibition of nitrate reduction observed in water stressed leaves was reversed by external CO₂ concentrations (10-15%) high enough to overcome stomatal resistance. (c) Nitrate reduction was also inhibited when turgid leaves were kept in CO₂-free air or at the CO₂-compensation point or in nitrogen. (d) When leaves were illuminated in CO₂-free air, activity of NR decreased rapidly. It increased again, when CO₂ was added back to the system. The half-time for a 50% change in activity was about 30 min. It thus appears that there is a rapid inactivation/activation mechanism of NR in leaves which couples nitrate reductase to net photosynthesis.     

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     

Water use efficiency as a function of leaf age and position within the cotton canopy

The gas exchange properties of whole plant canopies are an integral part of crop productivity and have attracted much attention in recent years. However, insufficient information exists on the coordination of transpiration and CO₂ uptake for individual leaves during the growing season. Single-leaf determinations of net photosynthesis (Pn), transpiration (E) and water use efficiency (WUE) for field-grown cotton (Gossypium hirsutum L.) leaves were recorded during a 2-year field study. Measurements were made at 3 to 4 day intervals on the main-stem and first three sympodial leaves at main-stem node 10 from their unfolding through senescence. Results indicated that all gas exchange parameters changed with individual main-stem and sympodial leaf age. Values of Pn, E and WUE followed a rise and fall pattern with maximum rates achieved at a leaf age of 18 to 20 days. While no significant position effects were observed for Pn, main-stem and sympodial leaves did differ in E and WUE particularly as leaves aged beyond 40 days. For a given leaf age, the main-stem leaf had a significantly lower WUE than the three sympodial leaves. WUE's for the main-stem and three sympodial leaves between the ages of 41 to 50 days were 0.85, 1.30, 1.36 and 1.95 μmol CO₂ mmol⁻¹ H₂O, respectively. The mechanisms which mediated leaf positional differences for WUE were not strictly related to changes in stomatal conductance (g_s·H₂O) since decreases in g_s·H₂O with leaf age were similar for the four leaves. However, significantly different radiant environments with distance along the fruiting branch did indicate the possible involvement of mutual leaf shading in determining WUE. The significance of these findings are presented in relation to light competition within the plant canopy during development.     

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; )     

Effect of temperature on net CO2 assimilation and photosystem II quantum yield of electron transfer of French bean (Phaseolus vulgaris L.) leaves during drought stress

The effect of leaf temperature on stomatal conductance and net CO₂ uptake was studied on French bean (Phaseolus vulgaris L.) using either dehydrated attached leaves (25–40% water deficit) or cut leaves supplied with 10^–4 M abscisic acid (ABA) solution to the transpiration stream. Decreasing leaf temperature caused stomatal opening and increased net CO₂ uptake (which was close to zero at around 25° C) to a level identical to that of control leaves (without water deficit) at around 15° C. (i) The ABA effect on stomatal closure was modulated by temperature and, presumably, ABA is at least partly responsible for stomatal closure of french bean submitted to a drought stress. (ii) For leaf temperatures lower than 15° C, net CO₂ uptake was no longer limited by water deficit even on very dehydrated leaves. This shows that dehydrated leaves retain a substantial part of their photosynthetic capacity which can be revealed at normal CO₂ concentrations when stomata open at low temperature. In contrast to leaves fed with ABA, decreasing the O₂ concentration from 21% to 1% O₂ did not increase either the rate of net CO₂ uptake or the thermal optimum for photosynthesis of dehydrated leaves. The quantum yield of PSII electron flow (measured by F/F_m) was lower in 1% O₂ than in 21% O₂ for each leaf pretreatment given (non-dehydrated leaves, dehydrated leaves, and leaves fed with ABA) even within a temperature range in which leaf photosynthesis at normal CO₂ concentration was the same in these two O₂ concentrations. It is concluded that this probably indicates an heterogeneity of photosynthesis, since this difference in quantum yield disappears when using high CO₂ concentrations during measurements.Abbreviations and Symbols ABA abscisic acid - F_m maximum chlorophyll fluorescence - F difference between steady-state chlorophyll fluorescence and F_m - PPFD photosynthetic photon flux density We would like to thank Dr. J.-M. Briantais (Laboratoire d'écologie végétale, Orsay, France) for help during fluorescence measurements and Ms. J. Liebert for technical assistance.     

Postillumination respiration of maize in relation to oxygen concentration and glycolic Acid metabolism

Prior illumination in CO₂-free air enhances a respiration from maize (Zea mays L.) leaves different in onset and duration from the postillumination burst of photorespiration. The course of respiration after brief illumination of attached leaves was measured as CO₂ efflux in darkness into CO₂-free atmospheres with four O₂ concentrations. The peak of CO₂ efflux following illumination was suppressed by 2.23% O₂, was completely eliminated by 0.04% O₂, and was not stimulated by 40% O₂ compared with air. Compared with air, steady dark respiration was suppressed by 0.04% O₂ but was not affected by 2.23% nor 40% O₂. Excision and subsequent uptake of distilled water through the vascular system nearly eliminated the enhanced respiration.     

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.     

Nutrition and the ozone sensitivity of birch (Betula pendula)

 Cuttings of a single birch clone (Betula pendula) were grown in field fumigation chambers throughout the growing season in either filtered air (control) or 90/40 nl O₃ l^–1 (day/night). Both regimes were split into plants under high and low nutrient supply (macro- and micronutrients). The stomatal density of leaves was increased by ozone but was lowered at high nutrition, while the inner air space was hardly affected by the treatments. Ozone induced macroscopic leaf injury regardless of nutrition, but leaf shedding was delayed in the low-fertilized plants, despite O₃ uptake being similar to that in high-fertilized plants. The leaf turn-over was enhanced in the O₃-exposed high-fertilized plants, but length growth and leaf formation of stems were not affected by ozone in either nutrient regime. Leaves of high-fertilized plants showed O₃-caused decline in photosynthetic capacity, water-use efficiency, apparent carbon uptake efficiency and quantum yield earlier as compared with low-fertilized plants, whereas chlorophyll fluorescence (F_V/F_M) and leaf nitrogen concentration were rather stable. CO₂ uptake rate and rubisco activity of young leaves compensated for the O₃ injury in the ageing leaves of the low-fertilized plants. In 8-week-old leaves, however, the O₃-induced decline in CO₂ uptake did not differ between the nutrient regimes and was associated with increased dark respiration rather than changed photorespiration. The balance between CO₂ supply and demand was lost, as was stomatal limitation on CO₂ uptake. High nutrition did not help leaves to maintain a high photosynthetic capacity and life span under O₃ stress. Received: 6 July 1996 / Accepted: 4 June 1997     

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     

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.     

Differentiation and structural decline in the leaves and bark of birch (Betula pendula) under low ozone concentrations

Summary Leaf and bark structure of a birch clone (Betula pendula Roth) continuously exposed to charcoal-filtered air or charcoal-filtered air plus ozone (0.05, 0.075, 0.1 l 1^-1) was investigated throughout one growing season. Increasing ozone dose influenced leaf differentiation by reducing leaf area and increasing inner leaf air space, density of cells developing into stomata, scales and hairs. When approximately the same ozone dose had been reached, macroscopical and microscopical symptoms appeared irrespective of the ozone concentration used during treatment. Structural decline began in mesophyll cells around stomatal cavities (droplet-like exudates on the cell walls), continued with disintegration of the cytoplasma and ended in cell collapse. Epidermal cells showed shrinkage of the mucilaginous layer (related to water loss). Their collapse marked the final stage of leaf decline. When subsidiary cells collapsed, guard cells passively opened for a transitory period before collapsing and closing. With increasing ozone dose starch remained accumulated along the small leaf veins and in guard cells. IIK-positive grains were formed in the epidermal cells. This contrasted with the senescent leaves, where starch was entirely retranslocated. Injury symptoms in stem and petiole proceeded from the epidermis to the cambium. Reduced tissue area indicated reduced cambial activity. In plants grown in filtered air and transferred into ozone on 20 August, injury symptoms developed faster than in leaves formed in the presence of ozone. Results are discussed with regard to O₃-caused acclimation and injury mechanisms.     

Cultivar differences in leaf photosynthesis of rice bred in Japan

The grain yield of rice (Oryza sativa L.), as well as of other cereal crops, is limited to a large extent, by the supply of photosynthates produced during grain filling period. In this study, flag leaf photosynthesis (LPS) after heading was compared among 32 cultivars bred during the past century in Japan, to determine if the improvement of LPS has occurred with the breeding advance of high yielding cultivars. Measurement of LPS was made for 5 consecutive years in the paddy field, on the flag leaf of the main stem, at heading (LPS-0), and 2 weeks (LPS-2) and 4 weeks (LPS-4) after heading. LPS decreased with advance of leaf senescence from LPS-0 to LPS-2, and then to LPS-4. However, if nitrogen was top-dressed at the heading time, high LPS-2 was maintained, particularly in the newer cultivars. A significant positive correlation between LPS and the released year of cultivar was found at LPS-2, especially in the nitrogen top-dressed plot, but not at LPS-0 or LPS-4. Cultivar difference in LPS of the senescing leaves were not stable through the different years, whereas LPS-0 was stable over years, suggesting that the LPS in the senescent leaf is susceptible to the environmental variation due to the effects on leaf senescence. Cultivar difference in LPS at any stage was closely associated with mesophyll conductance to CO₂, and stomatal conductance was also associated with cultivar difference in such a high LPS as LPS-0 and nitrogen top-dressed LPS-2. Significant correlation between LPS and specific leaf weight was not observed at any stage of the flag leaf.Abbreviations CV coefficient of variation - g_m mesophyll conductance - g_s stomatal conductance - LPS apparent photosynthetic rate per unit leaf area (leaf photosynthesis) - LPS-0 LPS at heading - LPS-2 LPS at active grain filling - LPS-4 LPS at maturity of grain - NT non-top dressed plot - PPFD photosynthetic photon flux density - r_m mesophyll resistance - r_s stomatal diffusion resistance against CO₂ - r_s(H₂O) stomatal diffusion resistance against H₂O - RuBisCO ribulose-1,5-bisphosphate carboxylase/oxygenase - SLW specific leaf weight - TD nitrogen top-dressed plot     

Energy Supply for Stomatal Opening in Epidermal Strips of Commelina benghalensis

The influence of light or darkness on stomatal opening in epidermal strips of Commelina benghalensis was evaluated in the presence or absence of O₂ and/or metabolic inhibitors. Opening was restricted in nitrogen and was promoted by NADH and acids of the tricarboxylic acid cycle (succinate and α-ketoglutarate) in CO₂-free air in light as well as in darkness. The enhancement by light of stomatal opening was prevalent under nitrogen or in the presence of the respiratory inhibitors (sodium azide and oligomycin). Respiratory inhibitors decreased the opening in light or darkness under CO₂-free air but exhibited no effect under nitrogen, whereas phosphorylation uncouplers were inhibitory in light or darkness under both CO₂-free air and nitrogen. The results suggest that oxidative phosphorylation is a basic source of energy for stomatal opening, although photophosphorylation could be an energy source.     

Carbohydrate and carbon metabolite accumulation responses in leaves of ozone tolerant and ozone susceptible spinach plants after acute ozone exposure

The objective of this study was to determine whether exposure of plants to ozone (O₃) increased the foliar levels of glucose, glucose sources, e.g., sucrose and starch, and glucose-6-phosphate (G6P), because in leaf cells, glucose is the precursor of the antioxidant, L-ascorbate, and glucose-6-phosphate is a source of NADPH needed to support antioxidant capacity. A further objective was to establish whether the response of increased levels of glucose, sucrose, starch and G6P in leaves could be correlated with a greater degree of plant tolerance to O₃. Four commercially available Spinacia oleracea varieties were screened for tolerance or susceptibility to detrimental effects of O₃ employing one 6.5 hour acute exposure to 25O nL O₃ L^-1 air during the light. One day after the termination of ozonation (29 d post emergence), leaves of the plants were monitored both for damage and for gas exchange characteristics. Cultivar Winter Bloomsdale (cv Winter) leaves were least damaged on a quantitative grading scale. The leaves of cv Nordic, the most susceptible, were approximately 2.5 times more damaged. Photosynthesis (Pn) rates in the ozonated mature leaves of cv Winter were 48.9% less, and in cv Nordic, 66.2% less than in comparable leaves of their non-ozonated controls. Stomatal conductance of leaves of ozonated plants was found not to be a factor in the lower Pn rates in the ozonated plants. At some time points in the light, leaves of ozonated cv Winter plants had significantly higher levels of glucose, sucrose, starch, G6P, G1P, pyruvate and malate than did leaves of ozonated cv Nordic plants. It was concluded that leaves of cv Winter displayed a higher tolerance to ozone mediated stress than those of cv Nordic, in part because they had higher levels of glucose and G6P that could be mobilized during diminished photosynthesis to generate antioxidants (e.g., ascorbate) and reductants (e.g., NADPH). Elevated levels of both pyruvate and malate in the leaves of ozonated cv Winter suggested an increased availability of respiratory substrates to support higher respiratory capacity needed for repair, growth, and maintenance.Abbreviations ADPG-PPiase ADPglucose pyrophosphorylase - ASC L-ascorbic acid - APX ascorbate peroxidase - Ce CO₂ concentration in air in the measuring cuvette during photosynthesis measurements - Ci CO₂ concentration in the leaf intercellular spaces during photosynthesis measurement - Chl chlorophyll - DHA dehydroascorbic acid - DHA reductase dehydroascorbate reductase - DHAP dihydroxyacetone phosphate - GAP glyceraldehyde-3-phosphate - Gluc glucose - GR glutathione reductase - G_sw stomatal conductance with units as mmol H₂O m^-2 s^-1 - GSSG oxidized glutathione - GSH reduced glutathione - G1P glucose-1-phosphate - G6P glucose-6-phosphate - G6P dehydrogenase glucose-6-phosphate dehydrogenase - 6PG 6-phosphogluconate - 6PG dehydrogenase 6-phosphogluconate dehydrogenase - F6P fructose-6-phosphate - FBP fructose-1,6-bisphosphate - MAL malate - MDHA reductase monodehydroascorbate reductase - PE post-emergence - PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - Pi orthophosphate - PYR pyruvate - Pn net CO₂ photoas-similation in leaves - PPFD photosynthetic photon flux density with units of mol photons m^-2 s^-1 - PPRC pentose phosphate reductive cycle - RuBP ribulose-1,5-bisphosphate - rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - SLW specific leaf weight - TCA cycle tricarboxylic acid cycle - Triose-P DHAP+GAP     

Carbon Dioxide Exchange and Acidity Levels in Detached Pineapple, Ananas comosus (L.), Merr., Leaves during the Day at Various Temperatures, Oxygen and Carbon Dioxide Concentrations

The effects of temperature, O₂, and CO₂ on titratable acid content and on CO₂ exchange were measured in detached pineapple (Ananas comosus) leaves during the daily 15-hour light period. Comparative measurements were made in air and in CO₂-free air. Increasing the leaf temperature from 20 to 35 C decreased the total CO₂ uptake in air and slightly increased the total CO₂ released into CO₂-free air. Between 25 and 35 C, the activation energy for daily acid loss was near 12 kcal mol⁻¹, but at lower temperatures the activation energy was much greater.