Inhibition of photosynthesis in olive trees (Olea europaea L.) during water stress and rewatering

The effect of high levels of natural light on leaf photosynthesisin olive trees (Olea europaea L. var. Coratina), grown in potsoutdoors in the summer and subjected to water, stress, was studied.Net photosynthetic rates reached maximum values early in themorning in both control and stressed plants and subsequentlydeclined gradually. This inactivation of photosynthetic activitywas accompanied by changes in the fluorescence characteristicsof the upper intact leaf surface. The maximum fluorescence yield(Fp) and the ratio Fv/Fp decreased at midday especially in water-stressedplants, but the initial fluorescence (Fo) rose to a maximumvalue at midday and declined again in the afternoon. In controlplants the values of maximum fluorescence Fp and the ratio Fv/Fpincreased again in the afternoon and had recovered almost completelyby 8 p.m. as the leaf water potential recovered. In stressedplants this diurnal recovery was not complete, so that the photosyntheticrates and the ratio Fv/Fp declined gradually during the developmentof water stress. These results indicate that in olive treessubjected to severe water stress the non-stomatal componentof photosynthesis was affected and perhaps a light-dependentinactivation of the primary photochemistry associated with photosystemII (PSII) occurred. Four to five days after rewatering severelystressed plants, the predawn leaf water potential, net photosyntheticrates and chlorophyll fluorescence indices recovered only partially. Key words: Olea europaea, photosynthesis, water stress, chlorophyll a fluorescence, inhibition of photosynthesis     

Primary sites of ozone-induced perturbations of photosynthesis in leaves: identification and characterization in Phaseolus vulgaris using high resolution chlorophyll fluorescence imaging

High resolution imaging of chlorophyll a fluorescence was used to identify the sites at which ozone initially induces perturbations of photosynthesis in leaves of Phaseolus vulgaris. Leaves were exposed to 250 and 500 nmol mol(-1) ozone at a photosynthetically active photon flux density of 300 micromol m(-2) s(-1) for 3 h. Images of fluorescence parameters indicated that large decreases in both the maximum and operating quantum efficiencies of photosystem II had occurred in cells adjacent to stomata in the upper, but not lower, leaf surfaces. However, this treatment did not produce any significant changes in the maximum or operating quantum efficiencies of photosystem II in the leaves when estimated from fluorescence parameters measured with a conventional, integrating fluorometer. The localized decreases in photosystem II photochemical efficiencies were accompanied by an increase in the minimal fluorescence level, which is indicative of photoinactivation of photosystem II complexes and a decrease in stomatal conductance. Perturbations of photochemical efficiencies were not observed in cells associated with all of the stomata on the upper leaf surface or within cells distant from the upper leaf surface. It is concluded that ozone penetrates the leaf through stomata and initially damages only cells close to stomatal pores.     

Temperature-induced alterations of in vivo chlorophyll a fluorescence induction in cucumber as affected by DCMU

Induction of chlorophyll a fluorescence and photosynthesis as affected by temperature were measured in cucumber leaf discs. Abrupt changes of the maximal variable fluorescence, Fv(p), and photosynthesis were observed around 9° and 21°C when the temperature was decreased from 30° to 0°C. The temperature-dependent maximal fluorescence of DCMU-treated leaf discs showed a single change around 21°C. Temperature-induced chlorophyll a fluorescence alterations are discussed in relation to electron transport activity of the two photosystems and photosynthetic activity of the cucumber leaf discs.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - Fm maximal fluorescence - Fv(p) maximal variable fluorescence - qE energy-dependent fluorescence quenching - qQ Qa-dependent fluorescence quenching     

Oscillations in levels of metabolites from the photosynthetic carbon reduction cycle in spinach leaf disks generated by the transition from air to 5% CO2

Variations in the endogenous concentrations of leaf metabolites were measured during the transition from steady-state photosynthesis in air to that in 5% CO2, 21% O2. The transition in the CO2 supply to the leaf caused a pronounced oscillation in the overall rate of photosynthesis, as indicated by chlorophyll a fluorescence, which was accompanied by large changes in the levels of metabolites associated with carbon metabolism. A dramatic increase in the ATP concentration occurred immediately following the gas transition and was maximal at the point at which the chlorophyll a fluorescence showed a transient decrease. During the first large fluorescence increase the ATP level rapidly declined. Oscillations in the level of 3-phosphoglycerate were the most pronounced of the photosynthetic metabolites measured. These accompanied the oscillations in chlorophyll a fluorescence which showed an inverse relationship to the oscillations in ATP/ADP ratio. The ribulose 1,5-bisphosphate concentration showed a small increase following the gas transition but subsequently fell much lower, enforced by the high level of CO2 supplied. Less pronounced oscillations in the intracellular concentrations of fructose 1,6-bisphosphate, fructose 6-phosphate, glucose 6-phosphate, and triose phosphate were also observed.     

Genetic modification of photosynthesis with E. coli genes for trehalose synthesis

Improvement in photosynthesis per unit leaf area has been difficult to alter by breeding or genetic modification. We report large changes in photosynthesis in Nicotiana tabacum transformed with E. coli genes for the trehalose pathway. Significantly, photosynthetic capacity (CO2 assimilation at varying light and CO2, and quantum yield of PSII electron transport) per unit leaf area and per leaf dry weight were increased in lines of N. tabacum transformed with the E. coli gene otsA, which encodes trehalose phosphate synthase. In contrast, transformation with otsB, which encodes trehalose phosphate phosphatase or Trec, encoding trehalose phosphate hydrolase, produced the opposite effect. Changes in CO2 assimilation per unit leaf area were closely related to the amount and activity of Rubisco, but not to the maximum activities of other Calvin cycle enzymes. Alterations in photosynthesis were associated with trehalose 6-phosphate content rather than trehalose. When growth parameters were determined, a greater photosynthetic capacity did not translate into greater relative growth rate or biomass. This was because photosynthetic capacity was negatively related to leaf area and leaf area ratio. In contrast, relative growth rate and biomass were positively related to leaf area. These results demonstrate a novel means of modifying Rubisco content and photosynthesis, and the complexities of regulation of photosynthesis at the whole plant level, with potential benefits to biomass production through improved leaf area.     

Impact of elevated temperatures on specific leaf weight,stomatal density,photosynthesis and chlorophyll fluorescence in soybean

High-temperature stress is a major environmental stress and there are limited studies elucidating its impact on soybean (Glycine max L. Merril.). The objectives of present study were to quantify the effect of high temperature on changes in leaf thickness, number of stomata on adaxial and abaxial leaf surfaces, gas exchange, chlorophyll fluorescence parameters and seed yield in soybean. Twelve soybean genotypes were grown at day/night temperatures of 30/22, 34/24, 38/26 and 42/28?°C with an average temperature of 26, 29, 32 and 35?°C, respectively, under greenhouse conditions. One set was also grown under ambient temperature conditions where crop season average maximum, minimum and mean temperatures were 28.0, 22.4 and 25.2?°C, respectively. Significant negative effect of temperature was observed on specific leaf weight (SLW) and leaf thickness. Rate of photosynthesis, stomatal conductance and water use efficiency declined as the growing temperatures increased; whereas, intercellular CO₂ and transpiration rate were increased. With the increase in temperature chlorophyll fluorescence parameters such as Fv/Fm, qP and PhiPSII declined while there was increase in qN. Number of stomata on both abaxial and adaxial surface of leaf increased significantly with increase in temperatures. The rate of photosynthesis, PhiPSII, qP and SPAD values were positively associated with leaf thickness and SLW. This indicated that reduction in photosynthesis and associated parameters appears to be due to structural changes observed at higher temperatures. The average seed yield was maximum (13.2 g/pl) in plants grown under ambient temperature condition and declined by 8, 14, 51 and 65% as the temperature was increased to 30/22, 34/24, 38/26 and 42/28?°C, respectively.     

Effect of solar ultraviolet-B radiation during springtime ozone depletion on photosynthesis and biomass production of Antarctic vascular plants

We assessed the influence of springtime solar UV-B radiation that was naturally enhanced during several days due to ozone depletion on biomass production and photosynthesis of vascular plants along the Antarctic Peninsula. Naturally growing plants of Colobanthus quitensis (Kunth) Bartl. and Deschampsia antarctica Desv. were potted and grown under filters that absorbed or transmitted most solar UV-B. Plants exposed to solar UV-B from mid-October to early January produced 11% to 22% less total, as well as above ground biomass, and 24% to 31% less total leaf area. These growth reductions did not appear to be associated with reductions in photosynthesis per se: Although rates of photosynthetic O(2) evolution were reduced on a chlorophyll and a dry-mass basis, on a leaf area basis they were not affected by UV-B exposure. Leaves on plants exposed to UV-B were denser, probably thicker, and had higher concentrations of photosynthetic and UV-B absorbing pigments. We suspect that the development of thicker leaves containing more photosynthetic and screening pigments allowed these plants to maintain their photosynthetic rates per unit leaf area. Exposure to UV-B led to reductions in quantum yield of photosystem II, based on fluorescence measurements of adaxial leaf surfaces, and we suspect that UV-B impaired photosynthesis in the upper mesophyll of leaves. Because the ratio of variable to maximal fluorescence, as well as the initial slope of the photosynthetic light response, were unaffected by UV-B exposure, we suggest that impairments in photosynthesis in the upper mesophyll were associated with light-independent enzymatic, rather than photosystem II, limitations.     

Dynamics of Vertical Leaf Nitrogen Distribution in a Vegetative Wheat Canopy. Impact on Canopy Photosynthesis

The development of vertical canopy gradients of leaf N has beenregarded as an adaptation to the light gradient that helps tomaximize canopy photosynthesis. In this study we report thedynamics of vertical leaf N distribution during vegetative growthof wheat in response to changes in N availability and sowingdensity. The question of to what extent the observed verticalleaf N distribution maximized canopy photosynthesis was addressedwith a leaf layer model of canopy photosynthesis that integratesN-dependent leaf photosynthesis according to the canopy lightand leaf N distribution. Plants were grown hydroponically attwo amounts of N, supplied in proportion to calculated growthrates. Photosynthesis at light saturation correlated with leafN. The vertical leaf N distribution was associated with thegradient of absorbed light. The leaf N profile changed duringcrop development and was responsive to N availability. At highN supply, the leaf N profiles were constant during crop development.At low N supply, the leaf N profiles fluctuated between moreuniform and steep distributions. These changes were associatedwith reduced leaf area expansion and increasing N remobilizationfrom lower leaf layers. The distribution of leaf N with respectto the gradient of absorbed irradiance was close to the theoreticaloptimum maximizing canopy photosynthesis. Sensitivity analysisof the photosynthesis model suggested that plants maintain anoptimal vertical leaf N distribution by balancing the capacityfor photosynthesis at high and low light. Copyright 2000 Annalsof Botany Company Canopy photosynthesis, leaf nitrogen distribution, nitrogen, Triticum aestivum L, wheat     

Photosynthesis in localised regions of oat leaves infected with crown rust (Puccinia coronata): quantitative imaging of chlorophyll fluorescence

Localised changes in photosynthesis in oat leaves infected with the biotrophic rust fungus Puccinia coronata Corda were examined at different stages of disease development by quantitative imaging of chlorophyll fluorescence. Following inoculation of oat leaves with crown rust the rate of whole-leaf gas exchange declined. However, crown rust formed discrete areas of infection which expanded as the disease progressed and these localised regions of infection gave rise to heterogeneous changes in photosynthesis. To quantify these changes, images of chlorophyll fluorescence were taken 5, 8 and 11 d after inoculation and used to calculate images representing two parameters; Φ_II, a measure of PSII photochemical efficiency and ΔFm/Fm′, a measure of non-photochemical energy dissipation (qN). Five days after inoculation, disease symptoms appeared as yellow flecks which were correlated with the extent of the fungal mycelium within the leaf. At this stage, Δ_II was slightly reduced in the infected regions but, in uninfected regions of the leaf, values of Φ_II were similar to those of healthy leaves. In contrast, qN (ΔFm/Fm′) was greatly reduced throughout the infected leaf in comparison to healthy leaves. We suggest that the low value of qN in an infected leaf reflects a high demand for ATP within these leaves. At sporulation, 8 d after inoculation, Φ_II was reduced throughout the infected leaf although the reduction was most marked in areas invaded by fungal mycelium. In the infected leaf the pattern of non-photochemical quenching was complex; qN was low within invaded regions, perhaps reflecting high metabolic activity, but was now much higher in uninfected regions of the infected leaf, in comparison to healthy leaves. Eleven days after inoculation “green islands” formed in regions of the leaf associated with the fungal mycelium. At this stage, photosynthesis was severely inhibited over the entire leaf; however, heterogeneity was still apparent. In the region not invaded by the fungal mycelium, Φ_II and qN were very low and these regions of the leaf were highly fluorescent, indicating that the photosynthetic apparatus was severely damaged. In the greenisland tissue, Φ_II was low but detectable, indicating that some photosynthetic processes were still occurring. Moreover, qN was high and fluorescence low, indicating that the cells in this region were not dead and were capable of significant quenching of chlorophyll fluorescence.     

Relative contributions of zeaxanthin-related and zeaxanthin-unrelated types of ;high-energy-state' quenching of chlorophyll fluorescence in spinach leaves exposed to various environmental conditions

We have identified two rapidly relaxing components of non-photochemical fluorescence quenching which suggests that dissipative processes occur in two different sites in the photochemical system of leaves. Under a variety of treatment conditions involving different leaf temperatures, photon flux densities (PFD), exposure times, and in the presence of 5% CO₂ or 2% O₂, no CO₂, the components of nonphotochemical fluorescence quenching were characterized with respect to their sensitivity to dithiothreitol (DTT, which completely inhibits zeaxanthin formation), the effect on instantaneous fluorescence, and the rapidity of relaxation upon darkening. Under most circumstances the DTT-sensitive component (associated with a quenching of instantaneous fluorescence and correlated with zeaxanthin) represented the majority of the rapidly relaxing portion of fluorescence quenching. A DTT-insensitive (zeaxanthin-independent) component, which also relaxed rapidly upon darkening but was not associated with a quenching of instantaneous fluorescence, became proportionally greater in an atmosphere of 2% O₂ and no CO₂, at elevated leaf temperatures, and to some degree during the induction of photosynthesis (1 minute after the onset of illumination). A third component which was also DTT-insensitive and was sustained upon darkening, was largely suppressed in 2% O₂, O% CO₂. We conclude that, under conditions favorable for photosynthesis, energy dissipation occurred mainly in the chlorophyll antennae whereas, under conditions less favorable for photosynthesis, a second dissipation process, probably in or around the reaction center of photosystem II, also developed. Furthermore, evidence is presented that the zeaxanthin-associated dissipation process prevents sustained inactivation of photochemistry by excessive light.     

Indirect suppression of photosynthesis on individual leaves by arthropod herbivory

BACKGROUND: Herbivory reduces leaf area, disrupts the function of leaves, and ultimately alters yield and productivity. Herbivore damage to foliage typically is assessed in the field by measuring the amount of leaf tissue removed and disrupted. This approach assumes the remaining tissues are unaltered, and plant photosynthesis and water balance function normally. However, recent application of thermal and fluorescent imaging technologies revealed that alterations to photosynthesis and transpiration propagate into remaining undamaged leaf tissue. SCOPE AND CONCLUSIONS: This review briefly examines the indirect effects of herbivory on photosynthesis, measured by gas exchange or chlorophyll fluorescence, and identifies four mechanisms contributing to the indirect suppression of photosynthesis in remaining leaf tissues: severed vasculature, altered sink demand, defence-induced autotoxicity, and defence-induced down-regulation of photosynthesis. We review the chlorophyll fluorescence and thermal imaging techniques used to gather layers of spatial data and discuss methods for compiling these layers to achieve greater insight into mechanisms contributing to the indirect suppression of photosynthesis. We also elaborate on a few herbivore-induced gene-regulating mechanisms which modulate photosynthesis and discuss the difficult nature of measuring spatial heterogeneity when combining fluorescence imaging and gas exchange technology. Although few studies have characterized herbivore-induced indirect effects on photosynthesis at the leaf level, an emerging literature suggests that the loss of photosynthetic capacity following herbivory may be greater than direct loss of photosynthetic tissues. Depending on the damage guild, ignoring the indirect suppression of photosynthesis by arthropods and other organisms may lead to an underestimate of their physiological and ecological impacts.     

Responses of Chlorophyll Fluorescence and Carotenoids Biosynthesis to High Light Stress in Rice Seedling Leaves at Different Leaf Position

In the present study, we investigated the changes of photosynthesis, chlorophyll fluorescence and the content of carotenoid pigments in rice ( Oryza sativa L.) seedling leaves and their responses to high light. The results showed that the rate of photosynthesis, the contents of individual and total carotenoids and the pool size of xanthophyll cycle decreased with age increasing of the leaf. When the leaves were exposed to high light for 2 h, the qN of mature leaf (5th leaf) increased more significantly than that of younger (6th leaf) and older leaves (3rd and 4th leaf). Comparing with the leaves before exposure to high light, the excitation pressure on PSⅡ (1-qP) increased by 44%, 57%, 19% and 45% in the 3rd, 4th, 5th and 6th leaf under high light, respectively. The highest content of carotenoids and the greatest conversion of violaxanthin to zeaxanthin were found in the 5th leaf, and it was consistent with the 5th leaf exhibiting the strongest resistance to high light. Our results suggested that the ability of rice leaf to resist photoinhibition is related to the level of carotenoids and the ability of carotenoids biosynthesis.      

Causes of reduced leaf‐level photosynthesis during strong El Niño drought in a Central Amazon forest

Sustained drought and concomitant high temperature may reduce photosynthesis and cause tree mortality. Possible causes of reduced photosynthesis include stomatal closure and biochemical inhibition, but their relative roles are unknown in Amazon trees during strong drought events. We assessed the effects of the recent (2015) strong El Niño drought on leaf‐level photosynthesis of Central Amazon trees via these two mechanisms. Through four seasons of 2015, we measured leaf gas exchange, chlorophyll a fluorescence parameters, chlorophyll concentration, and nutrient content in leaves of 57 upper canopy and understory trees of a lowland terra firme forest on well‐drained infertile oxisol. Photosynthesis decreased 28% in the upper canopy and 17% in understory trees during the extreme dry season of 2015, relative to other 2015 seasons and was also lower than the climatically normal dry season of the following non‐El Niño year. Photosynthesis reduction under extreme drought and high temperature in the 2015 dry season was related only to stomatal closure in both upper canopy and understory trees, and not to chlorophyll a fluorescence parameters, chlorophyll, or leaf nutrient concentration. The distinction is important because stomatal closure is a transient regulatory response that can reverse when water becomes available, whereas the other responses reflect more permanent changes or damage to the photosynthetic apparatus. Photosynthesis decrease due to stomatal closure during the 2015 extreme dry season was followed 2 months later by an increase in photosynthesis as rains returned, indicating a margin of resilience to one‐off extreme climatic events in Amazonian forests.     

Photosynthesis and chlorophyllafluorescence during flag leaf senescence of field-grown wheat plants

The characteristics of photosynthetic gas exchange, chlorophyll a fluorescence, and xanthophyll cycle pigments during flag leaf senescence of field-grown wheat plants were investigated. With senescence progressing, the light-saturated net CO₂ assimilation rate expressed either on a basis of leaf area or chlorophyll decreased significantly. The apparent quantum yield of net photosynthesis decreased when expressed on a leaf area basis but increased when expressed on a chlorophyll basis. The maximal efficiency of PSII photochemistry decreased very little while actual PSII efficiency, photochemical quenching, and the efficiency of excitation capture by open PSII centers decreased considerably. At the same time, non-photochemical quenching increased significantly. A substantial decrease in the contents of violaxanthin and zeaxanthin, but a slight decrease in the content of antheraxanthin were observed. However, the de-epoxidation status of the xanthophyll cycle was positively correlated with progressive senescence. This increase was due mainly to a smaller decrease in zeaxanthin than in violaxanthin. Our results suggest that PSII apparatus remained functional, but a down-regulation of PSII occurred under the steady state of photosynthesis in senescent flag leaves. Such a down-regulation was associated with the closure of PSII centers and an enhanced xanthophyll cycle-related thermal dissipation in the PSII antennae.     

Pictorial Demonstrations of Photosynthesis

Theodor Engelmann's experiments in 1882 provided the first recorded visual demonstration of light wavelengths that are absorbed by photosynthetic pigments. Later, starch images in intact leaves were used to demonstrate photosynthesis in green plants. Similarly, light-induced chloroplast movements can form images in leaves as a result of changes in light transmittance through leaves and photoinhibition can form images that can be visualized by whole leaf chlorophyll fluorescence. This paper provides a brief account of how photosynthesis has been used to create an assortment of 'living images' that offer stunning demonstrations of various aspects of photosynthesis.     

干旱和复水对大豆(Glycine max)叶片光合及叶绿素荧光的影响

选用河南省大面积种植的大豆品种豫豆29作为实验材料,通过研究逐步干旱和旱后复水条件下大豆叶片光合、叶绿素荧光等指标随土壤水分的动态变化规律,以期为大豆的水分高效利用提供理论依据。研究发现,在土壤相对含水量高于46.5%时,虽然随着土壤相对含水量的下降,豫豆29仍可以保持它的叶片水分状态;豫豆29的叶片净光合速率在土壤水分中等条件下最大,在土壤相对含水量为64.3%时,它比对照组高出11.2%(P<0.01);在实验的第3d,处理组的土壤相对含水量降为46.5%,叶片水势与对照组相比降低了7.2%(P>0.05),净光合速率为对照组的89.6%(P<0.05),但气孔导度却迅速下降为对照组的44.7%(P<0.01),这说明与叶片的光合和水分状况相比,豫豆29的气孔对土壤水分的匮缺更加敏感。复水后,豫豆29叶片的水势、净光合速率、气孔导度和叶绿素荧光等值都可以得到迅速的恢复,并在实验的最后接近对照组的水平,这表明豫豆29的叶片光合在水分胁迫解除后有迅速恢复的能力。     

八倍体小偃麦和普通小麦旗叶及叶鞘光合日变化比较研究

以温室大棚中栽培的普通小麦'中国春'(对照)及八倍体小偃麦'小偃7430'(染色体组为ABDE_1)、'小偃693'(染色体组为ABDF_1)为材料,采用TPS-1光合作用测定系统及FMS-2荧光仪测定了开花期旗叶和叶鞘的净光合速率、气孔导度和叶绿素荧光参数的日变化,以揭示普通小麦与八倍体小偃麦旗叶及其叶鞘的光合作用差异.结果表明:八倍体小偃麦与普通小麦'中国春'的旗叶和叶鞘光合作用均有午休现象,净光合速率于上午11:00左右出现高峰,午间下降,下午又呈现上升趋势,且'小偃693'叶片在上午时上升较快;气孔导度和荧光参数F_v/F_m、Fv/F_o、 Φ_(PS)Ⅱ、ETR的变化趋势与净光合速率相似,而NPQ的变化趋势则相反.各光合作用和叶绿素荧光参数在材料间表现为八倍体小偃麦高于普通小麦'中国春',器官间表现为旗叶高于相应叶鞘,而'小偃693'的叶鞘和'中国春'叶片的相似.研究发现,八倍体小偃麦旗叶和叶鞘的光合效率优于普通小麦'中国春';'小偃693'的光合效率因其较高的光反应活性表现得尤为突出,其叶鞘的光合能力也不容忽视.     

Cell density and airspace patterning in the leaf can be manipulated to increase leaf photosynthetic capacity

The pattern of cell division, growth and separation during leaf development determines the pattern and volume of airspace in a leaf. The resulting balance of cellular material and airspace is expected to significantly influence the primary function of the leaf, photosynthesis, and yet the manner and degree to which cell division patterns affect airspace networks and photosynthesis remains largely unexplored. In this paper we investigate the relationship of cell size and patterning, airspace and photosynthesis by promoting and repressing the expression of cell cycle genes in the leaf mesophyll. Using microCT imaging to quantify leaf cellular architecture and fluorescence/gas exchange analysis to measure leaf function, we show that increased cell density in the mesophyll of Arabidopsis can be used to increase leaf photosynthetic capacity. Our analysis suggests that this occurs both by increasing tissue density (decreasing the relative volume of airspace) and by altering the pattern of airspace distribution within the leaf. Our results indicate that cell division patterns influence the photosynthetic performance of a leaf, and that it is possible to engineer improved photosynthesis via this approach.     

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.     

Nitrogen supply as a factor influencing photoinhibition and photosynthetic acclimation after transfer of shade-grown Solanum dulcamara to bright light

We have compared the ability of shadegrown clones of Solamum dulcamara L. from shade and sun habitats to acclimate to bright light, as a function of nitrogen nutrition before and after transfer to bright light. Leaves of S. dulcamara grown in the shade with 0.6 mM NO ₃ ^- have similar photosynthetic properties as leaves of plants grown with 12.0 mM NO ₃ ^- . When transferred to bright light for 1–2 d the leaves of these plants show substantial photoinhibition which is characterized by about 50% decrease in apparent quantum yield and a reduction in the rate of photosynthesis in air at light saturation. Photoinhibition of leaf photosynthesis is associated with reduction in the variable component of low-temperature fluorescence emission, and with loss of in-vitro electron transport, especially of photosystem II-dependent processes.We find no evidence for ecotypic differentiation in the potential for photosynthetic acclimation among shade and sun clones of S. dulcamara, or of differentiation with respect to nitrogen requirements for acclimation. Recovery from photoinhibition and subsequent acclimation of photosynthesis to bright light only occurs in leaves of plants provided with 12.0 mM NO ₃ ^- . In these, apparent quantum yield is fully restored after 14 d, and photosynthetic acclimation is shown by an increase in light-saturated photosynthesis in air, of light-and CO₂-saturated photosynthesis, and of the initial slope of the CO₂-response curve. The latter changes are highly correlated with changes in ribulose-bisphosphate-carboxylase activity in vitro. Plants supplied with 0.6 mM NO ₃ ^- show incomplete recovery of apparent quantum yield after 14 d, but CO₂-dependent leaf photosynthetic parameters return to control levels.Symbols and abbreviations F_o initial level of fluorescence at 77 K - F_m maximum level of fluorescence at 77 K - F_v variable components of fluorescence at 77 K (F_v=F_m-F_o) - PSI, PSII photosystem I and II, respectively - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39)