Non-stomatal Limitation Of Photosynthesis Research Articles

The Effects of Enhanced Ultraviolet-B Radiation on Leaf Photosynthesis and Submicroscopic Structures in Mangifera indica L. cv. ‘Tainong No 1’

Objective: In the future, the stress of enhanced UV-B radiation on the Earth will first affect the photosynthesis of plants, including mangoes. Therefore, it is necessary to study the effects of enhanced UV-B radiation on the photosynthesis of mangoes. Methods: ‘Tainong No 1’ mango trees in the field were selected as the experimental material and divided into 2 groups: one group was shined under 96 kJ·m−2·d−1 UV-B lamps for artificially simulated treatment of enhanced UV-B radiation, and the other group was shined under sunshine directly as the control (CK). The main photosynthetic physiological indicators were measured with conventional methods, and the expression levels of the genes encoding large and small subunits of the Rubisco enzyme were measured with fluorescent qPCR. The changes in stomatal morphology and chloroplast structure were observed with scanning electron microscopy and transmission electron microscopy. Results: The content of malondialdehyde (MDA) and the relative conductivity in the leaves of the treatment tended to be significantly higher than those of the CK. The net photosynthetic rate (Pn) of the treatment tended to decrease and be lower than that of CK. The dynamics of intercellular CO2 concentration (Ci) of the treatment and CK changed differently from each other but generally tended to decrease, and that of the treatment tended to be significantly higher than that of CK. The stomatal conductance (Gs) of the treatment and CK both generally decreased, and that of the treatment was always significantly lower than that of CK. The contents of chlorophyll a, chlorophyll b and total chlorophyll and the ratio of chlorophyll a/b of the treatment were lower than those of CK, while the carotenoid content showed the opposite trend. The stomata and the surface of leaves of the treatment were sunken and damaged, respectively. The palisade tissue, spongy tissue and upper epidermis thickened more, and the total thickness significantly increased. Meanwhile, the ratio of palisade tissue to spongy tissue decreased. During treatment, the chloroplasts were swollen and shortened, the number of chloroplasts was reduced, the starch grains were degraded, and the grana lamella were distorted, loosely arranged and blurred. The expression of the genes encoding the Rubisco large subunit (rbcL) in the treatment was significantly inhibited, while that encoding the Rubisco small subunit (rbcS) decreased first and increased later. In conclusion, 96 kJ·m−2·d−1 enhanced UV-B radiation treatment caused damage to the leaf cell membrane system. This led to stomatal limitation of photosynthesis by destroying the stomatal structure and nonstomatal limitation of photosynthesis by damaging the submicrostructure of the chloroplasts and downregulating the expression of rbcL. The leaves may resist the photosynthetic damage caused by enhanced UV-B radiation by upregulating rbcS expression as much as possible.

The Photosynthesis of Populus euphratica Oliv. Is Not Limited by Drought Stress in the Hyper-Arid Zone of Northwest China

The Ejin Oasis is located in the lower reaches of the Heihe River Basin of northwestern China. It is one of the most arid regions in the world, and Populus euphratica Oliv. is the foundation species of the desert riparian forests there. The photosynthesis of P. euphratica is one of the first physiological processes that is most likely to be affected by the extremely arid climate conditions. The factors impacting photosynthesis can be divided into stomatal and non-stomatal limitations. In order to investigate whether the photosynthesis of P. euphratica was limited and, if so, whether this limitation was caused by drought stress in the P. euphratica Forest Reserve on the Ejin River, we analyzed stomatal, non-stomatal, and relative stomatal limitations (reflecting the relative importance of the stoma in controlling the processes of photosynthesis) of photosynthesis. The results show that, at the beginning of the midday depression of photosynthesis, the values of stomatal limitation of photosynthesis (Ls) peaked, with its predominance being supported by sub-stomatal CO2 concentrations (Ci) being at a minimum. Thereafter, Ls decreased and non-stomatal limitation (Ci/stomatal conductance (gs)) increased sharply, indicating that the non-stomatal limitation of photosynthesis was predominant. Both Ls and relative stomatal limitation of photosynthesis increased in the morning, and then decreased, whereas Ci/gs showed the opposite trend. We concluded that P. euphratica did not experience drought stress by analyzing leaf water potential, groundwater table, and the decoupling coefficient (a parameter characterizing the coupling degree between vegetation canopy and atmospheric water vapor flux); however, the Ls values of P. euphratica were much greater than those of other species. This was likely because P. euphratica has a relatively conservative water use strategy even when growing under favorable water conditions. Extremely high temperatures caused the closure of the stoma to reduce transpiration, resulting in more intense stomatal limitations of photosynthesis.

Diversity of responses to nitrogen deficiency in distinct wheat genotypes reveals the role of alternative electron flows in photoprotection.

Nitrogen (N) deficiency represents an important limiting factor affecting photosynthetic productivity and the yields of crop plants. Significant reported differences in N use efficiency between the crop species and genotypes provide a good background for the studies of diversity of photosynthetic and photoprotective responses associated with nitrogen deficiency. Using distinct wheat (Triticum aestivum L.) genotypes with previously observed contrasting responses to nitrogen nutrition (cv. Enola and cv. Slomer), we performed advanced analyses of CO2 assimilation, PSII, and PSI photochemistry, also focusing on the heterogeneity of the stress responses in the different leaf levels. Our results confirmed the loss of photosynthetic capacity and enhanced more in lower positions. Non-stomatal limitation of photosynthesis was well reflected by the changes in PSII and PSI photochemistry, including the parameters derived from the fast-fluorescence kinetics. Low photosynthesis in N-deprived leaves, especially in lower positions, was associated with a significant decrease in the activity of alternative electron flows. The exception was the cyclic electron flow around PSI that was enhanced in most of the samples with a low photosynthetic rate. We observed significant genotype-specific responses. An old genotype Slomer with a lower CO2 assimilation rate demonstrated enhanced alternative electron flow and photorespiration capacity. In contrast, a modern, highly productive genotype Enola responded to decreased photosynthesis by a significant increase in nonphotochemical dissipation and cyclic electron flow. Our results illustrate the importance of alternative electron flows for eliminating the excitation pressure at the PSII acceptor side. The decrease in capacity of electron acceptors was balanced by the structural and functional changes of the components of the electron transport chain, leading to a decline of linear electron transport to prevent the overreduction of the PSI acceptor side and related photooxidative damage of photosynthetic structures in leaves exposed to nitrogen deficiency.

How chrysanthemum (Chrysanthemum × grandiflorum) 'Palisade White' deals with long-term salt stress.

Salinity is a serious problem in the cultivation of ornamental plants. Chrysanthemum (Chrysanthemum × grandiflorum) ‘Palisade White’ was evaluated in order to examine its responses to long-term salt stress. Plants were grown in substrate supplemented with NaCl doses (g dm−3 of substrate) 0, 0.44, 0.96, 1.47, 1.98, 2.48 and 2.99. The initial electrical conductivity (EC) of the substrates was 0.3, 0.9, 1.4, 1.9, 2.6, 3.1 and 3.9 dS m−1, respectively. Plant growth, relative water content (RWC), Na, Cl, K, N and P concentrations, membrane injury (MI), chlorophyll and proline levels, as well as gas exchange parameters in leaves of chrysanthemum were determined. A dose-dependent significant reduction of growth and minor decrease of leaf RWC were observed. Foliar Na and Cl concentrations increased with the highest NaCl dose up to 6-fold. However, the concentration of K increased by about 14 %, N by about 5 % but P decreased by about 23 %. Membrane injury was rather low (11 %) even at the highest NaCl dose. Statistically significant decreases of stomatal conductance (20 %), transpiration rate (32 %) and photosynthesis (25 %) were already observed at the lowest NaCl dose and about 40 % decrease of all these parameters with the highest dose. A significant reduction in the intercellular CO2 concentration occurred at the lower NaCl doses and no changes with the highest dose. These results show that in plants grown with the highest NaCl dose, non-stomatal limitation of photosynthesis may occur. According to Maas and Hoffman tolerance assessment (1977) chrysanthemum ‘Palisade White’ may be considered as moderately sensitive to salt stress in terms of growth inhibition. However, it is able to cope with long-term salt stress without any signs of damage, such as chlorophyll depletion, leaf browning or necrotic spots probably due to maintenance of K homeostasis and proline accumulation, which alleviate the toxic effect of chloride.

Potassium supply promotes the mitigation of NaCl-induced effects on leaf photochemistry, metabolism and morphology of Setaria viridis

Soil salinity has the potential to severely affect crop performance. To maintain cell functioning and improve salt tolerance, the maintenance of K+ homeostasis is crucial in several plant metabolism processes. Besides, potassium fertilization can efficiently alleviate the perilous effects of salinity. We characterized impacts in Setaria viridis exposed to NaCl and KCl to underlying photochemistry mechanisms, K+ and Na+ shoot contents, enzymatic activity, electrolytic leakage, and morphological responses focusing on non-stomatal limitation of photosynthesis. Plants were exposed to sodium chloride (NaCl; 0, 150 and 250 mM) and potassium chloride (KCl; 0, 5, 9 mM). The exposure to NaCl affected S. viridis leaves morphological and physiologically. Plants submitted to 150 mM showed reductions in performance indexes (PIabs and PItotal; JIP-test), and the presence of positive K- and L-bands. Plants exposed to 250 mM exhibited blockage in electron flow further than QA within 48h and permanent photoinhibition at 96 h. The presence of 9 and 5 mM of KCl counteracted the effects of NaCl on plants submitted to 150 mM, concomitant with increases in K+ accumulation and cell turgidity conservation, causing positive effects in plant growth and metabolism. Neither KCl concentrations were effective in reducing NaCl-induced effects on plants exposed to 250 mM of NaCl. Our results support the conclusion that greater availability of K+ alleviates the harmful effects of salinity in S. viridis under moderate stress and that application of KCl as means of lightning saline stress has a concentration and a salt level limit that must be experimentally determined.

Photochemistry and proteomics of ginger (Zingiber officinale Roscoe) under drought and shading

Drought has become an increasingly serious ecological problem that limits crop production. However, little is known about the response of ginger (Zingiber officinale Roscoe) to drought and shading, especially with respect to photosynthetic electron transport. Here, differential proteomics was used to study the response of ginger to four experimental treatments: control, drought, 50% shading, and the combination of 50% shading and drought. Proteomic analysis suggested that ginger increased cyclic electron flow under drought stress by enhancing the expression of proteins related to photosystem I and cytochrome b6f. Shading significantly increased the expression of proteins related to the light harvesting complex, even under drought stress. In addition, shading increased the expression of proteins related to the oxygen evolution complex, plastocyanin, and ferredoxin–NADP+ reductase (FNR), thereby enhancing the efficiency of photosynthetic electron utilization. The shading and drought combination treatment appeared to enhance ginger's drought tolerance by reducing the expression of FNR and enhancing cyclic electron flow. Photosynthetic and fluorescence parameters showed that drought stress caused non-stomatal limitation of photosynthesis in ginger leaves. Drought stress also significantly reduced the quantum efficiency of photosystem II (Fv/Fm), the non-cyclic electron transfer efficiency of photosystem II (ϕPSII), and photochemical quenching (qP), while simultaneously increasing nonphotochemical quenching (NPQ). The addition of shading improved photosynthetic efficiency under drought. These results provide important baseline information on the photosynthetic mechanisms by which ginger responds to drought and shading. In addition, they provide a theoretical basis for the study of shade cultivation during the arid season.

Non-stomatal limitation of photosynthesis by soil salinity

Soil salinity is a major threat to agricultural sustainability and a global food security. Until now, most research has concentrated around stomatal limitation to photosynthesis, while non-stomatal limitations receiving much less attention. This work summarizes the current knowledge of impact of salinity on chloroplast metabolism and operation and finding viable solutions to minimize it. The major topics covered are: (1) the key targets of the photosynthetic apparatus under salt stress; (2) a tolerance of PSII to salt stress and its repair; (3) salinity effects on biochemistry of CO2 fixation and its regulation; (4) ionic requirements for optimal operation of chloroplasts; and (5) ion transport systems in chloroplasts that optimize chloroplast performance under hostile saline conditions. We show that enhancing plant capacity for protection by modifying PSI cyclic electron transport, redistribution of electron transport between photosystems, thylakoid membrane composition and photosynthetic antioxidant enzymes activity may be a promising way to improve tolerance to salt stress under real-field condition. It is concluded that revealing the molecular nature of chloroplast ion transporters and understanding the modes of their operation will ensure the future sustainability of the world agriculture and the prospects of biological phytoremediation of salinized land via using salt-tolerant crop germplasm.

Leaf carbon and water status control stomatal and nonstomatal limitations of photosynthesis in trees.

Photosynthetic rate is concurrently limited by stomatal limitations and nonstomatal limitations (NSLs). However, the controls on NSLs to photosynthesis and their coordination with stomatal control on different timescales remain poorly understood. According to a recent optimization hypothesis, NSLs depend on leaf osmotic or water status and are coordinated with stomatal control so as to maximize leaf photosynthesis. Drought and notching experiments were conducted on Pinus sylvestris, Picea abies, Betula Pendula and Populus tremula seedlings in glasshouse conditions to study the dependence of NSLs on leaf osmotic and water status, and their coordination with stomatal control, on timescales of minutes and weeks, to test the assumptions and predictions of the optimization hypothesis. Both NSLs and stomatal conductance followed power-law functions of leaf osmotic concentration and leaf water potential. Moreover, stomatal conductance was proportional to the square root of soil-to-leaf hydraulic conductance, as predicted by the optimization hypothesis. Though the detailed mechanisms underlying the dependence of NSLs on leaf osmotic or water status lie outside the scope of this study, our results support the hypothesis that NSLs and stomatal control are coordinated to maximize leaf photosynthesis and allow the effect of NSLs to be included in models of tree gas-exchange.

Physiological, biochemical and molecular responses to water stress and rehydration in Mediterranean adapted tomato landraces.

Mediterranean tomato landraces adapted to arid environments represent an option to counteract drought, and to address the complexity of responses to water deficit and recovery, which is a crucial component of plant adaptation mechanisms. We investigated physiological, biochemical and molecular responses of two Mediterranean tomato landraces, 'Locale di Salina' (Lc) and 'Pizzutello di Sciacca' (Pz) under two dehydration periods and intermediate rehydration in greenhouse pot experiments. Relationship between CO2 assimilation (A) and stomatal conductance under severe water stress (gs <0.05mol·m-2 ·s-1 ) indicated the occurrence of stomatal and non-stomatal limitations of photosynthesis. Gas exchange promptly recovered within 2-3days of rehydration. ABA and gs showed a strict exponential relationship. Both leaf ABA and proline peaked under severe water stress. Lc showed higher accumulation of ABA and higher induction of the expression of both NCED and P5CS genes than Pz. Poly(ADP-ribose) polymerase increased during imposition of stress, mainly in Lc, and decreased under severe water stress. The two landraces hardly differed in their physiological performance. Under severe water stress, gs showed low sensitivity to ABA, which instead controlled stomatal closure under moderate water stress (gs >0.15mol·m-2 ·s-1 ). The prompt recovery after rehydration of both landraces confirmed their drought-tolerant behaviour. Differences between the two landraces were instead observed at biochemical and molecular levels.

Photosynthetic Properties and Response to Drought in Cucumber Seedlings Acclimatized to Different Vapor-pressure-deficit Levels

Hardening with a high vapor-pressure-deficit (VPD) can decrease conductance (gs) and thereby enhance drought tolerance by reducing transpiration, which is particularly useful during transplant establishment. However, high-VPD hardening may decrease photosynthetic performance due to stomatal (diffusion) and non-stomatal (mesophyll) limitation. If gs can be lowered without significantly reducing photosynthesis, water-use efficiency (WUE = CO2 assimilation/transpiration), which is an important criterion for transplant quality, would improve. We investigated the photosynthetic properties of cucumber (Cucumis sativus L.) leaves acclimatized to different VPD levels (0.4 and 3.2 kPa at 28°C), and determined whether photosynthesis was limited by stomatal or non-stomatal factors at high VPD. The net photosynthetic rate (Pn) and gs were measured at a VPD of 0.8 kPa and a leaf temperature of 28°C under saturating light. The photosynthetic response to the intercellular CO2 concentration (Ci) was used to quantify the effects of VPD acclimatization on carboxylation efficiency. Pn did not differ significantly between acclimatization VPDs, but gs was much lower (×0.36) in high-VPD leaves at an ambient CO2 concentration of 400 μmol·mol−1. Thus, the intrinsic WUE (= Pn/gs) of the high-VPD leaves was much higher (×2.85). The Ci of high-VPD seedlings was lower than that of low-VPD seedlings, but, this did not cause any significant reduction in Pn in the high-VPD treatments because the decrease in Ci occurred within a range in which photosynthesis was not limited by ribulose-1,5-bisphosphate carboxylation in this experiment. The Pn–Ci curve did not differ between the VPD treatments, indicating that carboxylation efficiency was not affected. When VPD-acclimatized seedlings experienced a limited water supply, the low-VPD leaves showed lower leaf water potential and more severe wilting than the high-VPD leaves 30 min after water limitation began. The gs of the high-VPD leaves was significantly lower (approximately ×0.5 in average) than that of the low-VPD leaves throughout the water-supply-limitation treatment. The lower gs maintained the water status of the high-VPD leaves at less-wilting levels by decreasing transpiration. These results indicate that high-VPD hardening could enhance the tolerance to short-term drought without stomatal or non-stomatal limitation of photosynthesis when controlling gs effectively.

Nitrogen fertigation effect on photosynthesis, grain yield and water use efficiency of winter wheat

Excessive fertilization is common in North China Plain. It is essential to determine the extent to which reduction of nitrogen application rates has little impact on yield and understand its physiological basis. Two seasons of field experiments were conducted in a winter wheat field with three nitrogen (N) fertilization treatments, i.e. N3, traditional N application rates (290kgN hm−2), N2, ∼65% N application rates of N3 (190kgN hm−2) and N1, ∼40% N application rates of N3 (110kgN hm−2). Yield (Y) and water use efficiency (WUE) of N2 did not reduce significantly in both seasons despite that the instantaneous net photosynthetic rates (An) and stomatal conductance (gs) of N2 were significantly lower than those of N3. Y and WUE of N1 treatment reduced significantly in the second season. The reduction of Y was better indicated by photosynthetic capacity (Amax) in the grain filling stage (around 48–50 days after the irrigation of reviving stage, Dari 48–50) because the decrease of Amax in this stage was proportional to Y. The Amax and apparent quantum efficiency (α) of N2 measured on Dari 48–50 were not significantly lower than those of N3, which could be the reason of no significant reduction of Y for N2. Thus, the fertilization amount of N2 (190kgN hm−2) can be applied in this area to keep Y steady for at least two years. Differences of the relationships between Amax and biological factors among treatments and non-stomatal limitation of photosynthesis were also discussed. These results are important for understanding the mechanisms of yield reduction by reducing N application and provide scientific basis for application of fertigation.

Mitochondrial alternative oxidase maintains respiration and preserves photosynthetic capacity during moderate drought in Nicotiana tabacum.

The mitochondrial electron transport chain includes an alternative oxidase (AOX) that is hypothesized to aid photosynthetic metabolism, perhaps by acting as an additional electron sink for photogenerated reductant or by dampening the generation of reactive oxygen species. Gas exchange, chlorophyll fluorescence, photosystem I (PSI) absorbance, and biochemical and protein analyses were used to compare respiration and photosynthesis of Nicotiana tabacum 'Petit Havana SR1' wild-type plants with that of transgenic AOX knockdown (RNA interference) and overexpression lines, under both well-watered and moderate drought-stressed conditions. During drought, AOX knockdown lines displayed a lower rate of respiration in the light than the wild type, as confirmed by two independent methods. Furthermore, CO2 and light response curves indicated a nonstomatal limitation of photosynthesis in the knockdowns during drought, relative to the wild type. Also relative to the wild type, the knockdowns under drought maintained PSI and PSII in a more reduced redox state, showed greater regulated nonphotochemical energy quenching by PSII, and displayed a higher relative rate of cyclic electron transport around PSI. The origin of these differences may lie in the chloroplast ATP synthase amount, which declined dramatically in the knockdowns in response to drought. None of these effects were seen in plants overexpressing AOX. The results show that AOX is necessary to maintain mitochondrial respiration during moderate drought. In its absence, respiration rate slows and the lack of this electron sink feeds back on the photosynthetic apparatus, resulting in a loss of chloroplast ATP synthase that then limits photosynthetic capacity.

Foliar chemistry and tree ring δ13C of Pinus densiflora in relation to tree growth along a soil pH gradient

Soil acidification is known to be one of the constraints of tree growth; however, it is unclear how it affects tree growth at photosynthesis level (i.e., through affecting stomatal conductance vs. carboxylation rate) during the growth of trees. This paper studied the effects of soil acidification on Pinus densiflora foliar chemistry and tree ring C isotope ratio (13C/12C, expressed as δ13C) and their relationship with tree growth. Tree growth (diameter, annual growth ring area, and root biomass), soil chemistry (pH, mineral N, and exchangeable Ca and Al), foliage chemistry (N, Ca/Al, and δ13C), and tree ring δ13C in P. densiflora stands along a soil pH gradient (from 4.38 to 4.83, n = 9) in southern Korea were investigated. Overall, trees with relatively poor growth under more acidic soil conditions (low pH and Ca/Al) had lower values of foliar N concentration and δ13C and tree ring δ13C, suggesting that restricted N uptake under more acidic soil conditions caused N limitation for photosynthesis, leading to poor tree growth. In addition, relationships between mean annual area increment and carbon isotope discrimination of tree rings at five-yr intervals from 1968 to 2007 revealed that the impact of soil acidification on tree growth became severer during the last 15 yrs as negative correlations between them became significant after 1993. Reduced N uptake under acidic soil conditions resulted in lower radial growth of P. densiflora via non-stomatal limitation of photosynthesis.

Non-stomatal limitations of photosynthesis in grassland species under artificial drought in the field

As drought stress is expected to occur more frequently in future climate in central Europe, survival and productivity of grassland species are an important issue. Non-stomatal limitation processes related to the drought-stress inhibition of photosynthesis of selected grassland species were analysed at three locations using leaf gas exchange and chlorophyll fluorescence. The effect of an artificial drought on the non-stomatal limitations differed considerably between species present in the same grassland plot. The maximum efficiency of photosystem II (Fv/Fm), indicator for the intactness of the photosynthetic electron transport, showed only small differences under drought. On the other hand, more pronounced effects were observed for the carboxylation velocity of Rubisco (Vc,max). Vc,max was in Phleum pratense about 20% lower under drought than in control plants, while other species in the same plot were far less affected. The carboxylation velocity of Rubisco is highly sensitive to water deficit and might represent a tool to evaluate the drought response of various species in order to address the performance of grasslands.

Photosynthesis, chlorophyll fluorescence and non-structural carbohydrates changes in husk leaves of maize in black soils region of Northeast China.

Two varieties of maize (Zea mays L.) grown in fields in Black soils of Northeast China were tested to study the variation of chlorophyll (Chl) concentration, Chl fluorescence, photosynthetic parameters, and non-structural carbohydrates in husk leaves of maize. The results showed that Chl a, Chl b and total Chl concentrations increased dramatically from silking to grain filling period. Increased maximum quantum efficiency of Photosystem II primary photochemistry (Fv/Fm), potential photochemical efficiency (Fv/Fo), net photosynthetic rate (Pn), transpiration rate (E) and stomatal conductance (gs) were observed at grain filling period. The metabolic changes revealed that patterns of total soluble sugars (TSS) and sucrose were consistent with photosynthesis. Decline or a little increase of intercellular CO2 concentrations (Ci) was found for both varieties, suggesting the existence of a non-stomatal limitation of photosynthesis in husk leaves in maize. Our results suggest that PS II apparatus remained functional in senescent husk leaves and that there was a continuously efficient use of the light captured by the remaining PS II apparatus. Key words: Maize, husk leaves, chlorophyll concentration, chlorophyll fluorescence, photosynthesis, photoinhibition, total soluble sugar, sucrose.

Stomatal and non‐stomatal limitations of photosynthesis in trees of a tropical seasonally flooded forest

Trees in the flooded forest of the Mapire River in Venezuela suffer a decrease in photosynthetic rate (A) when flood begins, which is reverted at maximum flood. Changes in A are accompanied by similar changes in stomatal conductance (g(s)), and the possibility of changes in photosynthetic capacity is not ruled out. In order to understand how relative stomatal and non-stomatal limitations of photosynthesis are affected by flooding, we studied the seasonal changes in A and its response to intercellular CO(2) concentration in trees of Campsiandra laurifolia, Symmeria paniculata, Acosmium nitens and Eschweilera tenuifolia. Flooding caused in trees of C. laurifolia and S. paniculata a reduction in A, g(s), carboxylation efficiency and total soluble protein (TSP), whereas gas exchange in A. nitens and E. tenuifolia was more sensitive to drought. Under flooding, relative stomatal limitation (L(s)) was on average half the highest, and relative non-stomatal limitation (L(ns)) increased from the dry season to flooding. Under full flood, A, g(s) and TSP regained high values. A was positively correlated to light-saturated electron transport rate, suggesting that part of the decrease in A under flooding was due to impairment of photosynthetic capacity. Under flooding, not only stomatal closure but also increased L(ns) causes a reduction in photosynthesis of all four species, and a process of acclimation as flooding progresses allows gas exchange and related variables to regain high values.

Physiological behaviour, oxidative damage and antioxidative protection of olive trees grown under different irrigation regimes

Irrigation effects were investigated on an 8-year-old olive (Olea europaea L., cv. Cobrancosa) commercial orchard located in northeast Portugal. Trees were subjected to a rainfed control (T0) and three treatments (T1, T2, T3) that received a seasonal water amount equivalent to 30%, 60% and 100% of the estimated local evaporative demand by a drip irrigation system. Irrigation increases the photosynthetic activity of olive trees, in association with increases in water status, and reduces the midday and afternoon depression in gas exchange. The closely association between photosynthetic rate (A) and stomatal conductance (gs) revealed that the decline in net photosynthesis over the course of the day was largely a consequence of stomatal limitation. However, the ratio of intercellular to atmospheric CO2 concentration increased markedly from morning to midday in non-irrigated plants, in spite of lower gs, suggesting that non-stomatal limitations of photosynthesis also occur when environmental conditions become more stressful. The occurrence of perturbations at chloroplastic level in rainfed plants was demonstrated by a lower maximum photochemical efficiency of photosystem II during the afternoon. Chlorophyll fluorescence measurements also revealed the occurrence of a dynamic photoinhibition in irrigated trees, mainly in T2 and T3, which seemed to be effective in protecting the photosynthetic apparatus from photodamage. Irrigation enhances antioxidant protection and decreases the oxidative damage at leaf level. Leaves grown under rainfed conditions revealed symptoms of oxidative stress, like the reduction (14%) in chlorophyll concentration and the increased levels (57%) of lipid peroxidation. We also found that the scavenging function of superoxide dismutase was impaired in rainfed plants. In contrast, the low thiobarbituric acid reactive substances concentration in T3 indicates that irrigation enhances the repairing mechanisms and decreases the oxidative damage by lipid peroxidation. Accordingly, leaves in T3 treatment had high levels of –SH compounds and the highest antioxidant potential. Meanwhile, the finding that guaiacol peroxidase activity increased in rainfed plants, associated with the appearance of oxidative damage, suggests that this enzyme has no major antioxidative function in olive.

Seasonal and diurnal monitoring of leaf polyamine content in Quercus ilex L. resprouts after fire in relation to changes in irradiation and photosynthetic parameters

Seasonal variations in free putrescine, spermidine and spermine content, gas-exchange and chlorophyll fluorescence parameters were followed during winter and summer on leaves of a similar age from undisturbed holm oak trees (control, C) and resprouts (R) originated after fire. We observed a general trend of putrescine content decrease with increasing irradiance. Putrescine content decreased markedly from winter to summer, especially in R, which were located on a site with much higher irradiation. Daily summer variations in putrescine showed a decline at midday from morning values, and they were also more accentuated in R. Measurement of gas-exchange and chlorophyll fluorescence parameters showed marked differences between C and R under their respective light conditions. R showed higher values of PSII quantum yield (ΦPSII), photochemical quenching (qP) and intrinsic efficiency of open PSII centres (\(F^\prime _{\rm v} /F^\prime _{\rm m} \)) The ΦPSII/PPFD response curve showed that under the same irradiance, ΦPSII was enhanced in R and mainly under high light conditions. In spite of increasing irradiance from winter to summer, and especially in burned areas, the mentioned chlorophyll fluorescence parameters were maintained indicating the adaptation of the photosynthetic apparatus. Results derived from A/C i and A/PPFD response curves showed enhanced photosynthetic capacity and lower non-stomatal limitation of photosynthesis in R during summer stress. The contribution of putrescine decline in the photoadaptation of the photosynthetic apparatus of species growing in natural forest habitats is considered.

Dynamics of the enhanced emissions of monoterpenes and methyl salicylate, and decreased uptake of formaldehyde, by Quercus ilex leaves after application of jasmonic acid

Jasmonic acid (JA) is a signalling compound with a key role in both stress and development in plants, and is reported to elicit the emission of volatile organic compounds (VOCs). Here we studied the dynamics of such emissions and the linkage with photosynthetic rates and stomatal conductance. We sprayed JA on leaves of the Mediterranean tree species Quercus ilex and measured the photosynthetic rates, stomatal conductances, and emissions and uptake of VOCs using proton transfer reaction mass spectrometry and gas chromatography after a dark-light transition. Jasmonic acid treatment delayed the induction of photosynthesis and stomatal conductance by approx. 20 min, and decreased them 24 h after spraying. Indications were found of both stomatal and nonstomatal limitations of photosynthesis. Monoterpene emissions were enhanced (20-30%) after JA spraying. Jasmonic acid also increased methyl salicylate (MeSa) emissions (more than twofold) 1 h after treatment, although after 24 h this effect had disappeared. Formaldehyde foliar uptake decreased significantly 24 h after JA treatment. Both biotic and abiotic stresses can thus affect plant VOC emissions through their strong impact on JA levels. Jasmonic acid-mediated increases in monoterpene and MeSa emissions might have a protective role when confronting biotic and abiotic stresses.

Genotypic variation in drought response of silver birch (Betula pendula Roth): leaf and root morphology and carbon partitioning

Young trees of four clones of Betula pendula from origins with different amounts of precipitation were subjected to prolonged drought periods in two subsequent summers. The aims of this study were: (1) to investigate the drought response of silver birch in a multi-level approach and to elucidate the basis of the particularly wide physiological amplitude of this species and (2) to analyse variation in drought responses of different genotypes in order to identify potential genotypic differences with regard to their adaptability to drought. Responses of different physiological, biochemical, and morphological parameters to water shortage were regularly analysed: CO2 and H2O gas exchange were investigated under ambient and elevated CO2 concentration; leaf water status and osmotic adjustment were analysed with the pressure-chamber technique and pressure volume curves; leaves were analysed for chlorophyll and nitrogen content, and carbon isotopic composition; at the end of each drought period, trees were completely harvested and above- and belowground masses and areas determined.The following results were obtained: (1) Reduction of stomatal conductance was the first and most plastic response of silver birch to water shortage, thus effectively maintaining predawn leaf water potentials; non-stomatal limitations of photosynthesis were not detected. (2) Silver birch appeared to be capable of osmotic adjustment, which was mainly used during the phase of establishment. (3) Marked responses of total leaf area and the especially plastic phenology of this species have to be classified as adaptations to drought. (4) Both within-organ and between-organ allocation of biomass responded to drought. (5) Genotypes from drier locations displayed higher degrees of osmotic adjustment than those from origins with better water supply. (6) Genotypes from drier locations reached higher stomatal conductances under both well watered and drought stressed conditions. (7) Paticularly one genotype from a stand prone to summer drought displayed a more prodigal water use strategy than the genotypes from origins with higher precipitation. This strategy in combination with effective mechanisms of both drought avoidance and tolerance is classified as highly competitive and adapted to habitats with edaphic summer droughts.In conclusion, all genotypes responded to water shortage by marked mechanisms of drought avoidance on all levels of plant performance; The extent of response, however, appeared to be partly affected by adaptations to places of origin, with the genotypes from drier locations displaying a tendency for more prodigal water use strategies.