Leaf CO2 Research Articles

Effects of 5-HT on the cold resistance of mangrove Kandelia obovata seedlings.

5-hydroxytryptamine (5-HT) participates in plant growth and development, and can also delay senescence and cope with abiotic stress. To explore the role of 5-HT in regulating the abilities of mangrove in cold resis-tance, we examined the effects of cold acclimation and the spraying of p-chlorophenylalanine (p-CPA, 5-HT synthesis inhibitor) on leaf gas exchange parameters and CO2 response curves (A/Ca), as well as the endogenous phytohormone content levels in the mangrove species Kandelia obovata seedlings under low temperature stress. The results showed that low temperature stress significantly reduced the contents of 5-HT, chlorophyll, endogenous auxin (IAA), gibberellin (GA), and abscisic acid (ABA). It weakened the CO2 utilization abilities of plants and reduced net photosynthetic rate, which ultimately reduced carboxylation efficiency (CE). Under low temperature stress, exogenous p-CPA reduced the contents of photosynthetic pigments, endogenous hormones, and 5-HT in the leaves, which aggravated the damages caused by low temperature stress on photosynthesis. By enhancing cold acclimation abilities, the endogenous IAA content in the leaves could was reduced under low temperature stress, promoted the production of 5-HT, improved the contents of photosynthetic pigments, GA, and ABA, as well as enhanced photosynthetic carbon assimilation abilities, which would increase photosynthesis in the K. obovata seedlings. Under cold acclimation conditions, the spraying of p-CPA could significantly inhibit the synthesis of 5-HT, promote the production of IAA, and reduce the contents of photosynthetic pigments, GA, ABA, and CE, which would weaken the effects of cold acclimation by improving the cold resistance of mangroves. In conclusion, cold acclimation could improve the cold resistance abilities of K. obovata seedlings by regulating photosynthetic carbon assimilation capacity and the contents of endogenous phytohormone. 5-HT synthesis is one of the necessary conditions for improving the cold resistance abilities of mangroves.

The effects of growth regulators on the photosynthetic apparatus of the sweet pepper (Capsicum annuum L.) in relation to the productivity

The effects of growth stimulants 6-benzylaminopurine (6-BAP), gibberellic acid (GA3) and 1-naphthaleneacetic acid (1-NAA) as well as gibberellin inhibitors tebuconazole (EW-250), ethephon (2-chloroethylphosphonic acid, 2-CEPA) and chloromequate chloride (ССС-750), which differ in their action mechanism, on growth, development, leaf apparatus formation, CO2 and H2O gas exchange, photochemical activity of Photosystem II (PSII) and productivity traits of sweet pepper plants were studied. It was shown that treatment with growth stimulants increased, and gibberellin inhibitors decreased the linear sizes of sweet pepper plants of the Antei variety. It was established that all growth regulators, except for 2-CEPA, increased the number and the mass of leaves on the plant. Under the action of all preparations, except for 2-CEPA, the area of the leaves increased. Gibberellin inhibitors and 6-BAP significantly increased the amount of chlorophyll in pepper leaves. However, it decreased under the action of GA3 and did not practically change in treatment with 1-NAA. All growth substances, except GA3, increased the total chlorophyll content in the plant. The impact of growth regulators on the activity of photosynthetic processes was more pronounced at the stage of fruit formation than at the flowering stage. The CO2 assimilation rate at the flowering stage increased under the treatment of 1-NAA, 6-BAP, 2-CEPA and EW-250, but decreased under the action of GA3 and CCC-750. At the same time, all studied growth regulators increased the CO2 assimilation rate at the stage of fruit formation. Changes in the CO2 assimilation rate were closely correlated with changes in stomatal conductance (r = 0.79—0.85). Growth regulators increased transpiration in the light at fruit formation stage while the transpiration in the dark was reduced at the flowering stage. Growth regulators increased the operating quantum efficiency of PSII in the light, photochemical quenching of chlorophyll fluorescence, and intensity of linear electron transport in chloroplasts, and reduced non-photochemical quenching (NPQ) fluorescence. The specified morphological, physiological and biochemical changes in plants of sweet pepper of the Antei variety contributed to improvement of crop productivity traits. The use of growth stimulants 6-BAP and GA3, and retardants EW-250 and ССС-750 was most effective.

Photosynthetic characteristics of the invasive weed Chromolaena odorata and other co‐occurring species in KwaZulu‐Natal

AbstractIn this study, we tested the hypothesis that photosynthetic characteristics contributed to the success and spread of the invasive, C3 perennial weed, Chromolaena odorata (L.) R. M. King & H. Rob. Measurements of gas exchange, chlorophyll fluorescence and water relations parameters were taken in summer and winter, as well as in sun and shade plants. Typically, diurnal CO2 exchange increased from dawn to a mid‐morning maximum between 10:00 h and 11:00 h, and thereafter decreased gradually for the rest of the day. Light response curves indicated saturation of CO2 uptake and electron transport rate (ETR) through Photosystem II (PSII) at a photosynthetic photon flux density (PPFD) ≥1800 μmol m−2 s−1. Maximal CO2 exchange was 15.64 ± 0.87 (±SE) in summer and 13.15 ± 0.26 μmol m−2 s−1 in winter. Leaf conductance (g), CO2 uptake (A) and transpiration (E) followed trends similar to those for PPFD. Leaf water potential (Ψ) and water use efficiency (WUE) declined from the predawn maximum values to a minimum at midday and thereafter recovered at dusk. Diurnal trends in actual quantum yield (ΦPSII) and maximum quantum yield (Fv/Fm) decreased from dawn to a minimum at midday, followed by complete recovery at dusk. The relationship between A and g, and A and ETR was linear. Trends in A, ΦPSII and ETR through PSII were tightly coupled to those of incident PPFD. Efficient light utilisation is achieved through modifying plastic traits such as leaf size, specific leaf area and chlorophyll content. Chromolaena is a facultative shade‐tolerant weed that exhibits superior light utilisation to maximise carbon gain, while efficient photoprotective mechanisms minimise photoinhibition and photodamage to the photosystems. These adaptive architectural and physiological strategies may probably confer competitive advantage over other species.

Green compost amendment improves potato plant performance on Mars regolith simulant as substrate for cultivation in space

Background and aimsHigher plants represent an optimal tool to regenerate resources while producing food in Space. However, the configuration of fertile cultivation substrates based on extraterrestrial resources is still a challenge. We evaluated the adaptability of potato (Solanum tuberosum L., cv. ‘Colomba’) to the growth on six substrates: the MMS-1 Mars regolith simulant, alone (R100) and in mixture with 30% in vol. of green compost (R70C30), a fluvial sand, alone or mixed with 30% of compost (S100 and S70C30), a red soil from Sicily (RS), and a volcanic soil from Campania (VS).MethodsWe assessed the physicochemical properties of the substrates, and the physiological and biometric parameters of potato plants grown in pot on these substrates, in cold glasshouse.ResultsBoth R100 and S100 were alkaline (pH ≥ 8.6) and coarse-textured, lacking organic matter and pivotal macronutrients. The amendment with compost significantly lowered their alkaline pH and improved the chemical fertility. The sandy-loam textured VS was sub-alkaline, slightly calcareous, with higher organic C and nutrient availability than RS. This latter was neutral-to-sub-alkaline, clay textured, poorly calcareous, with significantly higher CEC than VS. Leaf CO2 assimilation rate was higher in plants grown in terrestrial soils and S100. Plant growth was greater in VS, R70C30 and S70C30, while it was reduced on R100. Plants produced healthy tubers on all the substrates.ConclusionMMS-1 regolith simulant was found poor in nutrients and unsuitable to sustain adequately the plant growth. Amendment with organic compost improved MMS-1 physiochemical properties and fertility and plant performance.

Unravelling δ13C signal in Scots pine trees for climate change and tree physiology studies

Stable carbon isotope composition (δ13C) recorded in trees responds sensitively to the changing environmental conditions and thus provides a powerful tool for paleoenvironmental reconstructions. The retrospective interpretation of tree δ13C signal depends on comprehensive understanding of how environmental and physiological signals are recorded in δ13C during photosynthesis and how the δ13C signal is modified after photosynthesis. This thesis aims to improve the understanding of photosynthetic and post-photosynthetic isotope fractionation processes, and to examine the suitability of tree ring δ13C for intra-seasonal reconstructions of intrinsic water use efficiency (iWUE). The former goal was studied by combining compound-specific isotope analysis of organic matter with isotope discrimination models and online δ13C measurements of leaf CO2 fluxes for field-grown mature Scots pine (Pinus sylvestris L.). The latter was achieved via comparing 18-year-long intra-seasonal iWUE chronologies estimated from laser ablation derived tree ring δ13C, gas exchange and eddy covariance data. Mesophyll conductance and time-integral effect of leaf assimilates had a clear impact on the intra-seasonal dynamics of leaf sugar δ13C. No significant use of reserves was observed for biomass growth of needles, stem or roots of Scots pine. Unlike sucrose, leaf bulk matters had a significant δ13C offset from new assimilates, leading to a distorted environmental signal documented in their δ13C. The reliability of tree ring δ13C data for intra-seasonal iWUE reconstructions was supported by an agreement of intra-seasonal patterns across the iWUE estimation methods. These results broaden our knowledge of the less well-known photosynthetic and post-photosynthetic isotopic fractionation processes, demonstrate the benefits of analysing sucrose δ13C for understanding plant physiological responses, and show that the tree ring δ13C based iWUE reconstructions can be extended to intra-seasonal scale. This information not only helps to better unravel δ13C signal in trees, but also improves reliable reconstructions of environmental and physiological signals from tree ring δ13C.

Development of Two-Dimensional Model of Photosynthesis in Plant Leaves and Analysis of Induction of Spatial Heterogeneity of CO2 Assimilation Rate under Action of Excess Light and Drought.

Photosynthesis is a key process in plants that can be strongly affected by the actions of environmental stressors. The stressor-induced photosynthetic responses are based on numerous and interacted processes that can restrict their experimental investigation. The development of mathematical models of photosynthetic processes is an important way of investigating these responses. Our work was devoted to the development of a two-dimensional model of photosynthesis in plant leaves that was based on the Farquhar-von Caemmerer-Berry model of CO2 assimilation and descriptions of other processes including the stomatal and transmembrane CO2 fluxes, lateral CO2 and HCO3- fluxes, transmembrane and lateral transport of H+ and K+, interaction of these ions with buffers in the apoplast and cytoplasm, light-dependent regulation of H+-ATPase in the plasma membrane, etc. Verification of the model showed that the simulated light dependences of the CO2 assimilation rate were similar to the experimental ones and dependences of the CO2 assimilation rate of an average leaf CO2 conductance were also similar to the experimental dependences. An analysis of the model showed that a spatial heterogeneity of the CO2 assimilation rate on a leaf surface should be stimulated under an increase in light intensity and a decrease in the stomatal CO2 conductance or quantity of the open stomata; this prediction was supported by the experimental verification. Results of the work can be the basis of the development of new methods of the remote sensing of the influence of abiotic stressors (at least, excess light and drought) on plants.

Water relations and gas exchange in <i>Ruilopezia atropurpurea</i> (Asteraceae), a giant rosette growing under contrasting microclimates in the high tropical Andes

Tropical high andean ecosystems, known as paramos, are unique because they are highly diverse, have a high number of endemic species, and play an essential role in different ecosystem services, but are especially susceptible to climate change. Most of the giant rosettes, a dominant growth-form in the paramos, depend on unique features like stems protected by marcescent leaves, voluminous stem pith, and leaf pubescence. However, Ruilopezia atropurpurea lacks these characteristics and must respond differently to endure the paramo extreme conditions. Additionally, unlike other rosettes, this species is found under contrasting exposed and understory microenvironments so that intraspecific plasticity is also expected. We evaluated the responses of R. atropurpurea in terms of leaf water relations, gas exchange, and morphological characteristics in temporal (seasonal and daily variations) and spatial (microsite differences) scales in a Venezuelan paramo. R. atropurpurea displayed lower leaf water potentials (minimum leaf water potentials of -1.5 MPa and -1.8 MPa at the turgor loss point), higher leaf conductance (620 mmol m-2s-1), transpiration (5 molm-2s-1), and CO2 assimilation (13 mmol m-2s-1) rates compared to other paramo giant rosettes. A reduction in leaf area and specific leaf area occurred from understory to exposed sites. R. atropurpurea diverges from the typical responses of most paramo giant rosettes to the extreme environmental conditions. This species’ morphological and physiological plasticity permits it inhabit under variable microclimatic conditions, but despite its confirmed plasticity, it is not able to reach higher elevations as other giant rosettes successfully have.

Consistent responses of vegetation gas exchange to elevated atmospheric CO2emerge from heuristic and optimization models

Abstract. Elevated atmospheric CO2 concentration is expected to increase leaf CO2 assimilation rates, thus promoting plant growth and increasing leaf area. It also decreases stomatal conductance, allowing water savings, which have been hypothesized to drive large-scale greening, in particular in arid and semiarid climates. However, the increase in leaf area could reduce the benefits of elevated CO2 concentration through soil water depletion. The net effect of elevated CO2 on leaf- and canopy-level gas exchange remains uncertain. To address this question, we compare the outcomes of a heuristic model based on the Partitioning of Equilibrium Transpiration and Assimilation (PETA) hypothesis and three model variants based on stomatal optimization theory. Predicted relative changes in leaf- and canopy-level gas exchange rates are used as a metric of plant responses to changes in atmospheric CO2 concentration. Both model approaches predict reductions in leaf-level transpiration rate due to decreased stomatal conductance under elevated CO2, but negligible (PETA) or no (optimization) changes in canopy-level transpiration due to the compensatory effect of increased leaf area. Leaf- and canopy-level CO2 assimilation is predicted to increase, with an amplification of the CO2 fertilization effect at the canopy level due to the enhanced leaf area. The expected increase in vapour pressure deficit (VPD) under warmer conditions is generally predicted to decrease the sensitivity of gas exchange to atmospheric CO2 concentration in both models. The consistent predictions by different models that canopy-level transpiration varies little under elevated CO2 due to combined stomatal conductance reduction and leaf area increase highlight the coordination of physiological and morphological characteristics in vegetation to maximize resource use (here water) under altered climatic conditions.

Contrasting Trends in Water Use Efficiency of the Alpine Grassland in Tibetan Plateau

AbstractAlpine grassland is the dominant land cover type of the Tibetan Plateau (TP). Previous studies suggested that TP's climate and vegetation condition have changed substantially in past few decades. However, the impacts of such changes on ecosystem water use efficiency (WUE), a vital index for depicting the tradeoff between water loss and carbon gain, of alpine grassland remain poorly understood. This study used a water‐carbon coupled model, Penman‐Monteith‐Leuning Version 2 (PML_V2), to investigate the variation in WUE of alpine grassland in the TP during 1982–2016 and its drivers. PML_V2 performs well in estimating WUE at nine eddy‐covariance flux sites, noticeably better than other main‐stream products. The long‐term mean WUE decreases from the southeastern to the northwestern TP, generally following the spatial pattens of surface greenness and climatic condition. During 1982–2016, WUE increased in most parts (71%) of alpine grassland but declined in certain regions of western TP. At the ecosystem scale, contrasting trends in WUE emerged between alpine meadow and alpine steppe over the last 35 years. The former's significant increasing WUE was primarily driven by increased leaf area index (LAI) and CO2 concentration (eCO2). While greening also occurred in the latter, its WUE remained stable because (a) the large negative effect of precipitation offset the positive effect of LAI on WUE, and; (b) the enhancement of photosynthesis resulted from elevated eCO2 was weaker because of low LAI. Our results are of importance for understanding the dynamics and mechanisms of water‐carbon coupling in high‐elevation ecosystems under a changing climate.

Plasticity in stomatal behaviour across a gradient of water supply is consistent among field‐grown maize inbred lines with varying stomatal patterning

Stomata regulate leaf CO2 assimilation (A) and water loss. The Ball–Berry and Medlyn models predict stomatal conductance (g s) with a slope parameter (m or g 1) that reflects the sensitivity of g s to A, atmospheric CO2 and humidity, and is inversely related to water use efficiency (WUE). This study addressed knowledge gaps about what the values of m and g 1 are in C4 crops under field conditions, as well as how they vary among genotypes and with drought stress. Four inbred maize genotypes were unexpectedly consistent in how m and g 1 decreased as water supply decreased. This was despite genotypic variation in stomatal patterning, A and g s. m and g 1 were strongly correlated with soil water content, moderately correlated with predawn leaf water potential (Ψpd), but not correlated with midday leaf water potential (Ψmd). This implied that m and g 1 respond to long‐term water supply more than short‐term drought stress. The conserved nature of m and g 1 across anatomically diverse genotypes and water supplies suggests there is flexibility in structure‐function relationships underpinning WUE. This evidence can guide the simulation of maize g s across a range of water supply in the primary maize growing region and inform efforts to improve WUE.

Shading Nets Reduce Canopy Temperature and Improve Photosynthetic Performance in ‘Pinkerton’ Avocado Trees during Extreme Heat Events

Frequent extreme heat events cause major financial losses for the avocado industry. Shading nets are used to protect crops from harsh environmental hazards. To determine their ability to improve photosynthetic performance under extreme heat in avocado, we examined the impact of a resilient high-density silver 60% shading net on mature ‘Pinkerton’ avocado trees during such conditions. We hypothesized that reduced solar irradiation will lower canopy temperature and improve tree performance. During extreme heat events, photosynthetic photon flux density (PPFD), air temperature (Tair) and leaf temperature (Tleaf) reached 1188 μmol m−2 s−1, 43.1 °C and 43.8 °C in the control plots, respectively. In the net-covered plots, these parameters significantly decreased to 401 μmol m−2 s−1, 40.3 °C and 39.8 °C, respectively. Interestingly, leaf CO2 assimilation, stomatal conductance to water vapor (gs) and substomatal internal CO2 concentration (Ci) were significantly higher, by 94%, 120% and 13%, respectively, than in controls. On days with regular fall temperatures, PPFD, Tair and Tleaf reached 814 μmol m−2 s−1, 31.2 °C and 31.6 °C in the control plots, respectively. In the net-covered plots, these parameters significantly decreased to 291 μmol m−2 s−1, 29.5 °C and 29.4 °C, respectively. However, leaf CO2 assimilation was significantly (20%) lower, gs was similar and Ci was 10% higher than in control trees. These findings suggest that silver 60% shading nets may potentially reduce heat stress during extreme heat events, calling for long-term studies on their effects on flowering and fruit set, load, size and quality.

Into the Shadows and Back into Sunlight: Photosynthesis in Fluctuating Light.

Photosynthesis is an important remaining opportunity for further improvement in the genetic yield potential of our major crops. Measurement, analysis, and improvement of leaf CO2 assimilation (A) have focused largely on photosynthetic rates under light-saturated steady-state conditions. However, in modern crop canopies of several leaf layers, light is rarely constant, and the majority of leaves experience marked light fluctuations throughout the day. It takes several minutes for photosynthesis to regain efficiency in both sun-shade and shade-sun transitions, costing a calculated 10-40% of potential crop CO2 assimilation. Transgenic manipulations to accelerate the adjustment in sun-shade transitions have already shown a substantial productivity increase in field trials. Here, we explore means to further accelerate these adjustments and minimize these losses through transgenic manipulation, gene editing, and exploitation of natural variation. Measurement andanalysis of photosynthesis in sun-shade and shade-sun transitions are explained. Factors limiting speeds of adjustment and how they could be modified to effect improved efficiency are reviewed, specifically nonphotochemical quenching (NPQ), Rubisco activation, and stomatal responses.

Prolonged drought affects the interaction of carbon and nitrogen metabolism in root and shoot of cotton

Maintaining assimilates metabolism is critical for root and shoot normal function, especially during drought. However, little is known about how plants coordinate assimilates transport and internal root and shoot assimilates usage under drought. To test these objectives, a greenhouse experiment was carried out in which two cotton cultivars (Dexiamian 1 and Yuzaomian 9110) with varied drought tolerance were subjected to a continuous soil water deficit. We used stable-isotope labeling to examine the changes in assimilates allocation, as well as the responses of metabolic activities in the root and leaf to drought. According to the findings, root nitrogen absorption was reduced as a result of insufficient ATP supply and the down-regulated GhNPF4.6 . Further, the limited root nitrate absorption and nitrate transport from root to shoot reduced leaf nitrogen accumulation. Moreover, less proportion of nitrogen invested in photosynthetic apparatus under drought limited the leaf photosynthetic performance. Drought suppressed leaf CO 2 fixation and photosynthates export, but the relative amount of photosynthates allocated in root increased due to the increasing expression of sucrose transporters. However, the stunted respiratory metabolism limited root function although the carbohydrates were sufficient. We conclude that drought disrupts assimilate partition between root and shoot, restricts root carbohydrates use, and reduces leaf photosynthetic nitrogen utilization, resulting in a functional imbalance between root and shoot. • Drought limited CO 2 fixation and promoted photoassimilates transport to root. • Root respiratory metabolism was inhibited by drought. • The inadequate energy supply and the down-regulated GhNPF4.6 reduced nitrate uptake. • The limited leaf photosynthetic nitrogen utilization intensified photosynthesis loss.

Modelling potential potato yields: Accounting for experimental differences in modern cultivars

Current potential yield simulations of potato crops can be lower than actual yields observed in fields and the potential yield simulations miss important cultivar differences, for instance in terms of cultivar earliness. This causes issues not only for potential yield simulations but also when estimating the limiting effects of water (and nitrogen) and yield gaps in farmers’ fields. To address this issue, we recalibrated potential yield simulations with WOFOST for five commercial Dutch potato cultivars. An experiment conducted during the 2020 season was used for model calibration while the 2019 version of the same experiment was used for evaluation. The focus of the calibration was on phenology, photosynthesis, and biomass allocation. To aid the latter, the WOFOST biomass allocation table functions were replaced with sigmoid functions and biomass reallocation from haulm to tubers was introduced. In total 19 parameters were adjusted and/or introduced. Twelve of these parameters were not cultivar-specific, e.g. the specific leaf area over the plant developmental stage (SLATB), and the maximum leaf CO2 assimilation rate as function of developmental time (AMAXTB). Seven parameters differed between the cultivars, e.g. the thermal time between emergence and tuber initiation (TSUM1), and most of these were correlated to cultivar earliness. For the calibration year, final yields were underestimated by on average 5.4%, while for the evaluation year the final yield was overestimated by 4.4%. In comparison, the final dry matter yields ranged from 16.4 to 18.5 t ha−1 in 2019 and 15.5–17.5 t ha−1 in 2020. The old WOFOST parameters for potato resulted in an underestimation of 23.7% and 28.7% for, respectively, 2019 and 2020. Improving the parameter sets greatly increased accuracy of the simulations and provided insight in which parameters are cultivar specific and related to cultivar-earliness.

Boron-mediated amelioration of copper-toxicity in sweet orange [Citrus sinensis (L.) Osbeck cv. Xuegan] seedlings involved reduced damage to roots and improved nutrition and water status

‘Xuegan’ (Citrus sinensis) seedlings were fertilized 6 times weekly for 24 weeks with 0.5 or 350 μM CuCl2 and 2.5, 10 or 25 μM H3BO3. Cu-toxicity increased Cu uptake per plant (UPP) and Cu concentrations in leaves, stems and roots, decreased water uptake and phosphorus, nitrogen, calcium, magnesium, potassium, sulfur, boron and iron UPP, and increased the ratios of magnesium, potassium, calcium and sulfur UPP to phosphorus UPP and the ratios of leaf magnesium, potassium and calcium concentrations to leaf phosphorus concentration. Many decaying and dead fibrous roots occurred in Cu-toxic seedlings. Cu-toxicity-induced alterations of these parameters and root damage decreased with the increase of boron supply. These results demonstrated that B supplementation lowered Cu uptake and its concentrations in leaves, stems and roots and subsequently alleviated Cu-toxicity-induced damage to root growth and function, thus improving plant nutrient (decreased Cu uptake and efficient maintenance of the other nutrient homeostasis and balance) and water status. Further analysis indicated that the improved nutrition and water status contributed to the boron-mediated amelioration of Cu-toxicity-induced inhibition of seedlings, decline of leaf pigments, large reduction of leaf CO2 assimilation and impairment of leaf photosynthetic electron transport chain revealed by greatly altered chlorophyll a fluorescence (OJIP) transients, reduced maximum quantum yield of primary photochemistry (Fv/Fm), quantum yield for electron transport (ETo/ABS) and total performance index (PIabs,total), and elevated dissipated energy per reaction center (DIo/RC). To conclude, our findings corroborate the hypothesis that B-mediated amelioration of Cu-toxicity involved reduced damage to roots and improved nutrient and water status. Principal component analysis showed that Cu-toxicity-induced changes of above physiological parameters generally decreased with the increase of B supply and that B supply-induced alterations of above physiological parameters was greater in 350 μM Cu-treated than in 0.5 μM Cu-treated seedlings. B and Cu had a significant interactive influence on C. sinensis seedlings.

Cell Wall Properties Determine Genotype-Specific Response to Cold in Miscanthus × giganteus Plants.

The cell wall plays a crucial role in plant growth and development, including in response to environmental factors, mainly through significant biochemical and biomechanical plasticity. The involvement of the cell wall in C4 plants’ response to cold is, however, still poorly understood. Miscanthus × giganteus, a perennial grass, is generally considered cold tolerant and, in contrast to other thermophilic species such as maize or sorgo, can maintain a relatively high level of photosynthesis efficiency at low ambient temperatures. This unusual response to chilling among C4 plants makes Miscanthus an interesting study object in cold acclimation mechanism research. Using the results obtained from employing a diverse range of techniques, including analysis of plasmodesmata ultrastructure by means of transmission electron microscopy (TEM), infrared spectroscopy (FTIR), and biomechanical tests coupled with photosynthetic parameters measurements, we present evidence for the implication of the cell wall in genotype-specific responses to cold in this species. The observed reduction in the assimilation rate and disturbance of chlorophyll fluorescence parameters in the susceptible M3 genotype under cold conditions were associated with changes in the ultrastructure of the plasmodesmata, i.e., a constriction of the cytoplasmic sleeve in the central region of the microchannel at the mesophyll–bundle sheath interface. Moreover, this cold susceptible genotype was characterized by enhanced tensile stiffness, strength of leaf wall material, and a less altered biochemical profile of the cell wall, revealed by FTIR spectroscopy, compared to cold tolerant genotypes. These changes indicate that a decline in photosynthetic activity may result from a decrease in leaf CO2 conductance due to the formation of more compact and thicker cell walls and that an enhanced tolerance to cold requires biochemical wall remodelling. Thus, the well-established trade-off between photosynthetic capacity and leaf biomechanics found across multiple species in ecological research may also be a relevant factor in Miscanthus’ tolerance to cold. In this paper, we demonstrate that M. giganteus genotypes showing a high degree of genetic similarity may respond differently to cold stress if exposed at earlier growing seasons to various temperature regimes, which has implications for the cell wall modifications patterns.

Temperature acclimation of net photosynthesis and its underlying component processes in four tropical tree species.

The effect of temperature change on leaf physiology has been extensively studied in temperate trees and to some extent in boreal and tropical tree species. While increased temperature typically stimulates leaf CO2 assimilation and tree growth in high-altitude ecosystems, tropical species are often negatively affected. These trees may operate close to their temperature optima and have a limited thermal acclimation capacity due to low seasonal and historical variation in temperature. To test this hypothesis, we studied the extent to which the temperature sensitivities of leaf photosynthesis and respiration acclimate to growth temperature in four common African tropical tree species. Tree seedlings native to different altitudes and therefore adapted to different growth temperatures were cultivated at three different temperatures in climate-controlled chambers. We estimated the acclimation capacity of the temperature sensitivities of light-saturated net photosynthesis, the maximum rates of Rubisco carboxylation (Vcmax) and thylakoid electron transport (J), and dark respiration. Leaf thylakoid membrane lipid composition, nitrogen content and leaf mass per area were also analyzed. Our results showed that photosynthesis in tropical tree species acclimated to higher growth temperatures, but that this was weakest in the species originating from the coolest climate. The temperature optimum of J acclimated significantly in three species and variation in J was linked to changes in the thylakoid membrane lipid composition. For Vcmax, there was only evidence of significant acclimation of optimal temperature in the lowest elevation species. Respiration acclimated to maintain homeostasis at growth temperature in all four species. Our results suggest that the lowest elevation species is better physiologically adapted to acclimate to high growth temperatures than the highest elevation species, indicating a potential shift in competitive balance and tree community composition to the disadvantage of montane tree species in a warmer world.

Leaf temperature and CO2 effects on photosynthetic CO2 assimilation and chlorophyll a fluorescence light responses during mid-ripening of Vitis vinifera cv. Shiraz grapevines grown in outdoor conditions.

Responses of CO2 assimilation and chlorophyll a fluorescence to light intensity for Shiraz leaves on vines grown outdoors were examined in relation to leaf temperature. The study aimed to assess whether perturbing the carbon source, by manipulating short-term CO2 concentrations, would affect photosynthetic responses to temperature. Strong interactions occurred between leaf temperature and CO2 on photosynthetic and electron transport light responses. Most responses to temperature occurred at low to moderate CO2 and little response to temperature occurred at high CO2 . While assimilation responses accorded with increasing substrate CO2 , electron transport was inhibited by elevated CO2 . By contrast, chlorophyll a fluorescence was not affected by a temperature×CO2 interaction and CO2 had no effect on PSII quantum efficiency or photochemical quenching; whereas there was a moderate effect of temperature. Quantum efficiency of PSII was most severely reduced at low temperatures. Most photochemical quenching also occurred at low temperatures and the least at 40°C, in keeping with the warm to hot growth climate and the apparent assimilation bias towards the higher temperatures of the growing season. No changes in temperature dependency of assimilation were detected at the different CO2 concentrations, confirming sinks have a greater effect on assimilation than does the source.

Exploring optimal stomatal control under alternative hypotheses for the regulation of plant sources and sinks.

Experimental evidence that nonstomatal limitations to photosynthesis (NSLs) correlate with leaf sugar and/or leaf water status suggests the possibility that stomata adjust to maximise photosynthesis through a trade-off between leaf CO2 supply and NSLs, potentially involving source-sink interactions. However, the mechanisms regulating NSLs and sink strength, as well as their implications for stomatal control, remain uncertain. We used an analytically solvable model to explore optimal stomatal control under alternative hypotheses for source and sink regulation. We assumed that either leaf sugar concentration or leaf water potential regulates NSLs, and that either phloem turgor pressure or phloem sugar concentration regulates sink phloem unloading. All hypotheses led to realistic stomatal responses to light, CO2 and air humidity, including conservative behaviour for the intercellular-to-atmospheric CO2 concentration ratio. Sugar-regulated and water-regulated NSLs are distinguished by the presence/absence of a stomatal closure response to changing sink strength. Turgor-regulated and sugar-regulated phloem unloading are distinguished by the presence/absence of stomatal closure under drought and avoidance/occurrence of negative phloem turgor. Results from girdling and drought experiments on Pinus sylvestris, Betula pendula, Populus tremula and Picea abies saplings are consistent with optimal stomatal control under sugar-regulated NSLs and turgor-regulated unloading. Our analytical results provide a simple representation of stomatal responses to above-ground and below-ground environmental factors and sink activity.

Mitigating negative effects of long-term treated wastewater irrigation: Leaf gas exchange and water use efficiency response of avocado trees (Persea americana Mill.)

Declining performance of avocado orchards growing in a clayey soil irrigated with treated wastewater (TWW) for more than 5 years has been observed in Israel. Measures studied to mitigate this were freshwater (FW), blended TWW:FW in a 1:1 ratio (MIX), low-frequency TWW-irrigation (LFI), TWW irrigated tuff trenches (TUF) and TWW as the control treatment. This study reports on the response of avocado leaf gas exchange, intrinsic water use efficiency, leaf water potential and leaf hydraulic conductance (Kleaf) to the mitigation measures applied to a 'Hass' avocado orchard (Persea americana Mill.) previously irrigated with TWW for 7 years. Stomatal conductance (gs) of FW, MIX, TUF and LFI was greater than that for TWW on most measurement dates, which increased net leaf CO2 assimilation rate (Aleaf) and intercellular CO2 concentration (Ci). Increased gs reduced intrinsic water use efficiency [WUE, determined from either 13C discrimination (iWUE) or gas exchange (WUEi)] as compared to TWW. Although the WUE in TWW was higher than that in mitigation treatments on most measurement dates, the lower assimilation rates at leaf (Aleaf) and whole tree (Atree) levels observed in TWW indicate that TWW irrigation had negative effects on tree physiological performance. A strong negative relationship between WUEi (Aleaf/gs) and gs was found in all treatments, with highest WUEi and lowest gs in TWW. Leaf hydraulic conductance was also lowest for TWW, which may indicate that lower gas exchange was linked to lower water availability to the leaves. Overall, the increased gs, Aleaf, Atree and Kleaf in FW, MIX and TUF relative to TWW indicate that these mitigation measures are good candidates for improving orchard performance negatively affected by long-term irrigation with TWW in clayey soils. Comparing all treatments, FW was the most effective.