The mechanism(s) involved in the photoprotection of PSII at elevated CO2 in nodulated alfalfa plants

In a previous study, we found that enhanced CO₂ subjected to nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4 °C) and water availability regimes could protect PSII from photodamage. The main objective of this study was to determine the mechanism(s) involved in the photoprotection of PSII at elevated CO₂ levels in this plant. Elevated CO₂ reduced carboxylation capacity-induced photosynthetic acclimation and reduced enzymatic and/or nonenzymatic antioxidant activities, suggesting that changes in electron flow did not cause any photooxidative damage (which was also confirmed by H₂O₂ and lipid peroxidation analyses). Enhanced nonphotochemical quenching and xanthophyll cycle pigments revealed that plants grown at 700 μmol mol⁻¹ CO₂ compensated for the reduction in energy sink with a larger capacity for nonphotochemical dissipation of excitation energy as heat, i.e., modulating the status of the VAZ components. Elevated CO₂ induced the de-epoxidation of violaxanthin to zeaxanthin, facilitating thermal dissipation and protecting the photosynthetic apparatus against the deleterious effect of excess excitation energy.     

Response of nodulated alfalfa to water supply,temperature and elevated CO2: productivity and water relations

Exposing plants to long-term CO₂ enrichment generally leads to increases in plant biomass, total leaf area and alterations on leaf net photosynthetic rates, stomatal conductance and water use efficiency. However, the magnitude of such effects is dependent on the availability of other potentially limiting resources. The aim of our study was to elucidate the effects of elevated CO₂, applied at different temperature and water availability regimes, on nodulated alfalfa plants. Regardless of water supply, elevated CO₂ enhanced plant growth, especially when combined with increased temperature although no differences were detected until 30 days of treatment. Absence of differences in leaf relative growth rate, and gas exchange measurements, suggested that plants grown in a low water regime adjusted their growth to the amount of available water. Elevated CO₂ enhanced water use efficiency because of reduced water consumption and a greater dry mass production. Increased dry matter production of plants grown under elevated CO₂ and temperature was the result of stimulated photosynthetic rates, greater leaf area and water use efficiency. Lack of CO₂ effect on photosynthesis of plants grown at ambient temperature might be consequence of down-regulation phenomena. Plants grown at 700 μmol mol⁻¹ CO₂ maintained control nitrogen levels, discarding enhanced nitrogen availability as the main factor explaining enhanced dry matter.     

Elevated CO2 and water-availability effect on gas exchange and nodule development in N2-fixing alfalfa plants

N₂-fixing alfalfa plants were grown in controlled conditions at different CO₂ levels (350 μmol mol^?1 versus 700 μmol mol^?1) and water-availability conditions (WW, watered at maximum pot water capacity versus WD, watered at 50% of control treatments) in order to determine the CO₂ effect (and applied at two water regimes) on plant growth and nodule activity in alfalfa plants. The CO₂ stimulatory effect (26% enhancement) on plant growth was limited to WW plants, whereas no CO₂ effect was observed in WD plants. Exposure to elevated CO₂ decreased Rubisco carboxylation capacity of plants, caused by a specific reduction in Rubisco (EC 4.1.1.39) concentration (11% in WW and 43% in WD) probably explained by an increase in the leaf carbohydrate levels. Plants grown at 700 μmol mol^?1 CO₂ maintained control photosynthetic rates (at growth conditions) by diminishing Rubisco content and by increasing nitrogen use efficiency. Interestingly, our data also suggest that reduction in shoot N demand (reflected by the TSP and especially Rubisco depletion) affected negatively nodule activity (malate dehydrogenase, EC 1.1.1.37, and glutamate-oxaloacetate transaminase, EC 2.6.1.1, activities) particularly in water-limited conditions. Furthermore, nodule DM and TSS data revealed that those nodules were not capable to overcome C sink strength limitations.     

Acclimation of peanut (Arachis hypogaea L.) leaf photosynthesis to elevated growth CO2 and temperature

Peanut (Arachis hypogaea L. cv. Florunner) was grown from seed sowing to plant maturity under two daytime CO₂ concentrations ([CO₂]) of 360 μmol mol⁻¹ (ambient) and 720 μmol mol⁻¹ (elevated) and at two temperatures of 1.5 and 6.0 °C above ambient temperature. The objectives were to characterize peanut leaf photosynthesis responses to long-term elevated growth [CO₂] and temperature, and to assess whether elevated [CO₂] regulated peanut leaf photosynthetic capacity, in terms of activity and protein content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), Rubisco photosynthetic efficiency, and carbohydrate metabolism. At both growth temperatures, leaves of plants grown under elevated [CO₂] had higher midday photosynthetic CO₂ exchange rate (CER), lower transpiration and stomatal conductance and higher water-use efficiency, compared to those of plants grown at ambient [CO₂]. Both activity and protein content of Rubisco, expressed on a leaf area basis, were reduced at elevated growth [CO₂]. Declines in Rubisco under elevated growth [CO₂] were 27–30% for initial activity, 5–12% for total activity, and 9–20% for protein content. Although Rubisco protein content and activity were down-regulated by elevated [CO₂], Rubisco photosynthetic efficiency, the ratio of midday light-saturated CER to Rubisco initial or total activity, of the elevated-[CO₂] plants was 1.3- to 1.9-fold greater than that of the ambient-[CO₂] plants at both growth temperatures. Leaf soluble sugars and starch of plants grown at elevated [CO₂] were 1.3- and 2-fold higher, respectively, than those of plants grown at ambient [CO₂]. Under elevated [CO₂], leaf soluble sugars and starch, however, were not affected by high growth temperature. In contrast, high temperature reduced leaf soluble sugars and starch of the ambient-[CO₂] plants. Activity of sucrose-P synthase, but not adenosine 5′-diphosphoglucose pyrophosphorylase, was up-regulated under elevated growth [CO₂]. Thus, in the absence of other environmental stresses, peanut leaf photosynthesis would perform well under rising atmospheric [CO₂] and temperature as predicted for this century.     

Ozone stress in Melissa officinalis plants assessed by photosynthetic function

Photosynthetic functions have been investigated in ozone stressed (200 ppb, 5 h) Melissa officinalis plants at the end of fumigation and 24 and 48 h after. Plants exhibited foliar injury and membrane permeability was significantly increased, indicating that there was membrane damage. After the end of treatment, CO₂ fixation capacity decreased and this lasted during the recovery period (until a maximum of −63% when compared to controls). These strong negative effects on photosynthetic ability were observed to be due both to stomatal and mesophyllic limitations, since stomatal conductance decreased (−23%) and intercellular CO₂ concentration significantly increased (+41%). Reduction in PSII efficiency is evidenced by (i) decrease of F_v/F₀ (−11.4%), indicating a partial inhibition at PSII donor side; (ii) significant correlation between the apparent electron transport rate through PSII and photosynthetic activity, suggesting that the O₃-induced effects are well established, as demonstrated by the development of leaf necrosis; (iii) increase in electrons required to fix one molecule of CO₂, showing a decrease in activity of photosynthetic enzymes and their ability to fix CO₂ in the presence of O₃; (iv) decrease of q_L, resulting in an increase in the PSII excitation pressure. On the other hand, a regulatory adjustment of PSII efficiency was highlighted by (i) higher value of q_NP, abling to counteract the negative effects of O₃ at chloroplast level because of their capacity to dissipate the excess of excitation energy; (ii) increase of the xanthophyll cycle pool size and DEPS index, showing a marked activation of photoprotective mechanisms. This represents an active response that M. officinalis initiates to cope with increased oxidative load.     

Pea plant responsiveness under elevated [CO2] is conditioned by the N source (N2 fixation versus NO3− fertilization)

The main goal of this study was to test the effect of [CO₂] on C and N management in different plant organs (shoots, roots and nodules) and its implication in the responsiveness of exclusively N₂-fixing and NO₃⁻-fed plants. For this purpose, exclusively N₂-fixing and NO₃⁻-fed (10 mM) pea (Pisum sativum L.) plants were exposed to elevated [CO₂] (1000 μmol mol⁻¹ versus 360 μmol mol⁻¹ CO₂). Gas exchange analyses, together with carbohydrate, nitrogen, total soluble proteins and amino acids were determined in leaves, roots and nodules. The data obtained revealed that although exposure to elevated [CO₂] increased total dry mass (DM) in both N treatments, photosynthetic activity was down-regulated in NO₃⁻-fed plants, whereas N₂-fixing plants were capable of maintaining enhanced photosynthetic rates under elevated [CO₂]. In the case of N₂-fixing plants, the enhanced C sink strength of nodules enabled the avoidance of harmful leaf carbohydrate build up. On the other hand, in NO₃⁻-fed plants, elevated [CO₂] caused a large increase in sucrose and starch. The increase in root DM did not contribute to stimulation of C sinks in these plants. Although N₂ fixation matched plant N requirements with the consequent increase in photosynthetic rates, in NO₃⁻-fed plants, exposure to elevated [CO₂] negatively affected N assimilation with the consequent photosynthetic down-regulation.     

Effects of gradual versus step increases in carbon dioxide on Plantago photosynthesis and growth in a microcosm study

This study investigated the effects of a gradual versus step increases in carbon dioxide (CO₂) on plant photosynthesis and growth at two nitrogen (N) levels. Plantago lanceolata were grown for 80 days and then treated with the ambient CO₂ (as the control), gradual CO₂ increase and step CO₂ increase as well as low and high N additions for 70 days. While [CO₂] were kept at constant 350 and 700 μmol mol⁻¹ for the ambient and step CO₂ treatments, respectively, [CO₂] in the gradual CO₂ treatment was raised by 5 μmol mol⁻¹ day⁻¹, beginning at 350 μmol mol⁻¹ and reaching 700 μmol mol⁻¹ by the end of experiment. The step CO₂ treatment immediately resulted in an approximate 50% increase in leaf photosynthetic carbon fixation at both the low and high N additions, leading to a 20–24% decrease in leaf N concentration. The CO₂-induced nitrogen stress, in return, resulted in partial photosynthetic downregulation since the third week at the low N level and the fourth week at the high N level after treatments. In comparison, the gradual CO₂ treatment induced a gradual increase in photosynthetic carbon fixation, leading to less reduction in leaf N concentration. In comparison to the ambient CO₂, both the gradual and step CO₂ increases resulted in decreases in specific leaf area, leaf N concentration but an increase in plant biomass. Responses of plant shoot:root ratio to CO₂ treatments varied with N supply. It decreased with low N supply and increased with high N supply under the gradual and step CO₂ treatments relative to that under the ambient CO₂. Degrees of those changes in physiological and growth parameters were usually larger under the step than the gradual CO₂ treatments, largely due to different photosynthetic C influxes under the two CO₂ treatments.     

Gas exchange acclimation to elevated CO2 in upper-sunlit and lower-shaded canopy leaves in relation to nitrogen acquisition and partitioning in wheat grown in field chambers

Growth at elevated CO₂ often decreases photosynthetic capacity (acclimation) and leaf N concentrations. Lower-shaded canopy leaves may undergo both CO₂ and shade acclimation. The relationship of acclimatory responses of flag and lower-shaded canopy leaves of wheat (Triticum aestivum L.) to the N content, and possible factors affecting N gain and distribution within the plant were investigated in a wheat crop growing in field chambers set at ambient (360 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂, and with two amounts of N fertilizer (none and 70 kg ha⁻¹ applied on 30 April). Photosynthesis, stomatal conductance and transpiration at a common measurement CO₂, chlorophyll and Rubisco levels of upper-sunlit (flag) and lower-shaded canopy leaves were significantly lower in elevated relative to ambient CO₂-grown plants. Both whole shoot N and leaf N per unit area decreased at elevated CO₂, and leaf N declined with canopy position. Acclimatory responses to elevated CO₂ were enhanced in N-deficient plants. With N supply, the acclimatory responses were less pronounced in lower canopy leaves relative to the flag leaf. Additional N did not increase the fraction of shoot N allocated to the flag and penultimate leaves. The decrease in photosynthetic capacity in both upper-sunlit and lower-shaded leaves in elevated CO₂ was associated with a decrease in N contents in above-ground organs and with lower N partitioning to leaves. A single relationship of N per unit leaf area to the transpiration rate accounted for a significant fraction of the variation among sun-lit and shaded leaves, growth CO₂ level and N supply. We conclude that reduced stomatal conductance and transpiration can decrease plant N, leading to acclimation to CO₂ enrichment.     

Leaf photosynthesis of Haberlea rhodopensis before and during drought

Haberlea rhodopensis is a homoiochlorophyllous resurrection plant that shows a low rate of leaf net CO₂ uptake (4–6 μmol m^?2 s^?1) under saturating photosynthetic photon flux densities in air (21% O₂ and about 390 ppm CO₂). However, leaf net CO₂ uptake reaches values of 17–18 μmol m^?2 s^?1 under saturating CO₂ and light. H. rhodopensis leaves have a very low mesophyll CO₂ conductance that can partly explain the low rate of leaf net CO₂ uptake in normal air. Experimental evidences suggest that mesophyll conductance is not sensitive to temperature in the 20–35 °C range. In addition, it is shown that the (1) transpiration rate of H. rhodopensis is nearly linearly related to the vapour pressure difference between the leaf and the ambient air within the interval from 0.5 kPa to 2.5 kPa at a leaf temperature of 25 °C and (2) leaf net CO₂ uptake in normal air under saturating light does not change much with leaf temperature (between 20 °C and 30 °C). At a leaf relative water content of between 90% and 30%, the decrease of leaf net CO₂ assimilation during drought can be explained by a decrease of leaf CO₂ diffusional conductance. Accordingly the non-photochemical chlorophyll fluorescence quenching decreases only at relative water contents lower than 20%, indicating that photosynthetic activity maintains a trans-thylakoidal proton gradient over a wide range of leaf water contents. Moreover, PSII photochemistry (as estimated by the Fv/Fm ratio and the thermoluminescence B band intensity) is only affected at leaf relative water contents lower than about 20%, thus confirming that primary photosynthetic reactions are resistant to drought. Interestingly, the effect of leaf desiccation on photosynthetic capacity, measured at very high ambient CO₂ molar ratios under saturating PPFD, is identical to that observed for three non-resurrection C₃ mesophytes. This demonstrates that the photosynthetic apparatus of H. rhodopensis is not more resistant to desiccation when compared to other C₃ plants. Since the leaf area decreases by more than 50% when the leaf relative water content is reduced to about 40% during drought it is supposed, following Farrant et al. [Farrant, J.M., Vander, W.C., Lofell, D.A., Bartsch, S., Whittaker, A., 2003. An investigation into the role of light during desiccation of three angiosperms resurrection plants. Plant Cell Environ. 26, 1275–1286], that H. rhodopensis leaf cells avoid mechanical stress.     

Impact of in vitro CO2 enrichment and sugar deprivation on acclimatory responses of Phalaenopsis plantlets to ex vitro conditions

We assessed the effect of growth at either 400 μmol mol^?1 (ambient) or 1000 μmol mol^?1 (elevated) CO₂ and 0 g L^?1 (deprivation) or 30 g L^?1 (supplementation) sugar on morphological traits, photosynthetic attributes and intrinsic elements of the CAM pathway using the CAM orchid Phalaenopsis ‘Amaglade’. The growth of shoot (retarded) and root (induced) was differently affected by CO₂ enrichment and mixotrophic regime (+sugar). The Fv/Fm ratio was 14% more in CO₂-enriched treatment than at ambient level during in vitro growth. At elevated level of CO₂ and sugar treatment, the content of Chl(a + b), Chl a/b and Chl/Car was enhanced while carotenoid content remained unaltered. During in vitro growth, gas-exchange analysis indicated that increased uptake of CO₂ accorded with the increased rate of transpiration and unchanged stomatal conductance at elevated level of CO₂ under both photo- and mixotrophic growth condition. At elevated level of CO₂ and sugar deprivation, activities of Rubisco (26.4%) and PEPC (74.5%) was up-regulated. Among metabolites, the content of sucrose and starch was always higher under CO₂ enrichment during both in vitro and ex vitro growth. Our results indicate that plantlets grown under CO₂ enrichment developed completely viable photosynthetic apparatus ready to be efficiently transferred to ex vitro condition that has far-reaching implications in micropropagation of Phalaenopsis.     

Interspecific variability of plant stomatal response to step changes of [CO2]

The kinetics of a stomatal response to sudden increases or decreases of CO₂ concentrations ([CO₂]) was studied in 13 plant species growing in the field. Plants were well supplied with water. In each plant, gas exchange measurements were made on a fully developed leaf that was first left to achieve steady-state stomatal conductance (g_s) at 400 μmol (CO₂) mol⁻¹) and then exposed to a step change of [CO₂] (to 700 μmol mol⁻¹ in one experiment; and to 700 and back to 400 μmol mol⁻¹ in a second experiment). Porometric data were captured in intervals of 3 s until a new steady state was reached.A comparison of t_1/2, the half-time needed to achieve new g_s, indicates similar responses of stomata in grasses when compared to herbs. The stomata of C₄ plants responded in approximately 5 min, the highest closure rate was detected in Echinochloa crus-galli and Digitaria sanguinalis. Opening rates were similar to closing rates and the response as a whole was rather symmetric. In C₃ plants, the full response of stomata was much slower. Analysis revealed differences in absolute rates of g_s change between C₃ and C₄ plants. These differences can be related to the specificities of the type of photosynthetic metabolism. C₄ photosynthesis enables plants to reduce g_s, which can hasten further changes of diffusivity in response to the environmental signals. A possible coupling of C₄ metabolism to the regulation of guard cells also has to be taken into account when explaining the observed results.     

PSII photochemistry and carboxylation efficiency in Liriodendron tulipifera under ozone exposure

Liriodendron tulipifera is an important forest plant which is commonly used in urban environments as a shade tree. Young plants have been exposed (under controlled conditions) to 120 ppb of O₃ for 45 consecutive days (5 h d⁻¹). The aim of this investigation was to clarify if O₃ limits the physiological performance of L. tulipifera. In treated plants, dynamics related to membrane injury, gas exchange and chlorophyll a fluorescence leads to: (i) increase in lipid peroxidation (maximum value of +78% 15 days after the fumigation, compared to controls); (ii) reduction of photosynthetic activity (up to 66% 28 days after the exposure), twinned with a partial stomatal closure and a store of CO₂ in substomatal chambers; (iii) reduction in carboxylation efficiency (−11% at the end of exposure); (iv) damage to PSII, as demonstrated by the increase in the PSII excitation pressure (−57% 28 days after the treatment). On this basis, O₃ should be considered very harmful to L. tulipifera, although the reduction of total chlorophylls content and the activation of xanthophyll cycle take place in order to attempt to regulate light absorbed energy limiting oxidative damage.     

Antagonism between elevated CO2, nighttime warming,and summer drought reduces the robustness of PSII performance to freezing events

Plant responses to warming, elevated CO₂, and changes in summer precipitation patterns involve complex interactions. In this study we aim to reveal the single factor responses and their interactive effects on photosystem II (PSII) performance during an autumn-to-winter period. The study was carried out in the CLIMAITE multifactor experiment, which includes the combined impact of elevated CO₂ (free air carbon enrichment; CO₂), warming (passive nighttime warming; T) and summer drought (rain-excluding curtains; D) in a temperate heath ecosystem. PSII performance was probed by the effective quantum yield in light, F_v′/F_m′, using the pulse amplitude methodology, and the total performance index, PI_total, which integrate changes of the chlorophyll-a fluorescence transient including the maximal quantum yield in darkness, F_v/F_m.Decreasing temperature during autumn linearly reduced PI_total, both in the wavy hair-grass, Deschampsia flexuosa, and in the evergreen dwarf shrub common heather, Calluna vulgaris, and following freezing events the PI_total and F_v′/F_m′ were reduced even more. Contrary to expected, indirect effects of the previous summer drought reduced PSII performance before freezing events, particularly in Calluna. In combinations with elevated CO₂ interactive effects with drought, D × CO₂ and warming, T × D × CO₂, were negatively skewed and caused the reduction of PSII performance in both species after occurrence of freezing events. Neither passive nighttime warming nor elevated CO₂ as single factors reduced PSII performance via incomplete cold hardening as hypothesized. Instead, the passive nighttime warming strongly increased PSII performance, especially after freezing events, and when combined with elevated CO₂ a strongly skewed positive T × CO₂ interactive effect was seen. This indicates that these plants take advantage of the longer growing season induced by the warming in elevated CO₂ until a winter frost period becomes permanent. However, if previously exposed to summer drought this positive effect reverses via interactive D × CO₂ and T × D × CO₂ effects immediately after freezing events, causing the full combination of TDCO₂ not to differ from the control.In a future warmer climate with high CO₂ and summer drought, the occurrence of freezing events thus seem highly decisive for reducing PSII performance in the autumn-to-winter period. Such a reduced robustness of PSII performance may be highly decisive for the magnitude of the late season photosynthetic carbon uptake and reduce the growing season length in these temperate heath plants.     

Increased feeding damage under elevated Co2: The case of the Russian wheat aphid

We report the results of a baseline study on the effects of Russian wheat aphid infestation on barley lines grown under ambient and elevated (450 and 550 μmol mol^− 1) CO₂ concentrations [CO₂]. Elevated CO₂ impacted on plant biomass, C:N ratios and leaf nitrogen concentrations. Visible manifestation of aphid feeding related damage was assessed by examining resultant chlorosis and leaf roll under ambient and two elevated [CO₂] levels using a control and three resistant barley host combinations. Elevated [CO₂] had a significant positive effect on the growth of the four barley lines that were not infested by the aphids. However under the same conditions aphid feeding under elevated CO₂ conditions caused very high biomass loss, which was more noticeable in experiments involving non-resistant PUMA than in the resistant barley lines. The results of this study demonstrate that CO₂ enrichment substantially increases aphid populations of RWASA1 and RWASA2 on the four barley lines investigated. Furthermore, aphid populations were higher on non-resistant PUMA than the three resistant lines and the RWASA2 biotype out-performed RWASA1 in each case. Under elevated [CO₂], aphid feeding, resulted in a significant increase in the leaf C:N ratios (as a percentage change) in most treatments, compared to levels recorded on uninfested plants. The resistant lines also showed a significant reduction in leaf nitrogen (~ 40% for PUMA and not less than 30% for the resistant STARS lines tested). C:N ratio changes and N loss correlated to [CO₂] and aphid biotype. By 28 days of infestation, most of the non-resistant PUMA line in particular showed significant irrecoverable levels of leaf chlorosis. At level 9 rating on the chlorosis scale (i.e. plant death when recovery was not possible), experiments were terminated. As aphid success is unlikely to be the sole product of [CO₂], but also of other limiting nutrients such as N, it may be worth further investigating the effect of plant quality and ultimately plant nutrition on the population growth of aphids.     

Quenching partitioning through light-modulated chlorophyll fluorescence: A quantitative analysis to assess the fate of the absorbed light in the field

Plants use a small part of the total absorbed light energy for net carboxylation, while the remaining amount is dissipated via alternative pathways involving thermal processes, fluorescence and non-carboxylation photochemistry in order to limit the formation of reactive oxygen species (ROS) and other photooxidative risks. The commonly used analysis of the Photosystem II (PSII) fluorescence signals gives qualitative information about absorbed light energy management by plants, but it is difficult to appreciate the relative contribution of each pathway in energy partitioning.This study reports the application of quenching partitioning through a chlorophyll fluorescence approach performed on peach leaves subjected to three different light intensities for four durations of exposure in absence of recovery from photo-damage. This methodology was compared with the P₇₀₀ redox kinetic method for determining the functional PSII fraction in leaves. In the absence of recovery processes the active PSII concentration decayed with an increase in photon exposure (the product of irradiance and the time of exposure), following an exponential pattern according to the reciprocity law. The photoprotective thermal dissipation (Φ_NPQ) was proportional to irradiance up to 30 min of photoinhibitory treatment. Afterwards Φ_NPQ was limited by the increasing competition for the absorbed energy re-emitted by the inactive PSII (Φ_NF). Φ_NF increased with the photon exposure dissipating up to 70% of the total incoming energy. The energy funnelled to photochemistry (Φ_PSII) decreased with increasing exposure time or intensity, becoming zero after 120 min of photoinhibitory treatment at the maximum irradiance (2100 μmol photon m⁻² s⁻¹). The relation between the fraction of energy dissipated by the inactive PSII (derived from the quenching partitioning) and the inactive PSII fraction (measured with the P₇₀₀ redox kinetic method) was linear.The quenching partitioning through light-modulated chlorophyll fluorescence is a useful tool to analyse plant energy management and gives also a reasonable estimation of the active PSII fraction. This methodology can easily be used in the field as measurements are rapid, non-destructive and detection devices are portable.     

The metabolic cost of bicarbonate use in the submerged plant Elodea nuttallii

The aquatic plant Elodea nuttallii (Planch.) St. John has been shown to express plasticity in the source of inorganic carbon it uses for photosynthesis. An investigation was undertaken to determine what effect the switch from CO₂ to HCO₃⁻ use had on the growth of E. nuttallii. Plants were grown under reduced CO₂ availability that favoured the switch, together with control plants (CO₂ at equilibrium with air) that continued to use CO₂ only. The extent to which both sets of plants could utilise HCO₃⁻ was determined (as the ratio of oxygen evolution at pH 9 and 6.5), and several measures of growth were made. Although reduced CO₂ availability produced an increase in HCO₃⁻ utilisation, no differences were found in the measured growth of the plants. Therefore, it was possible to estimate, from the difference between the estimated rate of photosynthesis of the plants utilising HCO₃⁻ and those using CO₂ only, the approximate cost of constructing, maintaining and running the bicarbonate utilisation mechanism in this species as 69 μmol photons m⁻² s⁻¹. This value can be used to estimate an irradiance of circa 80 μmol m⁻² s⁻¹ below which HCO₃⁻ use would not be expected in this species, an irradiance commonly experienced by submerged macrophytes in the field.     

Osmoregulation in Dunaliella,Part II: Photosynthesis and starch contribute carbon for glycerol synthesis during a salt stress in Dunaliella tertiolecta

In response to an osmotic stress, Dunaliella tertiolecta osmoregulates by metabolizing intracellular glycerol as compatible solute. Upon the application of a salt stress to 0.17 M or 0.7 M NaCl grown D. tertiolecta cells, rates of total glycerol synthesis were substantially higher than that arising from photosynthetic ¹⁴CO₂ fixation into glycerol. The source of this extra carbon is the reserve starch pool. The contribution of carbon from the starch breakdown to glycerol synthesis was estimated from the difference between the total glycerol synthesized and that arising from ¹⁴CO₂ fixation. The maximum observed flux of carbon from ¹⁴CO₂ to glycerol from photosynthesis was of the order of 15–20 μmol ¹⁴C-glycerol mg⁻¹ Chl h⁻¹, whereas the total glycerol synthesis reached about 70 μmol glycerol mg⁻¹ Chl h⁻¹. The contribution of products of starch breakdown to glycerol synthesis increased progressively with increasing salt stress. In light, contrary to prevailing assumptions, both the photosynthesis and the starch breakdown contribute carbon to glycerol biosynthesis. The relative contributions of these two processes in the light, while cells were actively photosynthesizing, depended on the magnitude of the salt stress. On application of dilution stress, the flux of carbon from newly photosynthetically fixed ¹⁴CO₂ into glycerol was reduced progressively with increasing dilution stress that was also accompanied by a decline in total glycerol contents of the cell. The maximum observed rate of glycerol dissimilation was about 135 μmol glycerol mg⁻¹ Chl h⁻¹.     

Effects of flooding on photosynthesis and root respiration in saltcedar (Tamarix ramosissima), an invasive riparian shrub

The introduced shrub Tamarix ramosissima invades riparian zones, but loses competitiveness under flooding. Metabolic effects of flooding could be important for T. ramosissima, but have not been previously investigated. Photosynthesis rates, stomatal conductance, internal (intercellular) CO₂, transpiration, and root alcohol dehydrogenase (ADH) activity were compared in T. ramosissima across soil types and under drained and flooded conditions in a greenhouse. Photosynthesis at 1500 μmol quanta m⁻² s⁻¹ (A₁₅₀₀) in flooded plants ranged from 2.3 to 6.2 μmol CO₂ m⁻² s⁻¹ during the first week, but A₁₅₀₀ increased to 6.4–12.7 μmol CO₂ m⁻² s⁻¹ by the third week of flooding. Stomatal conductance (g_s) at 1500 μmol quanta m⁻² s⁻¹ also decreased initially during flooding, where g_s was 0.018 to 0.099 mol H₂O m⁻² s⁻¹ during the first week, but g_s increased to 0.113–0.248 mol H₂O m⁻² s⁻¹ by the third week of flooding. However, photosynthesis in flooded plants was reduced by non-stomatal limitations, and subsequent increases indicate metabolic acclimation to flooding. Root ADH activities were higher in flooded plants compared to drained plants, indicating oxygen stress. Lower photosynthesis and greater oxygen stress could account for the susceptibility of T. ramosissima at the onset of flooding. Soil type had no effect on photosynthesis or on root ADH activity. In the field, stomatal conductance, leaf water potential, transpiration, and leaf δ¹³C were compared between T. ramosissima and other flooded species. T. ramosissima had lower stomatal conductance and water potential compared to Populus deltoides and Phragmites australis. Differences in physiological responses for T. ramosissima could become important for ecological concerns.     

Effects of shade treatments on the photosynthetic capacity,chlorophyll fluorescence,and chlorophyll content of Tetrastigma hemsleyanum Diels et Gilg

Tetrastigma hemsleyanum Diels et Gilg was grown under full sunlight and moderate and high levels of shade for one month to evaluate its photosynthetic and chlorophyll fluorescence response to different light conditions. The results showed that T. hemsleyanum attained greatest leaf size and P_n when cultivated with 67% shade. Leaves of seedlings grown with 90% shade were the smallest. Leaf color of plants grown under full sunlight and 50% shade was yellowish-green. The P_n value increased rapidly as PPFD increased to 200 μmol m^?2 s^?1 and then increased slowly to a maximum, followed by a slow decrease as PPFD was increased to 1000 μmol m^?2 s^?1. P_n was highest for the 67% shade treatment and the LSP for this shade treatment was 600 μmol m^?2 s^?1. Full sunlight and 50% shade treatments resulted in significant reduction of ETR and qP and increased NPQ. Chl a, Chl b and total chlorophyll content increased and Chl a/b values decreased with increased shading. Results showed that light intensity greater than that of 50% shade depressed photosynthetic activity and T. hemsleyanum growth. Irradiance less than that of 75% shade limited carbon assimilation and led to decreased plant growth. Approximately 67% shade is suggested to be the optimum light irradiance condition for T. hemsleyanum cultivation.     

Temperature dependence of growth,development, and photosynthesis in maize under elevated CO2

Global atmospheric carbon dioxide concentrations (C_a) are rising. As a consequence, recent climate models have projected that global surface air temperature may increase 1.4–5.8 °C with the doubling of C_a by the end of the century. Because, changes in C_a and temperature are likely to occur concomitantly, it is important to evaluate how the temperature dependence of key physiological processes are affected by rising C_a in major crop plants including maize (Zea mays L.), a globally important grain crop with C₄ photosynthetic pathway. We investigated the temperature responses of photosynthesis, growth, and development of maize plants grown at five temperature regimes ranging from 19/13 to 38.5/32.5 °C under current (370 μmol mol⁻¹) and doubled (750 μmol mol⁻¹) C_a throughout the vegetative stages using sunlit controlled environmental chambers in order to test if the temperature dependence of these processes was altered by elevated C_a. Leaf and canopy photosynthetic rates, C₄ enzyme activities, leaf appearance rates, above ground biomass accumulation and leaf area were measured. We then applied temperature response functions (e.g., Arrhenius and Beta distribution models) to fit the measured data in order to provide parameter estimates of the temperature dependence for modeling photosynthesis and development at current and elevated C_a in maize. Biomass, leaf area, leaf appearance rate, and photosynthesis measured at growth C_a was not changed in response to CO₂ enrichment. Carboxylation efficiency and the activities of C₄ enzymes were reduced with CO₂ enrichment indicating possible photosynthetic acclimation of the C₄ cycle. All measured parameters responded to growth temperatures. Leaf appearance rate and leaf photosynthesis showed curvilinear response with optimal temperatures near 32 and 34 °C, respectively. Total above ground biomass and leaf area were negatively correlated with growth temperature. The dependence of leaf appearance rate, biomass, leaf area, leaf and canopy photosynthesis, and C₄ enzyme activities on growth temperatures was comparable between current and elevated C_a. The results of this study suggest that the temperature effects on growth, development, and photosynthesis may remain unchanged in elevated C_a compared with current C_a in maize.