Stomatal conductance in mature deciduous forest trees exposed to elevated CO<Subscript>2</Subscript>

Stomatal conductance (g _s) of mature trees exposed to elevated CO₂ concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g _s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO₂ compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO₂ treatment. During the 2003 drought in the third treatment year, the CO₂ effect became reversed in Carpinus, resulting in higher g _s in trees exposed to elevated CO₂ compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO₂ enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO₂-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO₂. An erratum to this article can be found at     

Photosynthetic acclimation and leaf traits of <Emphasis Type="Italic">Stipa bungeana</Emphasis> in response to elevated CO<Subscript>2</Subscript> under five different watering conditions

Although plant performance under elevated CO₂ (EC) and drought has been extensively studied, little is known about the leaf traits and photosynthetic performance of Stipa bungeana under EC and a water deficiency gradient. In order to investigate the effects of EC, watering, and their combination, S. bungeana seedlings were exposed to two CO₂ regimes (ambient, CA: 390 ppm; elevated, EC: 550 ppm) and five levels of watering (?30%, ?15%, control, +15%, +30%) from 1 June to 31 August in 2011, where the control water level was 240 mm. Gas exchange and leaf traits were measured after 90-d treatments. Gas-exchange characteristics, measured at the growth CA, indicated that EC significantly decreased the net photosynthetic rate (P _N), water-use efficiency, nitrogen concentration based on mass, chlorophyll and malondialdehyde (MDA) content, while increased stomatal conductance (g _s), intercellular CO₂ concentration (C _i), dark respiration, photorespiration, carbon concentration based on mass, C/N ratio, and leaf water potential. Compared to the effect of EC, watering showed an opposite trend only in case of P _N. The combination of both factors showed little influence on these physiological indicators, except for g _s, C _i, and MDA content. Photosynthetic acclimation to EC was attributed to the N limitation, C sink/source imbalance, and the decline of photosynthetic activity. The watering regulated photosynthesis through both stomatal and nonstomatal mechanisms. Our study also revealed that the effects of EC on photosynthesis were larger than those on respiration and did not compensate for the adverse effects of drought, suggesting that a future warm and dry climate might be unfavorable to S. bungeana. However, the depression of the growth of S. bungeana caused by EC was time-dependent at a smaller temporal scale.     

Stomatal regulation in Douglas fir following a fungal-mediated chronic reduction in leaf area

Pathogens can cause chronic premature needle abscission in coniferous species. To assess the potential impacts on tree productivity, stomatal regulation was investigated in Douglas fir with chronic stomatal occlusion and defoliation from varying levels of the Swiss needle cast (SNC) fungus, Phaeocryptopus gaeumannii. Levels of SNC disease and subsequent defoliation were manipulated by choosing six sites with varying levels of disease and by foliar applications of fungicides on six trees per site. Diurnal measurements of leaf water potential (Ψ_leaf), stomatal conductance (g _s) and vapor pressure deficit (D) were made on six fungicide treated and six control trees per site. In addition, leaf specific hydraulic conductance was calculated on a single branch (K _{L_B}) from three trees per treatment per site. Stomatal conductance at D=1 kPa (g _sref) was negatively correlated with fungal colonization (number of fruiting bodies present in needle stomata) and positively correlated with K _{L_B}. Despite reduced needle retention in diseased trees, K _L declined due to a reduction in sapwood area and permeability (i.e., increasing presence of latewood in functional sapwood). In general, stomatal sensitivity to D for all foliage was consistent with stomatal regulation based on a simple hydraulic model [g _s=K _L(Ψ_soil?Ψ_leaf)/ D], which assumes strict stomatal regulation of Ψ_leaf. However, when fungal presence reduced maximum g _s below the potential maximum supported by hydraulic architecture, stomatal sensitivity was lower than expected based on the theoretical relationship: dg _s/dlnD=0.6·g _sref. The results indicate that losses in productivity associated with physical blockage of stomata and defoliation are compounded by additional losses in K _L and a reduction in g _s in remaining functional stomata.     

Differential influence of elevated CO<Subscript>2</Subscript> on gas exchange and water use efficiency of four indigenous shrub species distributed in different sandy environments in central Inner Mongolia

In view of the increase in global warming and carbon dioxide (CO₂) concentrations, it is essential to investigate the influences of climate change on plant growth and water use in arid and semi-arid grassland species. Experiments were conducted to understand the ecophysiological response of four indigenous species to elevated CO₂ in the semi-arid sandy grassland of central Inner Mongolia. Seedlings of the four species were grown for 8 weeks at four different consistently elevated CO₂ concentrations in the environment-controlled growth chambers. All four elevated CO₂ concentrations (400, 800, 1200, 1600 ppm) were found to result in decreased stomatal conductance (26–86%), decreased transpiration rate (21–80%), increased shoot water potential (1–42%) and increased water use efficiency (WUE) (10–412%) for two Artemisia species and Caragana korshinskii. Under our experimental conditions, the two Artemisia species and C. korshinskii would benefit more than Hedysarum laeve from exposure to elevated CO₂ in terms of higher shoot water potential and WUE combined with lower stomatal conductance and transpiration rate. The results indicate that in a warmer, CO₂-enriched future atmospheric environment, WUE in semi-arid grasslands may be higher than previously expected. Our findings will provide information for screening appropriate species for restoration of the degraded sandy grasslands in semi-arid areas under future climate change scenarios.     

Simple generalisation of a mesophyll resistance model for various intracellular arrangements of chloroplasts and mitochondria in C<Subscript>3</Subscript> leaves

The classical definition of mesophyll conductance (g _m) represents an apparent parameter (g _m,app) as it places (photo)respired CO₂ at the same compartment where the carboxylation by Rubisco takes place. Recently, Tholen and co-workers developed a framework, in which g _m better describes a physical diffusional parameter (g _m,dif). They partitioned mesophyll resistance (r _m,dif = 1/g _m,dif) into two components, cell wall and plasmalemma resistance (r _wp) and chloroplast resistance (r _ch), and showed that g _m,app is sensitive to the ratio of photorespiratory (F) and respiratory (R _d) CO₂ release to net CO₂ uptake (A): g _m,app = g _m,dif/[1?+?ω(F?+?R _d)/A], where ω is the fraction of r _ch in r _m,dif. We herein extend the framework further by considering various scenarios for the intracellular arrangement of chloroplasts and mitochondria. We show that the formula of Tholen et al. implies either that mitochondria, where (photo)respired CO₂ is released, locate between the plasmalemma and the chloroplast continuum or that CO₂ in the cytosol is completely mixed. However, the model of Tholen et al. is still valid if ω is replaced by ω(1?σ), where σ is the fraction of (photo)respired CO₂ that experiences r _ch (in addition to r _wp and stomatal resistance) if this CO₂ is to escape from being refixed. Therefore, responses of g _m,app to (F?+?R _d)/A lie somewhere between no sensitivity in the classical method (σ =1) and high sensitivity in the model of Tholen et al. (σ =0).     

Gas exchanges of an endangered species <Emphasis Type="Italic">Syringa pinnatifolia</Emphasis> and a widespread congener <Emphasis Type="Italic">S. oblata</Emphasis>

Net photosynthetic rate (P_N), transpiration rate (E), water use efficiency (WUE), stomatal conductance (g_s), and stomatal limitation (L_s) were investigated in two Syringa species. The saturation irradiance (SI) was 400 µmol m^-2s^-1 for S. pinnatifolia and 1 700 µmol m^-2s^-1 for S. oblata. Compared with S. oblata, S. pinnatifolia had extremely low g _s . Unlike S. oblata, the maximal photosynthetic rate (P_max) in S. pinnatifoliaoccurred around 08:00 and then fell down, indicating this species was sensitive to higher temperature and high photosynthetic photon flux density. However, such phenomenon was interrupted by the leaf development rhythms before summer. A relatively lower P_N together with a lower leaf area and shoot growth showed the capacity for carbon assimilation was poorer in S. pinnatifolia.     

Evolution of protein transport to the chloroplast envelope membranes

Comparing with other angiosperms, most members within the family Orchidaceae have lower photosynthetic capacities. However, the underlying mechanisms remain unclear. Cypripedium and Paphiopedilum are closely related phylogenetically in Orchidaceae, but their photosynthetic performances are different. We explored the roles of internal anatomy and diffusional conductance in determining photosynthesis in three Cypripedium and three Paphiopedilum species, and quantitatively analyzed their diffusional and biochemical limitations to photosynthesis. Paphiopedilum species showed lower light-saturated photosynthetic rate (A _N), stomatal conductance (g _s), and mesophyll conductance (g _m) than Cypripedium species. A _N was positively correlated with g _s and g _m. And yet, in both species A _N was more strongly limited by g _m than by biochemical factors or g _s. The greater g _s of Cypripedium was mainly affected by larger stomatal apparatus area and smaller pore depth, while the less g _m of Paphiopedilum was determined by the reduced surface area of mesophyll cells and chloroplasts exposed to intercellular airspace per unit of leaf area, and much thicker cell wall thickness. These results suggest that leaf anatomical structure is the key factor affecting g _m, which is largely responsible for the difference in photosynthetic capacity between those two genera. Our findings provide new insight into the photosynthetic physiology and functional diversification of orchids.     

Low concentrations of glycine inhibit photorespiration and enhance the net rate of photosynthesis in <Emphasis Type="Italic">Caragana korshinskii</Emphasis>

The inhibition of photorespiration can be used to improve plant carbon fixation. In order to compare the effects of three photorespiration inhibitors [glycine, NaHSO₃, and isonicotinyl hydrazide (INH)], photosynthetic parameters of leaves sprayed respectively with these chemicals were examined and their inhibiting efficiency was evaluated in Caragana korshinskii. Our results showed that 5 mM glycine could reduce the photorespiratory rate (P_R) effectively, while the net photosynthetic rate (P_N), stomatal conductance (g_s), and intercellular CO₂ concentration (Ci) significantly increased. The ratio of electron flow for ribulose-1,5-bisphosphate (RuBP) carboxylation to RuBP oxygenation was elevated markedly. NaHSO₃ and INH could also suppress the PR in some cases, whereas PN was not improved. The glyoxylate content increased considerably after application of low concentrations of glycine. These results suggested that low concentrations of glycine could suppress photorespiration by feed-back inhibition of glyoxylate and enhance photosynthesis by regulating g_s, C_i, and the distribution of electron flow in C. korshinskii.     

Impact of elevated CO<Subscript>2</Subscript> in <Emphasis Type="Italic">Casuarina equisetifolia</Emphasis> rooted stem cuttings inoculated with <Emphasis Type="Italic">Frankia</Emphasis>

Impact of different levels of elevated CO ₂ on the activity of Frankia (Nitrogen-fixing actinomycete) in Casuarina equisetifolia rooted stem cuttings has been studied to understand the relationship between C. equisetifolia, Frankia and CO₂. The stem cuttings of C. equietifolia were collected and treated with 2000 ppm of Indole Butyric Acid (IBA) for rooting. Thus vegetative propagated rooted stem cuttings of C. equisetifolia were inoculated with Frankia and placed in the Open top chambers (OTC) with elevated CO₂ facilities. These planting stocks were maintained in the OTC for 12 months under different levels of elevated CO₂ (ambient control, 600 ppm, 900 ppm). After 12 months, the nodule numbers, bio mass, growth, and photosynthesis of C. equisetifolia rooted stem cuttings inoculated with Frankia were improved under 600 ppm of CO₂. The rooted stem cuttings of C. equisetifolia inoculated with Frankia showed a higher number of nodules under 900 ppm of CO₂ and cuttings without Frankia inoculation exhibited poor growth. Tissue Nitrogen (N) content was also higher under 900 ppm of CO₂ than ambient control and 600 ppm levels. The photosynthetic rate was higher (17.8 μ mol CO₂ m^?2 s^?1) in 900 ppm of CO₂ than in 600 ppm (13.2 μ mol CO₂ m^?2 s^?1) and ambient control (8.3 μ mol CO₂ m^?2 s^?1). This study showed that Frankia can improve growth, N fixation and photosynthesis of C. equietifolia rooted stem cuttings under extreme elevated CO₂ level conditions (900 ppm).     

Evidences and magnitude of nighttime transpiration derived from <Emphasis Type="Italic">Populus euphratica</Emphasis> in the extreme arid region of China

Extensive research has found that nighttime transpiration (E _n) is positively correlated to the vapour pressure deficit (VPD), that suggested E _n was highest during the night under high temperatures and low humidity along with high soil water availability, typically for the riparian forest in the extreme arid region of China. This study used the heat ratio method to measure sap velocity (V _s) for mature and saplings Populus euphratica Oliv., and then E _n was conservatively calculated as total nocturnal sap flow (F _s, the product of V _s and sapwood area A _s) between 01:00 to 06:00. A gas exchange system was used to measure the leaf transpiration rate (T _r) and stomatal conductance (g _s) of saplings. For mature trees, nighttime V _s was extensive and logarithmic correlated to VPD (similar to daytime). For saplings, g _s and T _r was extensive in different months, and also a strong logarithmic relationship was found between V _s and VPD for both daytime and nighttime periods. Both of stem sap flow and leaf gas exchange suggusted the occurrence of E _n, whether mature or sapling trees. E _n contribution to daily transpiration (E _d) was high just as expected for P. euphratica, which was confirmed by proportional E _n to E _d (E _n/E _d) means taken in 2012 (24.99%) and 2013 (34.08%). Compared to mature trees, E _n/E _d of saplings in 2013 was lower with means of 12.06%, that supported further by the shorter duration times and less T _r,n (16.64%) and g _s,n (26.45%) of leaf, suggesting that E _n magnitude is associated to individual the tree size, that effect to stored water of individual trees, although this hypothesis requires further research.     

Chemical and hydraulic signals regulate stomatal behavior and photosynthetic activity in maize during progressive drought

The objectives of this study were to investigate stomatal regulation in maize seedlings during progressive soil drying and to determine the impact of stomatal movement on photosynthetic activity. In well-watered and drought-stressed plants, leaf water potential (Ψ _leaf), relative water content (RWC), stomatal conductance (g _s), photosynthesis, chlorophyll fluorescence, leaf instantaneous water use efficiency (iWUE_leaf), and abscisic acid (ABA) and zeatin-riboside (ZR) accumulation were measured. Results showed that g _s decreased significantly with progressive drought and stomatal limitations were responsible for inhibiting photosynthesis in the initial stages of short-term drought. However, after 5 days of withholding water, non-stomatal limitations, such as damage to the PSII reaction center, became the main limiting factor. Stomatal behavior was correlated with changes in both hydraulic and chemical signals; however, changes in ABA and ZR occurred prior to any change in leaf water status. ABA in leaf and root tissue increased progressively during soil drying, and further analysis found that leaf ABA was negatively correlated with g _s (R ² = 0.907, p < 0.05). In contrast, leaf and root ZR decreased gradually. ZR in leaf tissue was positively correlated with g _s (R ² = 0.859, p < 0.05). These results indicate that ABA could induce stomatal closure, and ZR works antagonistically against ABA in stomatal behavior. In addition, the ABA/ZR ratio also had a strong correlation with g _s, suggesting that the combined chemical signal (the interaction between ABA and cytokinin) plays a role in coordinating stomatal behavior. In addition, Ψ _leaf and RWC decreased significantly after only 3 days of drought stress, also affecting stomatal behavior.     

Effects of elevated atmospheric CO<Subscript>2</Subscript> and increased tidal flooding on leaf gas-exchange parameters of two common mangrove species: <Emphasis Type="Italic">Avicennia marina</Emphasis> and <Emphasis Type="Italic">Rhizophora stylosa</Emphasis>

In this study, we examined interactive effects of elevated atmospheric CO₂, concentrations, and increased tidal flooding on two mangroves species, Avicennia marina and Rhizophora stylosa. Leaf gas-exchange parameters (photosynthesis, transpiration rates, water-use efficiency, stomatal conductance, and dark respiration rates) were measured monthly on more than 1000 two-year-old seedlings grown in greenhouses for 1 year. In addition, stomatal density and light curve responses were determined at the end of the experiment. Under elevated CO₂ concentrations (800 ppm), the net photosynthetic rates were enhanced by more than 37% for A. marina and 45% for R. stylosa. This effect was more pronounced during the warm season, suggesting that an increase in global temperatures would further enhance the photosynthetic response of the considered species. Transpiration rates decreased by more than 15 and 8% for A. marina and R. stylosa, respectively. Consequently, water-use efficiency increased by 76% and 98% for A. marina and R. stylosa, respectively, for both species, which will improve drought resistance. These responses to elevated CO₂ were minimized (by 5%) with longer flooding duration. Consequently, future increases of atmospheric CO₂ may have a strong and positive effect on juveniles of A. marina and R. stylosa during the next century, which may not be suppressed by the augmentation of tidal flooding duration induced by sea-level rise. It is possible that this effect will enhance seedling dynamic by increasing photosynthesis, and therefore will facilitate their settlements in new area, extending the role of mangrove ecosystems in carbon sequestration and climate change mitigation.     

Interacting effects of elevated atmospheric CO<Subscript>2</Subscript> and hydrology on the growth and carbon sequestration of <Emphasis Type="Italic">Sphagnum</Emphasis> moss

Peatlands are a critical carbon store comprising 30% of the Earth’s terrestrial soil carbon. Sphagnum mosses comprise up to 90% of peat in the northern hemisphere but impacts of climate change on Sphagnum mosses are poorly understood, limiting development of sustainable peatland management and restoration. This study investigates the effects of elevated atmospheric CO₂ (eCO₂) (800 ppm) and hydrology on the growth of Sphagnum fallax, Sphagnum capillifolium and Sphagnum papillosum and greenhouse gas fluxes from moss–peat mesocosms. Elevated CO₂ levels increased Sphagnum height and dry weight but the magnitude of the response differed among species. The most responsive species, S. fallax, yielded the most biomass compared to S. papillosum and S. capillifolium. Water levels and the CO₂ treatment were found to interact, with the highest water level (1 cm below the surface) seeing the largest increase in dry weight under eCO₂ compared to ambient (400 ppm) concentrations. Initially, CO₂ flux rates were similar between CO₂ treatments. After week 9 there was a consistent three-fold increase of the CO₂ sink strength under eCO₂. At the end of the experiment, S. papillosum and S. fallax were greater sinks of CO₂ than S. capillifolium and the ? 7 cm water level treatment showed the strongest CO₂ sink strength. The mesocosms were net sources of CH₄ but the source strength varied with species, specifically S. fallax produced more CH₄ than S. papillosum and S. capillifolium. Our findings demonstrate the importance of species selection on the outcomes of peatland restoration with regards to Sphagnum’s growth and GHG exchange.     

Fish behavior in elevated CO<Subscript>2</Subscript>: implications for a movement barrier in flowing water

Preventing the spread of invasive fishes is an important aspect of management programs, but is challenging due to the behavior of fish and the nature of aquatic environments. The use of dissolved carbon dioxide (CO₂) has recently gained traction as a non-physical barrier for invasive fishes due to its ability to elicit avoidance behaviors in fish. Research to date has focused on the development of CO₂ barriers using static water environments. Because CO₂ barriers have been proposed for flowing water (i.e., in rivers or shipping canals), understanding the dynamics between fish and elevated CO₂ in flowing water is essential. Our study aims to define threshold levels required to alter behavior of bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides) in flowing water, and to quantify behavioral metrics of fish exposed to < 200 [ambient], 25,000, 50,000, and 100,000 µatm pCO₂. We also sought to quantify the impacts of repeated CO₂ exposure on fish behavior. Bluegill showed increased activity at 25,000 µatm, while largemouth bass showed increased activity at 100,000 µatm. When repeatedly exposed to cycles of 50,000 µatm pCO₂, bluegill exhibited increased activity followed by a diminished response after the second exposure. Results from this study define threshold levels required to elicit behavioral responses, and show that the effects that multiple exposures of elevated pCO₂ can decline, possibly due to habituation. Results will help shape the development and deployment of a CO₂ barrier to control the movements of invasive fishes.     

Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of <Emphasis Type="Italic">Cucumis sativus</Emphasis> seedlings under different CO<Subscript>2</Subscript> concentrations

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO₂ concentrations ([CO₂]; 400 and 1,000 μmol mol^–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO₂ concentrations. Under ambient [CO₂], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO₂] (C_i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P_N) at low salinity, but P_N was potentially limited by C_i at high salinity. At high [CO₂], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C_i occurred where slope of P_N–C_i curve was almost flat.     

Adaptive temperature regulation in the little bird in winter: predictions from a stochastic dynamic programming model

Stomatal CO₂ responsiveness and photosynthetic capacity vary greatly among plant species, but the factors controlling these physiological leaf traits are often poorly understood. To explore if these traits are linked to taxonomic group identity and/or to other plant functional traits, we investigated the short-term stomatal CO₂ responses and the maximum rates of photosynthetic carboxylation (V _cmax) and electron transport (J _max) in an evolutionary broad range of tropical woody plant species. The study included 21 species representing four major seed plant taxa: gymnosperms, monocots, rosids and asterids. We found that stomatal closure responses to increased CO₂ were stronger in angiosperms than in gymnosperms, and in monocots compared to dicots. Stomatal CO₂ responsiveness was not significantly related to any of the other functional traits investigated, while a parameter describing the relationship between photosynthesis and stomatal conductance in combined leaf gas exchange models (g ₁) was related to leaf area-specific plant hydraulic conductance. For photosynthesis, we found that the interspecific variation in V _cmax and J _max was related to within leaf nitrogen (N) allocation rather than to area-based total leaf N content. Within-leaf N allocation and water use were strongly co-ordinated (r ² = 0.67), such that species with high fractional N investments into compounds maximizing photosynthetic capacity also had high stomatal conductance. We conclude that while stomatal CO₂ responsiveness of tropical woody species seems poorly related to other plant functional traits, photosynthetic capacity is linked to fractional within-leaf N allocation rather than total leaf N content and is closely co-ordinated with leaf water use.     

Ameliorative effects of <Emphasis Type="Italic">Trichoderma harzianum</Emphasis> on monocot crops under hydroponic saline environment

Ameliorative effects of Trichoderma harzianum (Th-6) on monocot crops under saline environment using hydroponic system were examined. Both rice and maize seeds were coated with T. harzianum (Th-6) and used for the saline and non-saline treatment. Germination and seedling growth performance were studied. T. harzianum (Th-6)-treated seeds showed constantly faster and more uniform germination as compared to untreated seeds. Moreover, seeds treated with Trichoderma improved plants’ growth and physiological performance under hydroponic saline environment compared to control. The treatments showed higher relative water content (RWC), dark-adapted quantum yield (F _v/F _m ratio), performance index (PI_ABS), photochemical quenching (q _P), stomatal conductance (g _s), pigments concentrations and antioxidant enzymes as compared to untreated saline environment. Application of endophyte inhibited the Na⁺ and Cl^? ion uptake in leaves when plants were exposed to saline environment. However, H₂O₂ contents of both treated crops declined under hydroponic salt stress environment. Physiological mechanism of T. harzianum (Th-6) application in mitigating the salt-related consequences of both monocot crops was discussed.     

Pigment structure in the violaxanthin–chlorophyll-<Emphasis Type="Italic">a</Emphasis>-binding protein VCP

Salt-tolerant grasses of warm sub-tropical ecosystems differ in their distribution patterns with respect to salinity and moisture regimes. Experiments were conducted on CO₂ fixation and light harvesting processes of four halophytic C₄ grasses grown under different levels of salinity (0, 200 and 400 mM NaCl) under ambient environmental conditions. Two species were from a high saline coastal marsh (Aeluropus lagopoides and Sporobolus tremulus) and two were from a moderate saline sub-coastal draw-down tidal marsh (Paspalum paspalodes and Paspalidium geminatum). Analyses of the carbon isotope ratios of leaf biomass in plants indicated that carbon assimilation was occurring by C₄ photosynthesis in all species during growth under varying levels of salinity. In the coastal species, with increasing salinity, there was a parallel decrease in rates of CO₂ fixation (A), transpiration (E) and stomatal conductance (g _s), with no effect on water use efficiency (WUE). These species were adapted for photoprotection by an increase in the Mehler reaction with an increase in activity of PSII/CO₂ fixed accompanied by high levels of antioxidant enzymes, superoxide dismutase and ascorbate peroxidase. The sub-coastal species P. paspalodes and P. geminatum had high levels of carotenoid pigments and non-photochemical quenching by the xanthophyll cycle.