Altitudinal Change in the Photosynthetic Capacity of Tropical Trees: A Case Study from Ecuador and a Pantropical Literature Analysis

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO₂ concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO₂] (A _sat) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A _sat and leaf dark respiration (R _D) per leaf area remain constant with elevation. Stand-level means of A _sat were 8.8, 11.3, and 7.2?μmol?CO₂?m^?2?s^?1; those of R _D 0.8, 0.6, and 0.7?μmol?CO₂?m^?2?s^?1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A _sat and R _D of 20–53% (n?=?10–16 tree species per stand). Examining our data in the context of a pan-tropical A _sat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A _sat decreases in tropical mountains by, on average, 1.3?μmol?CO₂?m^?2?s^?1?per?km altitude increase (or by 0.2?μmol?CO₂?m^?2?s^?1 per K temperature decrease). The A _sat decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A _sat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A _sat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains.     

Higher Thermal Acclimation Potential of Respiration but Not Photosynthesis in Two Alpine Picea Taxa in Contrast to Two Lowland Congeners

The members of the genus Picea form a dominant component in many alpine and boreal forests which are the major sink for atmospheric CO₂. However, little is known about the growth response and acclimation of CO₂ exchange characteristics to high temperature stress in Picea taxa from different altitudes. Gas exchange parameters and growth characteristics were recorded from four year old seedlings of two alpine (Picea likiangensis vars. rubescens and linzhiensis) and two lowland (P. koraiensis and P. meyeri) taxa. Seedlings were grown at moderate (25°C/15°C) and high (35°C/25°C) day/night temperatures, for four months. The approximated biomass increment (ΔD²H) for all taxa decreased under high temperature stress, associated with decreased photosynthesis and increased respiration. However, the two alpine taxa exhibited lower photosynthetic acclimation and higher respiratory acclimation than either lowland taxon. Moreover, higher leaf dry mass per unit area (LMA) and leaf nitrogen content per unit area (N_area), and a smaller change in the nitrogen use efficiency of photosynthesis (PNUE) for lowland taxa indicated that these maintained higher homeostasis of photosynthesis than alpine taxa. The higher respiration rates produced more energy for repair and maintenance biomass, especially for higher photosynthetic activity for lowland taxa, which causes lower respiratory acclimation. Thus, the changes of ΔD²H for alpine spruces were larger than that for lowland spruces. These results indicate that long term heat stress negatively impact on the growth of Picea seedlings, and alpine taxa are more affected than low altitude ones by high temperature stress. Hence the altitude ranges of Picea taxa should be taken into account when predicting changes to carbon fluxes in warmer conditions.     

Temperature,water, and PAR influences on predicted and measured productivity of Agave deserti at various elevations

Summary To measure productivity of Agave deserti over its elevational range in the northwestern Sonoran Desert, leaf unfolding from its basal rosette was monitored on groups of 10 plants at 13 sites. Based on data from an intermediate elevation (840 m), leaf unfolding proved to be highly correlated (r ²=0.88) with an environmental productivity index (EPI) formed as the product of indices for water status, temperature, and photosynthetically active radiation (PAR); each of these latter indices indicated the fraction of maximum net CO₂ uptake expected for that parameter based on laboratory measurements of gas exchange and field microclimatic data. At 840 m, the main environmental variable influencing leaf unfolding for A. deserti over a 2-y period was soil water potential. On steep slopes, however, leaf unfolding during the winter ranged from 0.7 leaves per 10 plants for north-facing slopes to 7.3 for south-facing slopes, reflecting the importance of PAR. Summer and winter rainfall increased 3-fold from elevations of 300 m to 1,200 m. Temperatures were more optimal for net CO₂ uptake at high elevations in the summer and at low elevations in the winter. Hence EPI increased with elevation in the summer but was maximal at an intermediate elevation in the winter. Moreover, measured leaf unfolding in both the summer and the winter closely followed the changes in EPI with elevation, indicating that productivity could be closely predicted for A. deserti based on physiological CO₂ responses and changes in environmental conditions with elevation.     

Leaf carbon isotope ratio and water use efficiency of urban roadside trees in summer in Kyoto city

The effect of summer climate on leaf carbon isotope composition (δ¹³C) of the major roadside tree species Prunus × yedoensis (P. yedoensis) was investigated in Kyoto city, Japan, to explore the implications for alterations in urban environments. Temperature and the vapor pressure deficit were higher at sites of higher traffic volumes, possibly affected by a heat island effect. The leaf δ¹³C of P. yedoensis trees was affected strongly by leaf carbon isotope discrimination (Δ), with much less effect of δ¹³C on atmospheric CO₂. Leaf Δ values in the summer were smaller at sites of higher traffic volumes with high atmospheric temperatures, suggesting a higher long-term water use efficiency (WUE) at these sites. Gas exchange measurements of P. yedoensis leaves indeed suggested a higher intrinsic WUE at sites of higher traffic volumes with high atmospheric temperatures. These results suggest that leaf Δ is related to the response of WUE to summer climates, and that leaf δ¹³C in urban areas is a useful tracer for understanding the influences of urban environments on plant photosynthetic processes.     

The impact of recent increases in atmospheric CO2 on biomass production and vegetative retention of Cheatgrass (Bromus tectorum): implications for fire disturbance

Cheatgrass (Bromus tectorum) is a recognized, invasive annual weed of the western United States that reduces fire return times from decades to less than 5 years. To determine the interaction between rising carbon dioxide concentration ([CO₂]) and fuel load, we characterized potential changes in biomass accumulation, C : N ratio and digestibility of three cheatgrass populations from different elevations to recent and near‐term projections in atmospheric [CO₂]. The experimental CO₂ values (270, 320, 370, 420 μmol mol^?1) corresponded roughly to the CO₂ concentrations that existed at the beginning of the 19th century, that during the 1960s, the current [CO₂], and the near‐term [CO₂] projection for 2020, respectively. From 25 until 87 days after sowing (DAS), aboveground biomass for these different populations increased 1.5–2.7 g per plant for every 10 μmol mol^?1 increase above the 270 μmol mol^?1 preindustrial baseline. CO₂ sensitivity among populations varied with elevational origin with populations from the lowest elevation showing the greatest productivity. Among all populations, the undigestible portion of aboveground plant material (acid detergent fiber ADF, mostly cellulose and lignin) increased with increasing [CO₂]. In addition, the ratio of C : N increased with leaf age, with [CO₂] and was highest for the lower elevational population. These CO₂‐induced qualitative changes could, in turn, result in potential decreases in herbivory and decomposition with subsequent effects on the aboveground retention of cheatgrass biomass. Overall, these data suggest that increasing atmospheric [CO₂] above preambient levels may have contributed significantly to cheatgrass productivity and fuel load with subsequent effects on fire frequency and intensity.     

Photosynthetic characteristics of Amaranthus tricolor,a C4 tropical leafy vegetable

The gas exchange characteristics are reported for Amaranthus tricolor, a C₄ vegetable amaranth of southeastern Asia. Maximum photosynthetic capacity was 48.3±1.0μmol CO₂ m^?2s^?1 and the temperature optimum was 35°C. The calculated intercellular CO₂ concentration at this leaf temperature and an incident photon flux (400–700 mm) of 2 mmol m^?2s^?1 averaged 208±14 μl l^?1, abnormally high for a C₄ species. The photosynthetic rate, intercellular CO₂ concentration, and leaf conductance all decreased with an increase in water vapor pressure deficit. However, the decrease in leaf conductance which resulted in a decrease in intercellular CO₂ concentration accounted for only one fourth of the observed decrease in photosynthetic rate as water vapor pressure deficit was increased. Subsequent measurements indicated that the depence of net photosynthesis on intercellular CO₂ concetration changed with water vapor pressure deficit.     

Photosynthetic characteristics of Amaranthus tricolor,a C4 tropical leafy vegetable

The gas exchange characteristics are reported for Amaranthus tricolor, a C₄ vegetable amaranth of southeastern Asia. Maximum photosynthetic capacity was 48.3±1.0 μmol CO₂ m^-2 s^-1 and the temperature optimum was 35°C. The calculated intercellular CO₂ concentration at this leaf temperature and an incident photon flux (400–700 mm) of 2 mmol m^-2 s^-1 averaged 208±14 μl l^-1, abnormally high for a C₄ species. The photosynthetic rate, intercellular CO₂ concentration, and leaf conductance all decreased with an increase in water vapor pressure deficit. However, the decrease in leaf conductance which resulted in a decrease in intercellular CO₂ concentration accounted for only one fourth of the observed decrease in photosynthetic rate as water vapor pressure deficit was increased. Subsequent measurements indicated that the dependence of net photosynthesis on intercellular CO₂ concentration changed with water vapor pressure deficit.     

Changes in photosynthesis caused by adaptation of maize seedlings to short-term drought

Detached leaf is in the state of increasing water deficit; it is a good experimental model for looking into the hardening effect of adaptation of eight-day-old maize (Zea mays L.) seedlings to short-term drought (five days without watering). The light stage of photosynthesis and photosynthetic CO₂/H₂O exchange in detached leaves were studied. Specific surface density of leaf tissue (SSDL), the content of chlorophylls a and b, proline, MDA as well as photosynthetic parameters: quantum yield of photosystem II fluorescence, assimilation of CO₂, and transpiration at room temperature and light saturation (density of PAR quantum flux of 2000 μmol/(m² s)) at normal and half atmospheric CO₂ concentration were determined. The leaves of seedlings exposed to short-term drought differed from control material by a greater SSDL and higher content of proline. The hardening effect of the stress agent on the dark stage of photosynthesis was detected; it was expressed in the maintenance of the higher photosynthetic CO₂ assimilation against control material due to the elevation of stomatal conductance for CO₂ diffusing into the leaf. Judging from the lack of differences in the MDA content, short-term drought did not injure photosynthetic membranes. In detached leaves of experimental maize seedlings, photosynthesis was maintained on a higher level than in control material.     

A comparison of photosynthetic characteristics of encelia species possessing glabrous and pubescent leaves

Measurements of the dependence of photosynthesis on light, CO₂, and temperature are reported for two species of Encelia (Compositae) which differ in leaf pubescence and in geographical distribution. Encelia californica is glabrous and occurs in relatively mild, but arid habitats and Encelia farinosa is heavily pubescent and occurs in hot, arid habitats. Both species possess the C₃ photosynthetic pathway. Under high irradiances and normal atmospheric conditions the two species have high photosynthetic rates, exceeding 3 nanomoles of CO₂ per square centimeter per second (48 milligrams of CO₂ per square decimeter per hour) and complete light saturation does not occur by full noon sunlight. The high photosynthetic capacity is related to a high efficiency of utilization of intercellular CO₂ combined with high stomatal conductance. Leaf estimates of total soluble protein and fraction I protein are higher in these species than in most plants, although the proportion of fraction I protein is not higher. Both E. californica and E. farinosa attain a maximum rate of photosynthesis between 25 and 30 C, despite the fact that the two species grow in very different thermal habitats. Neither E. californica nor E. farinosa shows significant acclimation in the temperature dependence of photosynthesis when grown under different temperature regimes. The presence of leaf hairs which reduce leaf absorptance and consequently leaf temperature plays an important part in the ability of E. farinosa to survive in its native high temperature environment. When the effects of pubescence are taken into account, there are few if any significant differences in the photosynthetic characteristics of the two species.     

Global patterns of intraspecific leaf trait responses to elevation

Elevational gradients are often used to quantify how traits of plant species respond to abiotic and biotic environmental variations. Yet, such analyses are frequently restricted spatially and applied along single slopes or mountain ranges. Since we know little on the response of intraspecific leaf traits to elevation across the globe, we here perform a global meta‐analysis of leaf traits in 109 plant species located in 4 continents and reported in 71 studies published between 1983 and 2018. We quantified the intraspecific change in seven morpho‐ecophysiological leaf traits along global elevational gradients: specific leaf area (SLA), leaf mass per area (LMA), leaf area (LA), nitrogen concentration per unit of area (Narea), nitrogen concentration per unit mass (Nmass), phosphorous concentration per unit mass (Pmass) and carbon isotope composition (δ¹³C). We found LMA, Narea, Nmass and δ¹³C to significantly increase and SLA to decrease with increasing elevation. Conversely, LA and Pmass showed no significant pattern with elevation worldwide. We found significantly larger increase in Narea, Nmass, Pmass and δ¹³C with elevation in warmer regions. Larger responses to increasing elevation were apparent for SLA of herbaceous compared to woody species, but not for the other traits. Finally, we also detected evidences of covariation across morphological and physiological traits within the same elevational gradient. In sum, we demonstrate that there are common cross‐species patterns of intraspecific leaf trait variation across elevational gradients worldwide. Irrespective of whether such variation is genetically determined via local adaptation or attributed to phenotypic plasticity, the leaf trait patterns quantified here suggest that plant species are adapted to live on a range of temperature conditions. Since the distribution of mountain biota is predominantly shifting upslope in response to changes in environmental conditions, our results are important to further our understanding of how plants species of mountain ecosystems adapt to global environmental change.     

Preference of carbon absorption determines the competitive ability of algae along atmospheric CO2 concentration

Although many studies have focused on the effects of elevated atmospheric CO₂ on algal growth, few of them have demonstrated how CO₂ interacts with carbon absorption capacity to determine the algal competition at the population level. We conducted a pairwise competition experiment of Phormidium sp., Scenedesmus quadricauda, Chlorella vulgaris and Synedra ulna. The results showed that when the CO₂ concentration increased from 400 to 760 ppm, the competitiveness of S. quadricauda increased, the competitiveness of Phormidium sp. and C. vulgaris decreased, and the competitiveness of S. ulna was always the lowest. We constructed a model to explore whether interspecific differences in affinity and flux rate for CO₂ and HCO₃ ⁻ could explain changes in competitiveness between algae species along the gradient of atmospheric CO₂ concentration. Affinity and flux rates are the capture capacity and transport capacity of substrate respectively, and are inversely proportional to each other. The simulation results showed that, when the atmospheric CO₂ concentration was low, species with high affinity for both CO₂ and HCO₃ ⁻ (HCHH) had the highest competitiveness, followed by the species with high affinity for CO₂ and low affinity for HCO₃ ⁻ (HCLH), the species with low affinity for CO₂ and high affinity for HCO₃ ⁻ (LCHH) and the species with low affinity for both CO₂ and HCO₃ ⁻ (LCLH); when the CO₂ concentration was high, the species were ranked according to the competitive ability: LCHH > LCLH > HCHH > HCLH. Thus, low resource concentration is beneficial to the growth and reproduction of algae with high affinity. With the increase in atmospheric CO₂ concentration, the competitive advantage changed from HCHH species to LCHH species. These results indicate the important species types contributing to water bloom under the background of increasing global atmospheric CO₂, highlighting the importance of carbon absorption characteristics in understanding, predicting and regulating population dynamics and community composition of algae.     

Altitudinal variation in photosynthetic capacity, diffusional conductance and δ13C of butterfly bush (Buddleja davidii) plants growing at high elevations

In this study, we have examined several physiological, biochemical and morphological features of Buddleja davidii plants growing at 1300 m above sea level (a.s.l.) and 3400 m a.s.l., respectively, to identify coordinated changes in leaf properties in response to reduced CO₂ partial pressure (P_a). Our results confirmed previous findings that foliar δ¹³C, photosynthetic capacity and foliar N concentration on a leaf area basis increased, whereas stomatal conductance (g_s) decreased with elevation. The net CO₂ assimilation rate (A_max), maximum rate of electron transport (J_max) and respiration increased significantly with elevation, although no differences were found in carboxylation efficiency of Rubisco (V_cmax). Consequently, also the J_max to V_cmax ratio was significantly increased by elevation, indicating that the functional balance between Ribulose‐1,5‐biphosphate (RuBP) consumption and RuBP regeneration changes as elevation increases. Our results also indicated a homeostatic response of CO₂ transfer conductance inside the leaf (mesophyll conductance, g_m) to increasing elevation. In fact, with elevation, g_m also increased compensating for the strong decrease in g_s and, thus, in the P_i (intercellular partial pressure of CO₂) to P_a ratio, leading to similar chloroplast partial pressure of CO₂ (P_c) to P_a ratio at different elevations. Because there were no differences in V_cmax, also A measured at similar PPFD and leaf temperature did not differ statistically with elevation. As a consequence, a clear relationship was found between A and g_m, and between A and the sum of g_s and g_m. These data suggest that the higher dry mass δ¹³C of leaves at the higher elevation, indicative of lower long‐term P_c/P_a ratio, cannot be attributed to changes either in diffusional resistances or in carboxylation efficiency. We speculate that because temperature significantly decreases as the elevation increases, it dramatically affects CO₂ diffusion and hence P_c/P_a and, consequently, is the primary factor influencing ¹³C discrimination at high elevation.     

Estimation of Mesophyll Conductance to CO(2) Flux by Three Different Methods

The resistance to diffusion of CO₂ from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, ¹³C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO₂ diffusion determined in unstressed plants at high light. The relatively low CO₂ partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O₂ sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO₂ partial pressure upon switching to low O₂. We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO₂ and O₂ partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO₂-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO₂ level.     

Needle δ13C and mobile carbohydrates in Pinus koraiensis in relation to decreased temperature and increased moisture along an elevational gradient in NE China

A tree’s crown interacts with atmospheric variables such as CO₂, temperature, and humidity. Physioecology of leaves/needles (e.g. δ¹³C, mobile carbohydrates, and nitrogen) is, therefore, strongly affected by microclimate in and surrounding a tree crown. To understand the physiological responses of leaves to changes in air temperature and moisture, we measured δ¹³C, soluble sugars, starch, and total nitrogen (N) concentrations in current year and 1-yr-old needles of Pinus koraiensis trees, and compared the growing season air temperature and relative humidity within and outside P. koraiensis crowns along an elevational gradient from 760 to 1,420 m a.s.l. on Changbai Mountain, NE China. Our results indicated that needle N and mobile carbohydrates concentrations, as well as needle δ¹³C values changed continuously with increasing elevation, corresponding to a continuous decrease in air temperature and an increase in relative humidity. Needle carbon and nitrogen status is highly significantly negatively correlated with temperature, but positively correlated with relative humidity. These results indicate that increases in air temperature in combination with decreases in relative humidity may result in lower levels of N and mobile carbohydrates in P. koraiensis trees, suggesting that future climate changes such as global warming and changes in precipitation patterns will directly influence the N and carbon physiology at P. koraiensis individual level, and indirectly affect the competitive ability, species composition, productivity and functioning at the stand and ecosystem level in NE China. Due to the relatively limited range of the transect (760–1,420 m) studied, further research is needed to explain whether the present results are applicable to scales across large elevational gradients.     

Oxygen Inhibition of Photosynthesis: I. Temperature Dependence and Relation to O(2)/CO(2) Solubility Ratio

The magnitude of the percentage inhibition of photosynthesis by atmospheric levels of O₂ in the C₃ species Solanum tuberosum L., Medicago sativa L., Phaseolus vulgaris L., Glycine max L., and Triticum aestivum L. increases in a similar manner with an increase in the apparent solubility ratio of O₂/CO₂ in the leaf over a range of solubility ratios from 25 to 45. The solubility ratio is based on calculated levels of O₂ and CO₂ in the intercellular spaces of leaves as derived from whole leaf measurements of photosynthesis and transpiration. The solubility ratio of O₂/CO₂ can be increased by increased leaf temperature under constant atmospheric levels of O₂ and CO₂ (since O₂ is relatively more soluble than CO₂ with increasing temperature); by increasing the relative levels of O₂/CO₂ in the atmosphere at a given leaf temperature, or by increased stomatal resistance. If the solubility ratio of O₂/CO₂ is kept constant, as leaf temperature is increased, by varying the levels of O₂ or CO₂ in the atmosphere, then the percentage inhibition of photosynthesis by O₂ is similar. The decreased solubility of CO₂ relative to O₂ (decreased CO₂/O₂ ratio) may be partly responsible for the increased percentage inhibition of photosynthesis by O₂ under atmospheric conditions with increasing temperature.     

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.     

Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis

In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO₂ assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO₂ loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO₂ assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36°C and 50 millibars bar⁻¹, respectively). Studies on the relationship between CO₂ assimilation rate and intercellular CO₂ partial pressure at various temperatures and humidities showed that this decrease in CO₂ assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO₂ diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher ¹³C content of Welwitschia compared to the majority of C₃ species.     

C4 photosynthetic carbon metabolism in the leaves of aromatic tropical grasses — I. Leaf anatomy,CO2 compensation point and CO2 assimilation

A few species of Cymbopogon and Vetiveria are potentially important tropical grasses producing essential oils. In the present study, we report on the leaf anatomy and photosynthetic carbon assimilation in five species of Cymbopogon and Vetiveria zizanioides. Kranz-type leaf anatomy with a centrifugal distribution of chloroplasts and exclusive localization of starch in the bundle sheath cells were common among the test plants. Besides the Kranz leaf anatomy, these grasses displayed other typical C₄ characteristics including a low (0–5 µl/l) CO₂ compensation point, lack of light saturation of CO₂ uptake at high photon flux densities, high temperature (35°C) optimum of net photosynthesis, high rates of net photosynthesis (55–67 mg CO₂ dm^-2 leaf area h^-1), little or no response of net photosynthesis to atmospheric levels of O₂ and high leaf ¹³C/¹²C ratios. The biochemical studies with ¹⁴CO₂ indicated that the leaves of the above plant species synthesize predominantly malate during short term (5 s) photosynthesis. In pulse-chase experiments it was shown that the synthesis of 3-phosphoglycerate proceeds at the expense of malate, the major first formed product of photosynthesis in these plant species.     

Oxygen inhibition of photosynthesis

The effect of leaf temperature, O₂ and calculated O₂/CO₂ solubility ratio in the leaf on the quantum yield of photosynthesis was studied for the C₄ species, Zea mays L., and the C₃ species, Triticum aestivum L. Over a range of leaf temperatures of 16 to 35° C, the quantum yield of Z. mays was relatively constant and was similar under 1.5 and 21% O₂, being ca. 0.059 mol CO₂ mol^-1 quanta absorbed. Under 1.5% O₂ and atmospheric levels of CO₂, the quantum yield of T. aestivum was relatively constant (0.083 mol CO₂ mol^-1 quanta absorbed) at leaf temperatures from 15 to 35° C. Atmospheric levels of O₂ (21%) reduced the quantum yield of photosynthesis in T. aestivum and as leaf temperature increased, the quantum yield decreased from 0.062 at 15°C to 0.046 mol CO₂ mol^-1 quanta absorbed at 35°C. Increasing temperature decreases the solubility of CO₂ relatively more than the solubility of O₂, resulting in an increased solubility ratio of O₂/CO₂. Experimentally manipulating the atmospheric levels of O₂ or CO₂ to maintain a near-constant solubility ratio of O₂/CO₂ at varying leaf temperatures largely prevented the temperature-dependent decrease in quantum yield in t. aestivum. Thus, the decrease in quantum yield with increasing leaf temperature in C₃ species may be largely caused by a temperaturedependent change in the solubility ratio of O₂/CO₂.J and II=Ku and Edwards, 1977a, b     

Free-air CO2 enrichment in a wheat-weed assembly – effects on water relations

In a three-year free-air CO₂ enrichment study (Mini-FACE), spring wheat associated with typical arable weeds were grown under present and elevated atmospheric carbon dioxide concentrations [CO₂] (ambient air+150 μmol mol^?1). Analyses of plant stable carbon isotope ratios and in vivo measurements of leaf gas exchange were used to describe the CO₂ effects on water relations. For most species examined elevated [CO₂] significantly increased the intrinsic water-use efficiency (A/g_s) as derived from carbon isotope analyses. In some of the species, seasonal averages of the ratio between leaf internal to atmospheric CO₂ (c_i/c_a) were found to be significantly reduced by elevated [CO₂]. Periodic leaf gas exchange measurements confirmed the increased water-use efficiency, but significant CO₂ effects became evident only over the entire season by carbon isotope analysis. In both types of analysis conducted, spring wheat was found to react significantly different from all other species examined. The relation between A/g_s and biomass production was significantly influenced by elevated [CO₂] in all three years of the study. At the end of the drier growing seasons 2003 and 2004, the soil water content tended to be increased in the CO₂ enriched plots indicating a water saving effect. These observations demonstrate the impact of elevated [CO₂] on plant water relations with a likely positive feedback leading to higher soil water availability. Due to the differences in the CO₂ responses of spring wheat compared to the weeds we suggest that rising [CO₂] may cause shifts in the species composition of crop-weed communities.