Is the rate of photosynthesis under blue light altered in the 7B-1 tomato mutant?

Changes of photosynthesis under blue light were examined in the ABA-overproducing 7B-1 mutant in tomato. Net photosynthetic rate (P _N), stomatal conductance (g _s), intrinsic water-use efficiency (WUE_i) and chlorophyll (a+b) [Chl (a+b)] content in leaves of different insertion (1^st, 4^th and 9^th ones) were measured in 5-, 7- and 9-week-old plants. P _N, g _s, and Chl (a+b) content were mostly similar in young leaves of 7B-1 and wild type (WT) plants. With the aging of leaves, a blue-light-induced increase in P _N and g _s to steady-state was delayed and steady-state values of P _N and g _s were lower in 7B-1 plants compared with WT. Steady-state values of WUE_i were increased in 4^th and 9^th leaves of 7B-1 plants compared with WT. The results can be explained by the higher endogenous level of ABA in 7B-1 plants and their lower sensitivity to ABA in earlier growth stage.     

Growth, gas exchange, and water-use efficiency response of two young apple cultivars to drought stress in two scion-one rootstock grafting system

The combination of two scion-one rootstock was used for two apple cultivars, ‘Pink Lady’ and ‘Qinguan’, budded on the same, one-year-old Malus hupehensis (Pamp.) Rehd. to reduce the impact of root and pot size and in order to understand the growth, water-use efficiency (WUE), and chlorophyll fluorescence characteristics. The two-scion grafted trees were planted in plastic pots under two water regimes, i.e. 70% field capacity (FC) and 55% FC. Results indicated that different scions were affected differently by drought stress. ‘Pink Lady’ had higher net photosynthetic rate (P _N), stomatal conductance (g _s), and transpiration rate (E) compared with ‘Qinguan’ under both water treatments. However, ‘Qinguan’ had lower minimal fluorescence (F₀), higher maximum fluorescence (F_m), and higher maximum photochemical efficiency of photosystem II (F_v/F_m) than ‘Pink Lady’ at 55% FC. Moreover, ‘Qinguan’ had larger shoot dry mass (ShDM) and higher intrinsic WUE_I than ‘Pink Lady’ under both water status. Gas-exchange and growth parameters, except for P _N and scion diameter, were significantly affected by the cultivar and water treatment. At 70% FC, ShDM was significantly correlated with WUE_I. Moreover, WUE_I was negatively linearly correlated with g _s at either 70 or 55% FC. These results might indicate that ‘Pink Lady’ was more sensitive to drought than ‘Qinguan’. ‘Qinguan’ apple was able to improve WUE more than ‘Pink Lady’ under both well-watered and drought conditions. The growth parameters and photosynthetic capacity of two different scions showed that the combination of double scion-one rootstock might eliminate the influences of the rootstock and pot size.     

Differential responses of leaf water-use efficiency and photosynthetic nitrogen-use efficiency to fertilization in Bt-introduced and conventional rice lines

Leaf stomatal conductance (g _s), transpiration rate (E), and light-saturated net photosynthetic rate (P _Nmax) at three developmental stages (tillering stage, jointing-booting stage, and milking stage) and leaf total nitrogen concentration (LTNC) and δ¹³C value at milking stage were measured for a conventional rice line (Minghui 63) and its corresponding Bacillus thuringiensis (Bt)-gene [cry1A (b and c)] introduced line (Bt line) under three fertilizer levels. Compared to conventional line, Bt line showed lower g _s, which was associated with lower P _Nmax and E, but instantaneous water-use efficiency (WUE), measured as the ratio of P _Nmax to E, was higher in the Bt line than in the conventional line, particularly in the jointing-booting stage. However, δ¹³C values were not significantly different across treatments, suggesting that intrinsic water-use efficiency (WUE_in) might be indistinguishable between Bt and conventional lines. LTNC was higher but P _Nmax was lower in Bt line compared to conventional line, resulting in significantly lower photosynthetic nitrogen-use efficiency (PNUE). This might result from the additional cost of producing Bt protein in the Bt line due to the effect of competing nitrogen with photosynthetic machinery. Bt-gene introduction and expression does not significantly change WUE_in but may significantly decrease leaf PNUE. Thus we suggest that Bt rice should be carefully examined in relation to environmental risks (e.g. water-body pollution) before planting commercially.     

Rearrangement of leaf traits with changing source-sink relationship in blueberry (Vaccinium corymbosum L.) leaves

The source-sink relationship is one of major determinants of plant performance. The influence of reproductive sink demand on light-saturated photosynthesis (P_max), dark respiration (R_D), stomatal conductance (g_s), intrinsic water-use efficiency (WUE_i), contents of soluble sugar (SSC), nitrogen, carbon, and photosynthetic pigments was examined in blueberry (Vaccinium corymbosum L. cv. ‘Brigitta’) during the final stage of rapid fruit growth. Measurements were performed three times per day on developed, sun-exposed leaves of girdled shoots with 0.1, 1, and 10 fruit per leaf (0.1F:L, 1F:L, and 10F:L, respectively) and nongirdled shoots bearing one fruit per leaf (NG). Girdling and lower fruit amount induced lower P_max, g_s, N, and total chlorophyll (Chl) and higher WUE_i, SSC, R_D, Chl a/b ratio and carotenoids-to-chlorophylls ratio (Car/Chl) for the 1F:L and 0.1F:L treatments. The impact of girdling was counterbalanced by 10F:L, with NG and 10F:L having similar values. Variables other than P_max, R_D, g_s, WUE_i, and SSC were unaffected throughout the course of the day. P_max and g_s decreased during the course of the day, but g_s decreased more than P_max in the afternoon, while WUE_i was increasing in almost all treatments. SSC increased from the morning until afternoon, whereas R_D peaked at noon regardless of the treatment. Generally, P_max was closely and negatively correlated to SSC, indicating that sugar-sensing mechanisms played an important role in regulation of blueberry leaf photosynthesis. With respect to treatments, P_max and N content were positively related, while R_D was not associated to substrate availability. The enhanced Car/Chl ratio showed a higher photoprotection under the lower sink demand. Changes in the source-sink relationship in ‘Brigitta’ blueberry led to a rearrangement of physiological and structural leaf traits which allowed adjusting the daily balance between carbon assimilation and absorbed light energy.     

Responses of young ‘Pink lady’ apple to alternate deficit irrigation following long-term drought: growth, photosynthetic capacity, water-use efficiency, and sap flow

We studied photosynthetic capacity, growth, sap flow, and water-use efficiency in young trees of ‘Pink Lady’ apple (Malus domestica) that were exposed to 60 d of moisture stress. Three irrigation schemes were tested in the greenhouse: well-watered control; drought; or alternate deficit irrigation (ADI). Compared with the drought-stressed plants, those treated via ADI showed better height growth, larger scion diameters, and greater total leaf area, as well as significantly increased gains in dry biomass and rootstock diameters. However, their performance was still significantly lower than that demonstrated by continuously well-watered plants. Sap flow was greater under ADI than under drought, but less than under control conditions. The average rate of net photosynthesis, total amount of irrigation water applied, and dry biomass gain had highly significant and positive linear correlations with long-term water-use efficiency (WUE_L). The same was true between average stomatal conductance and WUE_L. By contrast, instantaneous water-use efficiency (WUE_I) was very significantly and negatively correlated with WUE_L. In addition, values for WUE_L were much higher from well-watered plants when compared with either drought-stressed trees or those treated per ADI. Therefore, our results indicate that, although ‘Pink Lady’ apple normally has high WUE, it still consumes a large amount of water. Therefore, the practice of ADI following a period of long-term drought could be used to improve growth and WUEL by this cultivar.     

Variation in bulk‐leaf 13C discrimination,leaf traits and water‐use efficiency–trait relationships along a continental‐scale climate gradient in Australia

Large spatial and temporal gradients in rainfall and temperature occur across Australia. This heterogeneity drives ecological differentiation in vegetation structure and ecophysiology. We examined multiple leaf‐scale traits, including foliar ¹³C isotope discrimination (Δ¹³C), rates of photosynthesis and foliar N concentration and their relationships with multiple climate variables. Fifty‐five species across 27 families were examined across eight sites spanning contrasting biomes. Key questions addressed include: (i) Does Δ¹³C and intrinsic water‐use efficiency (WUE_i) vary with climate at a continental scale? (ii) What are the seasonal and spatial patterns in Δ¹³C/WUE_i across biomes and species? (iii) To what extent does Δ¹³C reflect variation in leaf structural, functional and nutrient traits across climate gradients? and (iv) Does the relative importance of assimilation and stomatal conductance in driving variation in Δ¹³C differ across seasons? We found that MAP, temperature seasonality, isothermality and annual temperature range exerted independent effects on foliar Δ¹³C/WUE_i. Temperature‐related variables exerted larger effects than rainfall‐related variables. The relative importance of photosynthesis and stomatal conductance (g_s) in determining Δ¹³C differed across seasons: Δ¹³C was more strongly regulated by g_s during the dry‐season and by photosynthetic capacity during the wet‐season. Δ¹³C was most strongly correlated, inversely, with leaf mass area ratio among all leaf attributes considered. Leaf N_mass was significantly and positively correlated with MAP during dry‐ and wet‐seasons and with moisture index (MI) during the wet‐season but was not correlated with Δ¹³C. Leaf P_mass showed significant positive relationship with MAP and Δ¹³C only during the dry‐season. For all leaf nutrient‐related traits, the relationships obtained for Δ¹³C with MAP or MI indicated that Δ¹³C at the species level reliably reflects the water status at the site level. Temperature and water availability, not foliar nutrient content, are the principal factors influencing Δ¹³C across Australia.     

Physiological foundation for the differences of long-term water use efficiency among Populus deltoides clones

With randomized complete block design, long-term water use efficiency (WUE_L), photosynthetic parameters (net photosynthetic rate Pn, transpiration rate Tr, stomatal conductance Gs, instantaneous water use efficiency WUE_i, maximum photosynthetic rate P max, light saturation pointLSP, carboxylic efficiency Ce), stomatal properties (frequency, size, superior/inferior stoma amount ratio) and root/shoot ratio of twelve Populus deltoids clones were studied under water stress in a greenhouse. Four water treatments were designed, namely, well-watered condition, slight water stress, moderate water stress, and severe water stress. Although the volumes of irrigated water for different treatments were the same, the irrigation intervals were strictly controlled. The results showed that long-term water use efficiency (WUE_L) differences among tested clones were obvious and became greater with the increase in water stress. J₂, J₆, J₇, J₈, and J₉ were excellent clones with high WUE_L. Stomatal frequency, stomatal size, Gs, Tr, Pn, and root/shoot ratio of J₂et al were moderate, whereas the superior/inferior ratio, P_max, LSP, Ce of J2et al were higher than other clones. The interrelations between WUE_L and the physiological parameters were analyzed. The results revealed that the WUE_L differences were induced by a series of physiological parameters, and clones with higher WUE_L always had strong photosynthetic capacity, higher WUE_i, and optimum root/shoot ratio.     

An integrated diagnostic approach to understand drought tolerance in mulberry (Morus indica L.)

Four popular mulberry cultivars (Morus indica L. cvs.V-1, MR-2, S-36 and K-2) were assessed for drought tolerance with an integration of selective approaches. The potted plants were subjected to two watering treatments for 75 days: control pots were watered up to 100% field capacity (FC) and stressed pots were maintained at 25–30% FC. Net photosynthetic rate (P_n), stomatal conductance (g_s), transpiration rate (E) and instantaneous water use efficiency (WUE_i) were the key parameters to assess photosynthetic gas exchange performance. Drought caused marked down-regulation in leaf gas exchange in all cultivars (cvs) except V-1 which maintained better P_n, g_s, E and higher WUE_i under severe water deficit. All the four cvs also showed differential antioxidative responses under water stress. Higher concentrations of carotenoids, ascorbic acid, glutathione, α-tocopherol and proline were observed in the leaf extracts of V-1, while minimum accumulation of those metabolites was recorded with K-2 and S-36. An endogenous loss of α-tocopherol and higher lipid peroxidation were encountered in K-2, S-36 and MR-2, whereas V-1 showed minimum lipid peroxidation under water deficit regimes. Comparative morpho-anatomical analysis revealed a well-developed root system and a better anatomical architecture in V-1 which could further contribute tolerance during drought stress.     

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO<Subscript>2</Subscript>

Using a free-air CO₂ enrichment (FACE) experiment, poplar trees (Populus × euramericana clone I214) were exposed to either ambient or elevated [CO₂] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm⁻²) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (g _s, μmol m⁻² s⁻¹), photosynthetic CO₂ fixation (A, mmol m⁻² s⁻¹), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO₂ (C_i:C_a). Elevated [CO₂] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO₂] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO₂], g _s was reduced, A/E was stimulated, and C_i:C_a was reduced relative to ambient [CO₂]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO₂]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO₂], although the mechanism for this remains obscure.     

Internal conductance to CO2 transfer of adult Fagus sylvatica: Variation between sun and shade leaves and due to free-air ozone fumigation

Two separate objectives were considered in this study. We examined (1) internal conductance to CO₂ (g_i) and photosynthetic limitations in sun and shade leaves of 60-year-old Fagus sylvatica, and (2) whether free-air ozone fumigation affects g_i and photosynthetic limitations. g_i and photosynthetic limitations were estimated in situ from simultaneous measurements of gas exchange and chlorophyll fluorescence on attached sun and shade leaves of F. sylvatica. Trees were exposed to ambient air (1× O₃) and air with twice the ambient ozone concentration (2× O₃) in a free-air ozone canopy fumigation system in southern Germany (Kranzberg Forest). g_i varied between 0.12 and 0.24 mol m⁻² s⁻¹ and decreased CO₂ concentrations from intercellular spaces (C_i) to chloroplastic (C_c) by approximately 55 μmol mol⁻¹. The maximum rate of carboxylation (V_cmax) was 22–39% lower when calculated on a C_i basis compared with a C_c basis. g_i was approximately twice as large in sun leaves compared to shade leaves. Relationships among net photosynthesis, stomatal conductance and g_i were very similar in sun and shade leaves. This proportional scaling meant that neither C_i nor C_c varied between sun and shade leaves. Rates of net photosynthesis and stomatal conductance were about 25% lower in the 2× O₃ treatment compared with 1× O₃, while V_cmax was unaffected. There was no evidence that g_i was affected by ozone.     

黑杨无性系间长期水分利用效率差异的生理基础

试验于2003年6月10日至8月20日在温室中进行,采用完全随机区组设计,4个水分处理（充分供水、轻度胁迫、中度胁迫、重度胁迫）,6次重复.各水分处理严格控制浇水间隔：充分供水,每2d浇水1次;轻度胁迫,每3d浇水1次;中度胁迫,每4d浇水1次;重度胁迫,每5d浇水1次.每次浇水的量相同,500ml.试验材料为美洲黑杨杂交无性系,父本为哈佛杨（Populus deltoides Bartr.CI.‘Harvard＇）,母本为山海关杨（Populus deltoides Bartr.CV.‘shanhaiguanensis＇）,共10个无性系,编号为J1～J10.以当前优良品系108杨（Populus euramericana CV.‘114/69＇）和中林46杨（Populus deltoides Bartr.CV.‘Zhonglin 46＇）为对照,编号为CK1、CK2.测定了12个无性系间长期水分利用效率（WUEL）、光合参数（净光合速率Pn、蒸腾速率Tr、气孔导度Gs、瞬时水分利用效率WUEi、最大净光合速率Pmax、光饱和点LSP、羧化效率Ce）及气孔特征参数（气孔频度、气孔大小、上/下表皮气孔数目比）、根冠比的差异,并深入分析了光合参数与WUEL、气孔特征参数与WUEL、根冠比与WUEL及光合参数之间的相互关系.结果表明：无性系间WUEL存在差异,这种差异随水分胁迫的加剧而更加显著.无性系间WUEL的差异在4个水分处理下表现出一致性,即WUEL最优的都是J2,其次为J6、J7、J8、J9.高WUEL的J2等无性系其气孔频度、气孔大小、Gs、Tr、Pn和根冠比在所有无性系中都处于适中的位置.它们的上/下表皮气孔数目比大,Pmax、LSP、Ce明显优于对照.Pmax高、LSP高、Ce大,表明高WUEL的J2等无性系光合能力强.Gs、Tr、Pn适中,表明J2等无性系在保证具有高WUEi的同时能保持较强的光合能力.根冠比适中,表明J2等无性系的光合产物在根、冠之间分配合理,能有充足的根系分布保证水分的供应,同时又有较多的光合产物积累在地上部分.气孔特征参数的差异是导致无性系间光合能力和WUEi差异,并最终导致WUEL差异的一个主要因子.气孔频度和气孔大小适中的无性系,其Gs、Tr、Pn适中,WUEi较高;上/下表皮气孔数目比大的无性系则具有较强的光合能力.无性系间WUEL的差异是一系列具有差异的生理参数共同作用的结果,高WUEL的无性系通常光合能力强、WUEi高、根冠比适中.今后工作中选育高WUE的无性系时,除需关注单个生理参数的突出作用外,更应关注生理参数的协同作用.     

The relationship between the soil water storage and water-use efficiency of seven energy crops

The aim of this work was to determine two types of photosynthetic water-use efficiency in order to examine their utility as selection criteria for tolerance of energy crops to soil water deficit. Furthermore, effects of crop cultivation on soil water content and storage were investigated. Seven energy crops were examined: miscanthus, prairie cordgrass, willow, thorn-free rose, Virginia mallow, Bohemian knotweed, and topinambour. The highest values of instantaneous (WUE) and intrinsic (WUE_i) water-use efficiencies were found for miscanthus and prairie cordgrass. The reduction of WUE and/or WUE_i was caused mainly by a rapid rise in the transpiration rate and a greater stomatal conductance, respectively. Principal component analysis showed that neither WUE nor WUE_i could be recommended as universal selection criteria for the drought tolerance in different energy crops. The proper localization of soil with a good supply of water is most the important condition for energy crop plantations.     

Photosynthesis, water use efficiency and stable carbon isotope composition are associated with anatomical properties of leaf and xylem in six poplar species

Although fast‐growing Populus species consume a large amount of water for biomass production, there are considerable variations in water use efficiency (WUE) across different poplar species. To compare differences in growth, WUE and anatomical properties of leaf and xylem and to examine the relationship between photosynthesis/WUE and anatomical properties of leaf and xylem, cuttings of six poplar species were grown in a botanical garden. The growth performance, photosynthesis, intrinsic WUE (WUE_i), stable carbon isotope composition (δ¹³C) and anatomical properties of leaf and xylem were analysed in these poplar plants. Significant differences were found in growth, photosynthesis, WUE_i and anatomical properties among the examined species. Populus cathayana was the clone with the fastest growth and the lowest WUE_i/δ¹³C, whereas P. × euramericana had a considerable growth increment and the highest WUE_i/δ¹³C. Among the analysed poplar species, the highest total stomatal density in P. cathayana was correlated with its highest stomatal conductance (g_s) and lowest WUE_i/δ¹³C. Moreover, significant correlations were observed between WUE_i and abaxial stomatal density and stem vessel lumen area. These data suggest that photosynthesis, WUE_i and δ¹³C are associated with leaf and xylem anatomy and there are tradeoffs between growth and WUE_i. It is anticipated that some poplar species, e.g. P. × euramericana, are better candidates for water‐limited regions and others, e.g. P. cathayana, may be better for water‐abundant areas.     

Effects of fertilization on leaf photosynthetic characteristics and grain yield in tartary buckwheat Yunqiao1

Tartary buckwheat (Fagopyrum tataricum Gaertn) has been praised as one of green foods for humans in the 21_st century. Effects of fertilization on leaf photosynthetic characteristics and grain yield of tartary buckwheat has not been yet reported in detail. Our experiment was set as a split-plot factorial. The main plots and subplots were designed by fertilizer ratio and rate as: NPK 1:1:1 (A1), NPK 1:4:2 (A2), NPK 1:2:3 (A3), and 300 (B1), 450 (B2), and 600 (B3) kg (NPK) ha^–1. Our results showed that the grain yield was significantly and positively correlated with the net photosynthetic rate (P _N), stomatal conductance (g _s), transpiration rate (E), PAR, stomatal limitation value (L_s), chlorophyll content (SPAD value), and leaf area index (LAI), while significantly and negatively correlated with intercellular CO₂ concentration (C _i) and water-use efficiency (WUE). The grain yield, P _N, g _s, E, PAR, L_s, SPAD, and LAI increased and then decreased with enhanced fertilization, and their maximum values appeared in the A2B2 treatment. The C _i and WUE decreased and then increased with enhanced fertilization, and their minimum values appeared in the A2B2 treatment. Our results suggested that fertilization had significant effects on the leaf photosynthetic capacity and grain yield of tartary buckwheat Yunqiao 1, and the best fertilization strategy was 450 kg ha^–1 with NPK 1:4:2.     

Water-use efficiency and carbon isotope discrimination in two cultivars of upland rice during different developmental stages under three water regimes

A pot experiment was conducted in a glasshouse to clarify and quantify the effect of plant part, water regime, growth period, and cultivar on carbon isotope discrimination (CID), and to analyze the relationship between CID, stomatal behavior and water-use efficiency (WUE). The experiment was comprised of two upland rice (Oryza sativa L.) cultivars and three water regimes (100, 70, and 40% of saturation moisture) in a completely randomized design. Plants were harvested at tillering, flowering, and maturity. No significant cultivar differences in above-ground dry matter-based WUE (WUE_A) and total dry matter-based WUE (WUE_T) were observed. WUE_A (and WUE_T) increased with water stress up to tillering, but decreased with water stress after tillering. Significant cultivar differences in CID in all the analyzed plant parts were observed at all harvest times. Reduction in CID with water stress was greatest at tillering, and the effect was less pronounced at flowering and at maturity. At each harvest, the effect was most pronounced in newly developed plant parts. Root and grain tended to have the lowest CID values, and stem the highest, at all harvest times. A negative relationship was observed between CID measured at tillering and WUE_A (and WUE_T) measured over the period from seedling to tillering, whereas a reverse relationship was obtained between CID measured at flowering and WUE_A (and WUE_T) measured over the period from tillering to flowering, and an unclear relationship between CID measured at maturity and WUE_A (and WUE_T) measured over the period from flowering to maturity. The ratio of the intercellular and atmospheric concentration of CO₂ (C_i/C_a) were closely associated with CID throughout the water regimes when one cultivar was considered, however, cultivar differences in CID were not related to variations in C_i/C_a. The results indicate that significant cultivar difference existed in CID in all the analyzed plant parts at all harvest times, while corresponding difference in WUE_A (and WUE_T) between the cultivars was not necessarily consistent. Abbreviations: WUE – water-use efficiency; WUEi – instantaneous WUE (or leaf transpiration efficiency); ADM – above-ground dry matter; TDM – total dry matter; WUE_A– ADM-based WUE; WUE_T– TDM-based WUE} CID – carbon isotope discrimination; NL – the newest leaves; FEL – recently fully expanded leaves; FL – flag leaves; P – photosynthesis rate; g – leaf stomatal conductance to water vapor; C_i– intercellular CO₂ concentration; C_a– atmospheric CO₂ concentration; T – transpiration rate; gs – total conductance of CO₂     

Interactions of Elevated CO2 Concentration and Drought Stress on Photosynthesis in Eucalyptus Cladocalyx F. Muell

Response of net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (c _i), and photosynthetic efficiency (F_v/F_m) of photosystem 2 (PS2) was assessed in Eucalyptus cladocalyx grown for long duration at 800 (C₈₀₀) or 380 (C₃₈₀) µmol mol^-1 CO₂ concentration under sufficient water supply or under water stress. The well-watered plants at C₈₀₀ showed a 2.2 fold enhancement of P _N without any change in g _s. Under both C₈₀₀ and C₃₈₀, water stress decreased P _N and g _s significantly without any substantial reduction of c _i, suggesting that both stomatal and non-stomatal factors regulated P _N. However, the photosynthetic efficiency of PS2 was not altered.     

Photosynthesis and Leaf Nutrient Contents in Ochroma Pyramidale (Bombacaceae)

In Ochroma pyramidale (Cav. ex Lam.) Urb., photon-saturated photosynthetic capacity (P _Nmax) was 13 mol(CO₂) m^–2 s^–1. Average stomatal conductance (g _s) and water-use efficiency (WUE) were greater at high irradiance, about 260 mmol(H₂O) m^–2 s^–1 and 2.15 g(C) kg^–1(H₂O), respectively. In the dark, g _s values were about 30% of maximum g _s. Leaf nutrient contents on a leaf area basis were 131, 15, 36, 21, and 12 mmol m^–2 for N, P, K, Ca, and Mg, respectively. Ochroma also accumulated a greater amount of soluble saccharides than starch, 128 versus 90 g kg^–1 (DM). The availability of N and Mg, but not P, Ca, or K, may limit photosynthetic rates of Ochroma in this site.     

Water availability affects seasonal CO2‐induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO₂ (eCO₂) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (g_s) and increased soil water content (V_SWC) and second, through increased leaf internal CO₂ (C_i) and decreased stomatal limitations (S_lim). It is unclear if such findings from temperate grassland studies similarly pertain to warmer ecosystems with periodic water deficits. We tested these mechanisms in three important C₃ herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO₂‐induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%–50% with a 150 μmol mol^?1 increase in atmospheric CO₂ across seasons. This eCO₂‐induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily V_SWC. The highest photosynthetic enhancement by eCO₂ (>30%) was observed during the most water‐limited period, for example, with V_SWC <0.07 in this sandy surface soil. Under eCO₂ there was neither a significant decrease in g_s in the three herbaceous species, nor increases in V_SWC, indicating no “water‐savings effect” of eCO₂. Periods of low V_SWC showed lower g_s (less than ≈ 0.12 mol m^?2 s^?1), higher relative S_lim (>30%) and decreased C_i under the ambient CO₂ concentration (aCO₂), with leaf photosynthesis strongly carboxylation‐limited. The alleviation of S_lim by eCO₂ was facilitated by increasing C_i, thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO₂, controls g_s and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO₂ has the potential to alter vegetation functioning in a periodically dry woodland understory through changes in stomatal limitation to photosynthesis, not by the “water‐savings effect” usually invoked in grasslands.