Photosynthetic downregulation in leaves of the Japanese white birch grown under elevated CO2 concentration does not change their temperature‐dependent susceptibility to photoinhibition

To determine the effects of elevated CO₂ concentration ([CO₂]) on the temperature‐dependent photosynthetic properties, we measured gas exchange and chlorophyll fluorescence at various leaf temperatures (15, 20, 25, 30, 35 and 40°C) in 1‐year‐old seedlings of the Japanese white birch (Betula platyphylla var. japonica), grown in a phytotron under natural daylight at two [CO₂] levels (ambient: 400 µmol mol^?1 and elevated: 800 µmol mol^?1) and limited N availability (90 mg N plant^?1). Plants grown under elevated [CO₂] exhibited photosynthetic downregulation, indicated by a decrease in the carboxylation capacity of Rubisco. At temperatures above 30°C, the net photosynthetic rates of elevated‐CO₂‐grown plants exceeded those grown under ambient [CO₂] when compared at their growth [CO₂]. Electron transport rates were significantly lower in elevated‐CO₂‐grown plants than ambient‐CO₂‐grown ones at temperatures below 25°C. However, no significant difference was observed in the fraction of excess light energy [(1 ? q_P)× F_v′/F_m′] between CO₂ treatments across the temperature range. The quantum yield of regulated non‐photochemical energy loss was significantly higher in elevated‐CO₂‐grown plants than ambient, when compared at their respective growth [CO₂] below 25°C. These results suggest that elevated‐CO₂‐induced downregulation might not exacerbate the temperature‐dependent susceptibility to photoinhibition, because reduced energy consumption by electron transport was compensated for by increased thermal energy dissipation at low temperatures.     

Elevated CO2 and limited nitrogen nutrition can restrict excitation energy dissipation in photosystem II of Japanese white birch (Betula platyphylla var. japonica) leaves

Elevated atmospheric CO₂ concentration [CO₂] and different levels of nitrogen (N) nutrition can influence the amount of excess excitation energy in photosystem (PS) II and related photosynthetic properties. The interactive effect of two [CO₂] levels (ambient: 360 µM M⁻¹ and elevated: 720 µM M⁻¹) and two N levels (high: 700 mg N plant⁻¹ and low: 100 mg N plant⁻¹) on these properties was examined in seedlings of Japanese white birch (Betula platyphylla var. japonica) using simultaneous measurements of gas exchange and chlorophyll fluorescence. Photosynthetic acclimation to elevated [CO₂], as indicated by a decline in carboxylation efficiency (CE), was observed in plants grown at elevated [CO₂] especially under low N. Elevated [CO₂] resulted in a decrease in area-based leaf N content (N_area) irrespective of N treatment. The adverse effect of elevated [CO₂] and low N on CE may have been exacerbated by a greater accumulation of leaf sugar and starch contents in these plants leading to a lower electron transport rate (ETR). While these plants also showed higher non-photochemical quenching (Nq_P) that could offset the reduction in energy dissipation through ETR to some extent, they still have a higher risk of photoinhibition from excessive excitation energy in PSII as indicated by a decrease in photochemical quenching (q_P). However, chronic photoinhibition was not observed in plant grown at elevated [CO₂] and low N because they showed no difference in F_v/F_m (the maximum photochemical efficiency of PSII) from those grown at ambient [CO₂] and low N after an overnight dark adaptation. High levels of Nq_P in plants grown at elevated [CO₂] and low N reflect a near saturation of thermal energy dissipation. This impaired capacity of photoprotection would render these plants more vulnerable to photoinhibition in the event of additional environmental stresses such as drought, low or high temperature.     

Photosynthetic characteristics and nitrogen allocation in the black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L.) grown in a FACE system

Key message

The black locust is adapted to elevated [CO ₂ ] through changes in nitrogen allocation characteristics in leaves.

Abstract

The black locust (Robinia pseudoacacia L.) is an invasive woody legume within Japan. This prolific species has a high photosynthetic rate and growth rate, and undergoes symbiosis with N₂-fixing micro-organisms. To determine the effect of elevated CO₂ concentration [CO₂] on its photosynthetic characteristics, we studied the chlorophyll (Chl) and leaf nitrogen (N) content, and the leaf structure and N allocation patterns in the leaves and acetylene reduction activity after four growing seasons, in R. pseudoacacia. Our specimens were grown at ambient [CO₂] (370 μmol mol^?1) and at elevated [CO₂] (500 μmol mol^?1), using a free air CO₂ enrichment (FACE) system. Net photosynthetic rate at growth [CO₂] (A _growth) and acetylene reduction activity were significantly higher, but maximum carboxylation rate of RuBisCo (V _cmax), maximum rate of electron transport driving RUBP regeneration (J _max), net photosynthetic rate under enhanced CO₂ concentration and light saturation (A _max), the N concentration in leaf, and in leaf mass per unit area (LMA) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) content were significantly lower grown at elevated [CO₂] than at ambient [CO₂]. We also found that RuBisCo/N were less at elevated [CO₂], whereas Chl/N increased significantly. Allocation characteristics from N in leaves to photosynthetic proteins, N_L (Light-harvesting complex: LHC, photosystem I and II: PSI and PSII) and other proteins also changed. When R. pseudoacacia was grown at elevated [CO₂], the N allocation to RuBisCo (N_R) decreased to a greater extent but N_L and N remaining increased relative to specimens grown at ambient [CO₂]. We suggest that N remobilization from RuBisCo is more efficient than from proteins of electron transport (N_E), and from N_L. These physiological responses of the black locust are significant as being an adaptation strategy to global environmental changes.

     

Sustained Photosynthetic Performance of Coffea spp. under Long-Term Enhanced [CO2]

Coffee is one of the world’s most traded agricultural products. Modeling studies have predicted that climate change will have a strong impact on the suitability of current cultivation areas, but these studies have not anticipated possible mitigating effects of the elevated atmospheric [CO₂] because no information exists for the coffee plant. Potted plants from two genotypes of Coffea arabica and one of C. canephora were grown under controlled conditions of irradiance (800 μmol m^-2 s^-1), RH (75%) and 380 or 700 μL CO₂ L^-1 for 1 year, without water, nutrient or root development restrictions. In all genotypes, the high [CO₂] treatment promoted opposite trends for stomatal density and size, which decreased and increased, respectively. Regardless of the genotype or the growth [CO₂], the net rate of CO₂ assimilation increased (34-49%) when measured at 700 than at 380 μL CO₂ L^-1. This result, together with the almost unchanged stomatal conductance, led to an instantaneous water use efficiency increase. The results also showed a reinforcement of photosynthetic (and respiratory) components, namely thylakoid electron transport and the activities of RuBisCo, ribulose 5-phosphate kinase, malate dehydrogenase and pyruvate kinase, what may have contributed to the enhancements in the maximum rates of electron transport, carboxylation and photosynthetic capacity under elevated [CO₂], although these responses were genotype dependent. The photosystem II efficiency, energy driven to photochemical events, non-structural carbohydrates, photosynthetic pigment and membrane permeability did not respond to [CO₂] supply. Some alterations in total fatty acid content and the unsaturation level of the chloroplast membranes were noted but, apparently, did not affect photosynthetic functioning. Despite some differences among the genotypes, no clear species-dependent responses to elevated [CO₂] were observed. Overall, as no apparent sign of photosynthetic down-regulation was found, our data suggest that Coffea spp. plants may successfully cope with high [CO₂] under the present experimental conditions.     

Effects of Potato Virus YNTN Infection on Gas Exchange and Photosystem 2 Function in Leaves of Solanum tuberosum L.

Photosynthetic responses of potato (Solanum tuberosum L. cv. Chunzao) were examined during potato virus Y (PVY^NTN) infection. PVY^NTN infection significantly reduced net photosynthetic rate and stomatal conductance, but had little influence on intercellular CO₂ concentration. As the disease developed, the maximum carboxylation velocity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the maximum electron transport rate contributing to ribulose-1,5-bisphosphate regeneration gradually decreased, followed by substantial reductions in the relative quantum efficiency of photosystem 2 (PS2) electron transport, the efficiency of excitation energy capture by open PS2 reaction centres, and photochemical quenching, but not in sustained photoinhibition. Thus PVY^NTN depressed photosynthesis mainly by interfering with the enzymatic processes in the Calvin cycle which resulted in a down-regulation of electron transport.     

Interactive effects of elevated CO2 and drought on photosynthetic capacity and PSII performance in maize

Elevated atmospheric CO₂ concentration [CO₂] and the change of water distribution in arid and semiarid areas affect plant physiology and ecosystem processes. The interaction of elevated [CO₂] and drought results in the complex response such as changes in the energy flux of photosynthesis. The performance of photosystem (PS) II and the electron transport were evaluated by using OJIP induction curves of chlorophyll a fluorescence and the P _N-C _i curves in the two-factor controlled experiment with [CO₂] of 380 (AC) or 750 (EC) [μmol mol^?1] and water stress by 10% polyethylene glycol 6000. Compared to water-stressed maize (Zea mays L.) under AC, the EC treatment combined with water stress decreased the number of active reaction centers but it increased the antenna size and the energy flux (absorbed photon flux, trapping flux, and electron transport flux) of each reaction center in PSII. Thus, the electron transport rate was enhanced, despite the indistinctively changed quantum yield of the electron transport and energy dissipation. The combination of EC and the water-stress treatment resulted in the robust carboxylation rate without elevating the saturated photosynthetic rate (P _max). This study demonstrated that maize was capable of transporting more electrons into the carboxylation reaction, but this could not be used to increase P _max under EC.     

高大气CO2浓度下氮素对小麦叶片光能利用的影响

关于氮素对高大气CO₂浓度下C₃植物光合作用适应现象的调节机理已有较为深入的研究, 但对其光合作用适应现象的光合能量转化和分配机制缺乏系统分析。该文以大气CO₂浓度和施氮量为处理手段, 通过测定小麦(Triticum aestivum)抽穗期叶片的光合作用-胞间CO₂浓度响应曲线以及荧光动力学参数来测算光合电子传递速率和分配去向, 研究了长期高大气CO₂浓度下小麦叶片光合电子传递和分配对施氮量的响应。结果表明, 与正常大气CO₂浓度处理相比, 高大气CO₂浓度下小麦叶片较多的激发能以热量的形式耗散, 增施氮素可使更多的激发能向光化学反应方向的分配, 降低光合能量的热耗散速率; 大气CO₂浓度升高后小麦叶片光化学淬灭系数无明显变化, 高氮叶片的非光化学猝灭降低而低氮叶片明显升高, 施氮促进PSII反应中心的开放比例, 降低光能的热耗散; 高大气CO₂浓度下高氮叶片通过PSII反应中心的光合电子传递速率(J_F)较高, 而且参与光呼吸的非环式电子流速率(J₀)显著降低, 较正常大气CO₂浓度处理的高氮叶片下降了88.40%, 光合速率增加46.47%; 高大气CO₂浓度下小麦叶片J_F-J₀升高而J₀/J_F显著下降, 光呼吸耗能被抑制, 更多的光合电子分配至光合还原过程。因此, 大气CO₂浓度增高条件下, 小麦叶片激发能的热耗散速率增加, 但增施氮素后小麦叶片PSII反应中心开放比例提高, 光化学速率增加, 进入PSII反应中心的电子流速率明显升高, 光呼吸作用被抑制, 光合电子较多地进入光化学过程, 这可能是高氮条件下光合作用适应性下调被缓解的一个原因。     

Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida

To evaluate whether leaf spot disease and related effects on photosynthesis are influenced by increased nitrogen (N) input and elevated atmospheric CO₂ concentration ([CO₂]), we examined disease incidence and photosynthetic rate of Solidago rigida grown in monoculture under ambient or elevated (560 μmol mol⁻¹) [CO₂] and ambient or elevated (+4 g N m⁻² year⁻¹) N conditions in a field experiment in Minnesota, USA. Disease incidence was lower in plots with either elevated [CO₂] or enriched N (−57 and −37%, respectively) than in plots with ambient conditions. Elevated [CO₂] had no significant effect on total plant biomass, or on photosynthetic rate, but reduced tissue%N by 13%. In contrast, N fertilization increased both biomass and total plant N by 70%, and as a consequence tissue%N was unaffected and photosynthetic rate was lower on N fertilized plants than on unfertilized plants. Regardless of treatment, photosynthetic rate was reduced on leaves with disease symptoms. On average across all treatments, asymptomatic leaf tissue on diseased leaves had 53% lower photosynthetic rate than non-diseased leaves, indicating that the negative effect from the disease extended beyond the visual lesion area. Our results show that, in this instance, indirect effects from elevated [CO₂], i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher [CO₂].     

The photosynthetic irradiance-response of Norway spruce exposed to a long-term elevation of CO2 concentration

During an open-top chamber experiment performed in a mountain stand of young (12-year-old) Norway spruce (Picea abies [L.] Karst.), the trees were exposed to one of two CO² concentrations (ambient CO², AC, or AC + 350 μmol mol-1 = elevated CO₂, EC) continuously over three growing seasons. To evaluate the EC influence, measurements of the relations between the rate of net CO₂ uptake (P _N ) and incidental photosynthetically active photon flux density (PPFD), as well as the content of photosynthetic pigments and chlorophyll (Chl) a fluorescence were taken in the third growing season. The short-term response to EC was evident mainly on ribulose-1,5-bisphosphate carboxylase/oxygenase kinetics without any significant change to the utilization of radiant energy. The long-term effect of EC was responsible for a decrease in P _N , content of Chl a + b, Fv/Fm ratio, quantum yield of fluorescence, and photochemical quenching. Changes of stoichiometry between the electron transport, Calvin cycle and the end-product synthesis were confirmed for responses to the long-term import of EC and led to a definition of the photosynthetic acclimation to EC in Norway spruce. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrogen assimilation and transpiration: key processes conditioning responsiveness of wheat to elevated [CO2] and temperature

Although climate scenarios have predicted an increase in [CO₂] and temperature conditions, to date few experiments have focused on the interaction of [CO₂] and temperature effects in wheat development. Recent evidence suggests that photosynthetic acclimation is linked to the photorespiration and N assimilation inhibition of plants exposed to elevated CO₂. The main goal of this study was to analyze the effect of interacting [CO₂] and temperature on leaf photorespiration, C/N metabolism and N transport in wheat plants exposed to elevated [CO₂] and temperature conditions. For this purpose, wheat plants were exposed to elevated [CO₂] (400 vs 700 µmol mol^?1) and temperature (ambient vs ambient + 4°C) in CO₂ gradient greenhouses during the entire life cycle. Although at the agronomic level, elevated temperature had no effect on plant biomass, physiological analyses revealed that combined elevated [CO₂] and temperature negatively affected photosynthetic performance. The limited energy levels resulting from the reduced respiratory and photorespiration rates of such plants were apparently inadequate to sustain nitrate reductase activity. Inhibited N assimilation was associated with a strong reduction in amino acid content, conditioned leaf soluble protein content and constrained leaf N status. Therefore, the plant response to elevated [CO₂] and elevated temperature resulted in photosynthetic acclimation. The reduction in transpiration rates induced limitations in nutrient transport in leaves of plants exposed to elevated [CO₂] and temperature, led to mineral depletion and therefore contributed to the inhibition of photosynthetic activity.     

Elevated CO<Subscript>2</Subscript> reduces stomatal and metabolic limitations on photosynthesis caused by salinity in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

The future environment may be altered by high concentrations of salt in the soil and elevated [CO₂] in the atmosphere. These have opposite effects on photosynthesis. Generally, salt stress inhibits photosynthesis by stomatal and non-stomatal mechanisms; in contrast, elevated [CO₂] stimulates photosynthesis by increasing CO₂ availability in the Rubisco carboxylating site and by reducing photorespiration. However, few studies have focused on the interactive effects of these factors on photosynthesis. To elucidate this knowledge gap, we grew the barley plant, Hordeum vulgare (cv. Iranis), with and without salt stress at either ambient or elevated atmospheric [CO₂] (350 or 700 μmol mol⁻¹ CO₂, respectively). We measured growth, several photosynthetic and fluorescence parameters, and carbohydrate content. Under saline conditions, the photosynthetic rate decreased, mostly because of stomatal limitations. Increasing salinity progressively increased metabolic (photochemical and biochemical) limitation; this included an increase in non-photochemical quenching and a reduction in the PSII quantum yield. When salinity was combined with elevated CO₂, the rate of CO₂ diffusion to the carboxylating site increased, despite lower stomatal and internal conductance. The greater CO₂ availability increased the electron sink capacity, which alleviated the salt-induced metabolic limitations on the photosynthetic rate. Consequently, elevated CO₂ partially mitigated the saline effects on photosynthesis by maintaining favorable biochemistry and photochemistry in barley leaves.     

Mitochondrial uncoupling protein silence is compromised in photosynthesis and redox poise

Mitochondrial uncoupling proteins play important roles in generation of metabolic thermogenesis, response to stress situations, and regulation of energy metabolism. We demonstrated here that the absence of LeUCP in tomato inhibited photosynthesis using virus-induced gene-silencing approach. A significant decrease in the rate of CO₂ assimilation in LeUCP-silencing plants was observed over a range of different light intensities. Absence of LeUCP resulted in lower net photosynthetic rate, light-saturated rate of the CO₂ assimilation (A _sat), maximum carboxylation rates (V _cmax) and maximum RuBP regeneration rate (J _max). Activities of ribulose-1,5-bisphosphate carboxylase/oxygenase Rubisco and stromal fructose-1,6-bisphosphatase and genes expression levels encoded Calvin cycle enzymes of LeUCP gene-silencing plants were inhibited. However, silencing of LeUCP gene had no effect on F _v/F _m, but decreased photochemical quenching and electron transport rate. Meanwhile, non-photochemical quenching and Je (PSII), the distribution of Je (PCR) and Je (PCO), the content of AsA, NAD, and the ratios of NAD⁺/NADH, AsA/DHA were significantly reduced with increased reactive oxygen species while GSH and GSSG were unaltered. Taken together, these results strongly suggest that LeUCP gene in tomato leaves is crucial in maintaining the redox poise of the mitochondrial electron transport chain to facilitate photosynthetic metabolism.     

Limiting Factors in Photosynthesis: VI. Regeneration of Ribulose 1,5-Bisphosphate Limits Photosynthesis at Low Photochemical Capacity

Earlier work (SE Taylor, N Terry [1984] Plant Physiol 75: 82-86) has shown that the rate of photosynthesis may be colimited by photosynthetic electron transport capacity, even at low intercellular CO₂ concentrations. Here we monitored leaf metabolites diurnally and the activities of key Calvin cycle enzymes in the leaves of three treatment groups of sugar beet (Beta vulgaris L.) plants representing three different in vivo photochemical capacities, i.e. Fe-sufficient (control) plants, moderately Fe-deficient, and severely Fe-deficient plants. The results show that the decrease in photosynthesis with Fe deficiency mediated reduction in photochemical capacity was through a reduction in ribulose 1,5-bisphosphate (RuBP) regeneration and not through a decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Based on measurements of ATP and NADPH and triose phosphate/3-phosphoglycerate ratios in leaves, there was little evidence that photosynthesis and RuBP regeneration in Fe-deficient leaves were limited directly by the supply of ATP and NADPH. It appeared more likely that photochemical capacity influenced RuBP regeneration through modulation of enzymes in the photosynthetic carbon reduction cycle between fructose-6-phosphate and RuBP; in particular, the initial activity of ribulose-5-phosphate kinase was strongly diminished by Fe deficiency. Starch and sucrose levels changed independently of one another to some extent during the diurnal period (both increasing in the day and decreasing at night) but the average rates of starch or sucrose accumulation over the light period were each proportional to photochemical capacity and photosynthetic rate.     

<Emphasis Type="Italic">Artemisia frigida</Emphasis> and <Emphasis Type="Italic">Stipa krylovii</Emphasis>, two dominant species in Inner Mongolia steppe,differed in their responses to elevated atmospheric CO<Subscript>2</Subscript> concentration

Aims

Despite extensive studies on effects of elevated CO₂ concentration ([CO₂]_e) on plant growth, few studies have investigated the responses of native grassland plant species to [CO₂]_e in terms of nutrient acquisition.

Methods

The effects of [CO₂]_e (769 ± 23 ppm) on Artemisia frigida and Stipa krylovii, two dominant species in Inner Mongolia steppe were investigated by growing them for 7 weeks in Open-Top Chambers (OTC).

Results

Exposure to [CO₂]_e enhanced shoot and root growth of A. frigida and S. krylovii. Elevated [CO₂] increased photosynthetic rates (Pn) by 34 % in A. frigida but decreased Pn by 52 % in S. krylovii. Moreover, root-secreted acid phosphatase activity in A. frigida was stimulated by [CO₂]_e, while exudation of malate from roots of S. krylovii was suppressed by [CO₂]_e. Exposure to [CO₂]_e led to a decrease in P concentration in shoots and roots of A. frigida and S. krylovii, but total amount of P accumulated in shoots and roots of both species was increased by [CO₂]_e.

Conclusions

The two dominant species in temperate steppes differed in their responses to [CO₂]_e, such that A. frigida was more adapted to [CO₂]_e than S. krylovii under low availability of soil P.

     

Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur

The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO₂ concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m^?2 s^?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO₂ (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO₂ enhanced photosynthesis but leaf respiration rates were not affected by either the CO₂ or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO₂, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO₂. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO₂ and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.     

Seasonal Changes of Selected Parameters of CO2 Fixation Biochemistry of Norway Spruce Under the Long-Term Impact of Elevated CO2

Twelve-year-old Norway spruce (Picea abies [L.] Karst.) trees were exposed to ambient (AC) or elevated (EC) [ambient + 350 µmol(CO₂) mol^-1] CO₂ concentrations in open-top-chamber (OTC) experiment under the field conditions of a mountain stand. Short-term (4 weeks, beginning of the vegetation season) and long-term (4 growing seasons, end of the vegetation season) effects of this treatment on biochemical parameters of CO₂ assimilation were evaluated. A combination of gas exchange, fluorescence of chlorophyll a, and application of a mathematical model of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity was used. The analysis showed that the depression of photosynthetic activity by long-term impact of elevated CO₂ was mainly caused by decreased RuBPCO carboxylation rate. The electron transport rate as well as the rate of ribulose-1,5-bisphosphate (RuBP) formation were also modified. These modifications to photosynthetic assimilation depended on time during the growing season. Changes in the spring were caused mainly by local deficiency of nitrogen in the assimilating tissue. However, the strong depression of assimilation observed in the autumn months was the result of insufficient carbon sink capacity.     

Photosynthetic characteristics of diploid honeysuckle (<Emphasis Type="Italic">Lonicera japonica</Emphasis> Thunb.) and its autotetraploid cultivar subjected to elevated ozone exposure

In order to investigate the effect of chromosome doubling on ozone tolerance, we compared the physiological responses of a diploid honeysuckle (Lonicera japonica Thunb.) and its autotetraploid cultivar to elevated ozone (O₃) exposure (70 ng g⁻¹, 7 h d⁻¹ for 31 d). Net photosynthetic rate (P _N) of both cultivars were drastically (P<0.01) impaired by O₃. Although there were significantly positive correlation between P _N and stomatal conductance (g _s) in both cultivars under each treatment, the decreased g _s in O₃ might be the result rather than the cause of decreased P _N as indicated by stable or increasing the ratio of intercellular to ambient CO₂ concentration(C _i/C _a). P _N under saturating CO₂ concentration (P _Nsat) and carboxylation efficiency (CE) significantly decreased under O₃ fumigation, which indicated the Calvin cycle was impaired. O₃ also inhibited the maximum efficiency of photosystem II (PSII) photochemistry in the dark-adapted state (F_v/F_m), actual quantum yield of PSII photochemistry (Φ_PSII), electron transport rate (ETR), photochemical quenching coefficient (qP), non-photochemical quenching (NPQ), the maximum in vivo rate of Rubisco carboxylation (V_cmax) and the maximal photosynthetic electron transport rate (J_max) which demonstrated that the decrease in P _N of the honeysuckle exposed to elevated O₃ was probably not only due to impairment of Calvin cycle but also with respect to the light-harvesting and electron transport processes. Compared to the diploid, the tetraploid had higher relative loss in transpiration rate (E), (g _s), (P _Nsat), V_cmax and J_max. This result indicated that the Calvin cycle and electron transport in tetraploid was damaged more seriously than in diploid. A barely nonsignificant (P=0.086) interaction between O₃ and cultivar on P _N suggested a higher photosynthetic sensitivity of the tetraploid cultivar.     

Interactive effects of nitrogen and phosphorus on the acclimation potential of foliage photosynthetic properties of cork oak,Quercus suber,to elevated atmospheric CO2 concentrations

Leaf gas-exchange and chemical composition were investigated in seedlings of Quercus suber L. grown for 21 months either at elevated (700 μmol mol^–1) or normal (350 μmol mol^–1) ambient atmospheric CO₂ concentrations, [CO₂], in a sandy nutrient-poor soil with either ‘high’ N (0.3 mol N m^–3 in the irrigation solution) or with ‘low’ N (0.05 mol N m^–3) and with a constant suboptimal concentration of the other macro- and micronutrients. Although elevated [CO₂] yielded the greatest total plant biomass in ‘high’ nitrogen treatment, it resulted in lower leaf nutrient concentrations in all cases, independent of the nutrient addition regime, and in greater nonstructural carbohydrate concentrations. By contrast, nitrogen treatment did not affect foliar N concentrations, but resulted in lower phosphorus concentrations, suggesting that under lower N, P use-efficiency in foliar biomass production was lower. Phosphorus deficiency was evident in all treatments, as photosynthesis became CO₂ insensitive at intercellular CO₂ concentrations larger than ≈ 300 μmol mol^–1, and net assimilation rates measured at an ambient [CO₂] of 350 μmol mol^–1 or at 700 μmol mol^–1 were not significantly different. Moreover, there was a positive correlation of foliar P with maximum Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) carboxylase activity (V_cmax), which potentially limits photosynthesis at low [CO₂], and the capacities of photosynthetic electron transport (J_max) and phosphate utilization (P_max), which are potentially limiting at high [CO₂]. None of these potential limits was correlated with foliar nitrogen concentration, indicating that photosynthetic N use-efficiency was directly dependent on foliar P availability. Though the tendencies were towards lower capacities of potential limitations of photosynthesis in high [CO₂] grown specimens, the effects were statistically insignificant, because of (i) large within-treatment variability related to foliar P, and (ii) small decreases in P/N ratio with increasing [CO₂], resulting in balanced changes in other foliar compounds potentially limiting carbon acquisition. The results of the current study indicate that under P-deficiency, the down-regulation of excess biochemical capacities proceeds in a similar manner in leaves grown under normal and elevated [CO₂], and also that foliar P/N ratios for optimum photosynthesis are likely to increase with increasing growth CO₂ concentrations. Symbols: A, net assimilation rate (μmol m^–2 s^–1); A_max, light-saturated A (μmol m^–2 s^–1); α, initial quantum yield at saturating [CO₂] and for an incident Q (mol mol^–1); [CO₂], atmospheric CO₂ concentration (μmol mol^–1); C_i, intercellular CO₂ concentration (μmol mol^–1); C_a, CO₂ concentration in the gas-exchange cuvette (μmol mol^–1); F_B, fraction of leaf N in ‘photoenergetics’; F_L, fraction of leaf N in light harvesting; F_R, fraction of leaf N in Rubisco; Γ*, CO₂ compensation concentration in the absence of R_d (μmol mol^–1); J_max*, capacity for photosynthetic electron transport; J_mc, capacity for photosynthetic electron transport per unit cytochrome f (mol e^–[mol cyt f]^–1 s^–1); K_c, Michaelis-Menten constant for carboxylation (μmol mol^–1); K_o, Michaelis-Menten constant for oxygenation (mmol mol^–1); M_A, leaf dry mass per area (g m^–2); O, intercellular oxygen concentration (mmol mol^–1); [P_i], concentration of inorganic phosphate (mM); P_max*, capacity for phosphate utilization; Q, photosynthetically active quantum flux density (μmol m^–2 s^–1); R_d*, day respiration (CO₂ evolution from nonphotorespiratory processes continuing in the light); Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; RUBP, ribulose-1,5-bisphosphate; T_l, leaf temperature (°C); U_TPU*, rate of triose phosphate utilization; V_cmax*, maximum Rubisco carboxylase activity; V_cr, specific activity of Rubisco (μmol CO₂[g Rubisco]^–1 s^–1] *given in either μmol m^–2 s^–1 or in μmol g^–1 s^–1 as described in the text.     

Seedling survival in a northern temperate forest understory is increased by elevated atmospheric carbon dioxide and atmospheric nitrogen deposition

We tested the main and interactive effects of elevated carbon dioxide concentration ([CO₂]), nitrogen (N), and light availability on leaf photosynthesis, and plant growth and survival in understory seedlings grown in an N‐limited northern hardwood forest. For two growing seasons, we exposed six species of tree seedlings (Betula papyrifera, Populus tremuloides, Acer saccharum, Fagus grandifolia, Pinus strobus, and Prunus serotina) to a factorial combination of atmospheric CO₂ (ambient, and elevated CO₂ at 658 μmol CO₂ mol⁻¹) and N deposition (ambient and ambient +30 kg N ha⁻¹ yr⁻¹) in open‐top chambers placed in an understory light gradient. Elevated CO₂ exposure significantly increased apparent quantum efficiency of electron transport by 41% (P<0.0001), light‐limited photosynthesis by 47% (P<0.0001), and light‐saturated photosynthesis by 60% (P<0.003) compared with seedlings grown in ambient [CO₂]. Experimental N deposition significantly increased light‐limited photosynthesis as light availability increased (P<0.037). Species differed in the magnitude of light‐saturated photosynthetic response to elevated N and light treatments (P<0.016). Elevated CO₂ exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability (R²=0.26, P<0.0001). Experimental N deposition significantly increased average survival of all species by 48% (P<0.012). However, seedling survival was greatest (85%) under conditions of both high [CO₂] and N deposition (P<0.009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass (R²=0.39, P<0.001), and that carboxylation capacity (V_cmax) was a better predictor for seedling growth and survival than maximum photosynthetic rate (A_max). Our results suggest that increasing [CO₂] and N deposition from fossil fuel combustion could alter understory tree species recruitment dynamics through changes in seedling survival, and this has the potential to alter future forest species composition.