光呼吸条件下栾树和辣椒光合电子流的分配研究

用Li-6400光合仪同时测定了栾树和辣椒在温度为30℃、CO2浓度为380μmol·mol-1下的气体交换和叶绿素荧光数据。结果表明,栾树在饱和光合有效辐射时光合电子用于碳同化、光呼吸和其他途径的量分别为72.68、45.68和29.40μmol·m-2.s-1;辣椒在光合有效辐射为2000μmol·m-2.s-1时光合电子用于碳同化、光呼吸和其他途径的量分别为142.24、40.24和131.52μmol·m-2.s-1。揭示了在光呼吸条件下用Valentini和Epron等方法高估了辣椒和栾树的光合电子用于光呼吸的量,同时也高估了光呼吸在辣椒和栾树中的保护作用。     

Arundo donax L. (Poaceae) — a C3 Species with Unusually High Photosynthetic Capacity

Gas exchange characteristics, chlorophyll a fluorescence and leaf water potential were investigated in the giant reed, Arundo donax, under natural conditions in an estuarine mangrove swamp in Durban, South Africa. Maximum photosynthetic CO₂ uptake ranged between 19.8 and 36.7 μmol m^?2 s^?1, depending on irradiance, and appeared to be regulated by leaf conductance. There was no saturation of CO₂ uptake or electron transport through PSII (ETR) with increasing irradiance up to 2500 μmol photons m^?2 s^?1. A linear relationship between CO₂ uptake, corrected for respiration (A), and ETR has only been reported for C₄ species and C₃ species when photorespiration is eliminated. From this relationship, it was calculated that 8.5 electrons were transported through PSII for the fixation of one mole of CO₂. Predawn leaf water potential was about ?0.5 MPa and decreased to ?1.5 MPa on a cloudy day and to ?2.1 MPa on a clear day. Diurnal change in leaf water potential had little influence on leaf conductance and hence CO₂ uptake. The molar water use efficiency (WUE) ranged between 4.1 and 9.3 μmol mmol^?1. Percentage photorespiration was between 36 and 39%.     

Ecophysiological characteristics of two carrot (Daucus carota L.) cultivars in response to agroecological factors and nitrogen application

Plant density, planting time, harvest timing, and nitrogen influence on short-term gas-exchange properties of carrot cultivars, Topcut and Sugarsnax (Daucus carota L.) were investigated under field conditions. Net photosynthetic rate (P _N), stomatal conductance (g _s), and transpiration rate (E) differed significantly with the cultivars studied. Both planting and harvest timing changed the midday P _N rates. P _N increased as harvest timing advanced regardless of planting time. Late planting combined with late harvesting registered the maximum P _N rates (4.5 ??mol m^?2 s^?1). The water-use efficiency (WUE) was altered by temperature at different harvest timings along with the choice of cultivar. Early harvested Sugarsnax had a higher WUE (2.29 mmol mol^?1) than TopCut (1.64 mmol mol^?1) as Sugarsnax exhibited more stomatal conductance than TopCut. These changes were principally governed by fluctuations observed with air temperature and photosynthetic photon flux density (PPFD) and altered by the sensitivity of the cultivars to ecological factors. Plant density did not affect the photosynthetic gas-exchange parameters. Our results suggest that carrots manage high population density solely through morphological adaptations with no photosynthetic adjustments. Carrot leaves responded to N application in a curvilinear fashion in both cultivars. N did not alter g _s, E, or WUE in carrots. N, applied at a rate of 150 kg N ha^?1, increased foliar N up to 2.98%. We conclude that 2.98% of foliar N is sufficient to achieve the maximum photosynthetic rates in processing carrots.     

A model of isoprene emission based on energetic requirements for isoprene synthesis and leaf photosynthetic properties for Liquidambar and Quercus

We present a physiological model of isoprene (2-methyl-1,3-butadiene) emission which considers the cost for isoprene synthesis, and the production of reductive equivalents in reactions of photosynthetic electron transport for Liquidambar styraciflua L. and for North American and European deciduous temperate Quercus species. In the model, we differentiate between leaf morphology (leaf dry mass per area, M_A, g m ^{? 2}) altering the content of enzymes of isoprene synthesis pathway per unit leaf area, and biochemical potentials of average leaf cells determining their capacity for isoprene emission. Isoprene emission rate per unit leaf area ( μ mol m ^{? 2} s ^{? 1}) is calculated as the product of M_A, the fraction of total electron flow used for isoprene synthesis ( ? , mol mol ^{? 1}), the rate of photosynthetic electron transport (J) per unit leaf dry mass (J_m, μ mol g ^{? 1} s ^{? 1}), and the reciprocal of the electron cost of isoprene synthesis [mol isoprene (mol electrons ^{? 1})]. The initial estimate of electron cost of isoprene synthesis is calculated according to the 1-deoxy- D -xylulose-5-phosphate pathway recently discovered in the chloroplasts, and is further modified to account for extra electron requirements because of photorespiration. The rate of photosynthetic electron transport is calculated by a process-based leaf photosynthesis model. A satisfactory fit to the light-dependence of isoprene emission is obtained using the light response curve of J, and a single value of ? , that is dependent on the isoprene synthase activity in the leaves. Temperature dependence of isoprene emission is obtained by combining the temperature response curves of photosynthetic electron transport, the shape of which is related to long-term temperature during leaf growth and development, and the specific activity of isoprene synthase, which is considered as essentially constant for all plants. The results of simulations demonstrate that the variety of temperature responses of isoprene emission observed within and among the species in previous studies may be explained by different optimum temperatures of J and/or limited maximum fraction of electrons used for isoprene synthesis. The model provides good fits to diurnal courses of field measurements of isoprene emission, and is also able to describe the changes in isoprene emission under stress conditions, for example, the decline in isoprene emission in water-stressed leaves.     

High Light Intensity Augments Mercury Toxicity in Cyanobacterium Nostoc muscorum

The present study is aimed at investigating the role of growth irradiance in determining the extent of mercury (Hg) toxicity on various physiological parameters viz. growth, pigment contents, photosynthesis, respiration, ¹⁴CO₂ fixation, photosynthetic electron transport, photorespiration and enzyme activity of cyanobacterium Nostoc muscorum. A general decline was observed in all these parameters with increasing concentration of Hg except for carotenoids content and respiratory activity which exhibited significant enhancement. This effect was more pronounced in high light (130???mol photon m^?2?s^?1) exposed cells as compared to normal (70???mol photon m^?2?s^?1) and low (10???mol photon m^?2?s^?1) light exposed cells. Among the photosynthetic electron transport activities, whole chain was found to be more sensitive than photosystem II (PSII) and photosystem I (PSI). ¹⁴CO₂ fixation was more affected as compared to O₂ evolution when exposed to Hg and different light intensities. Photorespiratory activity, which is an index of protecting organisms from light-induced damage, also showed a similar declining trend. Enzyme assay revealed that among the carboxylating enzymes, activity of RUBISCO was more severely inhibited than PEPCase. Thus, these results suggest that Hg itself was toxic at all tested concentrations and high light intensity augmented its toxicity in N. muscorum inhibiting the growth, pigment contents and photosynthetic activity of the organism.     

Low temperature stress modifies the photochemical efficiency of a tropical tree species <Emphasis Type="Italic">Hevea brasiliensis</Emphasis>: effects of varying concentration of CO<Subscript>2</Subscript> and photon flux density

Two clones of Hevea brasiliensis (RRII 105 and PB 235) were grown for one year in two distinct agroclimatic locations (warmer and colder, W and C) in peninsular India. We simultaneously measured gas exchange and chlorophyll (Chl) fluorescence on fully mature intact leaves at different photosynthetic photon flux densities (PPFDs) and ambient CO₂ concentrations (C _a) and at constant ambient O₂ concentration (21 %). Net photosynthetic rate (P _N), apparent quantum yield for CO₂ assimilation (Φ_c), in vivo carboxylation efficiency (CE), and photosystem 2 quantum yield (Φ_PS2) were low in plants grown in C climate and these reductions were more predominant in RRII 105 than in PB 235 which was also reflected in their growth. We estimated in these clones the partitioning of photosynthetic electrons between CO₂ reduction (J_A) and processes other than CO₂ reduction (J^*) at low and high PPFDs and C _a. At high C _a (700 µmol mol⁻¹) most of the photosynthetic electrons were used for CO₂ assimilation and negligible amount went for other processes when PPFD was low (200–300 µmol m⁻² s⁻¹) both in the C and W climates. But at high PPFD (900-1 100 µmol m⁻² s⁻¹), J^* was appreciably high even at a high C _a. Hence at normal ambient C _a and high irradiance, electrons can be generated in the photosynthetic apparatus far in excess of what can be safely utilised for photosynthetic CO₂ reduction. However, at high C _a there was increased diversion of electrons to photosynthetic CO₂ reduction which resulted in improved photosynthetic parameters even in plants grown in C climate.     

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.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.     

Photosynthetic characteristics of two Cylindrospermopsis raciborskii strains differing in their toxicity

We studied the growth and photosynthetic characteristics of a toxic (CS506) and a nontoxic strain (CS509) of the bloom‐forming cyanobacterium Cylindrospermopsis raciborskii grown under identical experimental conditions. When exposed to light‐saturating growth conditions (100 μmol photons · m^?2 · s^?1), values for maximal photosynthetic capacity (P_max) and maximum quantum yield (F_v/F_m) indicated that both strains had an equal ability to process captured photons and deliver them to PSII reaction centers. However, CS506 grew faster than CS509. This was consistent with its higher light requirement for saturation of photosynthesis (I_k). Greater shade tolerance of CS509 was indicated by its higher ability to harvest light (α), lower photosynthetic light compensation point (I_c), and higher chlorophyll a to biovolume ratio. Strain‐specific differences were found in relation to non‐photochemical quenching, effective absorption cross‐sectional area of PSIIα‐centers (σPSIIα), and the antenna connectivity parameter of PSIIα (J_conPSIIα). These findings highlighted differences in the transfer of excitation from phycobilisome/PSII to PSI, on the dependence on different pigments for light harvesting and on the functioning of the PSII reaction centers between the two strains. The results of this study showed that both performance and composition of the photosynthetic apparatus are different between these strains, though with only two strains examined we cannot attribute the performance of strain 506 to its ability to produce cylindrospermopsins. The emphasis on a strain‐specific light adaptation/acclimation is crucial to our understanding of how different light conditions (both quantity and quality) can trigger the occurrence of different C. raciborskii strains and control their competition and/or dominance in natural ecosystems.     

Monitoring moderate Cu and Cd toxicity by chlorophyll fluorescence and P700 absorbance in pea leaves

We investigated the effect of moderate Cu²⁺ and Cd²⁺ stress by applying chlorophyll (Chl) fluorescence and P₇₀₀ absorbance measurements to monitor the photosynthetic electron transport activity of 3-week-old Pisum sativum L. cv. Petit Proven?al plants grown in a modified Hoagland solution containing 50 ??M CuSO₄ or 5 ??M CdCl₂. Both heavy metals caused a slight inhibition in PSII photochemistry as indicated by the decrease in the effective quantum efficiency of PSII (??_PSII), the maximum electron transport capacity (ETR_max), and the maximum quantum yield for electron transport (??). PSI photochemistry was also affected by these heavy metals. Cu²⁺ and Cd²⁺ decreased the quantum efficiency of PSI (??_PSI) as well as the number of electrons in the intersystem chain, and the Cu²⁺ treatment significantly reduced the number of electrons from stromal donors available for PSI. These results indicate that PSII and PSI photochemistry of pea plants are both sensitive to moderate Cu²⁺ and Cd²⁺ stress, which in turn is easily detected and monitored by Chl fluorescence and P₇₀₀ absorbance measurements. Therefore, monitoring the photochemistry of pea plants with these noninvasive, yet sensitive techniques offers a promising strategy to study heavy metal toxicity in the environment.     

Effects of low irradiation on photosynthesis and antioxidant enzyme activities in cucumber during ripening stage

In order to investigate the effects of low irradiation (LI) on cucumber (Cucumis sativus L. cv. Jinyou 35) during a ripening stage, our experiment was carried out in a climate chamber. Two levels of PAR were set for plants: normal irradiation [NI, 600 μmol(photon) m^?2 s^?1] and low irradiation [LI, 100 μmol(photon) m^?2 s^?1], respectively. The experiments lasted for 9 d; then both groups of plants were transferred under NI to recover for 16 d. The plants showed severe chlorosis after the LI treatment. Chlorophyll (Chl) a, initial slope, photosynthetic rate at saturating irradiation (P_max), light saturation point, maximal photochemical efficiency of PSII (F_v/F_m), electron transport rate of PSII (ETR), soluble protein content, and catalase (CAT) activity in cucumber leaves decreased under LI stress, while Chl b, carotenoids, light compensation point, nonphotochemical quenching (q_N), superoxide dismutase (SOD), and malondialdehyde (MDA) exhibited an increasing trend under LI. After 16 d of recovery, values of P_max, F_v/F_m, ETR, q_N, SOD, CAT, MDA, and soluble protein were close to those of the control after one, three, and five days of the LI treatment, while those kept under LI for 7 and 9 d could not return to the control level. Therefore, 7 d of LI stress was a meteorological disaster index for LI in cucumber at the fruit stage.     

Photosynthesis and photoprotection in mangroves under field conditions

Net CO₂ exchange and in vivo chlorophyll fluorescence were studied in mangrove (Rhizophora stylosa) leaves at a field site in Western Australia, and leaf samples were collected for the analysis of enzymes and substrates potentially involved in anti-oxidant photoprotection. Photosynthesis saturated at 900 μmol quanta m^?2 s^?1 and at no more than 7.5 μmol CO₂ m^?2 s^?1. However, fluorescence analysis indicated no chronic photoinhibition: F_v:F_m was 0.8 shortly after sunset, and quantum efficiencies of PSII were high up to 500 μmol quanta m^?2 s^?1. Electron flow through PSII was more than 3 times higher than electron consumption through Calvin cycle activity, however, even with photorespiration and temperature-dependent Rubisco specificities taken into account. Acknowledging the growing body of literature attributing a role to antioxidant systems in photoprotection, we also assayed the activities of superoxide dismutase (SOD) and several enzymes potentially involved in H₂O₂ metabolism. Their levels of maximal potential activity were compared with those in greenhouse-grown mangroves (R. mangle), and growth chamber-grown peas. Monodehydroascorbate reductase activities were similar in all species, and glutathione reductase was lower, and ascorbate peroxidase ～40% higher, in the mangroves. SOD activities in field-grown mangroves were more than 40 times those in peas. Our results support the hypothesis that O₂ may be a significant sink for photochemically derived electrons under field conditions, and suggest an important role for O₂^? scavenging in photoprotection. However, when relative patterns are compared between species, imbalances between SOD and the other enzymes in the mangroves suggest that more components of the system (e.g. phenolics or peroxidases) are yet to be identified.     

Effects of light intensity and temperature on the photosynthetic irradiance response curves and chlorophyll fluorescence in three picocyanobacterial strains of Synechococcus

Chrococcoid cyanobacteria of the genus Synechococcus are the important component of marine and freshwater ecosystems. Picocyanobacteria comprise even 80% of total cyanobacterial biomass and contribute to 50% of total primary cyanobacterial bloom production. Chlorophyll (Chl) fluorescence and photosynthetic light response (P-I) curves are commonly used to characterize photoacclimation of Synechococcus strains. Three brackish, picocyanobacterial strains of Synechococcus (BA-132, BA-124, BA-120) were studied. They were grown under 4 irradiances [10, 55, 100, and 145 μmol(photon) m^?2 s^?1] and at 3 temperatures (15, 22.5, and 30°C). Photosynthetic rate was measured by Clark oxygen electrode, whereas the Chl fluorescence was measured using Pulse Amplitude Modulation fluorometer. Based on P-I, two mechanisms of photoacclimation were recognized in Synechococcus. The maximum value of maximum rate of photosynthesis (P _max) expressed per biomass unit at 10 μmol(photon) m^?2 s^?1 indicated a change in the number of photosynthetic units (PSU). The constant values of initial slope of photosynthetic light response curve (α) and the maximum value of P _max expressed per Chl unit at 145 μmol(photon) m^?2 s^?1 indicated another mechanism, i.e. a change in PSU size. These two mechanisms caused changes in photosynthetic rate and its parameters (compensation point, α, saturation irradiance, dark respiration, P _max) upon the influence of different irradiance and temperature. High irradiance had a negative effect on fluorescence parameters, such as the maximum quantum yield and effective quantum yield of PSII photochemistry (φ_PSII), but it was higher in case of φ_PSII.     

Does elevated CO2 protect photosynthesis from damage by high temperature via modifying leaf water status in maize seedlings?

We hypothesized that decreased stomatal conductance (g _s) at elevated CO₂ might decrease transpiration (E), increase leaf water potential (Ψ_W), and thereby protect net photosynthesis rate (P _N) from heat damage in maize (Zea mays L) seedlings. To separate long-term effects of elevated CO₂, plants grew at either ambient CO₂ or elevated CO₂. During high-temperature treatment (HT) at 45°C for 15 min, leaves were exposed either to ambient CO₂ (380 μmol mol^?1) or to elevated CO₂ (560 μmol mol^?1). HT reduced P _N by 25 to 38% across four CO₂ combinations. However, the g _s and E did not differ among all CO₂ treatments during HT. After returning the leaf temperature to 35°C within 30 min, g _s and E were the same or higher than the initial values. Leaf water potential (Ψ_W) was slightly lower at ambient CO₂, but not at elevated CO₂. This study highlighted that elevated CO₂ failed in protecting P _N from 45°C via decreasing g _s and Ψ_W.     

Photosystem II recovery in the presence and absence of chloroplast protein repair in the symbionts of corals exposed to bleaching conditions

Increased seawater temperature causes photoinhibition due to accumulation of photodamaged photosystem II (PSII) in symbiotic algae (genus Symbiodinium) within corals, and it is assumed to be associated with coral bleaching. To avoid photoinhibition, photosynthetic organisms repair the photodamaged PSII through replacing the PSII proteins, primarily the D1 protein, with newly synthesised proteins. However, in experiments using cultured Symbiodinium strains, the PSII repair of Symbiodinium has been suggested not to be related to the synthesis of the D1 protein. In this study, we examined the relationship between the recovery of PSII photochemical efficiency (F _V/F _M) and the content of D1 protein after high-light and high-temperature treatments using the bleaching-sensitive coral species, Pocillopora damicornis and Acropora millepora, and the bleaching-tolerant coral species, Montipora digitata and Pavona decussata. When corals were exposed to strong light (600 µmol photons m^?2 s^?1) at elevated temperature (32 °C) for 8 h, significant bleaching occurred in bleaching-sensitive coral species although an almost similar extent of reduced PSII function was found across all coral species tested. During a subsequent 15-h recovery under low light (10 µmol photons m^?2 s^?1) at optimal temperature (22 °C), the reduced F _V/F _M recovered close to initial levels in all coral species, but the reduced D1 content recovered only in one coral species (Pavona decussata). D1 content was therefore not strongly linked to chloroplast protein synthesis-dependent PSII repair. These results demonstrate that the recovery of photodamaged PSII does not always correspond with the recovery of D1 protein content in Symbiodinium within corals, suggesting that photodamaged PSII can be repaired by a unique mechanism in Symbiodinium within corals.     

Effects of growth and measurement light intensities on temperature dependence of CO2 assimilation rate in tobacco leaves

Effects of growth light intensity on the temperature dependence of CO₂ assimilation rate were studied in tobacco (Nicotiana tabacum) because growth light intensity alters nitrogen allocation between photosynthetic components. Leaf nitrogen, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) contents increased with increasing growth light intensity, but the cyt f/Rubisco ratio was unaltered. Mesophyll conductance to CO₂ diffusion (g_m) measured with carbon isotope discrimination increased with growth light intensity but not with measuring light intensity. The responses of CO₂ assimilation rate to chloroplast CO₂ concentration (C_c) at different light intensities and temperatures were used to estimate the maximum carboxylation rate of Rubisco (V_cmax) and the chloroplast electron transport rate (J). Maximum electron transport rates were linearly related to cyt f content at any given temperature (e.g. 115 and 179 µmol electrons mol^?1 cyt f s^?1 at 25 and 40 °C, respectively). The chloroplast CO₂ concentration (C_trans) at which the transition from RuBP carboxylation to RuBP regeneration limitation occurred increased with leaf temperature and was independent of growth light intensity, consistent with the constant ratio of cyt f/Rubisco. In tobacco, CO₂ assimilation rate at 380 µmol mol^?1 CO₂ concentration and high light was limited by RuBP carboxylation above 32 °C and by RuBP regeneration below 32 °C.     

C3 plants enhance rates of photosynthesis by reassimilating photorespired and respired CO2

Photosynthetic carbon gain in plants using the C₃ photosynthetic pathway is substantially inhibited by photorespiration in warm environments, particularly in atmospheres with low CO₂ concentrations. Unlike C₄ plants, C₃ plants are thought to lack any mechanism to compensate for the loss of photosynthetic productivity caused by photorespiration. Here, for the first time, we demonstrate that the C₃ plants rice and wheat employ a specific mechanism to trap and reassimilate photorespired CO₂. A continuous layer of chloroplasts covering the portion of the mesophyll cell periphery that is exposed to the intercellular air space creates a diffusion barrier for CO₂ exiting the cell. This facilitates the capture and reassimilation of photorespired CO₂ in the chloroplast stroma. In both species, 24–38% of photorespired and respired CO₂ were reassimilated within the cell, thereby boosting photosynthesis by 8–11% at ambient atmospheric CO₂ concentration and 17–33% at a CO₂ concentration of 200 µmol mol^?1. Widespread use of this mechanism in tropical and subtropical C₃ plants could explain why the diversity of the world's C₃ flora, and dominance of terrestrial net primary productivity, was maintained during the Pleistocene, when atmospheric CO₂ concentrations fell below 200 µmol mol^?1.     

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.     

Photosynthetic response to genome methylation affects the growth of Chinese white poplar

Genome methylation plays a key role in regulating gene expression, but limited knowledge exists concerning the link between DNA methylation and economic traits in forest trees. We measured photosynthetic characteristics and growth traits in 130 intraspecific hybrids of Chinese white poplar (Populus tomentosa Carr.) and detected their genome methylation. The phenotypic data were normally distributed, and each trait had a significant difference among the hybrids. The net photosynthetic rate (Pn, 14.83?±?3.76???mol?m^?2?s^?1), stomatal conductance (Gs, 0.29?±?0.09?mol?m^?2?s^?1), and intercellular CO₂ concentration (Ci, 264.50?±?30.94???mol?mol^?1) showed similar trends. Positive correlations were found between Pn and height (H, 133.59?±?50.44?cm) and basal diameter (D, 16.29?±?5.20?mm), respectively. Using methylation-sensitive amplification polymorphism (MSAP) analysis, 32 primer-pair combinations generated 715 polymorphic markers. Positive correlations between photosynthetic characteristics, such as Pn and Gs, and total relative methylation level and relative hemimethylation (CNG methylation) level were investigated. Eighty-one candidate markers were associated with Pn, Gs, or Ci, 13 of which were also associated with growth traits using single MSAP molecular marker association. Sequencing and BLAST analysis showed that candidate markers were linked to genes encoding protochlorophyllide reductase and proteins of cytochrome P450 CYP4/CYP19/CYP26 subfamilies, and linked to genes taking part in, e.g., photosystem II. Therefore, the regions defined by the MSAP candidate markers are linked to genes that are essential for photosynthetic characteristics that respond to DNA methylation and subsequently affect growth traits.     

Structural and functional aspects of the Nuphar lutea (L.) Smith heterophylly: Ultrastructure and photosynthesis

The ultrastructure and functional characteristics of the photosynthetic apparatus of floating and submersed leaves of the heterophyllous plant Nuphar lutea (L.) Smith have been examined. Differences have been revealed in mesophyll cell chloroplasts, content of pigments, and chlorophyll fluorescence parameters between floating and submersed leaves and submersed leaves at different depths. A sharp decline in the PSII (photosystem II) efficiency of submersed leaves when exposed to an actinic light intensity of more than 60 ??mol m^?2 s^?1 has been noted. The described differences may be considered as an adaptation mechanism of submersed leaves to life in an aquatic environment with a reduced light intensity and changed light spectral composition.