Photosynthesis of <Emphasis Type="Italic">Hosta</Emphasis> under light and controlled-release nitrogen fertilizer

The interactive effects of light intensity and controlled-release nitrogen fertilizer (CRNF) supply on growth, gas exchange, and chlorophyll (Chl) fluorescence parameters of two species of potted Hosta seedlings, original species of the genus Hosta in China, were studied. N₄ (4 g of CRNF per pot), N₈ (8 g of CRNF per pot), and sometimes N₁₂ (12 g of CRNF per pot), significantly increased total dry weights, net photosynthetic rate (P _N), stomatal conductance (g _s), transpiration rate (E), the maximum quantum yield of PSII photochemistry (F _v/F _m), the maximum ratio of quantum yields of photochemical and concurrent nonphotochemical processes in PSII (F _v/F ₀), actual efficiency of photochemical energy conversion in PSII under light (Φ_PSII), and photochemical quenching coefficient (qP), but significantly decreased internal CO₂ concentration (C _i) and nonphotochemical Chl fluorescence quenching (NPQ) compared to control plants at different growth stages of the two Hosta species in two levels of light intensities (50% of natural light (L₅₀) and 70% of natural light (L₇₀)). Based on the available data, we concluded that the increments in total dry weights of Hosta clausa var. ensata and Hosta ventricosa by appropriate amount of CRNF supply treatments under L₅₀ and/or L₇₀ light conditions are directly related to the increments in the P _N, which may be due to both stomatal and nonstomatal improvements for a longer growing time. Furthermore, there was an interaction between light intensity and CRNF supply treatments on growth and photosynthetic characteristics of the two Hosta species. The adaptability of Hosta plants with obvious stoloniferous rootstock to stronger light was higher than that of Hosta plants without obvious stoloniferous rootstock.     

Genotypic differences in the responses of gas exchange,chlorophyll fluorescence,and antioxidant enzymes to aluminum stress in <Emphasis Type="Italic">Festuca arundinacea</Emphasis>

In this study, the gas exchange, chlorophyll fluorescence, and antioxidant activity in eight tall fescue cultivars were investigated under aluminum stress. The results showed that the net photosynthetic rate (P _N) and stomatal conductance (g _s) were decreased, while the intercellular CO₂ concentration (C_i) was stable or increased under Al stress conditions. The efficiency of excitation capture by open PSII reaction centers (F′_v/F′_m), the maximum quantum yield of PSII photochemistry (F _v/F _m), the quantum yield of PSII electron transport (Φ_PSII), and the photochemical quenching (qP) were also decreased after Al stress, while the non-photochemical quenching (NPQ) was increased. Moreover, Al stress increased the antioxidant activities and MDA contents in each tall fescue cultivars. However, there was a lot genotype differences between the Al-tolerant and Al-sensitive cultivars. Cv. Barrington was the most sensitive cultivar and cv. Crossfire 2 was the most tolerant cultivar. The excessive excitation energy could not be dissipated efficiently by antenna pigments, and reactive oxygen species could not be scavenged efficiently, thereby resulting in membrane lipid peroxidation in cv. Barrington under Al stress conditions.     

Caragana korshinskii seedlings maintain positive photosynthesis during short-term,severe drought stress

Seedling performance may determine plant distribution, especially in water-limited environments. Plants of Caragana korshinskii commonly grow in arid and semiarid areas in northwestern China, and endure water shortage in various ways, but little is known about their performance when water shortage occurs at early growth stages. The water relations, photosynthetic activity, chlorophyll (Chl) content and proline accumulation were determined in 1-year-old seedlings growing in a 1:1 mixture of Loess soil and Perlite and subjected to (1) a water deficit for 20 days and (2) kept adequately watered throughout. The water deficit induced low (−6.1 MPa) predawn leaf water potentials (LWP), but did not induce any leaf abscission. Stomatal conductance (g _s), leaf transpiration rate (E), and net photosynthetic rate (P _N) decreased immediately following the imposition of the water deficit, while the maximal photochemical efficiency of photosystem II (PSII) (F_v/F_m) and the effective quantum yield of PSII (Φ_PSII) decreased 15 days later. An early and rapid decrease in g _s, reduced E, increased Chl (a+b) loss, increased the apparent rate of photochemical transport of electrons through PSII (ETR)/P _N, as well as a gradual increase in non-photochemical quenching of fluorescence (NPQ) and proline may have contributed to preventing Φ_PSII from photodamage. C. korshinskii seedlings used a stress-tolerance strategy, with leaf maintenance providing a clear selective advantage, considering the occasional rainfall events during the growing season.     

Effects of water stress and high temperature on gas exchange and chlorophyll fluorescence in Triticum aestivum L.

Wheat plants grown in controlled growth chambers were exposed to drought stress (DS) and high temperature (HT) singly and in combination (DS+HT). The effects of these two stresses on net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), quantum efficiency of photosystem 2 (Φ_PS2), variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m), photochemical (q_p) and non-photochemical (NPQ) Chl fluorescence, and yield were investigated. Grain yield was decreased by 21 % due to DS, while it was increased by 26 % due to HT. P _N, g _s, C _i, and Chl fluorescence were dramatically reduced to DS, HT, and their interaction, except NPQ which showed an increase due to HT.     

Effects of exogenous putrescine on gas-exchange characteristics and chlorophyll fluorescence of NaCl-stressed cucumber seedlings

The effects of 10 mM putrescine (Put) treated by spraying on leaves on growth, chlorophyll content, photosynthetic gas-exchange characteristics, and chlorophyll fluorescence were investigated by growing cucumber plants (Cucumis sativus L. cv. ChangChun mici) using hydroponics with or without 65 mM NaCl as a salt stress. Salt stress caused the reduction of growth such as leaf area, root volume, plant height, and fresh and dry weights. Furthermore, net photosynthesis rate (P _n), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), and transpiration rate (T _r) were also reduced by NaCl, but water use efficiency (WUE; P _n/T _r) showed a tendency to be enhanced rather than reduced by NaCl. However, Put alleviated the reduction of P _n by NaCl, and showed a further reduction of C _i by NaCl. The reduction of g _s and T _r by NaCl was not alleviated at all. The enhancement of WUE by NaCl was shown to have no alleviation at day 1 after starting the treatment, but after that, the enhancement was gradually reduced till the control level. Maximum quantum efficiency of PSII (F _v/F _m) showed no effects by any conditions based on the combination of NaCl and Put, and in addition, kept constant values in plants grown in each nutrient solution during this experimental period. The efficiency of excitation energy capture by open photosystem II (PSII) (F _v′/F _m′), actual efficiency of PSII (Φ_PSII), and the coefficient on photochemical quenching (qP) of plants with NaCl were reduced with time, and the reduction was alleviated till the control level by treatment with Put. The F _v′/F _m′, Φ_PSII, and qP of plants without NaCl and/or with Put showed no variation during the experiment. Non-photochemical quenching of the singlet excited state of chlorophyll a (NPQ) showed quite different manner from the others as mentioned above, namely, continued to enhance during the experiment.     

Diurnal cycle of chlorophyll fluorescence in <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Chlorophyll (Chl) fluorescence of warm day/cool night temperature exposed Phalaenopsis plants was measured hourly during 48 h to study the simultaneous temperature and irradiance response of the photosynthetic physiology. The daily pattern of fluorescence kinetics showed abrupt changes of photochemical quenching (q_P), non-photochemical quenching (NPQ) and quantum yield of photosystem II electron transport (Φ_PSII) upon transition from day to night and vice versa. During the day, the course of Φ_PSII and NPQ was related to the air temperature pattern, while maximum quantum efficiency of PSII photochemistry (F_v/F_m) revealed a rather light dependent response. Information on these daily dynamics in fluorescence kinetics is important with respect to meaningful data collection and interpretation.     

Leaf gas exchange and grain yield of common bean exposed to spermidine under water stress

Three prevalent aliphatic polyamines (PAs) include putrescine, spermidine, and spermine; they are low-molecular-mass polycations involved in many physiological processes in plants, especially, under stressful conditions. In this experiment, three bean (Phaseolus vulgaris L.) genotypes were subjected to well-watered conditions and two moderate and severe water-stressed conditions with and without spermidine foliar application. Water stress reduced leaf relative water content (RWC), chlorophyll contents, stomatal conductance (gs), intercellular CO₂ concentration (C_i), transpiration rate, maximal quantum yield of PSII (F_v/F_m), net photosynthetic rate (P_N), and finally grain yield of bean plants. However, spermidine application elevated RWC, gs, C_i, F_v/F_m, and P_N, which caused an increase in the grain yield and harvest index of bean plants under water stress. Overall, exogenous spermidine could be utilized to alleviate water stress through protection of photosynthetic pigments, increase of proline and carotenoid contents, and reduction of malondialdehyde content.

     

Exogenous application of trehalose induced H2O2 production and stomatal closure in Vicia faba

Trehalose can reduce stomatal aperture by a hydrogen-peroxide-dependent pathway in Vicia faba L. (cv. Daqingpi) resulting in significantly lower values of net photosynthetic rate (P_N), stomatal conductance (g_s), and transpiration rate (E). At 8 and 24 h, the lower P_N in trehalose-treated plants was accompanied by significant decrease in intercellular CO₂ concentration (c_i) suggesting that the reduction of P_N was caused by stomatal limitation. At 48 and 72 h, trehalose decreased apparent carboxylation efficiency (P_N/c_i) and did not decrease c_i and g_s compared with controls; therefore the reduction in photosynthesis was caused by non-stomatal limitation. Trehalose treatment resulted in significantly higher effective photochemical efficiency of PS II (Φ_PSII) and did not affect maximum photochemical efficiency of PS II (F_v/F_m). At 24, 48, and 72 h, trehalose decreased non-photochemical quenching (NPQ) and increased photochemical quenching (qP). Our results suggest that trehalose did not damage photosynthetic reaction centers.     

Influence of irradiance on photosynthesis and PSII photochemical efficiency in maize during short-term exposure at high CO<Subscript>2</Subscript> concentration

This work aimed to evaluate if gas exchange and PSII photochemical activity in maize are affected by different irradiance levels during short-term exposure to elevated CO₂. For this purpose gas exchange and chlorophyll a fluorescence were measured on maize plants grown at ambient CO₂ concentration (control CO₂) and exposed for 4 h to short-term treatments at 800 μmol(CO₂) mol⁻¹ (high CO₂) at a photosynthetic photon flux density (PPFD) of either 1,000 μmol m⁻² s⁻¹ (control light) or 1,900 μmol m⁻² s⁻¹ (high light). At control light, high-CO₂ leaves showed a significant decrease of net photosynthetic rate (P _N) and a rise in the ratio of intercellular to ambient CO₂ concentration (C _i/C _a) and water-use efficiency (WUE) compared to control CO₂ leaves. No difference between CO₂ concentrations for PSII effective photochemistry (Φ_PSII), photochemical quenching (q_p) and nonphotochemical quenching (NPQ) was detected. Under high light, high-CO₂ leaves did not differ in P _N, C _i/C _a, Φ_PSII and NPQ, but showed an increase of WUE. These results suggest that at control light photosynthetic apparatus is negatively affected by high CO₂ concentration in terms of carbon gain by limitations in photosynthetic dark reaction rather than in photochemistry. At high light, the elevated CO₂ concentration did not promote an increase of photosynthesis and photochemistry but only an improvement of water balance due to increased WUE.     

田间条件下海岛棉和陆地棉花铃期叶片光保护的机制

研究海岛棉(Gossypium barbadense)和陆地棉(G. hirsutum)两个棉花栽培种的光合作用特性, 探讨两个栽培种光合机构的光抑制以及防御保护机制, 以期为新疆棉花高光效品种选育和高产高效栽培实践提供理论基础。在新疆生态气候条件下, 系统测定了海岛棉和陆地棉的叶片运动、叶片接受光量子通量密度(PFD)、叶片温度、叶绿素荧光参数、气体交换参数和光呼吸速率的日变化。研究结果表明: 陆地棉叶片的“横向日性”较强而海岛棉较弱, 导致海岛棉叶片接受PFD较低, 但其叶片温度较高。海岛棉叶片的光合速率和气孔导度均显著低于陆地棉。在8:00-10:00 (北京时间, 下同)海岛棉叶片的光呼吸速率略低于陆地棉, 其余时间段海岛棉和陆地棉叶片的光呼吸速率相似。不同栽培种间, 叶片的最大光化学效率和实际光化学效率的日变化均无明显差异。除14:00-16:00以外, 海岛棉叶片的表观电子传递速率和光化学猝灭系数均显著低于陆地棉。8:00以后, 海岛棉叶片的非光化学猝灭显著高于陆地棉。因此, 在新疆生态气候条件下, 海岛棉和陆地棉叶片“横向日性”运动能力和气孔导度的差异导致叶片所处的光温环境不同, 同时造成海岛棉叶片的碳同化能力较低。为阻止光合电子传递链的过度还原, 减轻光合机构的光抑制, 陆地棉叶片主要通过光合机构的电子流途径耗散激发能, 而海岛棉叶片通过热耗散途径和相对较高的光呼吸能力来耗散激发能。     

Up-regulation of cyclic electron flow and down-regulation of linear electron flow in antisense-<Emphasis Type="Italic">rca</Emphasis> mutant rice

To investigate how excess excitation energy is dissipated in a ribulose-1,5-bisphospate carboxylase/oxygenase activase antisense transgenic rice with net photosynthetic rate (P _N) half of that of wild type parent, we measured the response curve of P _N to intercellular CO₂ concentration (C _i), electron transport rate (ETR), quantum yield of open photosystem 2 (PS2) reaction centres under irradiation (F_v′/F_m′), efficiency of total PS2 centres (Φ_PS2), photochemical (q_P) and non-photochemical quenching (NPQ), post-irradiation transient increase in chlorophyll (Chl) fluorescence (PITICF), and P700⁺ re-reduction. Carboxylation efficiency dependence on C _i, ETR at saturation irradiance, and F_v′/F_m′, Φ_PS2, and q_P under the irradiation were significantly lower in the mutant. However, NPQ, energy-dependent quenching (q_E), PITICF, and P700⁺ re-reduction were significantly higher in the mutant. Hence the mutant down-regulates linear ETR and stimulates cyclic electron flow around PS1, which may generate the ΔpH to support NPQ and q_E for dissipation of excess excitation energy.     

Chill-induced inhibition of photosynthesis was alleviated by 24-epibrassinolide pretreatment in cucumber during chilling and subsequent recovery

To investigate whether brassinosteroids (BRs) could be used to alleviate chill-induced inhibition of photosynthesis in cucumber (Cucumis sativus L) during chilling and subsequent recovery, the effects of exogenously applied 24-epibrassinolide (EBR) on gas exchange, chlorophyll fluorescence parameters, and antioxidant enzyme activity were studied. Cucumber plants were exposed to chilling under low light (12/8°C and 100 μmol m⁻² s⁻¹ PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18°C and 600 μmol m⁻² s⁻¹ PPFD) for 6 days. Chilling significantly decreased the net photosynthetic rate (P _N) and stomatal conductance (g _s), and increased rate of O₂ ^·− formation and H₂O₂ and malondialdehyde (MDA) content in cucumber leaves, but did not influence the optimal quantum yield of PSII (F_v/F_m). Chilling also decreased the effective quantum yield of PSII photochemistry (Φ_PSII) and photochemical quenching (q_P), but induced an increase in nonphotochemical quenching (NPQ), and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). High irradiance (600 μmol m⁻² s⁻¹) further aggravated the decrease in P _N, g _s, Φ_PSII and q_P, and enhanced the increase in reactive oxygen species (ROS) generation and accumulation in the first day of recovery after chilling. However, high irradiance induced a sharp decrease in F_v/F_m and NPQ, as well as the activities of SOD and APX on the first day of recovery. EBR pretreatment significantly alleviated chill-induced inhibition of photosynthesis during chilling stress and subsequent recovery period, which was mainly due to significant increases in g _s, Φ_PSII, q_P and NPQ. EBR pretreatment also reduced ROS generation and accumulation, and increased the activities of SOD and APX during chilling and subsequent recovery. Those results suggest that EBR pretreatment alleviates the chill reduction in photosynthesis and accelerated the recovery rate mainly by increasing of the stomatal conductance, the efficiency of utilization and dissipation of leaf absorbed light, and the activity of the ROS scavenging system during chilling and subsequent recovery period.     

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.     

Changes in photosynthesis,fluorescence, and nitrogen metabolism of hawthorn (<Emphasis Type="Italic">Crataegus pinnatifida</Emphasis>) in response to exogenous glutamic acid

Photosynthesis, chlorophyll (Chl) a fluorescence, and nitrogen metabolism of hawthorn (Crataegus pinnatifida Bge.), subjected to exogenous L-glutamic acid (GLA) (200 mg l⁻¹, 400 mg l⁻¹, and 800 mg l⁻¹) that possibly affect secondary metabolic regulation, were measured. The results indicated that photosynthetic and fluorescence characteristics of hawthorn exhibited positive responses to the application of GLA. Different concentrations of GLA caused an increase in Chl content, net photosynthetic rate (P _N) and stomatal conductance (g _s) as well as transpiration rate (E), and improved the carboxylation efficiency (CE), apparent quantum yield (AQY) and maximum carboxylation velocity of Rubisco (V_cmax). Application of GLA could also enhance the maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PSII (F_v/F₀), the maximal quantum yield of PSII (F_v/F_m), the probability that an absorbed photon will move an electron into the electron transport chain beyond Q_A (Φ_Eo) as well as the performance index on absorption basis (PI_ABS), but decreased the intercellular CO₂ concentration (C _i) and the minimal fluorescence (F₀). Application of GLA also induced an increase in nitrate reductase (NR; EC 1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2) activities, and increased the soluble protein content, leaf nitrogen (N) content and N accumulation in leaves as well as the plant biomass. However, the effects were different among different concentrations of GLA, and 800 mg l⁻¹ GLA was better. This finding suggested that application of GLA is recommended to improve the photosynthetic capacity by increasing the light energy conversion and CO₂ transfer as well as the photochemical efficiency of PSII, and enhanced the nitrogen metabolism and growth and development of plants.     

Limitations to photosynthesis inCoffea canephoraas a result of nitrogen and water availability

Plants of C. canephora grown in pots under low nitrogen (LN) or high nitrogen (HN) applications were submitted to either cyclic water stress or daily irrigation. Water deficit led to marked decreases in net carbon assimilation rate (A) and, to a lesser extent, in stomatal conductance (g_s), regardless of the N treatments. In well-watered plants, A appreciably increased in HN plants relative to LN plants without significant changes in g_s. As a whole, changes in internal CO₂ concentration predominantly reflected changes in A rather than in g_s. Under irrigated conditions, A, but not g_s, correlated with leaf N concentration in a curvilinear way. Photosynthetic nitrogen-use efficiency was considerably low, and decreased with increasing leaf N concentration. Limited N, but not water, slightly decreased the maximum photochemical efficiency of photosystem II (PSII). Under continuous irrigation, LN plants had a smaller quantum yield of electron transport (_PSII) through slight decreases of photochemical quenching (q_p) and capture efficiency of excitation energy by open PSII reaction centres, and increases in Stern-Volmer non-photochemical quenching. Under water-stressed conditions, changes in PSII photochemistry were apparent only in HN plants, with a 25 % decrease in _PSII, due mainly to variations in q_p. Biochemical constraints, rather than stomatal or photochemical limitations, provoked the decreases in A under limited supply of either N or water.     

Effect of day/night temperature difference on chlorophyll content,photosynthesis and fluorescence parameters of tomato at fruit stage

In order to investigate the effect of day/night temperature difference (DIF) on photosynthetic characteristics of tomato plants (Solanum lycopersicum, cv. Jinguan 5) at fruit stage, an experiment was carried out in climate chambers. Five day/night temperature regimes (16/34, 19/31, 25/25, 31/19, and 34/16°C) with respective DIFs of -18, -12, 0, +12, and +18 were used and measured at mean daily temperature of 25°C. The results showed that chlorophyll (Chl) a, Chl b, net photosynthetic rate (P_N), stomatal conductance (g_s), maximum quantum yield of PSII photochemistry (F_v/F_m), effective quantum yield of PSII photochemistry (?_PSII), and photochemical quenching (qp) significantly increased under positive DIF, while they decreased with negative DIF. In contrast, the Chl a/b ratio and nonphotochemical quenching (NPQ) decreased under positive DIF, while increased with negative DIF. Chl a, Chl b, P_N, g_s, F_v/F_m, ?_PSII, and qp were larger under +12 DIF than those at +18 DIF, while Chl a/b and NPQ showed an opposite trend.     

Higher electron transport rate observed at low intercellular CO2 concentration in long-term drought-acclimated leaves of Japanese mountain birch (Betula ermanii)

The influence of long‐term drought stress on photosynthesis of Japanese mountain birch (Betula ermanii Cham.) was examined using chlorophyll fluorescence and gas exchange measurements. Drought stress was imposed in potted plants by reducing irrigation frequency from daily (control) to twice‐weekly and once‐weekly. Thirty‐day‐old leaves, which had developed under fully stressed conditions, were used for the measurements. The decline in net CO₂ assimilation rate (A) observed in situ in drought‐stressed plants resulted from a lower intercellular CO₂ concentration (C_i) due to stomatal closure but the carboxylation efficiency was not affected as there was no difference in the initial slope of the A/C_i response after watering. Although there were no treatment differences in A at C_i below 270 μmol mol^?1 (with ambient air at 360 μmol mol^?1 CO₂), higher electron transport rate (ETR), photochemical quenching (q_P) and the efficiency of energy conversion of open PSII (F_v′/F_m′), and similar or even lower non‐photochemical quenching (NPQ) were observed at a given C_i in drought‐stressed plants (of both twice‐ and once‐weekly irrigation), suggesting a higher fraction of open PSII resulting from energy dissipation achieved through higher electron flow rather than through thermal dissipation in PSII antennae. The once‐weekly watered plants showed a lower ratio of gross carbon assimilation rate to ETR (A*/ETR), suggesting an enhanced alternative pathway of electron flow probably involving the Mehler‐peroxidase (MP) reaction as indicated by a higher Φ_PSII at a given Φ_CO2 under non‐photorespiratory conditions. On the other hand, plants of twice‐weekly watering exhibited almost the same A*/ETR and Φ_PSII–Φ_CO2 relationship as control plants, indicating no enhanced alternative pathways under mild drought stress.     

PSII photochemistry, thermal energy dissipation, and the xanthophyll cycle in Kalanchoë daigremontiana exposed to a combination of water stress and high light

Kalanchoë daigremontiana, a CAM plant grown in a greenhouse, was subjected to severe water stress. The changes in photosystem II (PSII) photochemistry were investigated in water‐stressed leaves. To separate water stress effects from photoinhibition, water stress was imposed at low irradiance (daily peak PFD 150 μmol m^?2 s^?1). There were no significant changes in the maximal efficiency of PSII photochemistry (F_v/F_m), the traditional fluorescence induction kinetics (OIP) and the polyphasic fluorescence induction kinetics (OJIP), suggesting that water stress had no direct effects on the primary PSII photochemistry in dark‐adapted leaves. However, PSII photochemistry in light‐adapted leaves was modified in water‐stressed plants. This was shown by the decrease in the actual PSII efficiency (Φ_PSII), the efficiency of excitation energy capture by open PSII centres (F_v′/F_m′), and photochemical quenching (q_P), as well as a significant increase in non‐photochemical quenching (NPQ) in particular at high PFDs. In addition, photoinhibition and the xanthophyll cycle were investigated in water‐stressed leaves when exposed to 50% full sunlight and full sunlight. At midday, water stress induced a substantial decrease in F_v/F_m which was reversible. Such a decrease was greater at higher irradiance. Similar results were observed in Φ_PSII, q_P, and F_v′/F_m′. On the other hand, water stress induced a significant increase in NPQ and the level of zeaxanthin via the de‐epoxidation of violaxanthin and their increases were greater at higher irradiance. The results suggest that water stress led to increased susceptibility to photoinhibition which was attributed to a photoprotective process but not to a photodamage process. Such a photoprotection was associated with the enhanced formation of zeaxanthin via de‐epoxidation of violaxanthin. The results also suggest that thermal dissipation of excess energy associated with the xanthophyll cycle may be an important adaptive mechanism to help protect the photosynthetic apparatus from photoinhibitory damage for CAM plants normally growing in arid and semi‐arid areas where they are subjected to a combination of water stress and high light.     

Photosynthetic characterization of Australian pitcher plant <Emphasis Type="Italic">Cephalotus follicularis</Emphasis>

Australian carnivorous pitcher plant Cephalotus follicularis Labill. produces two types of leaves. During the spring time, the plant produces a foliage type of noncarnivorous leaf called lamina. Later, the second type of leaf is produced — carnivorous pitcher. Using simultaneous measurements of gas exchange and chlorophyll (Chl) fluorescence photosynthetic efficiency of these two distinct forms of leaves were compared. In addition stomatal density, an important component of gas exchange, and Chl concentration were also determined. Pitcher trap had lower net photosynthetic rate (P _N) in comparison to noncarnivorous lamina, whereas the rate of respiration (R _D) was not significantly different. This was in accordance with lower stomatal density and Chl concentration in the pitcher trap. On the other hand maximum quantum yield of PSII (F_v/F_m) and effective quantum yield of photochemical energy conversion in PSII (Φ_PSII) was not significantly different. Nonphotochemical quenching (NPQ) was significantly higher in the lamina at higher irradiance. These data are in accordance with hypothesis that changing the leaf shape in carnivorous plants to make it a better trap generally makes it less efficient at photosynthesis. However, the pitcher of Cephalotus had much higher P _N than it was expected from the data set of the genus Nepenthes. Because it is not possible to optimize for contrasting function such as photosynthesis and carnivory, it is hypothesized that Cephalotus pitchers are less elaborated for carnivorous function than the pitchers of Nepenthes.     

田间条件下棉花幼叶光合特性及光保护机制

通过比较棉花(Gossypium hirsutum)幼叶和完全展开叶气体交换参数及叶绿素荧光特性的差异, 探讨高光强下幼叶的光抑制程度及明确光保护机制间的协调机理。在田间自然条件下, 以棉花刚展平的幼嫩叶片(幼叶)和面积已达到最大的完全展开叶片为研究对象, 通过测定不同发育阶段叶片气体交换参数及叶绿素a荧光参数的变化, 并运用Dual-PAM100对不同发育阶段的叶片进行快速光响应曲线的拟合。结果表明: 幼叶和完全展开叶片在光合、荧光特性方面表现出明显的差异。与完全展开叶相比, 较低的叶绿素(Chl)含量和气孔导度(G_s)是幼叶较低净光合速率(P_n)的限制因素, 从而直接导致其光系统II (PSII)实际光化学效率(Φ_PSII)和光化学猝灭系数(q_P)的降低。在1800 μmol·m^-2·s^-1光强以下, 完全展开叶具有较强的围绕PSI循环的电子流(CEF), 有利于合成ATP, 是其具有较高光合能力的原因之一。相同光强下, 幼叶较低的光饱和点(LSP)更易受光抑制, 但其PSII原初光化学效率(F_v/F_m)的日变化幅度显著小于完全展开叶, 说明强光下幼叶通过类胡萝卜素(Car)猝灭单线态氧、光呼吸(P_r)、热耗散(NPQ)以及PSI-CEF等光保护机制能有效地耗散过剩的光能, 从而避免其光合机构发生光抑制。