Photosynthesis is improved by exogenous glycinebetaine in salt-stressed maize plants

The effects of exogenous application of glycinebetaine (GB) (10 m M ) on growth, leaf water content, water use efficiency, photosynthetic gas exchange, and photosystem II photochemistry were investigated in maize plants subjected to salt stress (50 and 100 m M NaCl). Salt stress resulted in the decrease in growth and leaf relative water content as well as net photosynthesis and the apparent quantum yield of photosynthesis. Stomatal conductance, evaporation rate, and water use efficiency were decreased in salt-stressed plants. Salt stress also caused a decrease in the actual efficiency of PSII ( Φ _PSII), the efficiency of excitation energy capture by open PSII reaction centers ( F _v'/ F _m'), and the coefficients of photochemical quenching ( q _P) but caused an increase in non-photochemical quenching (NPQ). Salt stress showed no effects on the maximal efficiency of PSII photochemistry ( F _v/ F _m). On the other hand, in salt-stressed plants, GB application improved growth, leaf water content, net photosynthesis, and the apparent quantum yield of photosynthesis. GB application also increased stomatal conductance, leaf evaporation rate, and water use efficiency. In addition, GB application increased Φ _PSII, F _v'/ F _m', and q _P but decreased NPQ. However, GB application showed no effects on F _v/ F _m. These results suggest that photosynthesis was improved by GB application in salt-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the actual PSII efficiency.     

Increased susceptibility to photoinhibition in pre-existing needles experiencing low temperature at spring budbreak in Sakhalin spruce (Picea glehnii) seedlings

The seasonal changes in photosynthetic properties in 1-year-old needles of Sakhalin spruce ( Picea glehnii ) were measured using the chlorophyll fluorescence technique at various temperatures (5, 10, 20, 25 and 30°C). In the course of seasonal change, a temporary decrease in the quantum yield of PSII electron transport (Φ_PSII) was observed just before budbreak. A decline in photochemical quenching ( q _P) was observed at the same time as that of Φ_PSII but only at the two lowest temperatures (5 and 10°C). Photochemical efficiency of open PSII ( F _v'/ F _m') also declined just before budbreak at 25 and 30°C. An increase in thermal energy dissipation as indicated by a decrease in F _v'/ F _m' before budbreak was not significant at lower temperatures (5 and 10°C) in spite of the declines in q _P. This implies that thermal energy dissipation necessitated by the decline in Φ_PSII might not be sufficiently strong to prevent a decline in q _P at lower temperatures. On the other hand, at higher temperatures no decline was observed in q _P because Φ_PSII decreased to a relatively small extent, therefore thermal energy dissipation is sufficient in coping with the excessive energy accumulation in PSII. Seedlings of Sakhalin spruce exposed to ambient air temperature below 10°C before budbreak exhibited photoinhibition indicated by a decrease in the maximal photochemical efficiency of PSII ( F _v/ F _m) after an overnight dark adaptation. The present study suggests that 1-year-old shoots of Sakhalin spruce have an increased susceptibility to photoinhibition at low temperature just before budbreak.     

Persistence of photosynthetic components and photochemical efficiency in ears of water-stressed wheat (Triticum aestivum)

Ear photosynthesis may be an important source of C for grain growth in water-stressed plants of cereals. The main objectives of this work were to determine the stability of the photosynthetic apparatus and the photochemical efficiency of ears in plants subjected to post-anthesis drought. Plants of wheat ( Triticum aestivum L. cv. Granero INTA) were grown in pots under a rain shelter and subjected to water stress (soil water potential around −0.6 to −0.8 MPa) starting 4  days after anthesis. Post-anthesis drought substantially accelerated the loss of chlorophyll, Rubisco and the light-harvesting complex of photosystem II (LHCII) in the flag leaf, but the degradation of these photosynthetic components was much less affected by water deficit in awns and ear bracts. Quantum yield of PSII (Φ_PSII) decreased in leaves of water-stressed plants. In contrast, ear bracts had a higher Φ_PSII than leaves, and Φ_PSII of ear bracts did not decrease at all in response to drought. Removing the grains immediately before fluorescence measurements (less than 30 min) slightly reduced Φ_PSII, indicating that CO₂ supplied by grain respiration may contribute to the high photochemical efficiency of ears in droughted plants. However, other factors may be involved in maintaining high Φ_PSII, since even in the absence of grains Φ_PSII remained much higher in ear bracts than in the flag leaf. The relative stability of ear photosynthetic components and their relatively high photochemical efficiency may help to maintain ear photosynthesis during the grain filling period in droughted plants.     

Effects of cadmium on growth of Helianthus annuus seedlings: physiological aspects

Sunflower seedlings ( Helianthus annuus hybrid Select) were grown in a complete nutrient solution in the absence or presence of Cd²⁺ (10 and 20 μM). Analyses were performed to establish whether there was a differential effect of Cd²⁺ on mature and young leaves. After 7 d the growth parameters as well as the leaf area had decreased in both mature and young leaves. Accumulation of Cd²⁺ in the roots exceeded that in the shoots. Seedlings treated with Cd²⁺ exhibited reduced contents of chlorophyll and CO₂ assimilation rate, with a greater decrease in young leaves. The photochemical efficiency of photosystem II (PSII) was not altered by Cd²⁺ treatment in either mature or young leaves, although during steady-state photosynthesis in young leaves there was a significant alteration in the following parameters: quantum yield of electron transport by PSII (Φ_PSII), photochemical quenching ( q _P), non-photochemical quenching ( q _NP), and excitation capture efficiency of PSII (Φ_exc).     

Photoinhibition at chilling temperatures and effects of freezing stress on cold acclimated spinach leaves in the field. A fluorescence study

The role of high light stress in a natural environment was studied on spinach plants ( Spinacia oleracea L. cv. Wolter) grown in the field during the winter season. Fluorescence induction (at 293 K and 77 K) of leaves was used to characterize the stress effects. Night frost with minimum temperatures between – 1.5°C and –7.5°C (i.e. above the'frost killing point'at ca. –11.5°C) led to impaired photosynthesis. This was seen as increased initial fluorescence (F_o), decreased ratio of variable to maximum fluorescence (F_V/F_M) and lowered rates of O₂ evolution. The freezing injury was reversible within several frostless days. Exposure to high light (about 900 mol m_–2 s_–1) at chilling temperatures in the field caused photoinhibition, manifested as decreased variable fluorescence (F_V) and F_V/F_M ratio without changes in F_O. The photoinhibitory fluorescence quenching was not stronger after frost than after frostless nights; synergism between light stress and preceding freezing stress was not observed. Fluorescence induction signals at 77 K showed that F_V of photosystems I and II decreased to the same extent, indicating increased thermal deactivation of excited chlorophyll. Photoinhibition was fully reversible at +4°C within 1 h in low light, but only partially in moderate light. Preceding night frosts did not affect the recovery. The photoinhibition observed here is regarded as a protective system of thermal dissipation of excess light energy.     

Photoinhibition of photosynthesis in intact willow leaves in response to moderate changes in light and temperature

When willow leaves were transferred from 270 to 650 μmol m^-2 s^-1 photosynthetic photon flux density (PPFD), partial photoinhibition developed over the next hours. This was manifested as roughly parallel inhibitions of the ratio of variable over maximal chlorophyll fluorescence (F_v/F_M), and of the maximal quantum yield and the capacity of photosynthesis. This occurred even though photosynthesis was operating well below its capacity and only about one fourth of the reaction centres of photosystem (PS) II were in the closed state. When the air temperature was lowered from 25 to 15°C (18°C leaf temperature) photoinhibition was markedly accelerated. This temperature effect is suggested to be mediated largely by a decrease in the rate of energy dissipation through photosynthesis and indicated by a 50% increase in the number of closed PSII reaction centres. The pool size of the carotcnoid zeaxanthin and the extent of inhibition of the F_v/F_M ratio were positively correlated during the treatment. However, the relaxation following imposition of darkness was much faster for zeaxanthin than for the F_v/F_M ratio, ruling out the possibility of a direct causal relationship. The energy distribution between PSII and PSI was unaltered upon photoinhibition. However, the functioning of the PSII reaction centres was altered, as indicated by a rise in the minimal fluorescence, Fa.     

NaCl胁迫增强杂交酸模(Rumex K-1)幼苗叶片光系统Ⅱ的耐热性

NaCl胁迫对杂交酸模幼苗光系统Ⅱ(PS Ⅱ)的最大光化学效率没有影响,但是增强了PS Ⅱ的耐热性.热胁迫条件下,与未经盐胁迫处理的叶片相比,经NaCl 200 mmol/L处理的杂交酸模幼苗叶片,其PS Ⅱ最大光化学效率下降较小,反映OEC受伤程度的指标Fk/Fj上升较小.此外,光化学猝灭系数(qP)、PS Ⅱ反应中心光能捕获效率(Fv1/Fm1)、PS Ⅱ光化学转换效率(ΦPS Ⅱ)的下降以及QB-非还原性反应PS Ⅱ反应中心的相对含量上升程度也较小.探讨了盐胁迫增强杂交酸模幼苗叶片PS Ⅱ耐热性的可能机理.     

Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat ( Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO₂ assimilation capacity. Analyses of fluorescence quenching under steady‐state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'_v/F'_m) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'_v/F'_m). At the same time, a significant increase in non‐photochemical quenching (q_N) and a considerable decrease in photochemical quenching (q_P) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of Q_A reduction and an increase in the proportion of Q_B‐non‐reducing PSII reaction during senescence. The decrease in both F'_v/F'_m and q_P explained the decrease in the actual quantum yield of PSII electron transport ((φ_PSII). We suggest that the modifications in PSII function, which led to the down‐regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.     

Partitioning of excitation energy in two wheat cultivars with different grain protein contents grown under three nitrogen applications in the field

Influences of different nitrogen applications on photosynthesis and utilization of excitation energy were explored by comparing two field-grown wheat ( Triticum aestivum L.) cultivars with high or low grain protein content [High protein cultivar '8901' (HC) and low protein cultivar '1391' (LC)]. High nitrogen application significantly decreased both CO₂ assimilation and photorespiration in both cultivars during the early stages after anthesis. However, the actual photosystem II (PSII) efficiency ( Ф _PSII) was not significantly different between high, moderate and low nitrogen applications in the HC. As a result, the ratio of Ф _PSII to the quantum yield of carbon metabolism ( Φ _CO2) measured under non-photorespiratory conditions in the HC was higher under high nitrogen application than under low or medium nitrogen application. The grain protein content of the HC was also increased by high nitrogen application. In contrast, high nitrogen application decreased the actual PSII efficiency in the flag leaves of the LC in the early stages after anthesis and different nitrogen applications did not significantly alter the Ф _PSII/ Φ _CO2 ratio or grain protein content in the LC. No significant difference was detected in the activity of superoxide dismutase or ascrobate peroxidase between different nitrogen treatments in either cultivar throughout the entire experimental period. These results indicate that more excitation energy is partitioned to nitrogen metabolism in the flag leaves of the HC under high nitrogen application whereas the partitioning of excitation energy in the LC was not affected by nitrogen application.     

Does calcium ameliorate the negative effect of NaCl on melon root water transport by regulating aquaporin activity?

The hydraulic conductance ( L ₀) of detached, exuding root systems from melon ( Cucumis melo cv. Amarillo oro) was measured. All plants received a half-strength Hoagland nutrient solution, and plants stressed either solely with NaCl (50 mM) or with NaCl (50 mM) following treatment (2 d) with CaCl₂ (10 mM) were compared with controls and CaCl₂-treated (10 mM) plants. The L ₀ of NaCl-treated plants was markedly decreased when compared to control and CaCl₂-treated plants, but the decrease was smaller when NaCl was added to plants previously treated with CaCl₂. A similar effect was observed when the flux of Ca²⁺ into the xylem and the Ca²⁺ concentration in the plasma membrane of the root cells were determined. In control, CaCl₂- and NaCl + CaCl₂-treated plants, HgCl₂ treatment (50 μM) caused a sharp decline in L ₀ to values similar to those of NaCl-stressed roots, but L ₀ was restored by treatment with 5 mM DTT. However, in NaCl roots only a slight effect of Hg²⁺ and DTT were observed. The effect of all treatments on L ₀ was similar to that on osmotic water permeability ( P _f) of individual protoplasts isolated from roots. The results suggest that NaCl decreased the passage of water through the membrane and roots by reducing the activity of Hg-sensitive water channels. The ameliorative effect of Ca²⁺ on NaCl stress could be related to water-channel function.     

Effects of salinity stress on photosystem II function in cyanobacterial Spirulina platensis cells

The changes in PSII photochemistry in Spirulina platensis cells exposed to salinity stress (0–0.8 M NaCl) for 12 h were studied. Salinity stress induced a decrease in oxygen evolution activity, which correlated with the decrease in the quantum yield of PSII electron transport ( Φ _PSII). Phycocyanin content decreased significantly while chlorophyll content remained unchanged in salt-stressed cells. Salinity stress induced an increase in non-photochemical quenching (q_N) and a decrease in photochemical quenching (q_P). Analyses of the polyphasic fluorescence transients (OJIP) showed that with the increase in salt concentration, the fluorescence yield at the phases J, I and P declined sharply and the transient almost levelled off at salt concentration of 0.8 M NaCl. The effects of DCMU on the polyphasic rise of fluorescence transients decreased significantly. Salinity stress resulted in a decrease in the efficiency of electron transfer from Q_A⁻ to Q_B. The slope at the origin of the relative variable fluorescence curves (dV/dt_o) and the relative variable fluorescence at phase J (V_J) increased in the absence of DCMU, but decreased in the presence of DCMU. The shape of the relative variable fluorescence transients in salt-stressed cells was comparable to that of the control cells incubated with DCMU. The results in this study suggest that salt stress inhibited the electron transport at both donor and acceptor sides of PSII, resulted in damage to phycobilisome and shifted the distribution of excitation energy in favour of PSI.     

NaCl treatment markedly enhances H2O2-scavenging system in leaves of halophyte Suaeda salsa

The C₃ halophyte Suaeda salsa L. grown under the high concentration of NaCl (200 m M ) was used to investigate the role of the hydrogen peroxide (H₂O₂)-scavenging system [catalase, ascorbate peroxidase, glutathione reductase (GR), ascorbic acid, and glutathione (GSH)] in removal of reactive oxygen species. The activity of catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), and GR (EC 1.6.4.2) increased significantly after 7 days of NaCl treatment. The isoform patterns of CAT and GR were not affected, but the staining intensities were significantly increased by NaCl treatment. Activities of both the thylakoid-bound APX or GR and stromal APX (S-APX) or GR in the chloroplasts were markedly enhanced under high salinity. Fifty percent of APX in the chloroplasts is thylakoid-bound APX. S-APX and GR activity represented about 74–78 and 64–71% of the total soluble leaf APX and GR activity, respectively. Salt treatment increased the contents of ascorbic acid and GSH. By contrast, a decreased content of H₂O₂ was found in the leaves of NaCl-treated S . salsa . The level of membrane lipid peroxidation decreased slightly after NaCl treatment. The plants grew well with high rate of net photosynthesis under high salinity. These data suggest that upregulation of the H₂O₂-scavenging system in plant cells, especially in the chloroplasts, is at least one component of the tolerance adaptations of halophytes to high salinity.     

Relationship between photosystem II activity and CO2 fixation in leaves

There is now potential to estimate photosystem II (PSII) activity in vivo from chlorophyll fluorescence measurements and thus gauge PSII activity per CO₂ fixed. A measure of the quantum yield of photosystem II, Φ_II (electron/photon absorbed by PSII), can be obtained in leaves under steady-state conditions in the light using a modulated fluorescence system. The rate of electron transport from PSII equals Φ_II times incident light intensity times the fraction of incident light absorbed by PSII. In C₄ plants, there is a linear relationship between PSII activity and CO₂ fixation, since there are no other major sinks for electrons; thus measurements of quantum yield of PSII may be used to estimate rates of photosynthesis in C₄ species. In C₃ plants, both CO₂ fixation and photorespiration are major sinks for electrons from PSII (a minimum of 4 electrons are required per CO₂, or per O₂ reacting with RuBP). The rates of PSII activity associated with photosynthesis in C₃ plants, based on estimates of the rates of carboxylation (v_o) and oxygenation (v_o) at various levels of CO₂ and O₂, largely account for the PSII activity determined from fluorescence measurements. Thus, in C₃ plants, the partitioning of electron flow between photosynthesis and photorespiration can be evaluated from analysis of fluorescence and CO₂ fixation.     

Photoinhibition and the xanthophyll cycle are not enhanced in the salt-acclimated halophyte Artimisia anethifolia

The effects of high salinity (up to 400 m M NaCl) on photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions. In order to examine the changes in PSII photochemistry, photoinhibition, thermal dissipation associated with the xanthophyll cycle in salt-acclimated plants, the experiments were conducted at midday on a clear day (maximal irradiance 1500 μmol m⁻¹ s⁻¹) and on a cloudy day (maximal irradiance 700 μmol m⁻¹ s⁻¹), respectively. With increasing salt concentration, the accumulation of sodium and chloride in leaves increased considerably while the relative growth rate and CO₂ assimilation rate decreased significantly. Salinity induced no effects on PSII photochemistry, thermal energy dissipation, and the contents of the xanthophyll cycle pigments either on a clear day or on a cloudy day. However, when compared with those on a cloudy day, PSII photochemistry decreased and thermal energy dissipation increased significantly in both control and salt-acclimated plants on a clear day. The levels of zeaxanthin and antheraxanthin at the expense of violaxanthin were higher on a clear day than on a cloudy day. The results suggest that photoinhibition and the xanthophyll cycle were not induced by high salinity but by high light only in A. anethifolia plants. The results also suggest that A. anethifolia showed high resistance not only to high salinity, but also to photoinhibition even when it was treated with high salinity and exposed to full sunlight.     

Oxygen-induced changes in the redox state of the cytochrome b559 in photosystem II depend on the integrity of the Mn cluster

The effect of oxygen and anaerobiosis on the redox properties of Cyt b ₅₅₉ was investigated in PSII preparations from spinach with different degree of disintegration of the donor side. Comparative studies were performed on intact PSII membranes and PSII membranes that were deprived of the 18-kDa peripheral subunit (0.25 NaCl washed), the 18- and 24-kDa peripheral subunits (1 M NaCl washed), the 18-, 24- and 33-kDa peripheral subunits (1.2 M CaCl₂ washed), Cl depleted and after complete depletion of the Mn cluster (Tris washed). In active PSII centers, about 75% of Cyt b ₅₅₉ was found in the high-potential form and the rest in the intermediate potential form. With decomposition of the donor side, the intermediate potential form started to dominate, reaching more than 90% after Tris treatment. The oxygen-dependent conversion of the intermediate potential form of Cyt b ₅₅₉ into the low-potential and high-potential forms was only observed after treatments that directly affect the Mn cluster. In PSII membranes, deprived of all three extrinsic subunits (CaCl₂ treatment), 21% of the intermediate potential form was converted into the low-potential form and 14% into the high-potential form by the removal of oxygen. In Tris-washed PSII membranes, completely lacking the Mn cluster, this conversion amounted to 60 and 33%, respectively. In intact PSII membranes, the oxygen-dependent conversion did not occur. The possible physiological role of this oxygen-dependent behavior of the Cyt b ₅₅₉ redox forms during the assembly/photoactivation cycle of PSII is discussed.     

Photosynthesis, carbohydrate levels and chlorophyll fluorescence-estimated intercellular CO2 in water-stressed Casuarina equisetifolia Forst. & Forst.

The effect of long-term water stress on photosynthetic carbon metabolism in Casuarina equisetifolia Forst. & Forst. was analysed by measuring CO₂ assimilation, stomatal conductance, the quantum yield of photosystem II ( Φ _PSII), enzyme activities, and the levels of photosynthetic intermediates and carbohydrates. CO₂ assimilation decreased under water stress while the intercellular CO₂ concentration ( C _i) as estimated by gas exchange measurements remained high. However, the estimates of C _i from measurements of Φ _PSII suggest that the decrease in photosynthesis can be explained in terms of stomatal closure. Water stress decreased total stromal fructose-1,6-bisphosphatase activity and did not alter the activities and activation states of ribulose bisphosphate carboxylase oxygenase and NADP-dependent malate dehydrogenase (NADP-MDH). The concentration of photosynthetic metabolites, glucose, fructose and sucrose decreased, whereas starch concentrations increased under drought conditions.     

Chlorophyll fluorescence and photoacoustic characteristics in relationship to changes in chlorophyll and Ca2+ content of a Cu-tolerant Silene compacta ecotype under Cu treatment

The effect of Cu toxicity on photosynthetic function, chlorophyll and Ca²⁺ content of Cu-tolerant Silene compacta plants grown in nutrient solution was studied. Since, in plants grown under 8 μ M Cu, the chlorophyll and Ca²⁺ concentration as well as the photosystem II (PSII) photochemistry were increased, compared to the control, the development of an adaptive mechanism of the Cu-tolerant ecotype of S. compacta to 8 μ M Cu is suggested. Increased Cu tolerance of the S. compacta ecotype reflects modulation of the photosynthetic apparatus to optimize photosynthesis. However, exposure of plants to 160 μ M Cu resulted in a marked increase of the fraction of closed PSII centres and decreased quantum yield of PSII electron transport (Φ_PSU) which was accompanied by a significant decline of relative quantum yield for O₂ evolution (A_ox/A_pt). The concentration of chlorophyll and Ca²⁺ in leaves also decreased significantly under 160 μ M Cu treatment. Photochemical quenching (q_p) displayed a reduction as a result of perturbation of the photosynthetic electron transfer chain, while non-photochemical quenching (q_N) increased. High Cu treatment reduced photosynthetic productivity of S. compacta plants which can be attributed, in part, to pertubation of photosynthetic process and photosynthetic pigments as well as to Ca²⁺ loss.     

Photosystem II inhibition by moderate light under low temperature in intact leaves of chilling-sensitive and -tolerant plants

Photosystem II (PSII) activity was examsined in leaves of chilling-sensitive cucumber ( Cucumis sativus L.), tomato ( Lycopersicum esculentum L.), and maize ( Zea mays L.), and in chilling-tolerant barley ( Hordeum vulgare L.) illuminated with moderate white light (300 µmol m⁻² s⁻¹) at 4°C using chlorophyll a fluorescence measurements. PSII activity was inhibited in leaves of all the four plants as suggested by the decline in F _v/ F _m, 1/ F _o − 1/ F _m, and F _v/ F _o values. The changes in initial fluorescence level ( F _o), F _v/ F _m, 1/ F _o − /1/ F _m, and F _v/ F _o ratios indicate a stronger PSII inhibition in cucumber, maize and tomato plants. The kinetics of chlorophyll a fluorescence rise showed complex changes in the magnitudes and rise of O-J, J-I, and I-P phases caused by photoinhibition. The selective suppression of the J-I phase of fluorescence rise kinetics provides evidence for weakened electron donation from the oxidizing side, whereas the accumulation of reduced Q_A suggests damage to the acceptor side of PSII. These findings imply that the process of chilling-induced photoinhibition involves damage to more than one site in the PSII complexes. Furthermore, comparative analyses of the decline in F _v/ F _o and photooxidation of P700 explicitly show that the extent of photoinhibitory damage to PSII and photosystem I is similar in leaves of cucumber plants grown at a low irradiance level.     

Photosynthetic rates and partitioning of absorbed light energy in photoinhibited leaves

Parameters for the evaluation of the effects of photoinhibition on photosynthetic carbon gain were studied in Chenopodium album leaves. The light-response curve of photosynthetic rate was determined at 36 Pa CO₂ partial pressure and fitted by a non-rectangular hyperbola. Both the initial slope of the curve and the light-saturated rate decreased in photoinhibited leaves, although the decrease in the latter was small. The convexity of the curve was also smaller in photoinhibited leaves. The capacities of ribulose-1,5-bisphosphate carboxylation ( V _cmax) and electron transport ( J _max) were estimated from the CO₂-response curves. V _cmax and J _max decreased similarly with increasing photoinhibition. Energy partitioning in photosystem II (PSII) was estimated using chlorophyll fluorescence parameters. The fraction of energy that was consumed by photochemistry decreased with increasing photoinhibition. However, an increase in inactive PSII, decreasing energy partitioning to active PSII, relaxed the excitation pressure in PSII, and led to a reduction in the fraction of excess energy that was neither consumed by photochemistry nor dissipated as heat.     

UV-B对设施桃叶片光合功能及叶绿体超微结构的影响

对温室栽培的油桃中油5号(Prunus persica var. nectarina cv. ‘Zhongyou5’)适量补充UV-B, 分析其对桃叶片光合功能及叶绿体超微结构的影响。结果表明, UV-B处理下各色素含量均有不同程度的增加, 其中叶绿素b的含量和净光合速率(P_n)提升幅度较大。相较于未补充UV-B的桃树(对照), UV-B处理的F_v/F_m无显著变化, F_v'/F_m'比值、光化学猝灭系数(qP)、非光化学猝灭系数(qN)以及PSII实际光化学量子效率(Φ_PSII)均有显著或极显著升高。透射电镜结果显示, UV-B处理下叶绿体基质片层空隙小, 堆叠紧密, 叶绿体外膜边缘清晰。可见, 温室内适量补充UV-B可快速改善叶片叶绿体的超微结构, 提升叶绿素分子捕获光能及向PSII传递的能力, 增大PSII反应中心的开放程度, 提高实际光能转化效率和PSII电子传递量子效率, 提高叶片的光合功能。该研究为设施果树光合性能改善和UV-B合理利用提供了理论依据。