Application of Lake-model based indices from chlorophyll fluorescence on sugarcane seedling Application of Lake-model based indices from chlorophyll fluorescence on sugarcane seedling

 为从能量平衡及分配的角度研究干旱胁迫下甘蔗(Saccharum officinarum)苗期光系统的运转状况, 进而为丰富不同甘蔗品种的抗旱性评价指标及实现对季节性干旱胁迫的快速诊断提供理论依据, 该研究通过对基于Lake模型的叶绿素荧光参数在不同入射光强下变化的动态分析, 研究光合电子传递链中能量平衡状态对不同水分梯度(40%、25%、10%、8%)的响应。结果表明: 两个供试品种(耐旱品种‘ROC22’和非耐旱品种‘ROC16’)的最大光能利用效率(F_v/F_m)、相对电子传递速率(rETR)、光系统II(PSII)量子效率(Φ_II)和光化学猝灭(q_L)均随着干旱胁迫程度的增加而下降, 可调节性能量耗散(Φ_NPQ)和非调节性能量耗散(Φ_NO)则随着干旱胁迫程度的增加而上升。除Φ_NO之外的叶绿素荧光参数的变化幅度均随着光合有效辐射(PAR)的增加而增大。在干旱胁迫的前中期, 相对于‘ROC22’, ‘ROC16’的PSII反应中心能够维持较高的开放程度; 但‘ROC22’调节能量耗散的能力和对干旱胁迫的敏感程度均高于‘ROC16’, 说明较强的光保护能力是‘ROC22’的抗旱性高于‘ROC16’的主要原因之一。对干旱胁迫敏感且在不同PAR下较为稳定的Φ_NO可作为甘蔗苗期抗旱性的快速诊断和评价指标。rETR对递增的PAR的响应表现为随着干旱胁迫程度的增加而提前出现峰值或下降趋势, 但是不同水分梯度下的rETR在PAR较低时并无显著差异, 表明干旱胁迫下光抑制现象的提早出现是造成光系统损伤的首要因素, 高光强对干旱胁迫信号起放大作用。     

Photosynthetic characteristics and effects of exogenous glycine of Chorispora bungeana under drought stress

We investigated the photosynthetic characteristics of Chorispora bungeana under conditions of drought stress caused by different concentrations of polyethylene glycol-6000 (PEG; 0, 5, 20, and 40%) and various concentrations of exogenous glycine (0, 5, 10, and 20 mM) with 20% PEG. We showed that moderate and severe drought stress of PEG reduced the chlorophyll (Chl) content (both Chl a and b), maximal quantum yield of PSII photochemistry (F_v/F_m), actual photochemical efficiency of PSII in light (Y_II), and quantum yield of regulated energy dissipation (Y_NPQ), while Chl a/b and quantum yield of nonregulated energy dissipation (Y_NO) increased. The low and moderate drought stress increased Mg²⁺ and Fe³⁺ contents, while a decrease in Mg²⁺ and Fe³⁺ was found under severe drought stress. Compared to sole PEG stress, the addition of exogenous 10 mM glycine increased Chl, Mg²⁺ and Fe³⁺ contents, F_v/F_m, Y_II, and Y_NPQ, and reduced Y_NO. On the contrary, 20 mM glycine showed an opposite effect, except for Y_NO. Our results proved that Chl contents and fluorescence parameters are reliable indicators for drought tolerance of C. bungeana. We suggest that a proper glycine content can relieve the effect of drought stress on C. bungeana.     

Effects of heat and high irradiance stress on energy dissipation of photosystem II in low irradiance-adapted peanut leaves

To increase crop yields and not to compete for land with food crops, intercropping agricultural cultivation approach was introduced into cultivation of peanut (Arachis hypogaca L.). This approach improves the total yield of the crop per unit area, but decreases the yield of a single crop compared with mono-cropped agricultural cultivation approach. In wheat-peanut relay intercropping system, peanut plants would suffer heat and high light (HI) stress after wheat harvest. In the present work, peanut seedlings were cultivated in low light to simulate wheat-peanut relay intercropping environments. Upon exposure to heat and HI stress, energy dissipation in PSII complexes was evaluated by comparing those cultivated in low irradiance conditions with the mono-cropped peanut. The maximal photochemical efficiency of PSII (F_v/F_m) and the net photosynthetic rate (P_n) decreased markedly in relay-cropped peanut (RP) after heat and HI stress, accompanied by higher degree of PSII reaction center closure (1–qP). After heat and HI stress, higher antioxidant enzyme activity and less ROS accumulation were observed in mono-cropped peanut (MP) seedlings. Meanwhile, higher content of D1 protein and higher ratio of (A + Z)/(V + A + Z) were also detected in MP plants under such stress. These results implied that heat and HI stress could induce photoinhibition of PSII reaction centers in peanut seedlings and both xanthophyll cycle-dependent thermal energy dissipation and the antioxidant system were down-regulated in RP compared to classical monocropping systems after heat and high irradiance stress.     

Differential responses of photosystems I and II to seasonal drought in two Ficus species

Hemiepiphytic Ficus species exhibit more conservative water use strategy and are more drought-tolerant compared with their non-hemiepiphytic congeners, but a difference in the response of photosystem I (PSI) and photosystem II (PSII) to drought stress has not been documented to date. The enhancement of non-photochemical quenching (NPQ) and cyclic electron flow (CEF) have been identified as important mechanisms that protect the photosystems under drought conditions. Using the hemiepiphytic Ficus tinctoria and the non-hemiepiphytic Ficus racemosa, we studied the water status and the electron fluxes through PSI and PSII under seasonal water stress. Our results clearly indicated that the decline in the leaf predawn water potential (ψ_pd), the maximum photosynthetic rate (A_max) and the predawn maximum quantum yield of PSII (F_v/F_m) were more pronounced in F. racemosa than in F. tinctoria at peak drought. The F_v/F_m of F. racemosa was reduced to 0.69, indicating net photoinhibition of PSII. Concomitantly, the maximal photo-oxidizable P700 (P_m) decreased significantly in F. racemosa but remained stable in F. tinctoria. The fraction of non-photochemical quenching [Y(NPQ)] and the ratio of effective quantum yield of PSI to PSII [Y(I)/Y(II)] increased for both Ficus species at peak drought, with a stronger increase in F. racemosa. These results indicated that the enhancement of NPQ and the activation of CEF contributed to the photoprotection of PSI and PSII for both Ficus species under seasonal drought, particularly for F. racemosa.     

Photosynthesis and PSII functionality of drought-resistant and drought-sensitive weeping lovegrass plants

World areas subject to drought are expected to increase under conditions of climate change. The purpose of this study is to clarify the response of grass species that can grow and produce under water stress. Therefore leaf photosynthesis, chlorophyll fluorescence and pigment content response to water stress were studied in two varieties of the C₄ grass Eragrostis curvula. Two-year-old plants of cv Ermelo and Consol were grown in plastic pots. Drought stress was imposed by withholding irrigation for 15 days and then rewatering for 5 days. During drought relative water content (RWC) decreased 65% in cv Ermelo, while lower reductions of RWC were observed in cv Consol. During the experiment in cv Ermelo increasing drought stress severity caused large decreases in photosynthetic rates, maximal PSII photochemical efficiency (F_V/F_M) and leaf pigment content. Cv Consol showed small variations in these parameters. Compared to cv Consol, after 15 days of drought, effective PSII quantum yield (Φ_II) was significantly lower in cv Ermelo. Reductions of Φ_II were related to significant reductions of open PSII energy capture efficiency (F′_V/F′_M). Photosynthetic response to increasing PPFD levels and to internal CO₂ concentration (C_i) were reduced by drought in cv Ermelo. Compared to well-watered control plants and to cv Consol, drought stressed plants of cv Ermelo showed also reductions of the initial slope of photosynthetic response to C_i and in the photosynthetic rate measured at saturating C_i. Moreover stomatal conductance (g) of both cvs decreased during drought. However, g was lower in drought stressed plants of cv Consol than in cv Ermelo. Water stress caused large reductions in leaf chlorophyll and carotenoid content in cv Ermelo, and small reductions in cv Consol. In drought-sensitive cv Ermelo water stress reduced the capabilities to down regulate PSII functionality through thermal energy dissipation. Results suggest that drought resistance of cv Consol, can be attributed to a higher water use efficiency.     

Exogenous proline induces soluble sugar accumulation and alleviates drought stress effects on photosystem II functioning of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> leaves

The effects of exogenous applied proline (Pro), on photosystem II (PSII) photochemistry of drought stressed (DS) 4-week old Arabidopsis thaliana plants, was studied by using chlorophyll (chl) fluorescence imaging. The maximum quantum yield of PSII photochemistry (F _v /F _m) in DS plants decreased significantly to 77% of that of the control value, suggesting that DS plants could not maintain PSII function, possibly due to accelerated photoinhibition of PSII. Free Pro and total soluble sugars (SS) increased, in response to DS. Exogenous foliar application of Pro by spraying, led to a remarkable increase in the accumulation of Pro and surprisingly also of SS. Both of them served to scavenge reactive oxygen species (ROS), as it was evident by the decreased lipid peroxidation level measured as malondialdehyde (MDA). DS plants sprayed with Pro showed a tolerance to photoinhibition, this indicated by F _v/F _m being close to values typical of healthy leaves by maintaining more than 98% of PSII function. Also the higher quantum efficiency of PSII photochemistry (Φ _PSΙΙ ) and the decreased excitation pressure (1 ? q _p ) recorded for stressed leaves with Pro, lead us to conclude that Pro appears to be involved in the protection of chloroplast structures by quenching ROS. The enhanced dissipation of excess light energy of PSII, in part accounts for the observed increased resistance to DS in A. thaliana leaves with Pro. Our data pointed out that Pro signalling interacts with SS signaling pathway and provided a new insight in Pro metabolism.     

Photosynthesis and activity of photosystem II in response to drought stress in Amur Grape (Vitis amurensis Rupr.)

The Amur Grape (Vitis amurensis Rupr.) cultivars ??shuangFeng?? and ??ZuoShanyi?? were grown in shelter greenhouse under natural sunlight and subjected to drought. Sap flow rate, net photosynthetic rate (P _N), and chlorophyll (Chl) fluorescence were measured on Amur Grape leaves subjected to different drought treatments. Significant decreases in P _N were associated with increasing intercellular CO₂ concentration (C _i), suggesting that the reduction in P _N was caused by nonstomatal limitation. Analysis of OJIP transients according to the JIP-test protocol revealed that specific (per PSII reaction center) energy fluxes for light absorption, excitation energy trapping and electron transport have significantly changed. The appearance of a pronounced K-step and J-step in polyphasic rise of fluorescence transient suggested the oxygen-evolving complex and electron transport were inhibited. Drought stress has relatively little effect on the parameter maximal quantum yield of PSII photochemistry (F_v/F_m), but the performance index (PIABS) is more sensitive in different drought treatment. There are cultivar differences in the response of PSII activity to drought, the photosynthetic apparatus of ??ZuoShanyi?? cultivar is more resistant to drought than that of ??ShuangFeng??, and JIP-test could be a useful indicator for evaluation and selection to drought tolerance.     

Effect of drought on pigments, osmotic adjustment and antioxidant enzymes in six woody plant species in karst habitats of southwestern China

Drought stress is one of the most important factors limiting the survival and growth of plants in the harsh karst habitats of southwestern China. Detailed knowledge about the ecophysiological responses of native plants with different growth forms to drought stress could contribute to the success of re-vegetation programs. Two shrubs, Pyracantha fortuneana and Rosa cymosa, and four trees, Broussonetia papyrifera, Cinnamomum bodinieri, Platycarya longipes and Pteroceltis tatarinowii, were randomly assigned to four drought treatments, i.e. well-watered, mild drought stress, moderate drought stress, and severe drought stress. Midday water potential, the maximum quantum efficiency of PSII photochemistry (F_v/F_m), pigments, osmotic solutes (soluble sugars and proline), cellular damages, and antioxidant enzymes (superoxide dismutase, catalase and peroxidase) were investigated. Drought stress significantly decreased pigments content, but increased the ratio of carotenoids to total chlorophylls in the studied species. After prolonged severe drought stress, the two shrubs exhibited higher F_v/F_m, less reductions of midday water potential, and lower increases of malondialdehyde content and ion leakage than the four trees. Prolonged severe drought stress largely decreased accumulations of osmotic solutes and activities of antioxidant enzymes in the four trees, but significantly increased proline content and superoxide dismutase activity in the two shrubs and peroxidase activity in P. fortuneana. The positive relationships were observed among activities of antioxidant enzymes, and between contents of osmotic solutes and activities of antioxidant enzymes. These findings suggested that the two shrubs had higher tolerance to severe drought stress than the four trees due to higher capacities of osmotic adjustment and antioxidant protection.     

Effects of Drought Stress on the Photosynthesis in Maize

To clarify how the components of the entire photosynthetic electron transport chain in response to drought stress in maize. The activities of photosystem II (PSII), photosystem I (PSI), and the electron transport chain between PSII and PSI of maize were investigated by prompt fluorescence (PF), delayed fluorescence (DF) and 820 nm modulated reflection (MR). Maize (Zea mays L.) plants were subjected to different levels of soil water availability including control, moderate and severe drought stress. A significant decrease in ?E₀, Ψ₀ and PI_ABS was found in maize treated with moderate drought stress. A significant increase in ABS/RC was observed, but there were no significant change in the fast MR phase and the amplitude of DF under moderate drought stress compared to the control. Under severe drought stress, the exchange capacity between Q_A to Q_B, reoxidation capacity of plastoquinol, and the oxidation and re-reduction rates of PC and P₇₀₀ all decreased. These results demonstrated that moderate drought stress reduced the photochemical activity of PSII from Q_A to PQH₂, while the photochemical activity of PSI was unscathed. However, severe drought stress inhibited the entire electron transport chain from the donor side of PSII to PSI-end electron acceptors. In addition, the photochemical activity of PSII is more sensitive to drought stress than PSI.     

Photosynthetic performance and growth traits in Pennisetum centrasiaticum exposed to drought and rewatering under different soil nutrient regimes

Responses of plants exposed to drought and rewatering have been well documented; however, little is known concerning strategies of psammophyte to drought and rewatering under different soil nutrient regimes. For this study, Pennisetum centrasiaticum under two soil nutrient regimes was subjected to progressive drought and subsequent rewatering. Soil water status, gas exchange characteristics, chlorophyll a fluorescence characteristics as well as biomass traits were measured to investigate ecophysiological responses. Net photosynthesis rate (P _n), stomatal conductance (g _s), water use efficiency, maximum quantum efficiency of photosynthesis system II (PSII, F _V/F _M), electron transport flux per cross section (ET₀/CS₀), and performance index on cross section basis (PI_CS) were suppressed during drought periods for both nutrient regimes. Meanwhile, leaf intercellular CO₂ concentration (C _i ), minimal fluorescence intensity (F ₀), and dissipated energy flux per cross section (DI₀/CS₀) increased. Reversible downregulation of PSII photochemistry and enhanced thermal dissipation of excess excitation energy (DI₀/CS₀) contributed to enhanced photo-protection in drought-stressed plants. Thus, the results indicate that P. centrasiaticum is capable of withstanding and surviving extreme drought events, and the recovery pattern of stressed P. centrasiaticum under both nutrient regimes was similar. However, fertilization increased the biomass and the variation in gas exchange and chlorophyll a fluorescence characteristics during drought periods. Additionally, fertilization accelerated the process of drought and aggravated stress under extreme drought events. Thus, the fertilization strategy used in P. centrasiaticum restoration should be carefully selected—fertilization may not always be beneficial.     

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.     

Response of chlorophyll fluorescence parameters to drought stress in sugar beet seedlings

We compared the parameters of chlorophyll fluorescence between two sugar beet (Beta vulgaris L.) species differing in drought tolerance. Our results indicated that there were different responses to the drought stress of these sugar beet species. In drought-tolerant sugar beet, the F ₀ increased slightly, while qN increased substantially, indicating that these plants can protect PSII reaction centers from the damage. F _v/F _m and qP decreased slightly during the initial period of drought stress; this suggests that there is a slight impact of drought stress on the openness of PSII reaction centers, and thus the plants did not suffer seriously. This was further shown by the decreased Yield and electron transfer rate. The parameters of chlorophyll fluorescence were stable and can be used as an important indicator for sugar beet seedlings in the early drought tolerance.     

PnF3H,a flavanone 3-hydroxylase from the Antarctic moss <Emphasis Type="Italic">Pohlia nutans</Emphasis>, confers tolerance to salt stress and ABA treatment in transgenic <Emphasis Type="Italic">Arabidopsis</Emphasis>

In the condition of prolonged drought stress during the reproductive stage, we addressed the photosynthetic performance in flag leaves of the high-yield hybrid rice (Oryza sativa L.) LYPJ. The chlorophyll a fluorescence transient dynamics analysis indicated a timely and constant responsive pattern involving in both PSI and PSII. For PSII functionality, uncoupling of oxygen evolving complex at the donor side and inhibition of electron transport from Q_A to Q_B at the accepter side were both accounted for the decrease of quantum yield of primary photochemistry at early stage (before 21 days after the onset of drought stress). Likewise, increased size of functional antenna may be primarily responsible for early reaction centers inactivation in drought stressed plants, but transformation to non-Q_A-reducing centers for the later. The consequent redundant excitation energy was predominantly eliminated by the increasing thermal dissipation. Advanced accumulation of drought stress (from 21 to 35 days) showed preferential impact on the donor side of PSII and significant loss of RC/CS₀ was induced during this period. In brief, up-regulation of thermal dissipation and possible cyclic electron transport, as well as down-regulation of activated reaction centers and linear electron transport was crucial for rebalance the energy distribution between the two photosystems from deviant stoichiometry resulting from the uncoupling of oxygen evolving complex.     

The contribution of drought-related decreases in foliar nitrogen concentration to decreases in photosynthetic capacity during and after drought in prairie grasses

While stomatal closure usually limits photosynthesis during drought, our previous results suggest that drought-related decreases in foliar nitrogen concentration (N_L) limit photosynthesis during recovery from drought in prairie grasses. Here we estimate the importance of decreases in N_L to decreased photosynthetic capacity (PS_cap) during drought and a subsequent recovery period in three perennial C₄ prairie grasses. PS_cap (O₂ evolution at light and CO₂ saturation) decreased 69 to 78% during drought in these grasses, and full recovery of PS_cap required 8 to 12 days, until younger leaves were expanded or older leaves were repaired, depending on species. Decreases in N_L explained 38 to 51% of the loss of PS_cap during drought and accounted for 51 to 69% of the total loss of PS_cap integrated over the post-drought recovery period. N-related loss of PS_cap appeared to result more from decreases in ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (4.1.1.31), and other soluble photosynthetic enzymes, than from decreases in thylakoid N-containing compounds. Decreases in quantum yield of O₂ evolution and F_v/F_m (variable-to-maximum fluorescence of dark-adapted leaves) during drought were small, so we assumed that little damage to photsystem II (PSII) and thylakoid membrane function occurred. Further, F₀ (minimum F) decreased or remained unchanged, dark F₀ was greater than light F₀, and decreases in photochemical quenching (the fraction of oxidized PSII) were reversed within 1–3 days after drought. Therefore, prolonged increases in non-photochemical quenching (q_n; thermal dissipation of excess light energy) during and after drought were indicative of protective downregulation and were likely associated with disproportionate loss of soluble photosynthetic proteins during drought. In support of this, post-drought recovery of q_n paralleled recovery of N_L and PS_cap. Thus, in C₄ prairie grasses, loss of PS_cap during drought is largely the result of decreases in shoot N_L and of associated protective downregulation, decreasing carbon assimilation for 1–2 weeks after drought.     

Xanthophyll cycle and light stress in nature: uniform response to excess direct sunlight among higher plant species

Photosystem II (PS II) efficiency, nonphotochemical fluorescence quenching, and xanthophyll cycle composition were determined in situ in the natural environment at midday in (i) a range of differently angled sun leaves ofEuonymus kiautschovicus Loesener and (ii) in sun leaves of a wide range of different plant species, including trees, shrubs, and herbs. Very different degrees of light stress were experienced by these leaves (i) in response to different levels of incident photon flux densities at similar photosynthetic capacities amongEuonymus leaves and (ii) as a result of very different photosynthetic capacities among species at similar incident photon flux densities (that were equivalent to full sunlight). ForEuonymus as well as the interspecific comparison all data fell on one single, close relationship for changes in intrinsic PSII efficiency, nonphotochemical fluorescence quenching, or the levels of zeaxanthin + antheraxanthin in leaves, respectively, as a function of the actual level of light stress. Thus, the same conversion state of the xanthophyll cycle and the same level of energy dissipation were observed for a given degree of light stress independent of species or conditions causing the light stress. Since all increases in thermal energy dissipation were associated with increases in the levels of zeaxanthin + antheraxanthin in these leaves, there was thus no indication of any form of xanthophyll cycle-independent energy dissipation in any of the twenty-four species or varieties of plants examined in their natural environment. It is also concluded that transient diurnal changes in intrinsic PSII efficiency in nature are caused by changes in the efficiency with which excitation energy is delivered from the antennae to PSII centers, and are thus likely to be purely photoprotective. Consequently, the possibility of quantifying the allocation of absorbed light into PSII photochemistry versus energy dissipation in the antennae from changes in intrinsic PSII efficiency is explored.Abbreviations A antheraxanthin - F actual level of fluorescence - F_a, F _o minimal fluorescence in the absence, presence of thylakoid energization - F_m, F _m maximal fluorescence in the absence, presence of thylakoid energization - F_m, - F)/F _m actual PSII efficiency ( = percent of absorbed light utilized in PSII photochemistry) - F_v/F_m, F _v /F_m/ PSII efficiency of open centers in the absence, presence of thylakoid energization - NPQ nonphotochemical fluorescence quenching - F_m/F _m - 1; q_p quenching coefficient for photochemical quenching - V violaxanthin - Z zeaxanthin     

Photosynthetic electron transport and specific photoprotective responses in wheat leaves under drought stress

The photosynthetic responses of wheat (Triticum aestivum L.) leaves to different levels of drought stress were analyzed in potted plants cultivated in growth chamber under moderate light. Low-to-medium drought stress was induced by limiting irrigation, maintaining 20 % of soil water holding capacity for 14 days followed by 3 days without water supply to induce severe stress. Measurements of CO₂ exchange and photosystem II (PSII) yield (by chlorophyll fluorescence) were followed by simultaneous measurements of yield of PSI (by P700 absorbance changes) and that of PSII. Drought stress gradually decreased PSII electron transport, but the capacity for nonphotochemical quenching increased more slowly until there was a large decrease in leaf relative water content (where the photosynthetic rate had decreased by half or more). We identified a substantial part of PSII electron transport, which was not used by carbon assimilation or by photorespiration, which clearly indicates activities of alternative electron sinks. Decreasing the fraction of light absorbed by PSII and increasing the fraction absorbed by PSI with increasing drought stress (rather than assuming equal absorption by the two photosystems) support a proposed function of PSI cyclic electron flow to generate a proton-motive force to activate nonphotochemical dissipation of energy, and it is consistent with the observed accumulation of oxidized P700 which causes a decrease in PSI electron acceptors. Our results support the roles of alternative electron sinks (either from PSII or PSI) and cyclic electron flow in photoprotection of PSII and PSI in drought stress conditions. In future studies on plant stress, analyses of the partitioning of absorbed energy between photosystems are needed for interpreting flux through linear electron flow, PSI cyclic electron flow, along with alternative electron sinks.     

Photoprotective mechanisms in photosystem II of Ephedra monosperma during development of frost tolerance

Dissipation of light energy absorbed by photosystem II (PSII) in assimilating shoots of an evergreen shrub Ephedra monosperma was investigated during its transition from the vegetative to frost-tolerant state under natural conditions of Central Yakutia. The dynamics of modulated chlorophyll fluorescence and carotenoid content was analyzed during seasonal decrease in ambient temperature. The seasonal cooling was accompanied by a stepwise decrease in photochemical activity of PSII (F _v/F _m = (F _m ? F ₀)/F _m). The decrease in F _v/F _m occurred from the beginning of September to the end of October, when the temperature was lowered from 10 to ?8°C. During winter period the residual activity of PSII was retained at about 30% of the summer values. The seasonal decrease in temperature was accompanied by a significant stimulation of pH-independent dissipative processes in reaction centers and antenna of PSII. The increase in energy losses was paralleled by a proportional increase in zeaxanthin content on the background of decreasing content of violaxanthin and β-carotene as possible zeaxanthin precursors. At the same time, inhibition of light-induced non-photochemical quenching in the PSII antenna was observed. The results suggest that principal photoprotective mechanisms during seasonal lowering of temperature are: (1) inactivation of PSII and dissipation of excitation energy in PSII reaction centers and (2) zeaxanthin-mediated energy dissipation in the antenna complexes. The first mechanism seems to prevail at early stages of seasonal cooling, whereas both mechanisms are recruited from the onset of sustained freezing temperatures.     

The increase in unsaturation of fatty acids of phosphatidylglycerol in thylakoid membrane enhanced salt tolerance in tomato

Overexpression of chloroplastic glycerol-3-phosphate acyltransferase gene (LeGPAT) in tomato increased cis-unsaturated fatty acid content in phosphatidylglycerol (PG) of thylakoid membrane. By contrast, suppressing the expression of LeGPAT decreased the content of cis-unsaturated fatty acid in PG. Under salt stress, sense transgenic plants exhibited higher activities of chloroplastic antioxidant enzymes, lower content of reactive oxygen species (ROS) and less ion leakage compared with the wild type (WT) plants. The net photosynthetic rate (P _N) and the maximal photochemical efficiency (F_v/F_m) of photosystem II (PSII) decreased more slightly in sense lines but more markedly in the antisense ones, compared to WT. D1 protein, located in the reactive center of the PSII, is the primary target of photodamage and has the highest turnover rate in the chloroplast. Under salt stress, compared with WT, the content of D1 protein decreased slightly in sense lines and significantly in the antisense ones. In the presence of streptomycin (SM), the net degradation of the damaged D1 protein was faster in sense lines than in other plants. These results suggested that, under salt-stress conditions, increasing cis-unsaturated fatty acids in PG by overexpression of LeGPAT can alleviate PSII photoinhibition by accelerating the repair of D1 protein and improving the activity of antioxidant enzymes in chloroplasts.