Photochemical efficiency of Photosystem II and xanthophyll cycle components in Zea mays leaves exposed to water stress and high light

The effects of two light treatments (photosynthetically active photon flux density of either 650 or 1950 µmol m^–2 s^–1) on the photochemical efficiency of Photosystem II (PS II) (measured as variable to maximum fluorescence ratio) and on the xanthophyll cycle components was studied in wilted Zea mays leaves. For comparison, these parameters were followed under the same light conditions in well-hydrated leaves maintained either in normal or CO₂-free air. The net CO₂ assimilation of dehydrated leaves declined rapidly as their relative water content (RWC) decreased from 100 to 60% while the PS II efficiency measured after a prolonged dark period of 16 h declined only when RWC leaves was lower than 60%. Furthermore, drought caused an increase in the pool size of the xanthophyll cycle pigments and the presence of a sustained elevated level of zeaxanthin and antheraxanthin at the end of the long dark period. The leaf water deficit enhanced the sensitivity of PS II efficiency to light exposure. During illumination, strong inhibition of PS II efficiency and large violaxanthin deepoxidation was observed in wilted leaves even under moderate photon flux density compared to control leaves in the same conditions. After 2 h of darkness following the light treatment, the PS II efficiency that is dependent on the previous PPFD, decreased with leaf water deficit. Moreover, zeaxanthin epoxidation led to an accumulation of antheraxanthin in dehydrated leaves. All these drought effects on PS II efficiency and xanthophyll cycle components were also obtained in well-hydrated leaves by short-term CO₂ deprivation during illumination. We conclude that the increased susceptibility of PS II efficiency to light in wilted maize leaves is mainly explained by the decrease of CO₂ availability and the resulting low net CO₂ assimilation.     

绿斑病藻寄生对夏橙叶片光合作用特性的影响

以盆栽的2年生奥灵达夏橙为试材,研究了绿斑病藻寄生对夏橙叶片光合作用特性的影响.结果表明,轻度病叶对叶绿素总量（Chl a+b）、类胡萝卜素含量（Car）、净光合速率（P_n）、胞间CO₂浓度（C_i）、原初光能转换效率（F_v/F_m）、光合电子传递量子效率（ΦPS Ⅱ）和光化学猝灭系数（qP）无显著影响;中度病叶和重度病叶的Chla+b、Car、P_n、F_v/F_m、ΦPS Ⅱ和qP较对照分别下降了23.85%、26.49%、43.3%、4.5%、35.1%、22.5%和37.61%、44.04%、64.5%、8.6%、63.6%、40.1%,与对照差异显著,而C_i较对照显著上升.绿斑病藻的大量寄生减弱了夏橙叶片的光合作用,而净光合速率的下降主要是由非气孔因素限制引起.     

Photo- and antioxidative protection,and a role for salicylic acid during drought and recovery in field-grown<Emphasis Type="Italic"> Phillyrea angustifolia</Emphasis> plants

Mechanisms of photo- and antioxidative protection, the extent of oxidative stress, and salicylic acid accumulation in leaves of Phillyrea angustifolia L. (Oleaceae) plants exposed to drought and recovery in Mediterranean field conditions were studied. The amounts of alpha-tocopherol increased up to 4-fold and those of zeaxanthin increased up to 3-fold at relative leaf water contents (RWCs) of ca. 60%, which caused up to 70% increases in the de-epoxidation state of the xanthophyll cycle (DPS). While alpha-tocopherol increased further in severe drought, zeaxanthin levels and DPS remained constant, beta-carotene decreased and malondialdehyde (MDA) levels increased at RWCs below 50%. Though this was associated with significant decreases in the maximum efficiency of photosystem II photochemistry (F(v)/ F(m)), the same leaves that suffered from drought recovered after rainfalls, and similar MDA levels and F(v)/ F(m) ratios to those observed before drought were attained. During recovery (i) the F(v)/ F(m) ratio and beta-carotene levels increased slowly, (ii) alpha-tocopherol levels decreased sharply, to increase again, and (iii) MDA levels in leaves increased to values 35% higher than those observed at maximum drought, and decreased later. Salicylic acid (SA) levels showed a strong negative correlation (r(2)=0.857) with the RWC, and increased progressively up to 5-fold, during drought. During recovery, SA levels decreased, but remained slightly higher than those observed before drought. SA levels were positively correlated with those of alpha-tocopherol during drought (r(2)=0.718), but not during recovery (r(2)=0.221). We conclude that (i) P. angustifolia plants activate several mechanisms of photo- and antioxidative protection to withstand drought stress during a Mediterranean summer, (ii) endogenous SA levels increase in leaves of drought-stressed plants, thus suggesting a role for SA in plant responses to drought, and (iii) plants suffer oxidative stress during recovery, and this stress is more severe as the previous drought is more intense.     

Photosystem II photochemical efficiency,zeaxanthin and antioxidant contents in the poikilohydric Ramonda serbica during dehydration and rehydration

Changes in photochemical efficiency, non-radiative energy dissipation (NRD), de-epoxidation state of xanthophyll cycle components (DPS) and contents of the antioxidants ascorbic acid and glutathione were studied in leaves of the poikilohydric Ramonda serbica Panc. (Gesneriaceae) during cycles of dehydration and subsequent rehydration. In drying leaves, the intrinsic efficiency of PS II photochemistry and the photon yield of PS II electron transport showed strong progressive decreases. Simultaneously, the fraction of excitation energy dissipated as heat in the PS II antenna increased markedly. The energy-dependent component of non-photochemical quenching (NPQ) showed an increase in dehydrating leaves down to relative water contents (RWC) values near 30%. Further decreases in RWC below these values caused a decrease in NPQ. Accordingly, DPS showed a similar behaviour, with a sharp increase and a subsequent decrease at very low RWC, although the maximum DPS was reached at slightly lower RWC than that for the maximum NPQ. The pools of reduced ascorbate and glutathione increased strongly when the RWC values fell below 40% and remained high in fully dehydrated leaves. When plants were re-watered photosynthetic efficiency, NRD, DPS and antioxidant contents recovered their initial control values. However, during rehydration, the zeaxanthin content showed a transient increase, as did NPQ, indicating an increasing demand for non-radiative dissipation. On the other hand, the contents of reduced ascorbate and reduced glutathione decreased but were still relatively high in the initial phase of rehydration, when the rate of photosynthetic electron transport, proton pumping and NRD were still relatively low. These results indicate that several photoprotective mechanisms are operating in R. serbica. Protection from photo-oxidation and photoinhibition appears to be achieved by co-ordinated contributions by ascorbate, glutathione and zeaxanthin-mediated NPQ. This variety of photoprotective mechanisms may be essential for conferring desiccation-tolerance.This revised version was published online in October 2005 with corrections to the Cover Date.     

Drought stress effects on photosystem I content and photosystem II thermotolerance analyzed using Chl a fluorescence kinetics in barley varieties differing in their drought tolerance

Drought stress has multiple effects on the photosynthetic system. Here, we show that a decrease of the relative contribution of the I-P phase, ΔV_IP = − V _I = ( F _M− F _I)/(F_M− F_o), to the fluorescence transient OJIP is observed in 10 drought-stressed barley and 9 chickpea varieties. The extent of the I-P loss in the barley varieties depended on their drought tolerance. The relative loss of the I-P phase seems to be related to a loss of photosystem (PS) I reaction centers as determined by 820-nm transmission measurements. In the second part of this study, the interaction of drought and heat stress in two barley varieties (the drought tolerant variety A¨t Baha and the drought sensitive variety Lannaceur) was studied using a new approach. Heat stress was induced by exposing the plant leaves to temperatures of 25–45°C and the inactivation of the O₂-evolving complex (OEC) was followed measuring chlorophyll a (Chl a ) fluorescence using a protocol consisting of two 5-ms pulses spaced 2.3 ms apart. In active reaction centers, the dark interval is long enough to allow the OEC to recover from the first pulse; whereas in heat-inactivated reaction centers it is not. In the latter category of reaction centers, no further fluorescence rise is induced by the second pulse. Lannaceur, under well-watered conditions, was more heat tolerant than Aït Baha. However, this difference was lost following drought stress. Drought stress considerably increased the thermostability of PS II of both varieties.     

低氧胁迫下黄瓜植株热耗散途径

采用营养液栽培,研究了低氧(营养液溶氧浓度为0.9～1.1 mg·L-1)胁迫下黄瓜幼苗光合作用热耗散与叶黄素循环的关系.结果表明:低氧胁迫下,黄瓜叶片PSⅡ的实际光化学效率(φPSⅡ)、饱和光强下的净光合速率(Pn)、表观量子效率(AQY)和PSⅡ的最大光化学效率(Fv/Fm)均显著降低,表明黄瓜植株的光合作用受到了光抑制;同时,光化学猝灭系数(qp)降低,而热耗散(NPQ)和天线耗散能量(D)的比值显著升高,说明黄瓜叶片热耗散增强;NPQ与叶黄素脱环氧化状态(DEPS)呈显著正相关,且两者均被抗坏血酸(AsA)所促进,被二硫苏糖醇(DTT)所抑制,说明低氧胁迫下,叶黄素循环是黄瓜植株光合作用热耗散的主要途径.     

甘蔗抗旱性生理生化鉴定指标

利用因子分析和灰色关联度分析方法研究了甘蔗叶片相对含水量、膜脂过氧化代谢、活性氧代谢、光合参数及蔗茎产量性状等指标与抗旱性的关系.结果表明,干旱胁迫下甘蔗叶片的MDA含量和PMP明显提高,而RWC、SOD活性、Chl含量、F_v/F_m、F_v/F_o、△F_v/F_t、△F_v/F_o和蔗茎单茎重(SSW)8个抗旱性指标均显著降低.SSW与其它9个生理生化指标的相关性大小依次为PMP＞SOD活性＞MDA含量＞RWC＞F_v/F_o＞F_v/F_m＞Chl含量＞F_v/F_o＞F_v/F_t,其中,SSW与F_v/F_o和ΔF_v/F_t相关性不显著.通过因子分析将10个甘蔗抗旱性指标用4个公共因子表示,累加方差贡献率达到92.08%.因子l主要是反映光合作用特性指标对甘蔗品种抗旱性起支配作用,因子2主要是反映叶片相对含水量及活性氧代谢指标对甘蔗品种抗旱性起支配作用,因子3和因子4分别只有SSW和Chl含量有较大载荷.灰色关联度分析表明,各抗旱性生理生化指标与SSW关联密切程度依次为F_v/F_m＞PMP＞F_v/F_o＞RWC＞MDA含量＞SOD活性＞ΔF_v/F_t＞Chl含量＞F_v/F_o.     

热锻炼对甘蓝幼苗叶片激发能分配的影响

以喜温凉的蔬菜甘蓝为试材,研究了热锻炼与对照甘蓝幼苗叶片光合速率和叶绿素荧光参数对高温胁迫的响应．结果表明,叶片温度在25-35℃之间,热锻炼苗和对照苗叶片叶绿素可变荧光(Ｆv)、光化学猝灭(qP)、非光化学猝灭(qN)、PSⅡ化学效率(ФPSⅡ)没有明显的变化;当叶温高于35℃时。热锻炼苗的Fv、qP和中ФPSⅡ均明显高于对照,37℃时Fv、qP和ФPSⅡ分别比对照高53％、24％和86％;qN较对照低22％,尤其是与光抑制(光破坏)有关的qNs明显降低,以维持较高的高能态猝灭(qNf)耗散过剩激发能。保护PSⅡ反应中心不受破坏。减轻了光抑制,这与热锻炼幼苗叶片在高温下具有较高的光合能力是一致的。     

外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ功能的影响

以叶绿素快相荧光动力学曲线(OJIP)为探针,研究了外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ(PSⅡ)功能的影响.结果表明:干旱胁迫降低了烤烟幼苗PSⅡ原初光能转换效率(Fv/Fm)和电子传递速率(ETR),抑制了光合作用的原初过程,烤烟幼苗叶片发生了明显的光抑制.叶面喷施10.0 mmol·L-1CaCl2溶液后烤烟叶片的光合电子传递能量比例(ФEo)在干旱胁迫下的降低幅度明显小于对照(喷施清水),电子转运效率(ET0/RC)在干旱胁迫下明显高于对照.叶面喷施CaC12溶液增加了PSⅡ捕获光能用于光合电子传递的比例、剩余有活性反应中心的效率和电子传递链中的能量传递,使烤烟叶片的光系统Ⅱ在干旱胁迫下保持相对较高的活性,从而提高了烤烟幼苗的抗旱能力.     

Low-night temperature (LNT) induced changes of photosynthesis in grapevine (Vitis vinifera L.) plants.

Changes of leaf pigments, ribulose-1,5-bisphosphate carboxylase (RuBPC) and photosynthetic efficiency were examined in grapevine (Vitis vinifera L.) plants grown under ambient irradiation (maximum daily PAR = 1500 micromol m(-2) s(-1)) for 7 days to low night temperature (LNT) of 5 degrees C (daily from 18:00 to 06:00). The contents of chlorophyll (Chl) and carotenoids (Car) per fresh mass were lower in LNT leaves than in control leaves. The contents of alpha + beta carotene and lutein-5,6-epoxide remained unaffected, but the de-epoxidation state involving the components of xanthophyll cycle increased. RuBPC activity and soluble proteins were also significantly reduced in LNT leaves. In isolated thylakoids, a marked inhibition of whole chain (PS I + PS II) and PS II activity were observed in LNT leaves. Smaller inhibition of PS I activity was observed in LNT leaves. The artificial exogenous electron donors, MnCl2, DPC and NH2OH did not restored the loss of PS II activity in LNT leaves. The same results were obtained when F(v)/F(m) was evaluated by Chl fluorescence measurements. The marked loss of PS II activity in LNT leaves was due to the marked loss of D1 protein which was determined by immunological studies.     

Effects of Drought Stress on Key Enzymes of Carbon Metabolism,Photosynthetic Characteristics and Agronomic Traits of Soybean at the Flowering Stage under Different Soil Substrates

Soybean is an important legume food crop, and its seeds are rich in nutrients, providing humans and animals with edible oil and protein feed. However, soybean is sensitive to water requirements, and drought is an important factor limiting soybean yield and quality. This study used Heinong 84 (drought resistant variety) and Hefeng 46 (intermediate variety) as tested varieties planted in chernozem, albic, and black soils. The effects of drought stress on the activities of key enzymes in carbon metabolism and photosynthetic characteristics of soybean were studied during the flowering stage, most sensitive to water. (1) The activities of SS-1, 6PGDH, and G6PDH enzymes in soybean leaves first increased and then decreased under drought stress. The enzyme activity was the highest under moderate drought stress and weakest in the blank group. (2) Drought stress increased Phi2, PhiNO, and Fm in soybean leaves and reached the highest value under severe drought; with the increase in drought stress, PhiNPQ and Fv/Fm of soybean leaves gradually decreased, reaching the lowest under severe drought. (3) With the increase in drought stress, F₀ and Fs of soybean leaves showed a single peak curve, and the maximum was at moderate drought. (4) Correlation analysis showed that F₀ was greatly affected by varieties and soil types; Fs, F0, and Fm soil varieties had a great influence, and chlorophyll fluorescence parameters were affected differently under drought stress with different drought degrees. (5) Drought stress changed the agronomic traits and yield of soybean. With the increase of drought degree, plant height, node number of main stem, effective pod number, 100-seed weight and total yield decreased continuously. (6) Drought stress affected the dry matter accumulation of soybean. With the increase of drought degree, the dry matter accumulation gradually decreased. Among them, the leaf was most seriously affected by drought, and SD decreased by about 55% compared with CK. Under the condition of black soil, the dry matter accumulation of soybean was least affected by drought.     

Identification of Loci Associated with Drought Resistance Traits in Heterozygous Autotetraploid Alfalfa (Medicago sativa L.) Using Genome-Wide Association Studies with Genotyping by Sequencing

Drought resistance is an important breeding target for enhancing alfalfa productivity in arid and semi-arid regions. Identification of genes involved in drought tolerance will facilitate breeding for improving drought resistance and water use efficiency in alfalfa. Our objective was to use a diversity panel of alfalfa accessions comprised of 198 cultivars and landraces to identify genes involved in drought tolerance. The panel was selected from the USDA-ARS National Plant Germplasm System alfalfa collection and genotyped using genotyping by sequencing. A greenhouse procedure was used for phenotyping two important traits associated with drought tolerance: drought resistance index (DRI) and relative leaf water content (RWC). Marker-trait association identified nineteen and fifteen loci associated with DRI and RWC, respectively. Alignments of target sequences flanking to the resistance loci against the reference genome of M. truncatula revealed multiple chromosomal locations. Markers associated with DRI are located on all chromosomes while markers associated with RWC are located on chromosomes 1, 2, 3, 4, 5, 6 and 7. Co-localizations of significant markers between DRI and RWC were found on chromosomes 3, 5 and 7. Most loci associated with DRI in this work overlap with the reported QTLs associated with biomass under drought in alfalfa. Additional significant markers were targeted to several contigs with unknown chromosomal locations. BLAST search using their flanking sequences revealed homology to several annotated genes with functions in stress tolerance. With further validation, these markers may be used for marker-assisted breeding new alfalfa varieties with drought resistance and enhanced water use efficiency.     

Water relations and nitrogen fixation in potassium fedVigna radiata nodules

Drought created by withholding the irrigation at 30 and 45 d after sowing significantly decreased relative water content (RWC) and osmotic potential (ψ_s) ofVigna radiata (L.) Wilczek cv. MH-83-30 nodules. Potassium fed plants showed higher RWC, whereas ψ_s was further declined irrespective of soil moisture levels. The nitrogenase activity and leghemoglobin content of nodules markedly decreased under drought and nodules of potassium fed plants showed better recovery after rehydration. The proline content significantly increased under drought but declined upon reirrigation. Also, the C, N and K contents of nodules significantly declined under drought.     

NO3-胁迫及恢复对黄瓜幼苗叶片叶绿素荧光参数及ATPase活性的影响

通过水培试验,探讨了不同NO₃^-浓度胁迫及恢复对黄瓜幼苗叶片叶绿素含量、叶绿素荧光参数及ATPase活性的影响.结果表明,胁迫7 d后,高浓度NO₃^-(168 mmol·L^-1)可极显著提高叶绿素a、叶绿素b、总叶绿素和类胡萝卜素含量,极显著提高初始荧光(F_o)、Mg ATPase和Ca-ATPase活性,而PSⅡ原初光能转化效率(F_v/F_m)、PSⅡ潜在活性(F_v/F_o)和PSⅡ光合电子传递量子效率(ΦPSⅡ),却随NO₃^-浓度的增加而降低.恢复7 d后,所有处理叶绿素和类胡萝卜素含量均低于对照;初始荧光基本都恢复至对照水平;PSⅡ原初光能转化效率和PSⅡ光合电子传递量子效率在NO₃^-浓度低于126 mmol·L^-1时,基本恢复至对照水平,而高于这一水平时,仍显著低于对照;PSⅡ潜在活性在NO₃^-浓度为42和126 mmol·L^-1的处理基本达对照水平,其它处理仍极显著低于对照;Mg-ATPase和Ca-ATPase活性均出现先降低后升高的变化趋势.     

High-light stress and photoprotection in Umbilicaria antarctica monitored by chlorophyll fluorescence imaging and changes in zeaxanthin and glutathione

The effect of high light on spatial distribution of chlorophyll (Chl) fluorescence parameters over a lichen thallus (Umbilicaria antarctica) was investigated by imaging of Chl fluorescence parameters before and after exposure to high light (1500 micro mol m (-2) s (-1), 30 min at 5 degrees C). False colour images of F (V)/F (M) and Phi (II) distribution, taken over thallus with 0.1 mm (2) resolution, showed that maximum F (V)/F (M) and Phi (II) values were located close to the thallus centre. Minimum values were typical for thallus margins. After exposure to high light, a differential response of F (V)/F (M) and Phi (II) was found. The marginal thallus part exhibited a loss of photosynthetic activity, manifested as a lack of Chl fluorescence signal, and close-to-centre parts showed a different extent of F (V)/F (M) and Phi (II) decrease. Subsequent recovery in the dark led to a gradual return of F (V)/F (M) and Phi (II) to their initial values. Fast (30 min) and slow (1 - 22 h) phase of recovery were distinguished, suggesting a sufficient capacity of photoprotective mechanisms in U. antarctica to cope with low-temperature photoinhibition. Glutathione and xanthophyll cycle pigments were analyzed by HPLC. High light led to an increase in oxidized glutathione (GSSG), and a conversion of violaxanthin to zeaxanthin, expressed as their de-epoxidation state (DEPS). The responses of GSSG and DEPS were reversible during subsequent recovery in the dark. GSSG and DEPS were highly correlated to non-photochemical quenching (NPQ), indicating involvement of these antioxidants in the resistance of U. antarctica to high-light stress. Heterogeneity of Chl fluorescence parameters over the thallus and differential response to high light are discussed in relation to thallus anatomy and intrathalline distribution of the symbiotic alga Trebouxia sp.     

Analysis of xanthophyll cycle carotenoids and chlorophyll fluorescence in light intensity-dependent chlorophyll-deficient mutants of wheat and barley

Three light intensity-dependent Chl b-deficient mutants, two in wheat and one in barley, were analyzed for their xanthophyll cycle carotenoids and Chl fluorescence characteristics under two different growth PFDs (30 versus 600 mol photons·m^–2 s^–1 incident light). Mutants grown under low light possessed lower levels of total Chls and carotenoids per unit leaf area compared to wild type plants, but the relative proportions of the two did not vary markedly between strains. In contrast, mutants grown under high light had much lower levels of Chl, leading to markedly greater carotenoid to Chl ratios in the mutants when compared to wild type. Under low light conditions the carotenoids of the xanthophyll cycle comprised approximately 15% of the total carotenoids in all strains; under high light the xanthophyll cycle pool increased to over 30% of the total carotenoids in wild type plants and to over 50% of the total carotenoids in the three mutant strains. Whereas the xanthophyll cycle remained fairly epoxidized in all plants grown under low light, plants grown under high light exhibited a considerable degree of conversion of the xanthophyll cycle into antheraxanthin and zeaxanthin during the diurnal cycle, with almost complete conversion (over 90%) occurring only in the mutants. 50 to 95% of the xanthophyll cycle was retained as antheraxanthin and zeaxanthin overnight in these mutants which also exhibited sustained depressions in PS II photochemical efficiency (F_v/F_m), which may have resulted from a sustained high level of photoprotective energy dissipation activity. The relatively larger xanthophyll cycle pool in the Chl b-deficient mutant could result in part from the reported concentration of the xanthophyll cycle in the inner antenna complexes, given that the Chl b-deficient mutants are deficient in the peripheral LHC-II complexes.Abbreviations A antheraxanthin - Chl chlorophyll - F_o and F_m minimal yield (at open PS II reaction centers) and maximal yield (at closed centers) of chlorophyll fluorescence in darkness - F level of fluorescence during illumination with photosynthetically active radiation - F_m maximal yield (at closed centers) of chlorophyll fluorescence during illumination with photosynthetically active radiation - (F_m–F)/F_m actual efficiency of PS II during illumination with photosynthetically active radiation - F_v/F_m+(F_m–F_o)/F_m intrinsic efficiency of PS II in darkness - LHC_II light-harvesting chlorophyll-protein complex of Photosystem II - PFD photon flux density (between 400 and 700 nm) - PS I Photosystem I - PS II Photosystem II - V violaxanthin - Z zeaxanthin     

Drought tolerance in three hybrid poplar clones submitted to different watering regimes

Aims Poplars grown in North China may experience water-deficient periods in their life cycle. The aim of the present paper was to quantify the response of three clones to different watering regimes and to determine which clone among the three is the best adapted to drought conditions.Methods Three hybrid poplar clones (clone DN-34, R-247 and OP-367) were used in the present experiment. The seedlings of the three clones were grown under four watering regimes: control (well watered, 100% field water capacity (FC)) and three drought treatments (drought stress I, 50% FC; drought stress II, 40% FC; drought stress III, 30% FC). Changes in morphological, physical and biochemical indicators of the three hybrid poplar clones were investigated.Important findings Drought treatment (50%, 40% and 30% FC) decreased net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (gs), shoot height, total biomass and chlorophyll (Chl) content in all the three clones and it increased activities of antioxidant enzymes and free proline content. The highest values of the above-mentioned morphological and physiological parameters were recorded in clone OP-367 under 30% FC, followed by clone DN-34 and R-247. Relative leaf water content (RWC) and stem diameter (sd) markedly declined in clone R-247 and DN-34 under drought stress I, II and III, whereas RWC and sd declined in clone OP-367 only under drought stress II and III. Clone OP-367 had more RWC and sd than DN-34 and R-247. Only the 30% FC induced an increase in the root-to-shoot ratio (rs) and water use efficiency (WUE) in all the three clones. OP-367 was the most efficient clone in water absorption and use, for plants of the clone had the highest values of rs and WUE. Our data demonstrate that among the three clones, OP-367 was better able to maintain photosynthesis and growth and lower the damage caused by drought.     

CPPU提高柚树苗抗旱性的研究

水分胁迫下,柚树苗叶片RWC、ψw、Pn及可溶性蛋白质,叶绿素含量下降,游离脯氨酸、MDA含量及SOD活性升高,CAT活性先上升后下降,APX活性、AsA和GSH含量明显下降,抗旱性强的品种具有较高的活性氧清除能力。CPPU处理提高柚树苗叶片RWC、Pn、叶绿素、游离脯氨酸、AsA、GSH含量,并增加SOD、APX活性,降低MDA水平,从而提高柚树苗抗旱性。实验结果表明,CPPU处理提高叶片对膜脂过氧化作用的保护能力可能是其提高柚树苗抗旱性的原因。     

Involvement of zeaxanthin and of the Cbr protein in the repair of photosystem II from photoinhibition in the green alga Dunaliella salina.

A light-sensitive and chlorophyll (Chl)-deficient mutant of the green alga Dunaliella salina (dcd1) showed an amplified response to irradiance stress compared to the wild-type. The mutant was yellow-green under low light (100 micromol photons m(-2) s(-1)) and yellow under high irradiance (2000 micromol photons m(-2) s(-1)). The mutant had lower levels of Chl, lower levels of light harvesting complex II, and a smaller Chl antenna size. The mutant contained proportionately greater amounts of photodamaged photosystem (PS) II reaction centers in its thylakoid membranes, suggesting a greater susceptibility to photoinhibition. This phenotype was more pronounced under high than low irradiance. The Cbr protein, known to accumulate when D. salina is exposed to irradiance stress, was pronouncedly expressed in the mutant even under low irradiance. This positively correlated with a higher zeaxanthin content in the mutant. Cbr protein accumulation, xanthophyll cycle de-epoxidation state, and fraction of photodamaged PSII reaction centers in the thylakoid membrane showed a linear dependence on the chloroplast 'photoinhibition index', suggesting a cause-and-effect relationship between photoinhibition, Cbr protein accumulation and xanthophyll cycle de-epoxidation state. These results raised the possibility of zeaxanthin and Cbr involvement in the PSII repair process through photoprotection of the partially disassembled, and presumably vulnerable, PSII core complexes from potentially irreversible photooxidative bleaching.