Enhanced photochemical light utilization and decreased chilling-induced photoinhibition of photosystem II in cotton overexpressing genes encoding chloroplast-targeted antioxidant enzymes

The aim of this study was to determine whether increases in stromal superoxide dismutase (SOD; EC 1.15.1.1), ascorbate peroxidase (APX; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) via transformation could reduce photosystem (PS) II photoinhibition at low temperature for cotton (Gossypium hirsutum L.) plants and to determine by what mechanism this protection may be realized. During 3-h exposures of lincomycin-treated leaf discs to 10 degrees C and a photon flux density of 500 &mgr;mol m-2 s-1, all transgenic plants exhibited significantly greater PSII activity and O2 evolution than did wild-type plants. Also, the rate constant of PSII photoinactivation was significantly lower for all transgenic plants than for wild-type plants. No significant differences existed between genotypes in non-photochemical quenching of chlorophyll a fluorescence and the regulated component of the thermal dissipation of excitation energy. The relationship between changes in variable to maximum chlorophyll fluorescence (Fv/Fm) and the time-dependent averaged excessive light exposure was similar for all genotypes. This observation excluded the possibility that differences in PSII photodamage were due to improvements in the direct protection of PSII from active oxygen by antioxidant enzyme overproduction. Similar decreases in Fv/Fm during the stress treatment for all genotypes when leaves were pre-treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the effect of overproduction involved events downstream of PSII in the electron transfer pathway. Since all transgenic plants exhibited a significantly higher photochemical quenching of chlorophyll fluorescence during the chilling treatment, we concluded that, under the conditions used in this study, the enhancement of the protection of PSII from photodamage by increasing the stromal antioxidant enzyme activity in cotton leaves was due to the maintenance of a higher rate of electron transport and, consequently, a lower reduction state of QA.     

棉花苞叶光呼吸和PSII热耗散对土壤水分的响应

在新疆气候生态条件下, 采用膜下滴灌植棉技术, 设置不同滴灌水分处理, 研究了不同滴灌量条件下棉花(Gossypium hirsutum)苞叶和叶片碳同化、光呼吸作用、光系统II (PSII)热耗散作用及其光破坏防御机制的差异, 以揭示滴灌节水条件下棉花苞叶缓解光抑制的机理及与棉花抗旱特性的关系。结果表明: 棉花开花后苞叶及叶片在高温强光下实际光化学效率(Φ_PSII)显著降低, 发生明显的光抑制现象, 但苞叶的光抑制程度较叶片轻; 与正常滴灌量处理相比, 节水滴灌条件下棉花水分亏缺, 叶片净光合速率(P_n)、Φ_PSII、光呼吸(P_r)、光化学猝灭系数(q_P)降低, 非光化学猝灭系数(NPQ)升高, 叶片光抑制程度加重, 而苞叶P_n、Φ_PSII、P_r、q_P、NPQ变化不大, 与正常滴灌量处理相比, 光抑制程度无显著差异。苞叶光呼吸速率与光合速率的比值(P_r/P_n)显著高于叶片; 滴灌节水条件下棉花适度水分亏缺对苞叶光呼吸及P_r/P_n无显著影响。高温强光下, 棉花节水滴灌对叶片PSII量子产量的转化与分配影响显著, 但对苞叶的影响不显著; 苞叶非调节性能量耗散的量子产量(Y(NPQ))高于叶片, 因此能有效地将PSII的过剩光能以热的形式耗散。综上所述, 与叶片相比, 苞叶对轻度水分亏缺不敏感, 是棉花适应干旱逆境较强的器官, 苞叶光呼吸和热耗散作用对光破坏防御具有重要意义。     

Response of the photosynthetic apparatus of cotton (Gossypium hirsutum) to the onset of drought stress under field conditions studied by gas-exchange analysis and chlorophyll fluorescence imaging.

The functioning of the photosynthetic apparatus of cotton (Gossypium hirsutum) grown during the onset of water limitation was studied by gas-exchange and chlorophyll fluorescence to better understand the adaptation mechanisms of the photosynthetic apparatus to drought conditions. For this, cotton was grown in the field in Central Asia under well-irrigated and moderately drought-stressed conditions. The light and CO(2) responses of photosynthesis (A(G)), stomatal conductance (g(s)) and various chlorophyll fluorescence parameters were determined simultaneously. Furthermore, chlorophyll fluorescence images were taken from leaves to study the spatial pattern of photosystem II (PSII) efficiency and non-photochemical quenching parameters. Under low and moderate light intensity, the onset of drought stress caused an increase in the operating quantum efficiency of PSII photochemistry (varphi(PSII)) which indicated increased photorespiration since photosynthesis was hardly affected by water limitation. The increase in varphi(PSII) was caused by an increase of the efficiency of open PSII reaction centers (F(v)'/F(m)') and by a decrease of the basal non-photochemical quenching (varphi(NO)). Using a chlorophyll fluorescence imaging system a low spatial heterogeneity of varphi(PSII) was revealed under both irrigation treatments. The increased rate of photorespiration in plants during the onset of drought stress can be seen as an acclimation process to avoid an over-excitation of PSII under more severe drought conditions.     

Loss of the precise control of photosynthesis and increased yield of non‐radiative dissipation of excitation energy after mild heat treatment of barley leaves

The after effects of a short exposure of intact barley leaves to moderately elevated temperature (40°C, 5 min) on the induction transients and the irradiance dependencies of photosynthesis and chlorophyll fluorescence are presented. This mild heat treatment strongly reduced the oscillations in the rate of photosynthesis and in the yield of chlorophyll fluorescence. However, only a 25% irreversible inhibition of maximum photosynthetic capacity of photosystem II (PSII) measured by oxygen evolution was produced and the intrinsic quantum yield of PSII measured by the chlorophyll fluorescence ratio (F_m‐ F_o)/F_m decreased by only 15%. In contrast, the above treatment increased radiationless dissipation processes in PSII by a factor of two. In heat‐treated leaves, photosynthesis was not saturated even by strong light. Both ΔpH‐dependent quenching of excitons in PSII (including formation of zeaxanthin) and state 1/state 2 transition were found to be stimulated. Heat exposure enhanced the control of PSII activity by PSI, as evidenced by a significant increase in the quenching effect of far‐red light on the maximum yield of chlorophyll fluorescence. It was deduced that after mild heat treatment, the photosynthetic apparatus in leaves lacks the precise coordinating control of electron transport and carbon metabolism owing to the inability of PSII to support electron transport at a level adequate for carbon metabolism. This effect was not related to the small irreversible thermal damage to PSII, but was rather due to a significant increase in non‐photochemical quenching of excitation energy.     

Characterization of PSII photochemistry and thermostability in salt-treated Rumex leaves

A study was conducted, using chlorophyll fluorescence, rapid fluorescence induction kinetics, and polyphasic fluorescence transients, to determine the effect of salt treatment and heat stress on PSII photochemistry in Rumex leaves. Salt treatment was accomplished by adding NaCl solutions of different concentrations ranging from 50 to 200 mmol/L. Heat stress was induced by exposing the plant leaves to temperatures ranging from 29 to 47 degrees C. The control plants were grown without NaCl treatment. The data acquired in this study showed that NaCl treatment alone had no effect on the maximal photochemistry of PSH or the polyphasic rise of chlorophyll fluorescence. However, the NaCl treatment modified heat stress on PSII photochemistry in Rumex leaves, which was manifested by a lesser heat-induced decrease in photochemical quenching (qP), efficiency of excitation energy capture by open PSII reaction centers (Fv'/Fm'), and quantum yield of PSII electron transport (phiPSII). The data also showed that NaCl treatment compromised the impact of heat stress on the capacity of transferring electrons from Q(A)- to Q(B). Furthermore, the NaCl treatment promoted heat resistance of O2-evolving complex (OEC). In summary, NaCl treatment enhanced the thermostability of PSII.     

Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO2 exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO2 assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO2 concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in non-photochemical quenching and a decrease in photo-chemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of QB-non-reducing PSII reaction centres and a decrease in the rate of QA reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant to thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was down-regulated by an increase in the proportion of QB-non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.Key words: Chlorophyll fluorescence, gas exchange, maize (Zea mays L.), photochemical and non-photochemical quenching, photosystem II photochemistry.      

Photorespiration and photoinhibition in the bracts of cotton under water stress

Gas exchange and chlorophyll fluorescence parameters of PSII were analyzed in the bracts and leaves of cotton plants after anthesis. Photosynthetic activity and photorespiration were measured in the leaves and bracts of cotton grown under either normal or reduced water-saving drip irrigation. The photosynthetic performance, amount of chlorophyll and Rubisco, and net photosynthesis were greater in the bracts than that in the leaves under water stress. The actual photochemical efficiency of PSII decreased in both the bracts and leaves after anthesis under reduced irrigation. However, the decrease was smaller in the bracts than in the leaves, indicating that the bracts experienced less severe photoinhibition compared to the leaves. The greater drought tolerance of bracts could be related to differences in relative water content, instantaneous water-use efficiency, and photorespiration rate. The ratio of photorespiration to net photosynthesis was much higher in the bracts than in leaves. Furthermore, water deficiency (due to the water-saving drip irrigation) had no significant effect on that ratio in the bracts. We hypothesized that photorespiration in the bracts alleviated photoinhibition and maintained photosynthetic activity.     

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.     

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。     

Impact of elevated ozone on chlorophyll a fluorescence in field-grown oat (Avena sativa)

Oat (Avena sativa) plants were grown in the field near the urban area of Valencia, Eastern Spain. The data on air quality showed that ozone was the main phytotoxic pollutant present in ambient air reaching a 7-h mean of 46 nl l(-1) and a maximum hourly peak of 322 nl l(-1). The effect of ambient ozone on PSII activity was examined by measurements of chlorophyll (Chl) a fluorescence. In leaves with visible symptoms, the function of PSII was changed at high actinic irradiances. Nonphotochemical quenching (NPQ) was higher and quantum efficiency of PSII (Phi(PSII)), photochemical quenching (q(p)), quantum efficiency of excitation capture and PSII electron flow (F(v)'/F(m)') were lower. An enhanced susceptibility to photoinhibition was observed for symptom-exhibiting leaves compared to leaves that remain free of visible symptoms. Both the lowering of photosynthesis efficiency and the increased sensitivity to photoinhibition probably contribute to reduced crop yield in the field, to different extents, depending on growth conditions. To our knowledge, this is the first report that demonstrates that quantum efficiency of exciton trapping in PSII is associated with foliar injury in oat leaves in response to ambient concentration of ozone.     

铅胁迫对玉米幼苗叶片光系统功能及光合作用的影响

通过同时测定玉米叶片的叶绿素荧光快速诱导动力学曲线和对820 nm光的吸收、分析叶片的气体交换过程以及叶绿体活性氧清除关键酶的活性,研究了不同浓度的铅(Pb)胁迫对玉米光系统Ⅰ(PSⅠ)、光系统Ⅱ(PSⅡ)的光化学活性和光合作用的影响,并分析了Pb胁迫下两个光系统的相互关系.结果表明:铅胁迫显著抑制了玉米地上部分和地下部分的生长、降低了叶片光合色素含量、并通过非气孔因素限制了光合作用、导致过剩激发能的增加;铅胁迫显著抑制了超氧化物歧化酶(SOD)和抗坏血酸过氧化物酶(APX)的活性、伤害了PSII反应中心、PSII的受体侧和供体侧(放氧复合体)以及PSI光化学活性.     

Photosynthetic responses of sun- and shade-grown barley leaves to high light: is the lower PSII connectivity in shade leaves associated with protection against excess of light?

In this study, we have compared photosynthetic performance of barley leaves (Hordeum vulgare L.) grown under sun and shade light regimes during their entire growth period, under field conditions. Analyses were based on measurements of both slow and fast chlorophyll (Chl) a fluorescence kinetics, gas exchange, pigment composition; and of light incident on leaves during their growth. Both the shade and the sun barley leaves had similar Chl a/b and Chl/carotenoid ratios. The fluorescence induction analyses uncovered major functional differences between the sun and the shade leaves: lower connectivity among Photosystem II (PSII), decreased number of electron carriers, and limitations in electron transport between PSII and PSI in the shade leaves; but only low differences in the size of PSII antenna. We discuss the possible protective role of low connectivity between PSII units in shade leaves in keeping the excitation pressure at a lower, physiologically more acceptable level under high light conditions.     

Interaction of phycobilisomes with photosystem II dimers and photosystem I monomers and trimers in the cyanobacterium Spirulina platensis.

Distribution of phycobilisomes between photosystem I (PSI) and photosystem II (PSII) complexes in the cyanobacterium Spirulina platensis has been studied by analysis of the action spectra of H2 and O2 photoevolution and by analysis of the 77 K fluorescence excitation and emission spectra of the photosystems. PSI monomers and trimers were spectrally discriminated in the cell by the unique 760 nm low-temperature fluorescence, emitted by the trimers under reductive conditions. The phycobilisome-specific 625 nm peak was observed in the action spectra of both PSI and PSII, as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 695 nm (PSII), 730 nm (PSI monomers), and 760 nm (PSI trimers). The contributions of phycobilisomes to the absorption, action, and excitation spectra were derived from the in vivo absorption coefficients of phycobiliproteins and of chlorophyll. Analyzing the sum of PSI and PSII action spectra against the absorption spectrum and estimating the P700:P680 reaction center ratio of 5.7 in Spirulina, we calculated that PSII contained only 5% of the total chlorophyll, while PSI carried the greatest part, about 95%. Quantitative analysis of the obtained data showed that about 20% of phycobilisomes in Spirulina cells are bound to PSII, while 60% of phycobilisomes transfer the energy to PSI trimers, and the remaining 20% are associated with PSI monomers. A relevant model of organization of phycobilisomes and chlorophyll pigment-protein complexes in Spirulina is proposed. It is suggested that phycobilisomes are connected with PSII dimers, PSI trimers, and coupled PSI monomers.     

3-D cell-level chlorophyll fluorescence imaging of ozone-injured sunflower leaves using a new passive light microscope system

A passive light microscope system has been developed, capable of reconstructing an extended-focus 3-D cell-level image of chlorophyll fluorescence and Phi(PSII) of intact attached leaves using a limited number of focal plane images of chlorophyll fluorescence. Using this system, the relationships between the depth of the mesophyll cells in spongy tissue and the intensity of the chlorophyll fluorescence and the Phi(PSII) were investigated in sunflower leaves exposed to 300 ppb ozone for 12 h at a PPFD of 300 micromol m(-2) s(-1) actinic light. After ozone exposure, fluorescence intensity (F) largely decreased in the cells just under the epidermal cells (within approximately 20 microm of the epidermal cells), but the sites where fluorescence intensity decreased had no relationship to the position of the stomata. By contrast, the distribution of Phi(PSII) showed no change after the ozone exposure. These findings suggest that ozone-induced inhibition occurs in the cells just under the epidermal cells by reducing the light absorption of the chloroplasts, while the operating quantum efficiency of PSII photochemistry is maintained.     

Resolution of the Photosystem I and Photosystem II contributions to chlorophyll fluorescence of intact leaves at room temperature

Green leaves illuminated with photosynthetically active light emit red fluorescence, whose time-dependent intensity variations reflect photosynthetic electron transport (the Kautsky effect). Usually, fluorescence variations are discussed by considering only the contribution of PSII-associated chlorophyll a, although it is known that the fluorescence of PSI-associated chlorophyll a also contributes to the total fluorescence [Aust. J. Plant Physiol. 22 (1995) 131]. Because the fluorescence emitted by each photosystem cannot be measured separately by selecting the emission wavelength in in vivo conditions, the contribution of PSI to total fluorescence at room temperature is still in ambiguity. By using a diode array detector, we measured fluorescence emission spectra corresponding to the minimal (F(O)) and maximal (F(M)) fluorescence states. We showed that the different shapes of these spectra were mainly due to a higher contribution of PSI chlorophylls in the F(O) spectrum. By exciting PSI preferentially, we recorded a reference PSI emission spectrum in the near far-red region. From the F(O) and F(M) spectra and from this PSI reference spectrum, we derived specific PSI and PSII emission spectra in both the F(O) and F(M) states. This enables to estimate true value of the relative variable fluorescence of PSII, which was underestimated in previous works. Accurate separation of PSI-PSII fluorescence emission spectra will also enable further investigations of the distribution of excitation energy between PSI and PSII under in vivo conditions.     

High resolution imaging of photosynthetic activities of tissues, cells and chloroplasts in leaves

Through imaging of chlorophyll fluorescence, it is possible to produce parameterized fluorescence images that estimate the operating quantum efficiency of photosystem II (PSII) photochemistry and which can be used to reveal heterogeneous patterns of photosynthetic performance within leaves. The operating quantum efficiency of PSII photochemistry is dependent upon the effective absorption cross-section of the light-harvesting system of PSII and the photochemical capacity of PSII. The effective absorption cross-section is decreased by the process of down-regulation, which is widely thought to operate within the pigment matrices of PSII and which results in non-photochemical quenching of chlorophyll fluorescence. The photochemical capacity is non-linearly related to the proportion of PSII centres in the 'open' state and results in photochemical quenching of chlorophyll fluorescence. Examples of heterogeneity of the operating quantum efficiency of PSII photochemistry during the induction of photosynthesis in maize leaves and in the chloroplast populations of stomatal guard cells of a leaf of Tradescantia albifora are presented, together with analyses of the factors determining this heterogeneity. A comparison of the operating quantum efficiency of PSII photochemistry within guard cells and adjacent mesophyll cells of Commelina communis is also made, before and after stomatal closure through a change in ambient humidity.     

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.     

Dynamic changes in photosynthesis and chlorophyll fluorescence in Nicotiana tabacum infested by Bemisia tabaci (Middle East–Asia Minor 1) nymphs

Bemisia tabaci Middle East–Asia Minor 1 (MEAM1) is a worldwide pest. To determine whether MEAM1 nymphs produce the same symptoms in different host plants, we measured the plant growth and chlorophyll content of tobacco and cotton plants that were infested by MEAM1 nymphs. Furthermore, to investigate the spatial and temporal changes in photosynthesis caused by MEAM1 nymphs, the net photosynthetic rate (Pn) and chlorophyll a fluorescence of local and systemic tobacco leaves were assayed at 8, 11, 14, and 20 days after MEAM1 adult removal, which represent the stages of 1st, 2nd, 3rd, and 4th instar nymphs, respectively. The results showed that MEAM1 nymph infestation reduced the plant height and internode length of tobacco at 14 and 20 days, as well as the dry weight of infested and systemic tobacco leaves. However, MEAM1 nymph infestation did not affect the plant height or internode length of cotton. Also, the dry weight and chlorophyll and carotenoid content of infested and systemic leaves of cotton plants were not influenced by MEAM1 nymph infestation. However, the contents of chlorophyll a and b and carotenoids in infested tobacco leaves decreased over time; the chlorophyll a content of systemic tobacco leaves decreased at 11, 14, and 20 days. The chlorophyll and carotenoid contents in infested and systemic leaves of cotton plants were not influenced by MEAM1 nymph infestation. In addition, the Pn of infested tobacco leaves decreased at 14 and 20 days, while the Pn in systemic tobacco leaves decreased after 11 days. The greatest decrease in performance index on absorption basis (PI _ABS ) of infested and systemic tobacco leaves occurred on day 14. The fluorescence intensity at 2 ms (J peak) and 300 μs (K peak) increased on day 14, which indicates that 3rd instar nymphs caused serious damage to the primary photochemical reactions and donor side of PSII. These results suggest that MEAM1 nymph infestation had different effects on tobacco and cotton plants. The infestation caused spatial and temporal changes in photosynthesis in tobacco plants. The lower chlorophyll a content may have been related to the lower net photosynthetic rate of systemic and infested tobacco leaves. The decreased stability of the oxygen-evolving complex and the reaction center of PSII and the decrease in electron transport were the main reasons for the decrease in the level of photosynthesis in tobacco leaves caused by MEAM1 nymphs during various stages of infestation.     

Spatio-temporal heterogeneity in Arabidopsis thaliana leaves under drought stress

Using chlorophyll (chl) fluorescence imaging, we studied the effect of mild (MiDS), moderate (MoDS) and severe (SDS) drought stress on photosystem II (PSII) photochemistry of 4-week-old Arabidopsis thaliana. Spatio-temporal heterogeneity in all chl fluorescence parameters was maintained throughout water stress. After exposure to drought stress, maximum quantum yield of PSII photochemistry (F(v)/F(m)) and quantum efficiency of PSII photochemistry (Φ(PSΙΙ)) decreased less in the proximal (base) than in the distal (tip) leaf. The chl fluorescence parameter F(v) /F(m) decreased less after MoDS than MiDS. Under MoDS, the antioxidant mechanism of A. thaliana leaves seemed to be sufficient in scavenging reactive oxygen species, as evident by the decreased lipid peroxidation, the more excitation energy dissipated by non-photochemical quenching (NPQ) and decreased excitation pressure (1-q(p)). Arabidopsis leaves appear to function normally under MoDS, but do not seem to have particular metabolic tolerance mechanisms under MiDS and SDS, as revealed by the level of lipid peroxidation and decreased quantum yield for dissipation after down-regulation in PSII (Φ(NPQ)), indicating that energy dissipation by down-regulation did not function and electron transport (ETR) was depressed. The simultaneous increased quantum yield of non-regulated energy dissipation (Φ(NO)) indicated that both the photochemical energy conversion and protective regulatory mechanism were insufficient. The non-uniform photosynthetic pattern under drought stress may reflect different zones of leaf anatomy and mesophyll development. The data demonstrate that the effect of different degrees of drought stress on A. thaliana leaves show spatio-temporal heterogeneity, implying that common single time point or single point leaf analyses are inadequate.     

Effects of water stress on photosystem II photochemistry and its thermostability in wheat plants

Modulated chlorophyll fluorescence, rapid fluorescence induction kinetics and the polyphasic fluorescence transients (OJIP) were used to evaluate PSII photochemistry in wheat plants exposed to water stress and/or heat stress (25-45C). Water stress showed no effects on the maximal quantum yield of PSII photochemistry (Fv/Fm), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, indicating that water stress had no effects on the primary photochemistry of PSII. However, in light-adapted leaves, water stress reduced the efficiency of excitation energycapture by open PSII reaction centres (F'v/F'm) and the quantum yield of PSII electron transport (PSII), increased the non-photochemical quenching (qN) and showed no effects on the photochemical quenching (qP). This suggests that water stress modified the PSII photochemistry in the light-adapted leaves and such modifications may be a mechanism to down-regulate the photosynthetic electron transport to match a decreased CO2 assimilation. In addition, water stress also modified the responses of PSII to heat stress. When temperature was above 35C, thermostability of PSII was strongly enhanced in water-stressed leaves, which was reflected in a less decrease in Fv/Fm, qP, F'v/F'm, and PSII in water-stressed leaves than in well-watered leaves. There were no significant variations in the above fluorescence parameters between moderately and severely water-stressed plants, indicating that the moderate water-stressed plants, indicating that the moderate water stress treatment caused the same effects on thermostability of PSII as the severe treatment. It was found that increased thermostability of PSII may be associated with an improvement of resistance of the O2-evolving complex and the reaction centres in water-stressed plants to high temperature.Key words: Chlorophyll fluorescence, heat stress, photosystem II photochemistry, water stress, wheat (Tritium aestivum L.).