Simultaneous measurement of stomatal conductance, non-photochemical quenching, and photochemical yield of photosystem II in intact leaves by thermal and chlorophyll fluorescence imaging

A new imaging system capable of simultaneously measuring stomatal conductance and fluorescence parameters, non-photochemical quenching (NPQ) and photochemical yield of photosystem II (Phi(PSII)), in intact leaves under aerobic conditions by both thermal imaging and chlorophyll fluorescence imaging was developed. Changes in distributions of stomatal conductance and fluorescence parameters across Phaseolus vulgaris L. leaves induced by abscisic acid treatment were analyzed. A decrease in stomatal conductance expanded in all directions from the treatment site, then mainly spread along the lateral vein toward the leaf edge, depending on the ABA concentration gradient and the transpiration stream. The relationships between stomatal conductance and fluorescence parameters depended on the actinic light intensity, i.e. NPQ was greater and Phi(PSII) was lower at high light intensity. The fluorescence parameters did not change, regardless of stomatal closure levels at a photosynthetically active photon flux (PPF) of 270 micro mol m(-2) s(-1); however, they drastically changed at PPF values of 350 and 700 micro mol m(-2) s(-1), when the total stomatal conductance decreased to less than 80 and 200 mmol m(-2) s(-1), respectively. This study has, for the first time, quantitatively analyzed relationships between spatiotemporal variations in stomatal conductance and fluorescence parameters in intact leaves under aerobic conditions.     

Effects of ozone exposure or fungal pathogen on white lupin leaves as determined by imaging of chlorophyll a fluorescence.

Chlorophyll fluorescence has been used routinely to investigate photosynthetic activity in plants subjected to both biotic and abiotic stresses. The aim of this work was to compare the perturbations in photosynthesis induced by ozone and by a pathogen. By using a conventional fluorometer a similar response pattern was observed in inoculated and O(3)-fumigated leaves. The application of chlorophyll fluorescence imaging provided further detailed information on the spatial-temporal heterogeneity of the response of white lupin leaves to fungal pathogen or to ozone fumigation. In particular, 48 h after artificial inoculation with the necrotrophic fungal pathogen Pleiochaeta setosa, the leaves showed a remarkable alteration in PSII operating efficiency (Phi(PSII)), which affected the whole surface. Afterwards, the infection site was surrounded by a ring of increased photosynthetic activity. The response of ozonated leaves was quite different. The reduction in Phi(PSII) was already evident 24h after fumigation; moreover, a distinct heterogeneity of the fluorescence yield was observed and the major veins displayed a lowered Phi(PSII).     

不同光强下高锰对黄瓜光合作用特性的影响

采用营养液培养的方法,研究了不同光强下高锰对黄瓜植株生长、叶绿素含量、叶绿素荧光参数和光合作用的影响.结果表明,高锰处理抑制了黄瓜植株的生长,与弱光处理相比强光下抑制幅度更加显著.强光下,高锰处理显著降低叶绿素含量,但降低光强却增加其含量.强光下,高锰处理显著降低原初光能转换效率(F_v/F_m)、光合电子传递量子效率(ΦPSII)和光化学猝灭系数(qP);弱光下,高锰处理对F_v/F_m和qP无显著影响.高锰处理使净光合速率(P_n)和气孔导度(G_s)下降.尤其是在强光下下降幅度更大.高锰处理使细胞间CO₂浓度(C_i)在强光下升高,而在弱光下则下降.与C_i相反,高锰处理使气孔限制值(L_s)在强光下下降,而在弱光下上升.因此,强光下高锰胁迫使净光合速率下降可能是由非气孔限制引起的,而弱光下高锰处理使净光合速率下降可能是由气孔因子限制引起的.     

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.     

Dynamics of spatial heterogeneity of stomatal closure in Tradescantia virginiana altered by growth at high relative air humidity

The spatial heterogeneity of stomatal closure in response to rapid desiccation of excised well-watered Tradescantia virginiana leaves grown at moderate (55%) or high (90%) relative air humidity (RH) was studied using a chlorophyll fluorescence imaging system under non-photorespiratory conditions. Following rapid desiccation, excised leaves grown at high RH had both a greater heterogeneity and a higher average value of PSII efficiency (Phi(PSII)) compared with leaves grown at moderate RH. Larger decreases in relative water content resulted in smaller decreases in water potential and Phi(PSII) of high RH-grown leaves compared with moderate RH-grown leaves. Moreover, the Phi(PSII) of excised high RH-grown leaves decreased less with decreasing water potential, implying that the stomata of high RH-grown leaves are less sensitive to decreases in leaf water potential compared with moderate RH-grown leaves. After desiccation, some non-closing stomata were distributed around the main vein in high RH-grown leaves. Direct measurements of stomatal aperture showed 77% stomatal closure in the margins after 2 h desiccation compared with 40% closure of stomata in the main-vein areas in high RH-grown leaves. Faster closure of stomata in leaf margins compared with main-vein areas of leaves grown at high RH was related to substantially lower relative water content in these areas of the leaves.     

Effect of the anthocyanic epidermal layer on Photosystem II and I energy dissipation processes in Tradescantia pallida (Rose) Hunt

The effect of anthocyanic cells of the epidermal layer was investigated on photosynthetic activity of the higher plant Tradescantia pallida. To determine the possible indirect role of anthocyanin in photosynthesis, analysis was done on intact leaves and leaves where anthocyanic epidermal layer was removed. Energy dissipation processes related to Photosystem II (PSII) and Photosystem I (PSI) activity was done using simultaneously Chlorophyll a (Chl a) fluorescence and P700 transmittance signals change. In anthocyanic epidermal-less leaves, PSII photochemical activity was more decreased in dependence to increasing light irradiance exposure. We found that photoinhibition of PSII decreased PSI activity by reducing the electron flow toward PSI, especially under high light intensities. Under those conditions, it resulted in the accumulation of oxidized PSI reaction centers, which was stronger in leaves where the anthocyanic epidermal layer was removed. In conclusion, our results showed that the anthocyanic epidermal layer had a photoprotective effect only on the PSII and not on the PSI of T. pallida leaves, supporting the role of anthocyanin pigments in the regulation of photosynthesis for excess absorbed light irradiance.     

Non-photochemical loss in PSII in high- and low-light-grown leaves of Vicia faba quantified by several fluorescence parameters including L(NP), F0/F'm, a novel parameter

Using the expression of fluorescence originated from the PSII open reaction center in the light by Oxborough and Baker (1997), we obtained a formula that expresses relationships between the quantum efficiency of PSII photochemistry in the dark (Phi(m)= F(v)/F(m)) and in the light Phi'(m)=F'(v)/F'(m):Phi'(m)=Phi(m)+L(NP), where L(NP)(=F(0)/F'(m)) denotes the quantum yield of light induced non-photochemical losses (heat dissipation and fluorescence de-excitation) in PSII. Using L(NP) and other conventional fluorescence parameters, we conducted quenching analyses with leaves of broad bean plants (Vicia faba L.) grown at 700 (high light; HL) and 80 mumol photons m(-2) s(-1) (low light; LL). We also examined whether behavior of q(0) quenching (q(0)=1-F'(0)/F(0)) is related to the reaction center quenching. When the actinic light (AL) was strong, Stern-Volmer quenching [NPQ=(F(m)-F'(m))/F'(m)] and L(NP) increased rapidly and then decreased slowly in HL leaves, while, in LL leaves, they increased slowly. It is probable that rapid formation of a large proton gradient was responsible for sharp rises in both parameters in HL leaves. The steady-state 'excess' parameter [Phi(Ex)= (1 - qP) Phi(m)/(Phi(m)+ L(NP))], fraction of energy migrating to closed PSII centers, increased with the photon flux density of AL in LL leaves. In contrast, in HL leaves, Phi(Ex) did not increase markedly. The examination of the relationship between Phi(Ex) and L(NP) obtained at various AL revealed that in LL leaves the increase in (1 - qP) with the increase in AL prevailed, while, in HL leaves, the increase in L(NP) suppressed the increase in (1 - qP). Using the difference between L(NP) and L(D) (Phi(ND)= L(NP)- L(D), where L(D)= F(0)/F(m)), q(0) and qN (=1-F'(v)/F(v)) were calculated without using measured F'(0). The relationships between q(0) and qN thus obtained for various AL levels were almost identical for both HL and LL leaves, implying no difference in the fluorescence origin between the HL and LL leaves. Usefulness of these equations expressing non-photochemical loss is discussed.     

Salinity treatment shows no effects on photosystem II photochemistry,but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa

Photosynthetic gas exchange, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients were used to evaluate PSII photochemistry in the halophyte Suaeda salsa exposed to a combination of high salinity (100-400 mM NaCl) and heat stress (35-47.5 degrees C, air temperature). CO(2) assimilation rate increased slightly with increasing salt concentration up to 300 mM NaCl and showed no decrease even at 400 mM NaCl. Salinity treatment showed neither effects on the maximal efficiency of PSII photochemistry (F(v)/F(m)), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, nor effects on the efficiency of excitation energy capture by open PSII reaction centres (F(v)'/F(m)') and the actual PSII effciency (Phi(PSII)), photochemical quenching (q(P)), and non-photochemical quenching (q(N)) in light-adapted leaves. The results indicate that high salinity had no effects on PSII photochemistry either in a dark-adapted state or in a light-adapted state. With increasing temperature, CO(2) assimilation rate decreased significantly and no net CO(2) assimilation was observed at 47.5 degrees C. Salinity treatment had no effect on the response of CO(2) assimilation to high temperature when temperature was below 40 degrees C. At 45 degrees C, CO(2) assimilation rate in control plants decreased to zero, but the salt-adapted plants still maintained some CO(2) assimilation capacity. On the other hand, the responses of PSII photochemistry to heat stress was modified by salinity treatment. When temperature was above 35 degrees C, the declines in F(v)/F(m), Phi(PSII), F(v)'/F(m)', and q(P) were smaller in salt-adapted leaves compared to control leaves. This increased thermostability was independent of the degree of salinity, since no significant changes in the above-described fluorescence parameters were observed among the plants treated with different concentrations of NaCl. During heat stress, a very clear K step as a specific indicator of damage to the O(2)-evolving complex in the polyphasic fluorescence transients appeared in control plants, but did not get pronounced in salt-adapted plants. In addition, a greater increase in the ratio (F(i)-F(o))/(F(p)-F(o)) which is an expression of the proportion of the Q(B)-non-reducing PSII centres was observed in control plants rather than in salt-adapted plants. The results suggest that the increased thermostability of PSII seems to be associated with the increased resistance of the O(2)-evolving complex and the reaction centres of PSII to high temperature.     

Lateral diffusion of CO2 from shaded to illuminated leaf parts affects photosynthesis inside homobaric leaves

Gas exchange is generally regarded to occur between the leaf interior and ambient air, i.e. in vertical (anticlinal) directions of leaf blades. However, inside homobaric leaves, gas movement occurs also in lateral directions. The aim of the present study was to ascertain whether lateral CO2 diffusion affects leaf photosynthesis when illuminated leaves are partially shaded. Measurements using gas exchange and chlorophyll fluorescence imaging techniques were performed on homobaric leaves of Vicia faba and Nicotiana tabacum or on heterobaric leaves of Glycine max and Phaseolus vulgaris. For homobaric leaves, gas exchange inside a clamp-on leaf chamber was affected by shading the leaf outside the chamber. The quantum yield of photosystem II (Phi(PSII)) was highest directly adjacent to a light/shade border (LSB). Phi(PSII) decreased in the illuminated leaf parts with distance from the LSB, while the opposite was observed for nonphotochemical quenching. These effects became most pronounced at low stomatal conductance. They were not observed in heterobaric leaves. The results suggest that plants with homobaric leaves can benefit from lateral CO2 flux, in particular when stomata are closed (e.g. under drought stress). This may enhance photosynthetic, instead of nonphotochemical, processes near LSBs in such leaves and reduce the photoinhibitory effects of excess light.     

Enhancement of cyclic electron flow around PSI at high light and its contribution to the induction of non-photochemical quenching of chl fluorescence in intact leaves of tobacco plants

Non-photochemical quenching (NPQ) of Chl fluorescence is a mechanism for dissipating excess photon energy and is dependent on the formation of a DeltapH across the thylakoid membranes. The role of cyclic electron flow around photosystem I (PSI) (CEF-PSI) in the formation of this DeltapH was elucidated by studying the relationships between O2-evolution rate [V(O2)], quantum yield of both PSII and PSI [Phi(PSII) and Phi(PSI)], and Chl fluorescence parameters measured simultaneously in intact leaves of tobacco plants in CO2-saturated air. Although increases in light intensity raised V(O2) and the relative electron fluxes through both PSII and PSI [Phi(PSII) x PFD and Phi(PSI) x PFD] only Phi(PSI) x PFD continued to increase after V(O2) and Phi(PSII) x PFD became light saturated. These results revealed the activity of an electron transport reaction in PSI not related to photosynthetic linear electron flow (LEF), namely CEF-PSI. NPQ of Chl fluorescence drastically increased after Phi(PSII) x PFD became light saturated and the values of NPQ correlated positively with the relative activity of CEF-PSI. At low temperatures, the light-saturation point of Phi(PSII) x PFD was lower than that of Phi(PSI) x PFD and NPQ was high. On the other hand, at high temperatures, the light-dependence curves of Phi(PSII) x PFD and Phi(PSI) x PFD corresponded completely and NPQ was not induced. These results indicate that limitation of LEF induced CEF-PSI, which, in turn, helped to dissipate excess photon energy by driving NPQ of Chl fluorescence.     

Effects of Strong Light and Active Oxygen on Photosynthesis in Soybean

With the use of chlorophyll fluorescence technique, it was found that the net photosynthetic oxygen evolution rate decreased after strong light (2 000 μmol· m-2·2-1 ) treatment for two hours in soybean ( Glycine max L. ) leaves. The chlorophyll fluorescence parameters, Fm/Fo, Fv/Fm, ФPSII, qp and qN decreased along with the increase of light intensity. In strong light, exogenous active oxygen H202、·OH and 'O2 were harmful to soybean leaves. The destruction of 'O2 and·OH to leaves was most evident, as was shown that Fv/Fm and PS H decreased significantly. The antioxidants DABCO, mannitol, ascorbate and histidine protected the leaves, but weakly, from strong light. In darkness, the SOD inhibitor sodium diethyldithiocar- bamate (DDC) had no significant effect on Fm/Fo and Fv/Fm, but NAN,, the ascorbate peroxidase (APX)inhibitor, significantly decreased Fm/Fo, Fv/Fm and ФPS II. In strong light, however, beth DDC and NaN3 reduced the above-mentioned fluorescence parameters, but NaN3 was more effective than DDC. The results suggested that photoinhibition did take place in soybean leaves under strong light, and it was related to active oxygen in vivo.     

烟草叶片中呼吸电子传递途径在缓解叶绿体PSⅡ光抑制中的作用

前期研究发现线粒体交替氧化酶(AOX)呼吸途径对叶绿体光系统II(PSII)的光抑制有明显的缓解作用。线粒体内另一条呼吸途径——细胞色素氧化酶(COX)呼吸途径是否也具有光保护作用尚不清楚。该文通过荧光快速诱导动力学和荧光淬灭分析,解析了烟草(Nicotiana tabacum)叶片中COX途径对PSII光保护的贡献及其与AOX途径的关系。结果表明,强光处理后PSII活性在所有叶片中均下降。AOX途径受抑明显加速了叶片PSII活性的下降。而当COX途径受抑后,叶片PSII活性的下降与水处理的对照叶片无明显差异。当AOX途径与COX途径同时受抑时,叶片PSII活性的下降比单独抑制AOX途径时更严重。此外,呼吸电子传递受抑均导致叶片非光化学淬灭(NPQ)增加,AOX途径受抑导致的NPQ上调比COX途径受抑时更明显,AOX和COX途径同时受抑时NPQ的增幅最大。上述结果表明,烟草叶片中COX途径和AOX途径均参与PSⅡ的光保护。当COX途径受抑时,其光保护功能可以被AOX途径和NPQ补偿,而AOX途径的光保护作用不能被COX途径和NPQ完全补偿。     

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.     

田间大豆叶片成长过程中的光合特性及光破坏防御机制

田间大豆叶片在成长进程中光饱和光合速率持续提高,但气孔导度的增加明显滞后.尽管叶片在成长初期就具有较高的最大光化学效率,但是仍略低于发育成熟的叶片.随着叶片的成长,光下叶片光系统Ⅱ实际效率增加;非光化学猝灭下降.幼叶叶黄素总量与叶绿素之比较高,随着叶面积的增加该比值下降,在光下,幼叶的脱环氧化程度较高.因此认为大豆叶片成长初期就能够有效地进行光化学调节;在叶片生长过程中依赖叶黄素循环的热耗散机制迅速建立起来有效抵御强光的破坏.     

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.     

Biogenesis of Thylakoid Membranes in Chlamydomonas reinhardtii y1 (A Kinetic Study of Initial Greening)

Initiation of thylakoid membrane assembly was examined in degreened cells of Chlamydomonas reinhardtii y1 cells depleted of thylakoid membranes and photosynthetic activity by growth in the dark for 3 to 4 d. Photoreductive activities of photosystem II (PSII) and photosystem I (PSI) increased with no apparent lag when degreened cells were exposed to light at 38[deg]C. However, fluorescence transients induced by actinic light, which reflect the functional state of PSII, changed only slightly during the first 2 h of greening. When these cells were treated with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or saturating light, fluorescence increased commensurate with the cellular content of chlorophyll. In similar experiments with greening cells of C. reinhardtii CC-2341 (ac-u-g-2.3), a PSI-minus strain, fluorescence increased with chlorophyll without treatment with DCMU. These data suggested that fluorescence of initial PSII centers in greening y1 cells was quenched by activity of PSI. Continuous monitoring of fluorescence in the presence or absence of DCMU showed that assembly of quenched PSII centers occurred within seconds after exposure of y1 cells to light. These results are consistent with initial assembly of PSI and PSII within localized domains, where their proximity allows efficient energy coupling.     

Role of cyanide-resistant respiration during light-induced attraction of predators to herbivore-infested leaves

The present work showed that spider mite-infested leaves placed in the light were more attractive to predatory mites than the infested leaves placed in the dark; furthermore, an increase in the light intensity enhanced this attractiveness. However, the increase of the light intensity did not change the attractiveness of the uninfested leaves to predatory mites. The capacity of cyanide-resistant respiration and the photosynthetic rates of both the infested and uninfested leaves increased with increasing light intensities, whereas the photosystem (PS) II chlorophyll (Chl) fluorescence decreased. The increase of the capacity of cyanide-resistant respiration in the infested leaves was more dramatic than that in the uninfested leaves, whereas the values of photosynthetic rates and Chl fluorescence were lower in the infested leaves than those in the uninfested leaves. Treatment of the infested and uninfested leaves with 1 mM salicylhydroxamic acid (SHAM, an inhibitor of cyanide-resistant respiration) decreased photosynthetic rates and caused further reductions in PSII fluorescence, especially under a higher light intensity. In contrast, the effects of SHAM on PSII fluorescence parameters and photosynthetic rates of the infested leaves were more dramatic than on those of the uninfested leaves. The treatment with SHAM did not significantly change the attractiveness of the infested or uninfested leaves to the predatory mites under all of the light intensities tested. These results indicated that cyanide-resistant respiration was not directly associated with the light-induced attraction of predators to plants, but it could play a role in the protection of photosynthesis. Such role might become relatively more important when photosynthesis is impaired by herbivores infestation.     

Light inactivation of functional photosystem II in leaves of peas grown in moderate light depends on photon exposure

To determine the dependence of in vivo photosystem (PS) II function on photon exposure and to assign the relative importance of some photoprotective strategies of PSII against excess light, the maximal photochemical efficiency of PSII (Fv/Fm) and the content of functional PSII complexes (measured by repetitive flash yield of oxygen evolution) were determined in leaves of pea (Pisum satlvum L.) grown in moderate light. The modulation of PSII functionality in vivo was induced by varying either the duration (from 0 to 3 h) of light treatment (fixed at 1200 or 1800 mol photons · m^-2 · s^-1) or irradiance (from 0 to 3000 mol photons · m^-2 · s^-1) at a fixed duration (1 h) after infiltration of leaves with water (control), lincomycin (an inhibitor of chloroplast-encoded protein synthesis), nigericin (an uncoupler), or dithiothreitol (an inhibitor of the xanthophyll cycle) through the cut petioles of leaves of 22 to 24-day-old plants. We observed a reciprocity of irradiance and duration of illumination for PSII function, demonstrating that inactivation of functional PSII depends on the total number of photons absorbed, not on the rate of photon absorption. The Fv/Fm ratios from photoinhibitory light-treated leaves, with or without inhibitors, declined pseudo-linearly with photon exposure. The number of functional PSII complexes declined multiphasically with increasing photon exposure, in the following decreasing order of inhibitor effect: lincomycin > nigericin > DTT, indicating the central role of D1 protein turnover. While functional PSII and Fv/Fm ratio showed a linear relationship under high photon exposure conditions, in inhibitor-treated leaves the Fv/Fm ratio failed to reveal the loss of up to 25% of the total functional PSII under low photon exposure. The loss of this 25% of less-stable functional PSII was accompanied by a decrease of excitation-energy trapping capacity at the reaction centre of PSII (revealed by the fluorescence parameter, 1/Fo-1/Fm, where Fo and Fm stand for chlorophyll fluorescence when PSII reaction centres are open and closed, respectively), but not by a loss of excitation energy at the antenna (revealed by the fluorescence parameter, 1/Fm). We conclude that (i) PSII is an intrinsic photon counter under photoinhibitory conditions, (ii) PSII functionality is mainly regulated by D1 protein turnover, and to a lesser extent, by events mediated via the transthylakoid pH gradient, and (iii) peas exhibit PSII heterogeneity in terms of functional stability during photon exposure.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - Fo chlorophyll fluorescence corresponding to open PSII reaction centres - Fv, Fm variable and maximum fluorescence after dark incubation, respectively - Fs, Fm steady-state and maximum fluorescence during illumination, respectively - P680 reactioncentre chlorophyll and primary electron donor of PSII - PS photosystem Financial support of this work by Department of Employment, Education and Training/Australian Research Council International Research Fellowships Program (Korea) is gratefully acknowledged.     

高温胁迫下不同光强对‘赤霞珠’葡萄PSII活性及恢复的影响

本文研究了高温与不同光强结合处理对‘赤霞珠’葡萄叶片PSII活性及恢复的影响。结果表明,高温黑暗处理（40℃,0μmaol·m-2．s-1）导致叶片PSII最大光化学效率（Fv/Fm）、反应中心吸收的光能用于电子传递的量子产额（ψEo）与单位反应中心光能的传递（ETo/RC）降低明显,且无恢复趋势,K点相对荧光（Vk）、单位反应中心光能的吸收（ABS／RC）与捕获（TRo／RC）显著升高。高温弱光处理（40℃,200μmol·m-2．s-1）后的叶片PSII活性明显恢复,ETo／RC降低明显,TRo/RC无显著变化。高温强光（40℃,1600μmol·m-2．S-1）处理导致单位面积有活性反应中心数量（RC/CSm）抑制程度最大,恢复程度较低。实验结果说明,高温处理下黑暗对葡萄PSII功能活性及恢复均会造成抑制,而弱光可以显著缓解高温对葡萄叶片的胁迫作用,并促进PSII的恢复,强光导致胁迫下的PSII功能抑制最明显。     

氮调控对盐环境下甜菜功能叶光系统Ⅱ荧光特性的影响

采用盆栽试验方法,以NaCl为盐分模拟不同盐度环境,研究了施氮(N)对盐环境下生长的甜菜(Beta vulgaris)功能叶光系统Ⅱ (PSⅡ)荧光特性的影响及光合色素含量的变化.结果表明:在轻度、中度及重度盐环境下,施N均能增大PSⅡ最大光化学效率(Fv/Fm)、PSⅡ潜在活性(Fv/Fo)、PSⅡ实际光量子产量(Y(Ⅱ))、非调节性能量耗散的量子产量(Y(NO))、相对电子传递速率(ETR)及光化学猝灭系数(qp),且在适宜的施N范围内(0-1.2 g·kg-1)上述参数随施N量的增加而增大.各叶绿素荧光参数光响应的结果表明,随着光强的增加,各处理下调节性能量耗散的量子产量(KNPQ))、ETR及非光化学猝灭系数(NPQ)旱上升趋势,相反,Y(Ⅱ)、Y(NO)及qp则呈下降趋势,在有效的光强范围内(0-1 000 μmol·m-2·s-1)施N提高了甜菜功能叶PSⅡ反应中心的开放程度,并且在高光强下调节PSⅡ耗散掉过剩的光能以避免对其反应中心造成伤害.各盐度环境下施N也显著增加了甜菜功能叶叶绿素与类胡萝卜素含量,增大了叶绿素a/叶绿素b值,且叶绿素与类胡萝卜素含量随施N水平的增加而增加.说明盐环境下施N能够增强甜菜功能叶PSⅡ的活性,提高PSⅡ光能利用率,从而增强其对盐渍环境的适应性.