Photo-inhibition and seasonal photosynthetic performance of the seaweed Laminaria saccharina during a simulated tidal cycle: chlorophyll fluorescence measurements and pigment analysis

Laminaria saccharina (Lamouroux) form the largest, most abundant and conspicuous seaweed populations along the French coast of the eastern English Channel. As they are located in the intertidal zone, they are exposed to considerable irradiance variations, mainly related to tidal cycles. The response of these macro‐algae to light variations over a simulated daily tidal cycle was investigated in the laboratory during spring, autumn and winter using chlorophyll fluorescence and pigment analysis. The maximum quantum yield of photosystem II (PSII) photochemistry (F_v/F_m) and the operating PSII efficiency (Φ_PSII) showed clear daily cycles according to the irradiance variation throughout the 12 h simulated tidal cycle, whereas the pattern of the relative photosynthetic electron transport rate (rETR) was not so obvious. The algae reacted to the light increase by developing photoprotective mechanisms able to dissipate the excess energy reaching PSII by the de‐epoxidation of violaxanthin into zeaxanthin. Because of their better acclimation to strong irradiance, spring populations were less affected by this light treatment than were winter populations. In particular, L. saccharina showed more pigments of the xanthophyll cycle in spring to cope with strong irradiance exposure. Alternatively, they developed their antenna complexes in winter to harvest a maximum of light.     

Effects of exogenous glycinebetaine on growth, CO2 assimilation, and photosystem II photochemistry of maize plants

Effects of exogenous glycinebetaine (GB, 2–50 mM) on growth, photosynthetic gas exchange, PSII photochemistry, and the activities of key enzymes involved in CO₂ fixation in maize plants were investigated. Growth, CO₂ assimilation rate, and stomatal conductance increased at low GB concentrations (2–20 mM) but decreased significantly at high GB concentrations (30–50 mM). Leaf relative water content and water potential remained unchanged at low GB concentrations but decreased at high GB concentrations. The maximal efficiency of PSII photochemistry was unchanged either at low or high GB concentrations. The actual PSII efficiency ( Φ _PSII) and photochemical quenching (q_P) increased at low GB concentrations but decreased at high GB concentrations. At low GB concentrations, there were no significant changes in the efficiency of excitation energy capture by open PSII reaction centres (F_v′/F_m′) and non‐photochemical quenching (q_N). At high GB concentrations, F_v′/F_m′ decreased while q_N increased significantly. There were no changes in the activities of phosphoenolpyruvate carboxylase, pyruvate phosphate dikinase, and ribulose‐1,5‐bisphosphate carboxylase in control and GB‐fed plants. However, there was a linear correlation between CO₂ assimilation rate and stomatal conductance in control and GB‐fed plants. Moreover, there were no significant differences in O₂ evolution rate between control and GB fed‐plants under saturated CO₂ conditions. The results suggest that exogenous GB application at certain concentrations can enhance CO₂ assimilation rate, which can be explained by an increased stomatal conductance.     

Photosystem II energy use,non-photochemical quenching and the xanthophyll cycle in Sorghumbicolor grown under drought and free-air CO2 enrichment(FACE) conditions

The present study was carried out to test the hypothesis thatelevated atmospheric CO₂ (Ca) will alleviate over‐excitationof the C₄ photosynthetic apparatus and decrease non‐photochemicalquenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescencewas monitored in Sorghum bicolor plants grown under a free‐aircarbon‐dioxide enrichment (FACE) by water‐stress (Dry) experiment.Under Dry conditions elevated Ca increased the quantum yield ofphotosystem II (φPSII) throughout the day throughincreases in both photochemical quenching coefficient (q_p)and the efficiency with which absorbed quanta are transferred toopen PSII reaction centres (F_v′/F_m′).However, in the well‐watered plants (Wets) FACE enhanced φPSIIonly at midday and was entirely attributed to changes in F_v′/F_m′_. Underfield conditions, decreases in φPSII under Dry treatmentsand ambient Ca corresponded to increases in NPQ but the de‐epoxidation stateof the xanthophyll pool (DPS) showed no effects. Water‐stress didnot lead to long‐term damage to the photosynthetic apparatus asindicated by φPSII and carbon assimilation measuredafter removal of stress conditions. We conclude that elevated Caenhances photochemical light energy usage in C₄ photosynthesisduring drought and/or midday conditions. Additionally,NPQ protects against photo‐inhibition and photodamage. However,NPQ and the xanthophyll cycle were affected differently by elevatedCa and water‐stress.     

ACCLIMATION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons·m^?2·s^?1. Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d^?1 for high PFD grown cells compared with 0.38 d^?1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (F_v/F_m) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (F_v′/F_m′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F_q′/F_m′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.     

Effects of groundwater depth variation on photosynthesis and photoprotection of <Emphasis Type="Italic">Elaeagnus angustifolia</Emphasis> L.

Physiological and photosynthetic responses were investigated at three different depths of groundwater (DGW: 1.4, 2.4, and 3.4 m) in Elaeagnus angustifolia L., a locally adapted tree to the arid region in northwest China. Predawn leaf water potential and chlorophyll content declined gradually with the increasing DGW, whereas there was little effect on predawn variable-to-maximum chlorophyll fluorescence ratio F _v/F _m and leaf carotenoid compositions (xanthophyll cycle pool, neoxanthin, lutein, and β-carotene). Net photosynthetic rate (P _n), quantum yield of PSII electron transport (Φ_PSII), stomatal conductance (Gs), and intercellular CO₂ concentration (Ci) declined obviously; however, P _n decreased more than Φ_PSII at deeper DGW. The photoinhibition of PSII at all three DGW occurred at midday in summer and increased as DGW increased. The ΔpH-dependent thermal dissipation and the level of de-epoxidation of the xanthophyll cycle at all three DGW reached their maxima at midday with the increase of light intensity. However, the fraction of functional PSII and light intensity at deeper DGW (2.4, 3.4 m) showed a negative correlation. This correlation suggested that most of violaxanthin was converted into zeaxanthin at midday, and the reversible inactivation of partial PSII reaction centers took place at deeper DGW. These results together suggest that both the xanthophyll cycle-dependent thermal dissipation and the reversible inactivation of partial PSII might have played important roles in avoiding the excess light-induced energy damage in leaves of this tree species at deeper DGW.     

Effects of salt stress on photosynthesis,PSII photochemistry and thermal energy dissipation in leaves of two corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) varieties

The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (F_v/F_m) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕ_PSII), photochemical quenching coefficient (q_p), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (g _s) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.     

Influence of High Salt on Protein Contents and Lipid Peroxidation and their Interaction with High Photon Flux Density on Isolated Chloroplasts of Mustard

The salinity and its interaction with high photon flux density (PFD) on in vivo chlorophyll fluorescence were investigated in isolated chloroplasts of mustard (Brassica juncea L. cv. Pusa Bold). Increase in salt stress decreases the protein contents of leaves and causes increase in lipid peroxidation. F_v/F_mratios suggesting that the efficiency of the photochemistry of PSII was not affected alone with the salt stressed plants. With high PFD, F_v/F_m ratio decreased with increased salt concentration. Our results indicate that salt stress enhances the photoinhibition of isolated chloroplasts.     

Midday photoinhibition of two newly developed super-rice hybrids

Super-rice hybrids are two-line hybrid rice cultivars with 15 to 20 % higher yields than the raditional three-line hybrid rice cultivars. Response of photosynthetic functions to midday photoinhibition was compared between seedlings of the traditional hybrid rice (Oryza sativa L.) Shanyou63 and two super-rice hybrids, Hua-an3 and Liangyoupeijiu. Under strong midday sunlight, in comparison with Shanyou63, the two super-rice hybrids were less photoinhibited, as indicated by the lower loss of the net photosynthetic rate (P _N), the quantum yield of photosystem 2 (Φ_PS2), and the maximum and effective quantum yield of PS2 photochemistry (F_v/F_m and F_v′/F_m′). They also had a much higher transpiration rate. Hence the super-rice hybrids could protect themselves against midday photoinhibition at the cost of water. The photoprotective de-epoxidized xanthophyll cycle components, antheraxanthin (A) and zeaxanthin (Z), were accumulated more in Hua-an3 and Liangyoupeijiu than in Shanyou63, but the size of xanthophyll cycle pool of the seedlings was not affected by midday photoinhibition. Compared to Shanyou63, the super-rice hybrids were better photoprotected under natural high irradiance stress and the accumulation of Z and A, not the size of the xanthophyll pool protected the rice hybrids against photoinhibition.     

Photoinhibition in outdoor Spirulina platensis cultures assessed by polyphasic chlorophyll fluorescence transients

Photoinhibition in outdoor cultures of Spirulina platensis was studied by measuring the polyphasic rise of chlorophyll fluorescence transients, which provide information on the primary photochemistry of PSII. The maximum efficiency of PSII photochemustry (F_v/F_m) declined in response to daily increasing irradiance and recovered as daily irradiance decreased. The greatest inhibition (15%) in F_v/F_m was observed at 12:00 hr which responded to the highest irradiance. The absorption flux, the trapping flux, and the electron transport flux per PSII reaction center increased in response to daily increasing irradiance and decreased as irradiance decreased. The daily change in the concentration of PSII reaction centers followed the same pattern as F_v/F_m. However, no significant changes in the probability of electron transport beyond Q_A (Ψ_o) were observed during the day. The results suggest that the decrease in F_v/F_m induced by photoinhibition in outdoor Spirulina cultures was a result of the inactivation of PSII reaction centers. The results also suggest that the measurement of polyphasic fluorescence transients is a powerful tool to study the mechanism of photoinhibition in outdoor Spirulina cultures and to screen strains for photoinhibition tolerance. This revised version was published online in June 2006 with corrections to the Cover Date.     

Involvement of betacyanin in chilling-induced photoinhibition in leaves of <Emphasis Type="Italic">Suaeda salsa</Emphasis>

Seeds of Suaeda salsa were cultured in dark for 3 d and betacyanin accumulation in seedlings was promoted significantly. Then the seedlings with accumulated betacyanin (C+B) were transferred to 14/10 h light/dark and used for chilling treatment 15 d later. Photosystem 2 (PS2) photochemistry, D1 protein content, and xanthophyll cycle during the chilling-induced photoinhibition (exposed to 5 °C at a moderate photon flux density of 500 μmol m⁻² s⁻¹ for 3 h) and the subsequent restoration were compared between the C+B seedlings and the control (C) ones. The maximal efficiency of PS2 photochemistry (F_v/F_m), the efficiency of excitation energy capture by open PS2 centres (F_v′/F_m′), and the yield of PS2 electron transport (Φ_PS2) of the C+B and C leaves both decreased during photoinhibition. However, smaller decreases in F_v/F_m, F_v′/F_m′, and Φ_PS2 were observed in the C+B leaves than in C ones. At the same time, the deepoxidation state of xanthophyll cycle, indicated by (A+Z)/(V+A+Z) ratio, increased rapidly but the D1 protein content decreased considerably during the photoinhibition. The increase in rate of (A+Z)/(V+A+Z) was higher but the D1 protein turnover was slower in C+B than C leaves. After photoinhibition treatment, the plants were transferred to a dim irradiation (10 μmol m⁻² s⁻¹) at 25 °C for restoration. During restoration, the chlorophyll (Chl) fluorescence parameters, D1 protein content, and xanthophyll cycle components relaxed gradually, but the rate and level of restoration in the C+B leaves was greater than those in the C leaves. The addition of betacyanins to the thylakoid solution in vitro resulted in similar changes of F_v/F_m, D1 protein content, and (A+Z)/(V+A+Z) ratio during the chilling process. Therefore, betacyanin accumulation in S. salsa seedlings may result in higher resistance to photoinhibition, larger slowing down of D1 protein turnover, and enhancement of non-radiative energy dissipation associated with xanthophyll cycle, as well as in greater restoration after photoinhibition than in the control when subjected to chilling at moderate irradiance.     

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency in pumpkin plants as affected by ozone fumigation

CO₂ assimilation, xanthophyll cycle pigments and PSII efficiency were analyzed in two different ages of pumpkin leaves (Cucurbita pepo L. cv. Ambassador) exposed to 150 nmol mol^-1 of ozone (5 days, 5 h day^-1). Gas-exchange measurements revealed a reduction in CO₂ assimilation and stomatal conductance, accompanied by an increase in the intercellular CO₂ concentration both in young and in mature leaves as compared to their respective controls. In both leaves, F₀ remained unchanged, while F_m and the F_v/F_m ratio decreased after O₃ fumigation, indicating that ozone may induce an alteration in the capability of photosystem II (PSII) to reduce the primary acceptor Q_A. In the mature leaves the photochemical quenching (q_p) was significantly lowered by the pollutant, but this was not the case in the young leaves where q_p did not change. In both mature and young ozonated pumpkin leaves, the development of non-photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between F_v/F_m and the de-epoxidation state of dark-adapted leaves. Decreases in the F_v/F_m ratio are generally attributed to damage to the PSII reaction centre, apart from the down-regulation of the capacity of PSII electron transport. While in young ozonated leaves the decrease in the F_v/F_m ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the F_v/F_m was accompanied by a significant decline in the D1 content. In conclusion, ozone exposure induces alterations in the light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xanthophyll cycle appears to counteract ozone effects against the photosynthetic apparatus as demonstrated by the absence of damage to the D1 protein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not sufficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress.     

Acclimation of winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>, cv. Yangmai 13) to low levels of solar irradiance

Winter wheat is a grass species widely planted in northern and central China, where the increase of aerosols, air pollutants and population density are causing significant reduction in solar irradiance. In order to investigate the adaptation of winter wheat (Triticum aestivum L., cv. Yangmai 13) to low irradiance conditions occurring in the downstream plain of the Yangtze River (China), plants were subjected to four solar irradiance treatments (100%, 60%, 40%, and 20% of environmental incident solar irradiance). Significant increases in chlorophyll (Chl) and xanthophyll (Xan) pigments, and decreases in Chl a/b and Xan/Chl ratios were observed in plants under low light. Light-response curves showed higher net photosynthetic rates (P _N) in fully irradiated plants, that also showed a higher light-compensation point. Shaded plants maintained high values of minimal fluorescence of dark-adapted state (F_o) and maximum quantum efficiency of PSII photochemistry (F_v/F_m) that assess a lower degree of photoinhibition under low light. Reduced irradiance caused decreases in effective quantum yield of PSII photochemistry (Φ_PSII), electron transport rate (ETR), and nonphotochemical quenching coefficient (q_N), and the promotion of excitation pressure of PSII (1 − q_P). The activities of the antioxidant enzymes superoxide dismutase and peroxidase were high under reduced light whereas no light-dependent changes in catalase activity were observed. Thiobarbituric acid reactive species content and electrolyte leakage decreased under shaded plants that showed a lower photooxidative damage. The results suggest that winter wheat cv. Yangmai 13 is able to maintain a high photosynthetic efficiency under reduced solar irradiance and acclimates well to shading tolerance. The photosynthetic and antioxidant responses of winter wheat to low light levels could be important for winter wheat cultivation and productivity.     

Application of trehalose ameliorates heat stress and promotes recovery of winter wheat seedlings

Trehalose was supplied to wheat (Triticum aestivum L.) seedlings just before a high temperature (40 °C) treatment and some physiological parameters were measured during the heat stress and recovery. The application of trehalose decreased the net photosynthetic rate (P_N) of wheat seedlings under the heat stress, but to a small extent increased the dry mass (DM) and leaf water content (LWC) after recovery from the heat stress. The trehalose-induced decrease in PN under the heat stress was not associated with a stomatal response. The heat stress slightly decreased the maximal efficiency of photosystem II (PS II) photochemistry (the variable to maximum chlorophyll a fluorescence ratio, F_v/F_m) similarly in the trehalose treated or non-treated plants. Under the heat stress, the actual efficiency of PS II photochemistry (Φ_PSII) and the efficiency of excitation energy capture by open reaction centers (F_v′/F_m′) were lower in the trehalose-pretreated seedlings, whereas they were higher after the recovery. The patterns of changes in nonphotochemical quenching (NPQ) were contrary to those of ?_{PS II} and F_v′/F_m′. The chlorophyll content was lower, whereas the β-carotene content and the degree of de-epoxidation (DEPS) of xanthophyll cycle pigments were higher in the trehalose-pretreated wheat seedlings under the heat stress. These results suggest that exogenous trehalose partially promotes recovery of wheat by the increase of NPQ, β-carotene content, and DEPS.     

PHOTOINHIBITION OF PSII IN EMILIANIA HUXLEYI (HAPTOPHYTA) UNDER HIGH LIGHT STRESS: THE ROLES OF PHOTOACCLIMATION,PHOTOPROTECTION, AND PHOTOREPAIR1

The response of the coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler to acute exposure to high photon flux densities (PFD) was examined in terms of PSII photoinhibition, photoprotection, and photorepair. The time and light dependencies of these processes were characterized as a function of the photoacclimation state of the alga. Low‐light (LL) acclimated cells displayed a higher degree of photoinhibition, measured as decline in F_v/F_m, than high‐light (HL) acclimated cells. However, HL cultures were more susceptible to photodamage but also more capable of compensating for it by performing a faster repair cycle. The relation between gross photoinhibition (observed in the presence of an inhibitor of repair) and PFD to which the algae were exposed deviated from linearity at high PFD, which calls into question the universality of current concepts of photoinhibition in mechanistic models. The light dependence of the de‐epoxidation state (DPS) of the xanthophyll cycle (XC) pigments on the timescale of hours was the same in cells acclimated to LL and HL. However, HL cells were more efficient in realizing nonphotochemical quenching (NPQ) on short timescales, most likely due to a larger XC pool. LL cells displayed an increase in the PSII effective cross‐section (σ_PSII) as a result of photoinhibition, which was observed also in HL cells when net photoinhibition was induced by blocking the D1 repair cycle. The link between σ_PSII and photoinhibition suggests that the population of PSII reaction centers (RCIIs) of E. huxleyi shares a common antenna, according to a “lake” organization of the light‐harvesting complex.     

Effect of phosphorus deficiency on the photosynthetic characteristics of rice plants

The effects of phosphorus deficiency on the photosynthetic characteristics were studied in rice seedlings (Oryza sativa L.) every 8 days after treatment. P deficiency caused a significant reduction in the net photosynthesis rate (P _N) in rice plants. During the first 16 days of P deficiency, the maximum efficiency of PSII photochemistry (F _v/F _m), the effective PSII quantum yield (ϕ_PSII), the electron transport rate (ETR) as well as photochemical quenching (qP) in the P-limited rice plants kept close to the control, but the excitation energy capture efficiency of PSII reaction centers (F′_v/F′_m) was significantly declined in the P-deficient rice leaves. Meanwhile, in the stressed leaves, we also found a significant increase in nonphotochemical quenching (NPQ) as well as in the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). It was indicated that a series of photoprotective mechanisms had been initiated in rice plants in response to short-term P deficiency. Therefore, PSII functioning was not affected significantly under such stress. As P deficiency continued, the excess excitation energy was accumulated in excess of the capacity of photoprotection systems. When the rice suffered from P deficiency more than 16 days, ϕ_PSII, ETR, and qP were decreased more rapidly than that in the control plants, although NPQ still kept higher in the stressed plants. These results were also consistent with the data on the distribution of excitation energy. The excess energy induced the generation of reactive oxygen species, which might lead to the further damage to PSII functioning. This text was submitted by the authors in English.     

Photoinhibition and photoprotection in senescent leaves of field-grown wheat plants

Photosynthesis, photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle in the senescent flag leaves of wheat (Triticum aestivum L.) plants grown in the field were investigated. Compared to the non-senescent leaves, photosynthetic capacity was significantly reduced in senescent flag leaves. The light intensity at which photosynthesis was saturated also declined significantly. The light response curves of PSII photochemistry indicate that a down-regulation of PSII photochemistry occurred in senescent leaves in particular at high light. The maximal efficiency of PSII photochemistry in senescent flag leaves decreased slightly when measured at predawn but substantially at midday, suggesting that PSII function was largely maintained and photoinhibition occurred in senescent leaves when exposed to high light. At midday, PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centers decreased considerably, while non-photochemical quenching increased significantly. Moreover, compared with the values at early morning, a greater decrease in CO₂ assimilation rate was observed at midday in senescent leaves than in control leaves. The levels of antheraxanthin and zeaxanthin via the de-epoxidation of violaxanthin increased in senescent flag leaves from predawn to midday. An increase in the xanthophyll cycle pigments relative to chlorophyll was observed in senescent flag leaves. The results suggest that the xanthophyll cycle was activated in senescent leaves due to the decrease in CO₂ assimilation capacity and the light intensity for saturation of photosynthesis and that the enhanced formation of antheraxanthin and zeaxanthin at high light may play an important role in the dissipation of excess light energy and help to protect photosynthetic apparatus from photodamage. Our results suggest that the well-known function of the xanthophyll cycle to safely dissipate excess excitation energy is also important for maintaining photosynthetic function during leaf senescence.     

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.     

Artificially increased ascorbate content affects zeaxanthin formation but not thermal energy dissipation or degradation of antioxidants during cold-induced photooxidative stress in maize leaves

Infiltrating detached maize (Zeamays L.) leaves with L-galactono-1,4-lactone (L-GAL) resulted in a 4-fold increase in the content of leaf ascorbate. Upon exposure to high irradiance (1000 μmol photons m⁻² s⁻¹) at 5 °C, L-GAL leaves de-epoxidized the xanthophyll-cycle pigments faster than the control leaves; the maximal ratio of de-epoxidized xanthophyll-cycle pigments to the whole xanthophyll-cycle pool was the same in both leaf types. The elevated ascorbate content, together with the faster violaxanthin de-epoxidation, did not affect the degree of photoinhibition and the kinetics of the recovery from photoinhibition, assayed by monitoring the maximum quantum efficiency of photosystem II primary photochemistry (F_v/F_m). Under the experimental conditions, the thermal energy dissipation seems to be zeaxanthin-independent since, in contrast to the de-epoxidation, the decrease in the efficiency of excitation-energy capture by open photosystem II reaction centers (F_v′/F_m′) during the high-irradiance treatment at low temperature showed the same kinetic in both leaf types. This was also observed for the recovery of the maximal fluorescence after stress. Furthermore, the elevated ascorbate content did not diminish the degradation of pigments or α-tocopherol when leaves were exposed for up to 24 h to high irradiance at low temperature. Moreover, a higher content of ascorbate appeared to increase the requirement for reduced glutathione. Received: 20 May 1999 / Accepted: 29 October 1999     

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.