Leaf Wilting Movement Can Protect Water-Stressed Cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.) Plants Against Photoinhibition of Photosynthesis and Maintain Carbon Assimilation in the Field

Under severe water stress, leaf wilting is quite general in higher plants. This passive movement can reduce the energy load on a leaf. This paper reports an experimental test of the hypothesis that leaf wilting movement has a protective function that mitigates against photoinhibition of photosynthesis in the field. The experiments exposed cotton (Gossypium hirsutum L.) to two water regimes: water-stressed and well-watered. Leaf wilting movement occurred in water-stressed plants as the water potential decreased to −4.1 MPa, reducing light interception but maintaining comparable quantum yields of photosystem II (PS II; Yield for short) and the proportion of total PS II centers that were open (qP). Predrawn F _v/F _m (potential quantum yield of PS II) as an indicator of overnight recovery of PS II from photoinhibition was higher than or similar to that in well-watered plants. Compared with water-stressed cotton leaves for which wilting movement was permitted, water-stressed cotton leaves restrained from such movement had significantly increased leaf temperature and instantaneous CO₂ assimilation rates in the short term, but reduced Yield, qP, and F _v/F _m. In the long term, predrawn F _v/F _m and CO₂ assimilation capacity were reduced in water-stressed leaves restrained from wilting movement. These results suggest that, under water stress, leaf wilting movement could reduce the incident light on leaves and their heat load, alleviate damage to the photosynthetic apparatus due to photoinhibition, and maintain considerable carbon assimilation capacity in the long term despite a partial loss of instantaneous carbon assimilation in the short term.     

How to correctly determine the different chlorophyll fluorescence parameters and the chlorophyll fluorescence decrease ratio R<Subscript>Fd</Subscript> of leaves with the PAM fluorometer

This contribution is a practical guide to the measurement of the different chlorophyll (Chl) fluorescence parameters and gives examples of their development under high-irradiance stress. From the Chl fluorescence induction kinetics upon irradiation of dark-adapted leaves, measured with the PAM fluorometer, various Chl fluorescence parameters, ratios, and quenching coefficients can be determined, which provide information on the functionality of the photosystem 2 (PS2) and the photosynthetic apparatus. These are the parameters F_v, F_m, F₀, F_m′, F_v′, NF, and ΔF, the Chl fluorescence ratios F_v/F_m, F_v/F₀, ΔF/F_m′, as well as the photochemical (q_P) and non-photochemical quenching coefficients (q_N, q_CN, and NPQ). q_N consists of three components (q_N = q_E + q_T + q_I), the contribution of which can be determined via Chl fluorescence relaxation kinetics measured in the dark period after the induction kinetics. The above Chl fluorescence parameters and ratios, many of which are measured in the dark-adapted state of leaves, primarily provide information on the functionality of PS2. In fully developed green and dark-green leaves these Chl fluorescence parameters, measured at the upper adaxial leaf side, only reflect the Chl fluorescence of a small portion of the leaf chloroplasts of the green palisade parenchyma cells at the upper outer leaf half. Thus, PAM fluorometer measurements have to be performed at both leaf sides to obtain information on all chloroplasts of the whole leaf. Combined high irradiance (HI) and heat stress, applied at the upper leaf side, strongly reduced the quantum yield of the photochemical energy conversion at the upper leaf half to nearly zero, whereas the Chl fluorescence signals measured at the lower leaf side were not or only little affected. During this HL-stress treatment, q_N, q_CN, and NPQ increased in both leaf sides, but to a much higher extent at the lower compared to the upper leaf side. q_N was the best indicator for non-photochemical quenching even during a stronger HL-stress, whereas q_CN and NPQ decreased with progressive stress even though non-photochemical quenching still continued. It is strongly recommended to determine, in addition to the classical fluorescence parameters, via the PAM fluorometer also the Chl fluorescence decrease ratio R_Fd (F_d/F_s), which, when measured at saturation irradiance is directly correlated to the net CO₂ assimilation rate (_{P N}) of leaves. This R_Fd-ratio can be determined from the Chl fluorescence induction kinetics measured with the PAM fluorometer using continuous saturating light (cSL) during 4–5 min. As the R_Fd-values are fast measurable indicators correlating with the photosynthetic activity of whole leaves, they should always be determined via the PAM fluorometer parallel to the other Chl fluorescence coefficients and ratios.     

Changes in leaf photosynthesis of transgenic rice with silenced <Emphasis Type="Italic">OsBP-73</Emphasis> gene

In comparison with its wild type (WT), the transgenic (TG) rice with silenced OsBP-73 gene had significantly lower plant height, grain number per panicle, and leaf net photosynthetic rate (P _N). Also, the TG rice showed significantly lower chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), RuBPCO activase, and RuBP contents, photosystem 2 (PS2) photochemical efficiency (F_v/F_m and ΔF/F_m′), apparent quantum yield of carbon assimilation (Φ_c), carboxylation efficiency (CE), photosynthetic electron transport and photophosphorylation rates as well as sucrose phosphate synthase activity, but higher intercellular CO₂ concentration, sucrose, fructose, and glycerate 3-phosphate contents, and non-photochemical quenching of Chl fluorescence (NPQ). Thus the decreased P _N in the TG rice leaves is related to both RuBP carboxylation and RuBP regeneration limitations, and the latter is a predominant limitation to photosynthesis.     

Photoinhibition of colonial and unicellular <Emphasis Type="Italic">Microcystis</Emphasis> cells in a summer bloom in Lake Taihu

To evaluate the photoinhibition of colonial and unicellular cells of Microcystis aeruginosa under natural conditions, the maximum and effective quantum yields of photosystem II were measured from variable chlorophyll a fluorescence in samples from Lake Taihu during a summer bloom from June 19 to 21, 2006. Diurnal changes in the photoinhibition of Microcystis cells incubated immediately below the surface in clear bottles for 30 min and in situ samples under natural conditions were measured. At solar noon during the three days, the mean values of maximum quantum yield (F _v/F _m) and effective quantum yield (ΔF/F _m′) for unicellular cells (F _v/F _m = 0.15, ΔF/F _m′ = 0.10) were lower than those for colonial cells (F _v/F _m = 0.25, ΔF/F _m′ = 0.15). For in situ samples, the values of F _v/F _m and ΔF/F _m′ for colonial cells at solar noon on the three days (F _v/F _m 0.30, 0.25, 0.29; ΔF/F _m′ 0.24, 0.21, 0.22) were also higher than those of unicellular cells (F _v/F _m 0.26, 0.18, 0.25; ΔF/F _m′ 0.15, 0.11, 0.14). The results indicate that colony formation has a protective effect on Microcystis cells by reducing the occurrence of photoinhibition under high light intensities.     

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     

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.     

Effect of daily photon receipt on the susceptibility of dwarf bean (Phaseolus vulgaris L.) leaves to photoinhibition of photosynthesis

Bean (Phaseolus vulgaris L.) plants were grown at two light periods of 8 and 13 h with a similar photon flux density (PFD) giving a daily photon receipt (DPR) of 17.9 and 38.2 mol · m–2, respectively. Shoot growth and leaf area development were followed at regular intervals and diurnal whole-plant photosynthesis measured. Single mature trifoliate leaves were exposed to photoinhibitory treatments at PFDs of 800 and 1400 mol · m–2 · s–1 and at temperatures of 12 and 20°C. Chlorophyll fluorescence and photon yields were measured at regular intervals throughout each treatment. Plants grown in 13 h had significantly greater leaf areas than those grown in 8 h. There were no differences in maximum rates of photosynthesis, photon yields and only minor but significant differences in F_v/F_m for plants in the two treatments, showing photosynthetic characteristics were dependent on PFD but not DPR. A significant decline in photosynthesis and F_v/F_m occurred over the 13-h but little change in photosynthesis for plants in the 8 h, indicating some feedback inhibition of photosynthesis was occurring. Plants grown in 8 h were consistently more susceptible to photoinhibition of photosynthesis at all treatments than 13-h plants. Nevertheless, photoinhibition was exacerbated by increases in PFD, and by decreases in temperature for leaves from both treatments. However, for plants from the 8-h day, exposing leaves to 12°C and 1400 mol · m–2 · s–1 caused photo-oxidation and severe bleaching but no visible damage on leaves from 13-h-grown plants. Closure of the photosystem II reaction-centre pool was partially correlated with increasing extents of photoinhibition but the relationship was similar for plants from both treatments. There remains no clear explanation for their wide differences in susceptibility to photoinhibition.Abbreviations and Symbols DPR daily photon receipt - F₀ and F_m initial and maximal fluorescence - F_v/F_m fluorescence ratio in dark-treated leaves - F/F_m intrinsic efficiency of PSII during illumination - PFD photon flux density - _i photon yield (incident basis) - _psi quantum yield of PSII electron transport - P_max maximum rate of photosynthesis - q_N non-photochemical quenching coefficient - q_P photochemical quenching coefficient Many thanks to my colleague William Laing who spent a considerable effort in developing the programme to run the photosynthesis apparatus. I am also indebted to one reviewer with whom I corresponded to resolve some issues in the paper. This project was funded by the New Zealand Foundation for Research, Science and Technology.     

Effect of 24-Epibrassinolide on Chlorophyll Fluorescence and Photosynthetic CO<Subscript>2</Subscript> Assimilation in <Emphasis Type="Italic">Vicia faba</Emphasis> Plants Treated with the Photosynthesis-Inhibiting Herbicide Terbutryn

Simultaneous measurements of chlorophyll (Chl) fluorescence and CO₂ assimilation (A) in Vicia faba leaves were taken during the first weeks of growth to evaluate the protective effect of 24-epibrassinolide (EBR) against damage caused by the application of the herbicide terbutryn (Terb) at pre-emergence. V. faba seeds were incubated for 24 h in EBR solutions (2 × 10⁻⁶ or 2 × 10⁻⁵ mM) and immediately sown. Terb was applied at recommended doses (1.47 or 1.96 kg ha⁻¹) at pre-emergence. The highest dose of Terb strongly decreased CO₂ assimilation, the maximum quantum yield of PSII photochemistry in the dark-adapted state (F _V/F _M), the nonphotochemical quenching (NPQ), and the effective quantum yield (ΔF/F′_M) during the first 3–4 weeks after plant emergence. Moreover, Terb increased the basal quantum yield of nonphotochemical processes (F ₀/F _M)_, the degree of reaction center closure (1 − q _p), and the fraction of light absorbed in PSII antennae that was dissipated via thermal energy dissipation in the antennae (1 − F′_V/F′_M). The herbicide also significantly reduced plant growth at the end of the experiment as well as plant length, dry weight, and number of leaves. The application of EBR to V. faba seeds before sowing strongly diminished the effect of Terb on fluorescence parameters and CO₂ assimilation, which recovered 13 days after plant emergence and showed values similar to those of control plants. The protective effect of EBR on CO₂ assimilation was detected at a photosynthetic photon flux density (PFD) of 650 μmol m⁻² s⁻¹ and the effect on ΔF/F′_M and photosynthetic electron transport (J) was detected under actinic lightings up to 1750 μmol m⁻² s⁻¹. The highest dose of EBR also counteracted the decrease in plant growth caused by Terb, and plants registered the same growth values as controls.     

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.     

Photoinhibition of isolated mesophyll cells from cold-hardened and non-hardened winter rye

Cold-hardened rye leaves have been shown to be more resistant to low temperature photoinhibition than non-hardened rye leaves. Isolated mesophyll cells from winter rye (Secale cereale L. cv. Musketeer) were exposed to photoinhibitory light conditions to estimate the importance of leaf morphology and leaf optical properties in the resistance of cold-hardened rye leaves to photoinhibition. Cold-hardened rye cells showed more resistance to photoinhibition than non-hardened rye cells when monitored with chlorophyll a variable to maximal fluorescence ratio (F_v/F_m). Thus, leaf morphology does not contribute to the resistance of cold-hardened rye leaves to low temperature photoinhibition. However, cold-hardened and non-hardened rye cells showed a similar extent of photoinhibition when photsynthetic CO₂ fixation rates were measured. They also showed the same capacity to recover from photoinhibition. During both photoinhibition and recovery, F_v/F_m and light limited CO₂ fixation rates showed different kinetics. We propose that inactivation and subsequent reactivation during recovery of some light activated Calvin cycle enzymes explain the greater extent of photoinhibition of light limited CO₂ fixation and its faster recovery compared to F_v/F_m kinetics during photoinhibition.     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: effect of growth temperature on photoinhibition and recovery

Intact leaves of kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) from plants grown in a range of controlled temperatures from 15/10 to 30/25°C were exposed to a photon flux density (PFD) of 1500 μmol·m⁻²·s⁻¹ at leaf temperatures between 10 and 25°C. Photoinhibition and recovery were followed at the same temperatures and at a PFD of 20 μmol·m⁻²·s⁻¹, by measuring chlorophyll fluorescence at 77 K and 692 nm, by measuring the photon yield of photosynthetic O₂ evolution and light-saturated net photosynthetic CO₂ uptake. The growth of plants at low temperatures resulted in chronic photoinhibition as evident from reduced fluorescence and photon yields. However, low-temperature-grown plants apparently had a higher capacity to dissipate excess excitation energy than leaves from plants grown at high temperatures. Induced photoinhibition, from exposure to a PFD above that during growth, was less severe in low-temperature-grown plants, particularly at high exposure temperatures. Net changes in the instantaneous fluorescence,F ₀, indicated that little or no photoinhibition occurred when low-temperature-grown plants were exposed to high-light at high temperatures. In contrast, high-temperature-grown plants were highly susceptible to photoinhibitory damage at all exposure temperatures. These data indicate acclimation in photosynthesis and changes in the capacity to dissipate excess excitation energy occurred in kiwifruit leaves with changes in growth temperature. Both processes contributed to changes in susceptibility to photoinhibition at the different growth temperatures. However, growth temperature also affected the capacity for recovery, with leaves from plants grown at low temperatures having moderate rates of recovery at low temperatures compared with leaves from plants grown at high temperatures which had negligible recovery. This also contributed to the reduced susceptibility to photoinhibition in low-temperature-grown plants. However, extreme photoinhibition resulted in severe reductions in the efficiency and capacity for photosynthesis.     

Nitrogen supply as a factor influencing photoinhibition and photosynthetic acclimation after transfer of shade-grown Solanum dulcamara to bright light

We have compared the ability of shadegrown clones of Solamum dulcamara L. from shade and sun habitats to acclimate to bright light, as a function of nitrogen nutrition before and after transfer to bright light. Leaves of S. dulcamara grown in the shade with 0.6 mM NO ₃ ^- have similar photosynthetic properties as leaves of plants grown with 12.0 mM NO ₃ ^- . When transferred to bright light for 1–2 d the leaves of these plants show substantial photoinhibition which is characterized by about 50% decrease in apparent quantum yield and a reduction in the rate of photosynthesis in air at light saturation. Photoinhibition of leaf photosynthesis is associated with reduction in the variable component of low-temperature fluorescence emission, and with loss of in-vitro electron transport, especially of photosystem II-dependent processes.We find no evidence for ecotypic differentiation in the potential for photosynthetic acclimation among shade and sun clones of S. dulcamara, or of differentiation with respect to nitrogen requirements for acclimation. Recovery from photoinhibition and subsequent acclimation of photosynthesis to bright light only occurs in leaves of plants provided with 12.0 mM NO ₃ ^- . In these, apparent quantum yield is fully restored after 14 d, and photosynthetic acclimation is shown by an increase in light-saturated photosynthesis in air, of light-and CO₂-saturated photosynthesis, and of the initial slope of the CO₂-response curve. The latter changes are highly correlated with changes in ribulose-bisphosphate-carboxylase activity in vitro. Plants supplied with 0.6 mM NO ₃ ^- show incomplete recovery of apparent quantum yield after 14 d, but CO₂-dependent leaf photosynthetic parameters return to control levels.Symbols and abbreviations F_o initial level of fluorescence at 77 K - F_m maximum level of fluorescence at 77 K - F_v variable components of fluorescence at 77 K (F_v=F_m-F_o) - PSI, PSII photosystem I and II, respectively - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39)     

Effect of temperature on net CO2 assimilation and photosystem II quantum yield of electron transfer of French bean (Phaseolus vulgaris L.) leaves during drought stress

The effect of leaf temperature on stomatal conductance and net CO₂ uptake was studied on French bean (Phaseolus vulgaris L.) using either dehydrated attached leaves (25–40% water deficit) or cut leaves supplied with 10^–4 M abscisic acid (ABA) solution to the transpiration stream. Decreasing leaf temperature caused stomatal opening and increased net CO₂ uptake (which was close to zero at around 25° C) to a level identical to that of control leaves (without water deficit) at around 15° C. (i) The ABA effect on stomatal closure was modulated by temperature and, presumably, ABA is at least partly responsible for stomatal closure of french bean submitted to a drought stress. (ii) For leaf temperatures lower than 15° C, net CO₂ uptake was no longer limited by water deficit even on very dehydrated leaves. This shows that dehydrated leaves retain a substantial part of their photosynthetic capacity which can be revealed at normal CO₂ concentrations when stomata open at low temperature. In contrast to leaves fed with ABA, decreasing the O₂ concentration from 21% to 1% O₂ did not increase either the rate of net CO₂ uptake or the thermal optimum for photosynthesis of dehydrated leaves. The quantum yield of PSII electron flow (measured by F/F_m) was lower in 1% O₂ than in 21% O₂ for each leaf pretreatment given (non-dehydrated leaves, dehydrated leaves, and leaves fed with ABA) even within a temperature range in which leaf photosynthesis at normal CO₂ concentration was the same in these two O₂ concentrations. It is concluded that this probably indicates an heterogeneity of photosynthesis, since this difference in quantum yield disappears when using high CO₂ concentrations during measurements.Abbreviations and Symbols ABA abscisic acid - F_m maximum chlorophyll fluorescence - F difference between steady-state chlorophyll fluorescence and F_m - PPFD photosynthetic photon flux density We would like to thank Dr. J.-M. Briantais (Laboratoire d'écologie végétale, Orsay, France) for help during fluorescence measurements and Ms. J. Liebert for technical assistance.     

The Influence of Leaf Windows on the Utilization and Absorption of Radiant Energy in Seven Desert Succulents

Four fluorescence parameters [F_v/F_m = the intrinsic efficiency of energy conversion via photosystem 2 (PS2); F_v/F_m= the efficiency of energy conversion via PS2 in the light; P = fraction of absorbed radiant energy utilized for photosynthesis; and D = fraction of absorbed radiant energy dissipated as heat] were measured on leaves of seven species of succulents having epidermal windows. While the function of leaf windows has reportedly been to increase absorption of radiant energy and, hence, the rate of photosynthesis in these species, recent evidence indicates that this translucent portion of epidermal tissue, lacking chlorophyll, may also result in photoinhibition in these species, especially for those with growth habits aboveground. Species with aboveground and belowground growth habits were compared with their leaf windows covered with reflective tape and with windows unobstructed. Results showed no increase in photoinhibition for these species resulting from the radiant energy penetrating the window tissue. Although the efficiency of the photosynthetic mechanism was not significantly influenced by the additional radiant energy provided by the window for individual species, there were significant differences in the efficiencies of radiant energy capture (F_v/F_m) and utilization (P) between the two growth habits. Species with an aboveground growth habit were less efficient in radiant energy utilization compared with the species having a belowground growth habit.     

The acceptor side of photosystem II is damaged more severely than the donor side of photosystem II in ‘Honeycrisp’ apple leaves with zonal chlorosis

To investigate the photoinhibition of photosynthesis in ‘Honeycrisp’ apple (Malus domestica Borkh. cv. Gala) leaves with zonal chlorosis, we compared pigments, CO₂ assimilation and chlorophyll (Chl) a fluorescence (OJIP) transient between chlorotic leaves and normal ones. Chl and carotenoids (Car) contents, Chl a/b ratio, and absorptance were lower in chlorotic leaves than in normal ones, whereas Car/Chl ratio was higher in the former. Although CO₂ assimilation and stomatal conductance were lower in chlorotic leaves, intercellular CO₂ concentration did not differ significantly between the two leaf types. Compared with normal leaves, chlorotic ones had increased deactivation of oxygen-evolving complexes (OEC), minimum fluorescence (F _o), dissipated energy, relative variable fluorescence at L-, W-, J- and I-steps, and decreased maximum fluorescence (F _m), maximum quantum yield for primary photochemistry (F _v /F _m or TR_o/ABS), quantum yield for electron transport (ET_o/ABS), quantum yield for the reduction of end acceptors of photosystem I (PSI) (φ_Ro and RE_o/ABS), maximum amplitude of IP phase, amount of active photosystem II (PSII) reaction centers (RCs) per cross section (CS) and total performance index (PI_tot,abs). In conclusion, photoinhibition occurs at both the donor (i.e., the OEC) and the acceptor sides of PSII in chlorotic leaves. The acceptor side is damaged more severely than the donor side, which possibly is the consequence of over-reduction of PSII due to the slowdown of Calvin cycle. In addition to decreasing light absorptance by lowering Chl level, energy dissipation is enhanced to protect chlorotic leaves from photo-oxidative damage.     

Photosynthetic induction in leaves of co-occurring <Emphasis Type="Italic">Fagus lucida</Emphasis> and <Emphasis Type="Italic">Castanopsis lamontii</Emphasis> saplings grown in contrasting light environments

Photosynthetic induction times and photoinhibition in relation to simulated sunflecks (sudden increase of irradiance from 20 to 1,500 μmol m⁻² s⁻¹) were examined in leaves of co-occurring Fagus lucida (a deciduous tree) and Castanopsis lamontii (an evergreen tree) saplings grown either in a beech forest understory or in an adjacent open site during a late rainy season. Two hypotheses were tested: (1) understory leaves would display faster photosynthetic induction times and greater photoinhibition than open-grown leaves; and (2) evergreen species would have slower photosynthetic induction times and lighter photoinhibition than deciduous species. Times to reach 90% of maximal CO₂ assimilation rate (t _90%A ) and stomatal conductance did not differ between species, but showed faster by 3–5 min in open-grown leaves than understory leaves due to higher initial stomatal conductance (g _{s initial}) and induction state 1 min into simulated sunflecks (IS_1min) in the former. Our analysis across the published data on photosynthetic induction of 48 broad-leaved woody species again revealed the negative correlations between t _90%A and either g _{s initial} or IS_1min, and the similarity of t _90%A and between evergreen and deciduous species. Measurements of maximum PSII photochemical efficiency (F _v/F _m) indicated that photoinhibition occurred in saplings in any of the growth habitats during sunfleck-induced photosynthetic induction. Despite no interspecific differences in the degree of photoinhibition, understory leaves of both species suffered heavier photoinhibition than open-grown leaves, as indicated by a stronger decrease of F _v/F _m in the former. Dynamic changes in the quantum yields of PSII photochemistry and ΔpH- and xanthophyll-regulated thermal dissipation and adjustments in the partitioning of electron flow between assimilative and non-assimilative processes were functional to resist photoinhibition. However, such photoinhibition, together with stomatal and biochemical limitations, would decrease carbon gain during simulated sunflecks, particularly in understory leaves.     

Growth and physiological changes in saplings of Minquartia guianensis and Swietenia macrophylla during acclimation to full sunlight

Low light availability under a forest canopy often limits plant growth; however, sudden increase in light intensity may induce photoinhibition of photosynthesis. The aim of this study was to evaluate the ecophysiological changes that occur in potted plants of Minquartia guianensis and Swietenia macrophylla during the acclimation process to full sunlight. We used six full-sun independent acclimation periods (30, 60, 90, 120, 150, and 180 days) and a control kept in the shade. Shading was obtained by placing plants under the canopy of a small forest. The F_v/F_m ratio, net photosynthetic rate (P _N), the maximum carboxylation velocity of Rubisco (V _cmax), maximum electron transport rate (J _max), specific leaf area (SLA), and growth were assessed at the end of each of the six acclimation periods. Plant exposure to full sunlight caused a sudden decrease in the F_v/F_m ratio (photoinhibition) particularly in Minquartia. Photooxidation (necrotic patches) of the leaf tissue was observed in upper leaves of Minquartia. The higher P _N values were observed in Swietenia under full sun, about 12 μmol(CO₂) m⁻² s⁻¹. V _cmax25 values were higher after 90 days of acclimation, about 14 μmol(CO₂) m⁻² s⁻¹ for Minquartia, and 35 μmol(CO₂) m⁻² s⁻¹ for Swietenia. At the end of a 180-d acclimation period J _max25 was 35 μmol(electron) m⁻² s⁻¹ for Minquartia and 60 μmol(electron) m⁻² s⁻¹ for Swietenia. SLA was higher in Swietenia than in Minquartia. In Minquartia, monthly rate of leaf production per plant (MRLP) was positive (0.22 leaf month⁻¹) after four months in the open. Whereas, in Swietenia MRLP was positive (0.56 leaf month⁻¹) after an acclimation period of two months. After six months in the open, height growth rates were 3.5 and 28 mm month⁻¹ for Minquartia and Swietenia, respectively. The greater acclimation capacity of Swietenia was associated to an enhanced photosynthetic plasticity under full sun. In Minquartia, transition to full-sun conditions and lack of physiological adjustment resulted in severe photoinhibition and loss of leaves.     

Effect of water deficit on photosynthetic oxygen exchange measured using 18O2 and mass spectrometry in Solanum tuberosum L. leaf discs

The effect of leaf dehydration on photosynthetic O₂ exchange of potato (Solanum tuberosum L., cv. Haig) leaf discs was examined using ¹⁸O₂ as a tracer and mass spectrometry. In normal air (350 μl·l^?1CO₂) and under an irradiance of 390 μmol photons·m^?2·s¹, a relative water deficit (RWD) of about 30% severely decreased net O₂ evolution and increased O₂ uptake by about 50%, thus indicating an enhancement of photorespiration. Increasing CO₂ concentrations diminished O₂ uptake and stimulated net O₂ evolution both in well-hydrated and in dehydrated (RWD of about 30%) leaves. Much higher CO₂ concentrations (up to 4%) were required to observe a complete effect of CO₂ in dehydrated leaves. The chloroplastic CO₂ concentration at the ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) level (C_c) was calculated from O₂-exchange data in both well-hydrated and dehydrated leaves, assuming that the specificity factor of Rubisco was unaffected by desiccation. When plotting net O₂ photosynthesis as a function of C_c, a similar relationship was obtained for well-hydrated and waterstressed leaf discs, thus showing that the main effect of water deficit is a decrease of the chloroplastic CO₂ concentration. At saturating CO₂ levels, the non-cyclic electron-transport rate, measured either as gross O₂ photosynthesis or as the chlorophyll fluorescence ratio (F_m -F_s)/F_m, was insensitive to water deficit, provided RWD was below 40%. In this range of RWD, the decrease in gross O₂ photosynthesis observed in normal air was attributed to the inability of oxidative processes to sustain the maximal electron-flow rate at low chloroplastic CO₂ concentration. The maximal efficiency of photosystem II, estimated as the chlorophyll fluorescence ratio (F_m -F₀)/F_m measured in dark-adapted leaves, was not affected by water deficits up to 60%.     

Effects of the interaction between ozone and carbon dioxide on gas exchange,photosystem II and antioxidants in rice leaves

To understand the interactive effects of O₃ and CO₂ on rice leaves; gas exchange, chlorophyll (Chl) fluorescence, ascorbic acid and glutathione were examined under acute (5 h), combined exposures of O₃ (0, 0.1, or 0.3 cm³ m⁻³, expressed as O₀, O_0.1, or O_0.3, respectively), and CO₂ (400 or 800 cm³ m⁻³, expressed as C⁴⁰⁰ or C⁸⁰⁰, respectively) in natural-light gas-exposure chambers. The net photosynthetic rate (P _N), maximum (F_v/F_m) and operating (F_q′/F_m′) quantum efficiencies of photosystem II (PSII) in young (8^th) leaves decreased during O₃ exposure. However, these were ameliorated by C⁸⁰⁰ and fully recovered within 3 d in clean air (O⁰ + C⁴⁰⁰) except for the O^0.3 + C⁴⁰⁰ plants. The maximum PSII efficiency at 1,500 μmol m⁻² s⁻¹ PPFD (F_v′/F_m′) for the O^0.3 + C₄₀₀ plants decreased for all measurement times, likely because leaves with severely inhibited P _N also had a severely damaged PSII. The P _N of the flag (16^th) leaves at heading decreased under O₃ exposure, but the decline was smaller and the recovery was faster than that of the 8^th leaves. The F_q′/F_m′ of the flag leaves in the O^0.3 + C⁴⁰⁰ and O^0.3 + C⁸⁰⁰ plants decreased just after gas exposure, but the F_v/F_m was not affected. These effects indicate that elevated CO₂ interactively ameliorated the inhibition of photosynthesis induced by O₃ exposure. However, changes in antioxidant levels did not explain the above interaction.     

Effects of photoinhibition on whole-plant carbon gain assessed with a photosynthesis model

A canopy photosynthesis model was modified to assess the effect of photoinhibition on whole‐plant carbon gain. Photoinhibitory changes in maximum quantum yield of photosystem II (F_v/F_m) could be explained solely from a parameter (Lflux) calculated from the light micro‐environment of the leaves. This relationship between F_v/F_m and the intercepted cumulative light dose, integrated and equally weighted over several hours was incorporated into the model. The effect of photoinhibition on net photosynthesis was described through relationships between photoinhibition and the shaping parameters of the photosynthetic light‐response curve (quantum use efficiency, convexity, and maximum capacity). This new aspect of the model was then validated by comparing measured field data (diurnal courses of F_v/F_m) with simulation results. Sensitivity analyses revealed that the extent of photoinhibitory reduction of whole‐plant photosynthesis was strongly dependent on the structural parameters (LAI and leaf angle). Simulations for a Mediterranean evergreen oak, Quercus coccifera, under climatic conditions which cause mild photoinhibition revealed a daily loss of 7·5–8·5% of potential carbon gain in the upper sunlit canopy layers, a 3% loss in the bottom canopy, and an overall loss of 6·1%. Thus, this canopy photoinhibition model (CANO‐PI) allows the quantitative evaluation of photoinhibition effects on primary production.