Gradual Disassembly of Photosystem 2 In Vivo Induced by Excess Irradiance. A Hypothesis Based on Changes in 77 K Fluorescence Spectra of Chlorophyll a in Barley Leaves

Effects of short-term exposure to different irradiances on the function of photosystem 2 (PS2) were studied for barley grown at low (LI; 50 μmol m⁻² s⁻¹) and high (HI; 1 100 μmol m⁻² s⁻¹) irradiances. HI barley revealed higher ability to down-regulate the light-harvesting within PS2 after exposure to high irradiance as compared to LI plants. This ability was estimated from the light-induced decreases of F685/F742 and E476/E436 in emission and excitation spectra of 77 K chlorophyll (Chl) a fluorescence in vivo which was 65 and 10 % for HI plants as compared to 30 and 2 % for LI plants, respectively. For LI plants this protective down-regulation of the light-harvesting of PS2 was saturated at 430 μmol m⁻² s⁻¹, and progressive PS2 photodamage was induced at higher irradiances. After exposure of LI segments to 2 200 μmol m⁻² s⁻¹ a pronounced maximum at 700 nm appeared in emission spectrum of 77 K Chl a fluorescence. Based on complementary analysis of 77 K excitation spectra measured at the emission wavelength 685 nm we suggest that this emission maximum may be attributed to the formation of aggregates of light-harvesting complexes of PS2 (LHC2) with part of PS2 core during progressive PS2 photodamage. Our results can be explained assuming different contributions of LHC2 and PS2 core to the total nonradiative dissipation of absorbed excitation energy for the LI and HI barley. This revised version was published online in August 2006 with corrections to the Cover Date.     

The development of antenna complexes of barley (Hordeum vulgare cv. Akcent) under different light conditions as judged from the analysis of 77 K chlorophyll a fluorescence spectra

Using 77 K chlorophyll a (Chl a) fluorescence spectra in vivo, the development was studied of Photosystems II (PS II) and I (PS I) during greening of barley under intermittent light followed by continuous light at low (LI, 50 μmol m⁻² s⁻¹) and high (HI, 1000 μmol m⁻² s⁻¹) irradiances. The greening at HI intermittent light was accompanied with significantly reduced fluorescence intensity from Chl b excitation for both PS II (F685) and PS I (F743), in comparison with LI plants, indicating that assembly of light-harvesting complexes (LHC) of both photosystems was affected to a similar degree. During greening at continuous HI, a slower increase of emission from Chl b excitation in PS II as compared with PS I was observed, indicating a preferred reduction in the accumulation of LHC II. The following characteristics of 77 K Chl a fluorescence spectra documented the photoprotective function of an elevated content of carotenoids in HI leaves: (1) a pronounced suppression of Soret region of excitation spectra (410–450 nm) in comparison with the red region (670–690 nm) during the early stage of greening indicated a strongly reduced excitation energy transfer from carotenoids to the Chl a fluorescing forms within PS I and PS II; (2) changes in the shape of the excitation band of Chl b and carotenoids (460–490 nm) during greening under continuous light confirmed that the energy transfer from carotenoids to Chl a within PS II remained lower as compared with the LI plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chlorophyll a Fluorescence Response of Norway Spruce Needles to the Long-Term Effect of Elevated CO2 in Relation to Their Position Within the Canopy

The long-term impact of elevated CO₂ concentration on photosynthetic activity of sun-exposed (E) versus shaded (S) foliage was investigated in a Picea abies stand (age 12 years) after three years of cultivation in adjustable-lamella-domes (ALD). One ALD is supplied with either ambient air [ca. 350 µmol(CO₂) mol^–1; AC-variant) and the second with elevated CO₂ concentration [ambient plus 350 µmol(CO₂) mol^–1; EC-variant). The pronounced vertical profile of the photosynthetically active radiation (PAR) led to the typical differentiation of the photosynthetic apparatus between the S- and E-needles in the AC-variant estimated from the irradiance-responses of various parameters of the room temperature chlorophyll (Chl) a fluorescence parameters. Namely, electron transport rate (ETR), photochemical efficiency of photosystem 2, PS2 (_PS2), irradiance-saturated values of non-photochemical quenching of minimum (SV₀) and maximum (NPQ) fluorescence levels, and photochemical fluorescence quenching (q_p) at higher irradiances were all significantly higher for E-needles as compared with the S-ones. The prolonged exposure to EC did not cause any stimulation of ETR for the E-needles but a strongly positive effect of EC on ETR was observed for the S-needles resulting in more than doubled ETR capacity in comparison with S-needles from the AC-variant. For the E-needles in EC-variant a slightly steeper reduction of the _PS2 and q_p occurred with the increasing irradiance as compared to the E-needles of AC-variant. On the contrary, the S-needles in EC variant revealed a significantly greater capacity to maintain a high _PS2 at irradiances lower than 200 µmol m^–2 s^–1 and to prevent the over-reduction of the PS2 reaction centres. Moreover, compared to the AC-variant the relation between SV₀ and NPQ exhibited a strong decrease (up to 72 %) of the SV₀-NPQ slope for the E-needles and an increase (up to 76 %) of this value for the S-needles. Hence the E- and S-foliage responded differently to the long-term impact of EC. Moreover, this exposure was responsible for the smoothing of the PAR utilisation vertical gradient in PS2 photochemical and non-photochemical reactions within the canopy.     

Excess irradiance causes early symptoms of senescence during leaf expansion in photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis> grown tobacco plants

Photosynthetic parameters, growth, and pigment contents were determined during expansion of the fourth leaf of in vitro photoautotrophically cultured Nicotiana tabacum L. plants at three irradiances [photosynthetically active radiation (400–700 nm): low, LI 60 μmol m⁻² s⁻¹; middle, MI 180 μmol m⁻² s⁻¹; and high, HI 270 μmol m⁻² s⁻¹]. During leaf expansion, several symptoms usually accompanying leaf senescence appeared very early in HI and then in MI plants. Symptoms of senescence in developing leaves were: decreasing chlorophyll (Chl) a+b content and Chl a/b ratio, decreasing both maximum (F_V/F_M) and actual (Φ_PS2) photochemical efficiency of photosystem 2, and increasing non-photochemical quenching. Nevertheless, net photosynthetic oxygen evolution rate (P _N) did not decrease consistently with decrease in Chl content, but exhibited a typical ontogenetic course with gradual increase. P _N reached its maximum before full leaf expansion and then tended to decline. Thus excess irradiance during in vitro cultivation did not cause early start of leaf senescence, but impaired photosynthetic performance and Chl content in leaves and changed their typical ontogenetic course.     

Differential Photosynthetic Acclimation Pattern to Limiting Growth-Irradiance in Two Types of C4 Plants

Photosynthetic acclimation to reduced growth irradiances (650 and 200 µmol m^–2 s^–1) in Eleusine coracana (L.) Garten, a nicotinamide adenine dinucleotide-malic enzyme (NAD-ME) C₄ species and Gomphrena globosa L., a nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C₄ species were investigated. E. coracana plants acclimated in 4 and 8 d to 650 and 200 µmol m^–2 s^–1, respectively, whereas G. globosa plants took 8 and 10 d, respectively, to acclimate to the same irradiances. The acclimation to reduced irradiance was achieved in both species by greater partitioning of chlorophyll towards the light-harvesting antennae at the expense of functional components. However, magnitude of increase in the light-harvesting antenna was higher in E. coracana as compared to G. globosa. Superior photosynthetic acclimation to reduced irradiance in G. globosa was due to the smaller change in functions of the cytochrome b ₆/f complex, photosystem (PS) 1 and PS2 leading to the higher carbon fixation rates compared to E. coracana.     

Enhancement of susceptivity to photoinhibition and photooxidation in rice chlorophyll <Emphasis Type="Italic">b</Emphasis>-less mutants

Two rice chlorophyll (Chl) b-less mutants (VG28-1, VG30-5) and the respective wild type (WT) plant (cv. Zhonghua No. 11) were analyzed for the changes in Chl fluorescence parameters, xanthophyll cycle pool, and its de-epoxidation state under exposure to strong irradiance, SI (1 700 μmol m⁻² s⁻¹). We also examined alterations in the chloroplast ultrastructure of the mutants induced by methyl viologen (MV) photooxidation. During HI (0–3.5 h), the photoinactivation of photosystem 2 (PS2) appeared earlier and more severely in Chl b-less mutants than in the WT. The decreases in maximal photochemical efficiency of PS2 in the dark (F_v/F_m), quantum efficiency of PS2 electron transport (Φ_PS2), photochemical quenching (q_P), as well as rate of photochemistry (P_rate), and the increases in de-epoxidation state (DES) and rate of thermal dissipation of excitation energy (D_rate) were significantly greater in Chl b-mutants compared with the WT plant. A relatively larger xanthophyll pool and 78–83 % conversion of violaxanthin into antheraxanthin and zeaxanthin in the mutants after 3.5 h of HI was accompanied with a high ratio of inactive/total PS2 (0.55–0.73) and high 1–q_P (0.57–0.68) which showed that the activities of the xanthophyll cycle were probably insufficient to protect the photosynthetic apparatus against photoinhibition. No apparent difference of chloroplast ultrastructure was found between Chl b-less mutants and WT plants grown under low, LI (180 μmol m⁻² s⁻¹) and high, HI (700 μmol m⁻² s⁻¹) irradiance. However, swollen chloroplasts and slight dilation of thylakoids occurred in both mutants and the WT grown under LI followed by MV treatment. These typical symptoms of photooxidative damage were aggravated as plants were exposed to HI. Distorted and loose scattered thylakoids were observed in particular in the Chl b-less mutants. A greater extent of photoinhibition and photooxidation in these mutants indicated that the susceptibility to HI and oxidative stresses was enhanced in the photosynthetic apparatus without Chl b most likely as a consequence of a smaller antenna size.     

Effects of different irradiances on the photosynthetic process during ex-vitro acclimation of Anoectochilus plantlets

Six months old in vitro-grown Anoectochilus formosanus plantlets were transferred to ex-vitro acclimation under low irradiance, LI [60 μmol(photon) m⁻² s⁻¹], intermediate irradiance, II [180 μmol(photon) m⁻² s⁻¹], and high irradiance, HI [300 μmol(photon) m⁻² s⁻¹] for 30 d. Imposition of II led to a significant increase of chlorophyll (Chl) b content, rates of net photosynthesis (P _N) and transpiration (E), stomatal conductance (g _s), electron transfer rate (ETR), quantum yield of electron transport from water through photosystem 2 (Φ_PS2), and activity of ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBPCO, EC 4.1.1.39). This indicates that Anoectochilus was better acclimated at II compared to LI treatment. On the other hand, HI acclimation led to a significant reduction of Chl a and b, P _N, E, g _s, photochemical quenching, dark-adapted quantum efficiency of open PS2 centres (F_v/F_m), probability of an absorbed photon reaching an open PS2 reaction centre (F_v′/F_m′), ETR, Φ_PS2, and energy efficiency of CO₂ fixation (Φ_CO2/Φ_PS2). This indicates that HI treatment considerably exceeded the photo-protective capacity and Anoectochilus suffered HI induced damage to the photosynthetic apparatus. Imposition of HI significantly increased the contents of antheraxanthin and zeaxanthin (ZEA), non-photochemical quenching, and conversion of violaxanthin to ZEA. Thus Anoectochilus modifies its system to dissipate excess excitation energy and to protect the photosynthetic machinery.     

Short-term effect of elevated CO2 concentration and high irradiance on the antioxidant enzymes in bean plants

The effect of short-term exposure to elevated CO₂ concentration and high irradiance on the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidases (GPX) and catalase (CAT), and on the extent of the lipid peroxidation was studied in bean (Phaseolus vulgaris L.) plants. Plants were exposed for 4 d (8 h a day) to irradiance of 100 (LI) or 1000 (HI) μmol m⁻² s⁻¹ at ambient (CA, 350 μmol mol⁻¹) or elevated (CE, 1300 μmol mol⁻¹) CO₂ concentration. Four-day exposure to CE increased the leaf dry mass in HI plants and RuBPC activity and chlorophyll content in LI plants. Total soluble protein content, leaf dry matter and RuBPC activity were higher in HI than in LI plants, although the HI and CE increased the contents of malonyldialdehyde and H₂O₂. Under CA, exposure to HI increased the activity of APX and decreased the total SOD activity. Under CE, HI treatment also activated APX and led to reduction of both, SOD and GPX, enzymes activities. CE considerably reduced the CAT activity at both irradiances, possibly due to suppressed rate of photorespiration under CE conditions.     

Low temperature stress modifies the photochemical efficiency of a tropical tree species <Emphasis Type="Italic">Hevea brasiliensis</Emphasis>: effects of varying concentration of CO<Subscript>2</Subscript> and photon flux density

Two clones of Hevea brasiliensis (RRII 105 and PB 235) were grown for one year in two distinct agroclimatic locations (warmer and colder, W and C) in peninsular India. We simultaneously measured gas exchange and chlorophyll (Chl) fluorescence on fully mature intact leaves at different photosynthetic photon flux densities (PPFDs) and ambient CO₂ concentrations (C _a) and at constant ambient O₂ concentration (21 %). Net photosynthetic rate (P _N), apparent quantum yield for CO₂ assimilation (Φ_c), in vivo carboxylation efficiency (CE), and photosystem 2 quantum yield (Φ_PS2) were low in plants grown in C climate and these reductions were more predominant in RRII 105 than in PB 235 which was also reflected in their growth. We estimated in these clones the partitioning of photosynthetic electrons between CO₂ reduction (J_A) and processes other than CO₂ reduction (J^*) at low and high PPFDs and C _a. At high C _a (700 µmol mol⁻¹) most of the photosynthetic electrons were used for CO₂ assimilation and negligible amount went for other processes when PPFD was low (200–300 µmol m⁻² s⁻¹) both in the C and W climates. But at high PPFD (900-1 100 µmol m⁻² s⁻¹), J^* was appreciably high even at a high C _a. Hence at normal ambient C _a and high irradiance, electrons can be generated in the photosynthetic apparatus far in excess of what can be safely utilised for photosynthetic CO₂ reduction. However, at high C _a there was increased diversion of electrons to photosynthetic CO₂ reduction which resulted in improved photosynthetic parameters even in plants grown in C climate.     

Acclimation of Two Distinct Plant Species,Spring Barley and Norway Spruce,to Combined Effect of Various Irradiance and CO2 Concentration During Cultivation in Controlled Environment

The short-term acclimation (10-d) of Norway spruce [Picea abies (L.) Karst] to elevated CO₂ concentration (EC) in combination with low irradiance (100 mol m^–2 s^–1) resulted in stimulation of CO₂ assimilation (by 61 %), increased total chlorophyll (Chl) content (by 17 %), significantly higher photosystem 2 (PS2) photochemical efficiency (F_v/F_m; by 4 %), and reduced demand on non-radiative dissipation of absorbed excitation energy corresponding with enhanced capacity of photon utilisation within PS2. On the other hand, at high cultivation irradiance (1 200 mol m^–2 s^–1) both Norway spruce and spring barley (Hordeum vulgare L. cv. Akcent) responded to EC by reduced photosynthetic capacity and prolonged inhibition of F_v/F_m accompanied with enhanced non-radiative dissipation of absorbed photon energy. Norway spruce needles revealed the expressive retention of zeaxanthin and antheraxanthin (Z+A) in darkness and higher violaxanthin (V) convertibility (yielding even 95 %) under all cultivation regimes in comparison with barley plants. In addition, the non-photochemical quenching of minimum Chl a fluorescence (SV₀), expressing the extent of non-radiative dissipation of absorbed photon energy within light-harvesting complexes (LHCs), linearly correlated with V conversion to Z+A very well in spruce, but not in barley plants. Finally, a key role of the Z+A-mediated non-radiative dissipation within LHCs in acclimation of spruce photosynthetic apparatus to high irradiance alone and in combination with EC was documented by extremely high SV₀ values, fast induction of non-radiative dissipation of absorbed photon energy, and its stability in darkness.     

Relationship Between Photosystem 2 Electron Transport and Photosynthetic CO2 Assimilation Responses to Irradiance in Young Apple Tree Leaves

The responses to irradiance of photosynthetic CO₂ assimilation and photosystem 2 (PS2) electron transport were simultaneously studied by gas exchange and chlorophyll (Chl) fluorescence measurement in two-year-old apple tree leaves (Malus pumila Mill. cv. Tengmu No.1/Malus hupehensis Rehd). Net photosynthetic rate (P _N) was saturated at photosynthetic photon flux density (PPFD) 600-1 100 (mol m^-2 s^-1, while the PS2 non-cyclic electron transport (P-rate) showed a maximum at PPFD 800 mol m^-2 s^-1. With PPFD increasing, either leaf potential photosynthetic CO₂ assimilation activity (F_d/F_s) and PS2 maximal photochemical activity (F_v/F_m) decreased or the ratio of the inactive PS2 reaction centres (RC) [(F_i – F_o)/(F_m – F_o)] and the slow relaxing non-photochemical Chl fluorescence quenching (q_s) increased from PPFD 1 200 mol m^-2 s^-1, but cyclic electron transport around photosystem 1 (RFp), irradiance induced PS2 RC closure [(F_s – F_o)/F_m – F_o)], and the fast and medium relaxing non-photochemical Chl fluorescence quenching (q_f and q_m) increased remarkably from PPFD 900 (mol m^-2 s^-1. Hence leaf photosynthesis of young apple leaves saturated at PPFD 800 mol m^-2 s^-1 and photoinhibition occurred above PPFD 900 mol m^-2 s^-1. During the photoinhibition at different irradiances, young apple tree leaves could dissipate excess photons mainly by energy quenching and state transition mechanisms at PPFD 900-1 100 mol m^-2 s^-1, but photosynthetic apparatus damage was unavoidable from PPFD 1 200 mol m^-2 s^-1. We propose that Chl fluorescence parameter P-rate is superior to the gas exchange parameter P _N and the Chl fluorescence parameter F_v/F_m as a definition of saturation irradiance and photoinhibition of plant leaves.     

The Effect of In Vitro Culture Conditions on the Pattern of Photoinhibition during Acclimation of Gardenia Plantlets to Ex Vitro Conditions

We tested the effect of growing conditions during micropropagation on the fast kinetics of chlorophyll (Chl) fluorescence of Gardenia jasminoides Ellis plantlets during a 4-week acclimation to ex vitro. We studied whether photoautotrophic growing in vitro produced plantlets with less photoinhibition impairment during acclimation. Of the growing conditions stimulating photoautotrophy in vitro, only loose tube caps had a positive effect, whereas low sucrose or sucrose-free content in the medium and high PPFD showed a negative effect. Thus, plantlets cultured with 3 % (m/v) of sucrose were subsequently less photoinhibited throughout acclimation than those cultured with low sucrose (0.5 %) or sucrose-free media. Moreover, at the end of acclimation the former plantlets showed F_v/F_m and F_v/F₀ ratios typical of unstressed ex vitro plants as well as a higher Chl content and ratio of Chls to carotenoids. Plantlets cultured at a photosynthetic photon fluence density (PPFD) of 50 µmol m^–2 s^–1 also showed a better performance at the end of acclimation than those cultured at a higher (110 µmol m^–2 s^–1) PPFD. Thus except in the case of loose-tube closure, gardenia plantlets cultured in vitro under conventional sucrose concentration and PPFD are the least photoinhibited during acclimation. Nevertheless, significant interactions between the in vitro growing factors were observed at the end of acclimation.     

Effect of Long Term Exposure to High CO2 Concentrations on Photosynthetic Characteristics of Prunus Avium L. Plants

The effect of two elevated carbon dioxide concentrations, 700 µmol(CO₂) mol^–1 (C700) and 1 400 µmol(CO₂) mol^–1 (C1400), on photosynthetic performances of 1-year-old Prunus avium L. plant was studied. Plants grown at C700 were characterised by increased net photosynthetic rate (P _N) as compared to those grown at C1400. Plant photosynthetic adjustment to C1400 resulted in 27 % higher P _N than in control at atmospheric CO₂ concentration (C _a) at the beginning of the experiment (3–4 weeks) with a consequent decline to the end of the experiment. Thus, 1 400 µmol(CO₂) mol^–1 had short-term stimulatory effect on plant P _N. Both chlorophyll (Chl) a and b concentrations dramatically decreased during exposure to C1400. Compensation irradiance was increased by 57 % in C700 and by 87 % in C1400. Photochemical efficiency () was affected by balloon environment, however, a clear stimulatory effect of C700 was detected. Opposite influence of both elevated CO₂ concentrations on P _Nmax was established: slight increase by C700 (2.7 % at C_a), but considerable decrease by C1400 (63 % at C_a). Exposure to C700 enhanced compensation irradiance by 42 %, while C1400 by only 21 %. Either C700 or C1400 did not reduce stomatal conductance (g _s). Leaf area per plant (LAR) was more stimulated by C700 than by C1400. High unit area leaf mass, specific leaf area, and dry matter accumulation in roots without affecting tissue density characterised plants grown in C1400. However, when considering the root : shoot ratio, these plants allocated less carbon to the roots than plants from other treatments.     

Effect of light intensity and ammonium-N on carotenogenesis ofTrentepohlia odorata andDunaliella bardawil

AxenicTrentepohlia odorata was cultured at three different NH₄Cl levels (3.5 × 10^–2, 3.5 × 10^–3, 3.5 × 10^–4 M) and three different light intensities (48, 76, 122 µmol m^–2 s^–1). Chloride had no effect on growth over this range of concentration. High light intensity and high NH₄Cl concentration enhanced the specific growth rate. The carotenoid content increased under a combination of high light intensity and low N concentration. WhenD. bardawil was exposed to the same combination of growth conditions, there was an increase in its carotenoid content. The light saturation and the light inhibition constants (K _s andK _i, respectively) for growth, and the saturation constant (K _m) for NH₄Cl were determined. TheK _s andK _i values were higher inT. odorata (66.7 and> 122 mol m^–2 s^–1, respectively) than inD. bardawil (5.1 and 14.7 µmol m^–2 s^–1, respectively). TheK _m value determined at 122 µmol m^–2 s^–1, however, was lower inT. odorata (0.048 µM) than inD. bardawil (0.062 µM).Author for correspondence     

Significance of the Leaf Area Ratio in Hevea brasiliensis Under High Irradiance and Low Temperature Stress

Adjustment in leaf area : mass ratio called leaf area ratio (LAR) is one of the strategies to optimize photon harvesting. LAR was recorded for 10 genotypes of Hevea brasiliensis under high irradiance and low temperature and the genotypes were categorized into two groups, i.e. high LAR and low LAR types. Simultaneously, the growth during summer as well as winter periods, photosynthetic characteristics, and in-vitro oxidative damage were studied. Low LAR (19.86±0.52 m² kg^–1) types, recorded an average of 18.0 % chlorophyll (Chl) degradation under high irradiance and 7.1 % Chl degradation under low temperature. These genotypes maintained significantly higher net photosynthetic rate (P _N) of 10.4 mol(CO₂) m^–2 s^–1 during winter season. On the contrary, the high LAR (24.33±0.27 m² kg^–1) types recorded significantly lower P _N of 4 mol(CO₂) m^–2 s^–1 and greater Chl degradation of 37.7 and 13.9 % under high irradiance and low temperature stress, respectively. Thus LAR may be one of the physiological traits, which are possibly involved in plant acclimation process under both stresses studied.     

Response of Photosynthetic Apparatus of Spring Barley (Hordeum vulgare L.) to Combined Effect of Elevated CO2 Concentration and Different Growth Irradiance

The response of barley (Hordeum vulgare L. cv. Akcent) to various photosynthetic photon flux densities (PPFDs) and elevated [CO₂] [700 μmol (CO₂) mol⁻¹; EC] was studied by gas exchange, chlorophyll (Chl) a fluorescence, and pigment analysis. In comparison with barley grown under ambient [CO₂] [350 μmol (CO₂) mol⁻¹; AC] the EC acclimation resulted in a decrease in photosynthetic capacity, reduced stomatal conductance, and decreased total Chl content. The extent of acclimation depression of photosynthesis, the most pronounced for the plants grown at 730 μmol m⁻² s⁻¹ (PPFD₇₃₀), may be related to the degree of sink-limitation. The increased non-radiative dissipation of absorbed photon energy for all EC plants corresponded to the higher de-epoxidation state of xanthophylls only for PPFD₇₃₀ barley. Further, a pronounced decrease in photosystem 2 (PS2) photochemical efficiency (given as F_V/F_M) for EC plants grown at 730 and 1 200 μmol m⁻² s⁻¹ in comparison with AC barley was related to the reduced epoxidation of antheraxanthin and zeaxanthin back to violaxanthin in darkness. Thus the EC conditions sensitise the photosynthetic apparatus of high-irradiance acclimated barley plants (particularly PPFD₇₃₀) to the photoinactivation of PS2. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of Cold-Hardening on Chilling-Induced Photoinhibition of Photosynthesis and on Xanthophyll Cycle Pigments in Sweet Pepper

Two cultivars of Capsicum annuum L. were acclimated for 5 d at sub-optimal temperature (14 °C) and irradiance of 250 µmol m^–2 s^–1. This cold-hardening resulted in some reduction in the extent of photoinhibition during an 8 h exposure to high irradiance at 4 °C. Obvious differences were observed between non-hardened leaves (NHL) and cold-hardened leaves (CHL) in the recovery under low irradiance at room temperature. The CHL of both cultivars recovered faster than NHL, especially during the initial fast phase of recovery. Compared with NHL, the total content of carotenoids (Cars), based on chlorophyll, Chl (a+b), and the proportions of xanthophyll cycle pigments referred to total Cars increased in CHL, mainly due to an increase of violaxanthin (V) + antheraxanthin (A) + zeaxanthin (Z) content per mol Chl (a+b). Faster development and a higher non-photochemical quenching (NPQ) of Chl fluorescence, related to a stronger deepoxidation of the larger xanthophyll cycle pool in NHL, could act as a major defence mechanism to reduce the formation of reactive oxygen species during severe chilling. This is suggested by higher content of Z or Z+A in photoinhibition as well as by its rapid decline during the initial fast phase of recovery. In contrast to the chilling-sensitive cv. 0004, the chilling-tolerant cv. 1141 did more easily acclimate its photosynthetic apparatus to low temperatures.     

Combined Effect of Irradiance and Water Regime on Sorghum Photosynthesis

Grain sorghum [Sorghum bicolor (L.) Moench. cvs. TX430 and KS82] was grown in a Haynie very fine sandy loam (coarse-silty, mixed, superactive, calcareous, mesic Mollic Udifluvents) under constant 47 % shade or full irradiance in a greenhouse under two watering regimes to see the combined and individual effects of low irradiance (LI) and low water (LW) on the sorghum genotypes. Under the high-irradiance (HI) and high-water (HW) treatment (control) and the LI-HW treatment, TX430 grew taller than KS82. Both LI and LW reduced several times the fresh and dry masses. Under the control conditions, TX430 reached its maximum net photosynthetic rate (P _Nmax) of 28.93 mol m^–2 s^–1 at a photosynthetic photon flux density (PPFD) of 1 707 mol m^–2 s^–1, and KS82 reached its P _Nmax of 28.32 mol m^–2 s^–1 at a PPFD of 2 973 mol m^–2 s^–1. The fact that TX430 had P _Nmax under a lower PPFD than KS82 may relate to its taller growth under LI conditions. Hence genotypes of sorghum might be selected for low irradiance using curves relating P _N to PPFD.     

Adaptation of the obligate CAM plant Clusia alata to light stress: Metabolic responses

In the Crassulacean acid metabolism (CAM) plants Clusia alata Triana and Planch., decarboxylation of citrate during phase III of CAM took place later than malate decarboxylation. The interdependence of these two CO₂ and NADPH sources is discussed. High light accelerated malate decarboxylation during the day and lowered citrate levels. Strong light stress also activated mechanisms that can protect the plant against oxidative stress. Upon transfer from low light (200 μmol m⁻² s⁻¹) to high light (650–740 μmol m⁻² s⁻¹), after 2 days, there was a transient increase of non-photochemical quenching (NPQ) of fluorescence of chlorophyll a of photosystem II. This indicated acute photoinhibition, which declined again after 7 days of exposure. Conversely, after 1 week exposure to high light, the mechanisms of interconversion of violaxanthin (V), antheraxanthin (A), zeaxanthin (Z) (epoxydation/de-epoxydation) were activated. This was accompanied by an increase in pigment levels at dawn and dusk.     

Impact of in vitro Cultivation Conditions on Stress Responses and on Changes in Thylakoid Membrane Proteins and Pigments of Tobacco during ex vitro Acclimation

Four physiologically and phenotypically diversified tobacco (Nicotiana tabacum L. cv. Samsun) plantlet variants had been generated by cultivation on media either lacking or containing sucrose (0 and 3 %, m/v) under two different photon flux densities (PFD), 50 µmol m^–2 s^–1 (LL) and 200 µmol m^–2 s^–1 (HL). Plantlets were transferred into soil without any pre-acclimation and grown either under PFD of 200 µmol m^–2 s^–1 or 700 µmol m^–2 s^–1. Sucrose feeding in vitro resulted in reduced degree and duration of wilting after transfer. The highest readiness for ex vitro acclimation was found in 3 % HL plants, in which changes of photosynthetic apparatus and stress responses were the smallest. On the contrary, the steepest decline of Fv/Fm ratio on the first day after transplantation, doubled chlorophyll content and almost tripled D1/LHC 2 ratio after 7 d of ex vitro growth under 700 µmol m^–2 s^–1 characterized 0 % HL plants, which had suffered chronic photoinhibition in vitro. Remarkably high abscisic acid content at the end of in vitro cultivation and during acclimation as well as increased synthesis of both D1 and LHC 2 proteins even at the end of analyzed acclimation period were found only in 0 % LL plants. Increase of D1/LHC 2 ratio and chlorophyll contents demonstrate that in vitro developed leaves of all plant variants are able to acclimate to new environment. The most surprising result in the whole study is the drop of D1 protein synthesis in all plants on the 3^rd day. Five times decline of photoprotection level of xanthophylls in plants after ex vitro transfer into the same PFD showed stress character of in vitro cultures.