Effects of Two Strains of Tobacco Mosaic Virus on Photosynthetic Characteristics and Nitrogen Partitioning in Leaves of Nicotiana tabacum cv Xanthi during Photoacclimation under Two Nitrogen Nutrition Regimes

Photoacclimation was studied in tobacco leaves (Nicotiana tabacum cv Xanthi) infected with two strains of tobacco mosaic virus (TMV) and grown under different light and nitrogen nutrition regimes. Photosynthetic acclimation measured by the quantum yield and the maximum rate in saturating light of CO2-saturated photosynthesis was impaired to a greater extent in tobacco leaves infected with TMV strain PV230 than in those infected with TMV strain PV42. Infection with TMV strain PV230 severely impaired photosynthetic acclimation at high light/low nitrogen and during transfer from low to high light. Expanding leaves showing chlorotic-mosaic symptoms had greatly reduced capacity to acclimate to high light compared with controls and with developed leaves without visible symptoms. We conclude that the failure of expanding leaves to acclimate was largely due to the destruction of chloroplasts in yellow areas of the tissue, accompanied by severe reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase levels, and corresponding reduction in photosynthesis on a leaf-area basis. When corrected for areas of healthy green tissue, photoacclimation of infected leaves was the same as that of controls. Visible symptom development was greatest in high light/low nitrogen treatments. In developed leaves without visible symptoms, virus accumulation, which was as extensive as in expanding leaves, accelerated senescence and impaired photoacclimation during transfer from low light to high light. Generally, infection with TMV strain PV42 did not impair photosynthetic acclimation and even enhanced it in some treatments, even though virus accumulated to the same concentration as in PV230-infected leaves. These data show that TMV does not simply impair photoacclimation in tobacco by competing with chloroplasts for leaf nitrogen reserves. Rather, specific properties of severe strains, such as PV230, which lead to visible symptom development and patchy loss of photosynthetic activity in expanding leaves as well as general acceleration of chloroplast senescence in developed leaves, contribute to impaired photoacclimation, which is generally exacerbated by low nitrogen nutrition.     

Recovery from Photoinhibition in Peas (Pisum sativum L.) Acclimated to Varying Growth Irradiances (Role of D1 Protein Turnover)

D1 protein turnover and restoration of the photochemical efficiency of photosystem II (PSII) after photoinhibition of pea leaves (Pisum sativum L. cv Greenfeast) acclimated to different light intensities were investigated. All peas acclimated to different light intensities were able to recover from photoinhibition, at least partially, at light intensities far above their growth light irradiance. However, the capacity of pea leaves to recover from photoinhibition under increasing high irradiances was strictly dependent on the light acclimation of the leaves; i.e. the higher the irradiance during growth, the better the capacity of pea leaves to recover from photoinhibition at moderate and high light. In our experimental conditions, mainly D1 protein turnover-dependent recovery was monitored, since in the presence of an inhibitor of chloroplast-encoded protein synthesis, lincomycin, only negligible recovery took place. In darkness, neither the restoration of PSII photochemical efficiency nor any notable degradation of damaged D1 protein took place. In low light, however, good recovery of PSII occurred in all peas acclimated to different light intensities and was accompanied by fast degradation of the D1 protein. The rate of degradation of the D1 protein was estimated to be 3 to 4 times faster in photoinhibited leaves than in nonphotoinhibited leaves under the recovery conditions of 50 [mu]mol of photons m-2 s-1. In moderate light of 400 [mu]mol of photons m-2 s-1, the photoinhibited low-light peas were not able to increase further the rate of D1 protein degradation above that observed in nonphotoinhibited leaves, nor was the restoration of PSII function possible. On the other hand, photoinhibited high-light leaves were able to increase the rate of D1 protein degradation above that of nonphotoinhibited leaves even in moderate and high light, ensuring at least partial restoration of PSII function. We conclude that the capacity of photoinhibited leaves to restore PSII function at different irradiances was directly related to the capacity of the leaves to degrade damaged D1 protein under the recovery conditions.     

Light Stress and Oxidative Cell Damage in Photoautotrophic Cell Suspension of Euphorbia characias L

A photoautotrophic cell-suspension culture of Euphorbia characias L. grown at 70 [mu]mol photons m-2 s-1 was very sensitive to light stress: the gross photosynthesis measured by using a mass spectrometric 16O2/18O2 isotope technique showed a fast decrease at a rather low light intensity of 100 [mu]mol photons m-2 s-1, far below the photosynthetic saturation level. The contribution of activated oxygen species on photosystem II photoinhibition was examined for a given light intensity. A protective effect on gross photosynthesis was observed with 1% oxygen. When light stress was applied to a methyl viologen-adapted cell suspension, photoinhibition was reduced. When 50 [mu]mol L-1 methyl viologen was added, photoinhibition was slightly enhanced. These responses suggested an involvement of superoxide radicals in the photoinhibition process of E. characias photoautotrophic cells. The long-term (16 h) effects of photoinhibition were then studied. Aldehyde (malondialdehyde and 4-hydroxyalcenals) production resulting from lipid peroxidation was stimulated in long-term stressed cells. When 50 [mu]mol L-1 methyl viologen were added, increased aldehyde production was measured. Under 1% oxygen, the aldehyde production was comparable to that of nonstressed cells. The relationship among lipid peroxidation, light intensity, and net photosynthesis suggests that aldehyde production may result from cell death provoked by a prolonged energy deficit due to the inhibition of photosynthesis.     

Chloroplast Movement in the Shade Plant Tradescantia albiflora Helps Protect Photosystem II against Light Stress

The role of high-light-induced chloroplast movement in the photoprotection of the facultative shade plant Tradescantia albiflora was investigated by comparison with pea (Pisum sativum L.) leaves, both grown in 50 [mu]mol photons m-2 s-1. Photoinactivation of photosystem II (PSII) in vivo was induced in 1.1% CO2 by varying either duration (0-2 h) of illumination (fixed at 1800 [mu]mol m-2 s-1) or irradiance (0-3000 [mu]mol m-2 s-1) at a fixed duration (1 h) after infiltration of leaves with water or lincomycin (an inhibitor of chloroplast-encoded protein synthesis). At all photon exposures, PSII of T. albiflora leaves showed a greater resistance to light stress than pea leaves, although both utilization of absorbed light by photosynthesis and psbA gene product synthesis were smaller than for pea leaves. This greater tolerance was not due to differences in PSII antenna size or the index of susceptibility of PSII to light stress, because these two parameters were comparable in both plants. However, the transmittance increase mediated by chloroplast movement was greater in T. albiflora than pea, resulting in a 10% decrease of absorbed light at high light. We suggest that the greater tolerance of PSII against light stress in T. albiflora may be partly ascribed to its light-induced chloroplast rearrangement.     

Photosystem II Excitation Pressure and Development of Resistance to Photoinhibition (I. Light-Harvesting Complex II Abundance and Zeaxanthin Content in Chlorella vulgaris)

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 - qp (photochemical quenching). We established that Chlorella vulgaris exposed to either 5[deg]C/150 [mu]mol m-2 s-1 or 27[deg]C/2200 [mu]mol m-2 s-1 experienced a high PSII excitation pressure of 0.70 to 0.75. In contrast, Chlorella exposed to either 27[deg]C/150 [mu]mol m-2 s-1 or 5[deg]C/20 [mu]mol m-2 s-1 experienced a low PSII excitation pressure of 0.10 to 0.20. Chlorella grown under either regime at high PSII excitation pressure exhibited: (a) 3-fold higher light-saturated rates of O2 evolution; (b) the complete conversion of PSII[alpha] centers to PSII[beta] centers; (c) a 3-fold lower epoxidation state of the xanthophyll cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll a/b; and (e) a lower abundance of light-harvesting polypeptides than Chlorella grown at either regime at low PSII excitation pressure. In addition, cells grown at 5[deg]C/150 [mu]mol m-2 s-1 exhibited resistance to photoinhibition comparable to that of cells grown at 27[deg]C/2200 [mu]mol m-2 s-1 and were 3- to 4-fold more resistant to photoinhibition than cells grown at either regime at low excitation pressure. We conclude that increased resistance to photoinhibition upon growth at low temperature reflects photosynthetic adjustment to high excitation pressure, which results in an increased capacity for nonradiative dissipation of excess light through zeaxanthin coupled with a lower probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.     

Paraheliotropic leaf movement in Siratro as a protective mechanism against drought-induced damage to primary photosynthetic reactions: damage by excessive light and heat

Damage to primary photosynthetic reactions by drought, excess light and heat in leaves of Macroptilium atropurpureum Dc. cv. Siratro was assessed by measurements of chlorophyll fluorescence emission kinetics at 77 K (-196°C). Paraheliotropic leaf movement protected waterstressed Siratro leaves from damage by excess light (photoinhibition), by heat, and by the interactive effects of excess light and high leaf temperatures. When the leaves were restrained to a horizontal position, photoinhibition occurred and the degree of photoinhibitory damage increased with the time of exposure to high levels of solar radiation. Severe inhibition was followed by leaf death, but leaves gradually recovered from moderate damage. This drought-induced photoinhibitory damage seemed more closely related to low leaf water potential than to low leaf conductance. Exposure to leaf temperatures above 42°C caused damage to the photosynthetic system even in the dark and leaves died at 48°C. Between 42 and 48°C the degree of heat damage increased with the time of exposure, but recovery from moderate heat damage occurred over several days. The threshold temperature for direct heat damage increased with the growth temperature regime, but was unaffected by water-stress history or by current leaf water status. No direct heat damage occurred below 42°C, but in water-stressed plants photoinhibition increased with increasing leaf temperature in the range 31–42°C and with increasing photon flux density up to full sunglight values. Thus, water stress evidently predisposes the photosynthetic system to photoinhibition and high leaf temperature exacerbates this photoinhibitory damage. It seems probable that, under the climatic conditions where Siratro occurs in nature, but in the absence of paraheliotropic leaf movement, photoinhibitory damage would occur more frequently during drought than would direct heat damage.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosyntem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - PLM paraheliotropic leaf movement; all data of parameter of variation are mean ± standard error     

Influence of high light and UV-B radiation on photosynthesis and D1 turnover in atrazine-tolerant and -sensitive cultivars of Brassica napus

An atrazine-tolerant mutant and an atrazine-sensitive cultivar of Brassica napus L. were grown under visible radiation (400 mumol m-2 s-1, photosynthetically active radiation, PAR) and then subjected to treatment conditions. These included short-term high PAR (1600 mumol m-2 s-1) which was given for 4 h either alone or in combination with an enhanced level of UV-BBE radiation (4.6 kJ m-2 h-1 biologically effective UV-B, 280-320 nm). Recovery from the radiation treatment was studied for 4 h under the light conditions for growth. Since it is known that the atrazine-tolerant mutant is susceptible to photoinhibition, one of the aims of the present study was to determine the effects of a supplemental, enhanced level of UV-B radiation with regard to the mutant. The results indicate an additive effect of UV-B radiation on Fv/Fm, photochemical yield and photosynthetic oxygen evolution during both exposure and recovery, and also a higher susceptibility of the mutant to photoinhibitory PAR conditions alone and in combination with UV-B, which may have implications in a changing environment. Both cultivars also showed a higher D1 turnover during the radiation stress than during recovery, as shown by immunoblotting and 35S-methionine incorporation measurements.     

Diagnosis of the Earliest Strain-Specific Interactions between Tobacco Mosaic Virus and Chloroplasts of Tobacco Leaves in Vivo by Means of Chlorophyll Fluorescence Imaging

Fluorescence imaging was used to diagnose early stages of the strain-specific interactions between tobacco mosaic virus (strain PV230) and chloroplasts following infection of tobacco leaves (Nicotiana tabacum cv Xanthi). The earliest indication of interaction in tissues that ultimately become chlorotic was a reduction in chlorophyll fluorescence, and there was little fluorescence quenching compared with adjacent healthy tissues. Subsequently, fluorescence increased but remained unquenched. In the late stages fluorescence declined again in chlorotic regions as the chloroticmosaic symptoms developed. These in vivo data showing altered fluorescence yields confirm strain-specific interaction of virus coat protein with photosystem II (PSII) components in vitro, leading to photoinhibition and photooxidation of chlorophyll in infected cells and the development of visible chlorotic-mosaic symptoms. Although mechanisms leading to the low, unquenched fluorescence condition are not known, the intermediate high, unquenched fluorescence condition is consistent with impaired PSII electron transport as measured in vitro. Fluorescence lesions appear more rapidly and develop more extensively in high light, consistent with the faster and larger extent of symptom formation in high-light-grown leaves than in low-light-grown leaves.     

Photosystem II Reaction Center Damage and Repair in Dunaliella salina (Green Alga) (Analysis under Physiological and Irradiance-Stress Conditions)

Mechanistic aspects of the photosystem II (PSII) damage and repair cycle in chloroplasts were investigated. The D1/32-kD reaction center protein of PSII (known as the psbA chloroplast gene product) undergoes a frequent light-dependent damage and turnover in the thylakoid membrane. In the model organism Dunaliella salina (green alga), growth under a limiting intensity of illumination (100 [mu]mol of photons m-2 s-1; low light) entails damage, degradation, and replacement of D1 every about 7 h. Growth under irradiance-stress conditions (2000 [mu]mol of photons m-2 s-1; high light) entails damage to and replacement of D1 about every 20 min. Thus, the rate of damage and repair of PSII appears to be proportional to the light intensity during plant growth. Low-light-grown cells do not possess the capacity for high rates of repair. Upon transfer of low-light-grown cells to high-light conditions, accelerated damage to reaction center proteins is followed by PSII disassembly and aggregation of neighboring reaction center complexes into an insoluble dimer form. The accumulation of inactive PSII centers that still contain the D1 protein suggests that the rate of D1 degradation is the rate-limiting step in the PSII repair cycle. Under irradiance-stress conditions, chloroplasts gradually acquire a greater capacity for repair. The induction of this phenomenon occurs with a half-time of about 24 h.     

Structural changes and lateral redistribution of photosystem II during donor side photoinhibition of thylakoids

The structural and topological stability of thylakoid components under photoinhibitory conditions (4,500 microE.m-2.s-1 white light) was studied on Mn depleted thylakoids isolated from spinach leaves. After various exposures to photoinhibitory light, the chlorophyll-protein complexes of both photosystems I and II were separated by sucrose gradient centrifugation and analysed by Western blotting, using a set of polyclonals raised against various apoproteins of the photosynthetic apparatus. A series of events occurring during donor side photoinhibition are described for photosystem II, including: (a) lowering of the oligomerization state of the photosystem II core; (b) cleavage of 32-kD protein D1 at specific sites; (c) dissociation of chlorophyll-protein CP43 from the photosystem II core; and (d) migration of damaged photosystem II components from the grana to the stroma lamellae. A tentative scheme for the succession of these events is illustrated. Some effects of photoinhibition on photosystem I are also reported involving dissociation of antenna chlorophyll-proteins LHCI from the photosystem I reaction center.     

Photosystem II Excitation Pressure and Development of Resistance to Photoinhibition (II. Adjustment of Photosynthetic Capacity in Winter Wheat and Winter Rye)

Winter wheat (Triticum aestivum L. cv Monopol), spring wheat (Triticum aestivum L. cv Katepwa), and winter rye (Secale cereale L. cv Musketeer) grown at 5[deg]C and moderate irradiance (250 [mu]mol m-2 s-1) (5/250) exhibit an increased tolerance to photoinhibition at low temperature in comparison to plants grown at 20[deg]C and 250 [mu]mol m-2 s-1 (20/250). However, 5/250 plants exhibited a higher photosystem II (PSII) excitation pressure (0.32-0.63) than 20/250 plants (0.18-0.21), measured as 1 - qP, the coefficient of photochemical quenching. Plants grown at 20[deg]C and a high irradiance (800 [mu]mol m-2 s-1) (20/800) also exhibited a high PSII excitation pressure (0.32-0.48). Similarly, plants grown at 20/800 exhibited a comparable tolerance to photoinhibition relative to plants grown at 5/250. In contrast to a recent report for Chlorella vulgaris (D.P. Maxwell, S. Falk, N.P.A. Huner [1995] Plant Physiol 107: 687-694), this tolerance to photoinhibition occurs in winter rye with minimal adjustment to polypeptides of the PSII light-harvesting complex, chlorophyll a/b ratios, or xanthophyll cycle carotenoids. However, Monopol winter wheat exhibited a 2.5-fold stimulation of sucrosephosphate synthase activity upon growth at 5/250, in comparison to Katepwa spring wheat. We demonstrate that low-temperature-induced tolerance to photoinhibition is not a low-temperature-growth effect per se but, instead, reflects increased photosynthetic capacity in response to elevated PSII excitation pressure, which may be modulated by either temperature or irradiance.     

杂交酸模叶片线粒体交替氧化酶呼吸途径在光破坏防御中的作用

以杂交酸模(Rumex K-1)为试材,研究了不同光强下线粒体交替氧化酶呼吸途径(AOX途径)对酸模叶片光破坏的防御作用.结果表明:在200 μmol·m-2·s-1弱光下,用水杨基羟肟酸抑制AOX途径后,Rumex K-1叶片的PSⅡ实际光化学效率、光合线性电子传递速率以及光合放氧速率均显著下降,非还原性QB反应中心显著升高,加重了叶片的光抑制,而活性氧清除机制上调,避免了活性氧的过量积累,部分缓解了Rumex K-1叶片的光抑制;在800 μmol·m-2·s-1强光下,AOX途径受抑,导致Rumex K-1叶片发生严重的光抑制,而此时活性氧清除机制的上调不足以缓解活性氧过量的积累.无论在强光还是弱光下,AOX途径在Rumex K-1叶片的光破坏防御过程中都起着重要作用,而且在强光下,AOX途径对叶片的光破坏防御作用是叶绿体内其他光破坏防御途径所不能代替的.     

Light-dependent damage to photosynthesis in olive leaves during chilling and high temperature stress

Abstract The leaves of olive are long lived and likely to experience both chilling and high temperature stress during their life. Changes in photosynthetic CO₂ assimilation resulting from chilling and high temperature stress, in both dim and high light, are investigated. The quantum yield (φ) of photosynthesis at limiting light levels was reduced following chilling (at 5°C for 12 h), in dim light by approximately 10%, and in high light by 75%; the difference being attributed to photoinhibition. Similar reductions were observed in the light-saturated rate of CO₂ uptake (A_max). Decrease in A_max correlated with a halving of the leaf internal CO₂ concentration (c_i), suggesting an increased limitation by stomata following photoinhibition. Leaves were apparently more susceptible to photoinhibitory damage if the whole plant, rather than the leaf alone, was chilled. On return to 26 °C, I he photosynthetic capacity recovered to pre-stress levels within a few hours if leaves had been chilled in high light for 8 h or less, but did not fully recover from longer periods of chilling when loss of chlorophyll occurred. Leaves which were recovering from chilling in high light showed far more damage on being chilled a second time in high light. Three hours in high light at 38 °C reduced φ by 80%, but φ recovered within 4h of return to 26 °C. Although leaves of Olive are apparently less susceptible to photoinhibitory damage during chilling stress than the short-lived leaves of chilling-sensitive annual? crops, the results nevertheless show that photoinhibition during temperature stress is potentially a major factor influencing the photosynthetic productivity of Olive in the field.     

Restriction of Chlorophyll Synthesis Due to Expression of Glutamate 1-Semialdehyde Aminotransferase Antisense RNA Does Not Reduce the Light-Harvesting Antenna Size in Tobacco

The formation of 5-aminolevulinate is a key regulatory step in tetrapyrrole biosynthesis. In higher plants, glutamate 1-semialdehyde aminotransferase (GSA-AT) catalyzes the last step in the sequential conversion of glutamate to 5-aminolevulinate. Antisense RNA synthesis for GSA-AT leads to reduced GSA-AT protein levels in tobacco (Nicotiana tabacum L.) plants. We have used these transgenic plants for studying the significance of chlorophyll (Chl) availability for assembly of the light-harvesting apparatus. To avoid interfering photoinhibitory stress, plants were cultivated under a low photon flux density of 70 [mu]mol photons m-2 s-1. Decreased GSA-AT expression does not seem to suppress other enzymic steps in the Chl pathway, indicating that reduced Chl content in transgenic plants (down to 12% of the wild-type level) is a consequence of reduced GSA-AT activity. Chl deficiency correlated with a drastic reduction in the number of photosystem I and photosystem II reaction centers and their surrounding antenna on a leaf area basis. Different lines of evidence from the transgenic plants indicate that complete assembly of light-harvesting pigment-protein complexes is given preference over synthesis of new reaction center/core complexes, resulting in fully assembled photosynthetic units with no reduction in antenna size. Photosynthetic oxygen evolution rates and in vivo Chl fluorescence showed that GSA-AT antisense plants are photochemically competent. Thus, we suggest that under the growth conditions chosen during this study, plants tend to maintain their light-harvesting antenna size even under limited Chl supply.     

Photoinhibition and Photoprotection in Ginkgo biloba leaves: Influence of Temperature,CO2 and O2

In midday ginkgo ( Ginkgo biloba L. ) leaves have to bear photon flux density over 1 400 μmol·m-2·s-l in combination with high temperatures around 35℃ at natural habitat. They show typical midday depression of stomatal conductance and of CO2 assimilation rate. The zeaxanthin changes with light intensity during the day. The influence of the combination of strong light and temperature on photoinhibition was also examined in the laboratory. A low CO2 internal conductance (31 mmol· m- 2·s- 1 ) was found in ginkgo leaves, which had been exposed to excessive light at temperature between 15 ℃ and 35 ℃ with reduced CO2 (80 μL·L-l) or oxygen (2%) for 2 h, causing a low CO2 concentration at the carboxylation site and a high proportion of photorespimtion. The ratio of electron transport to CO2 fixation was rather high in ginkgo ( 16 e- /CO2 at 25 ℃ ) as compared with other plants. It increased with temperature also in 2% 02 which could not be explained solely as due to change of photorespimtion. The reduction of oxygen in 340 or 80 μL·L- 1 CO2 had no effect on the extent of photoinhibition at all temperatures, which indicated that eleetron flow caused by photorespiration in excess light was negligible in protective effect in ginkgo leaves. However, a decreased CO2 coneentration increased photoinhibition, especially at high temperature. It is concluded that the dissipation of excessive excitation energy in the PS II antennae through the xanthophyll cycle may be the major protective mechanism to preventing from the deteriorated effects of strong light in ginkgo leaves.     

四种重楼属植物光合作用特征

Species of Paris (Trilliaceae) have often been used as medicinal plants. Because of excessive exploitation,in this regard the wild resource of Paris is almost exhausted. Some species of Paris were transplanted for use in photosynthesis research and for conservation purposes. In the present study, light and CO2 photosynthetic response curves were investigated in four Paris taxa: P.polyphylla var. yunnanensis and var. alba, P.mairei, and P.marmorata. Our results showed that P.marmorata had the highest maximum photosynthetic rate (Pmax; 8.6μmol·m-2·s-1), light saturation point (LSP; 827μmol·m-2·s-1), maximum electron transport rate (Jmax; 39.9mol·m-2·s-1), a relatively high maximum carboxylation rate (Vcmax; 28.9μmol·m-2·s-1) and carbon dioxide saturation point (Cisat; 726μmol·mol-1), but a lower light compensation point (LCP; 6.23μmol·m-2·s-1) and the lowest carbon dioxide compensation point (Г*; 20.7μmol·mol-1). This suggests that P.marmorata is well adapted to light and CO2; however it has a low ability to acclimate to environmental stress as indicated by low water use efficiency (WUE) in high light conditions. P.polyphylla var. yunnanensis had the highest light compensation point (LCP; 10.1μmol·m-2·s-1), carbon dioxide compensation point (Г*; 35.3μmol·mol-1), carbon dioxide saturation point (Cisat; 727μmol·mol-1), relatively high maximum photosynthetic rate (Pmax; 7.5μmol·m-2·s-1) and light saturation point (LSP; 728μmol·m-2·s-1), maximum light saturated electron transfer rate (Jmax; 37.7μmol·m-2·s-1), suggesting that it is suitable for conditions of higher light and CO2 concentration. This taxon can adapt to adverse conditions, as suggested by high WUE under increased CO2 concentration. In contrast, P.polyphylla var. alba exhibited a relatively lower apparent quantum yield (AQY; 0.037μmol·mol-1) and poorer growth performance than the other taxa. We suggest from our results that different light and water conditions are suitable for the growth of the different taxa. Photosynthesis assimilation efficiency and production can be increased by raising humidity for P.polyphylla var. yunnanensis and P.marmorata. To protect plants of P.polyphylla var. alba from strong sunshine, they should be shaded from March to mid June.     

Antenna Size Dependency of Photoinactivation of Photosystem II in Light-Acclimated Pea Leaves

Utilization of absorbed light energy by photosystem (PS) II for O2 evolution depends on the light-harvesting antenna size, but the role of antenna size in the photoinactivation of PSII seems controversial. To address this controversy, pea (Pisum sativum L.) plants were grown in low (50 [mu]mol m-2 s-1) or high (650 [mu]mol m-2 s-1) light. The doubled functional antenna size of PSII in low light allows each PSII to utilize twice as many photons at given flash light energies for O2 evolution. The application of a target theory to depict the photon dose dependency of PSII photoinactivation measured by repetitive-flash O2 yield and the ratio of variable to maximal chlorophyll fluorescence indicates that photoinactivation of PSII is probably a single-hit process in which repair or photoprotective mechanisms are only slightly involved. Furthermore, the exacerbation of photoinactivation of PSII with greater antenna size under anaerobic conditions strongly indicates that photoinactivation of PSII depends on antenna size.     

Photoinhibition of Photosynthesis Without Net Loss of D1 Protein in Wheat Leaves Under Field Conditions

To determine whether the net loss of D1 protein is the main cause of photoinhibition of photosynthesis in wheat leaves under field conditions in the absence of any environmental stress other than strong sunlight, the D1 protein content, photosynthetic evolution of oxygen and chlorophyll a fluorescence parameters were measured in field grown wheat leaves. After exposure to midday strong light for about 3 h, apparent photosynthetic quantum efficiency (Φ), Fv/Fm and Fo in wheat leaves declined, and these parameters recovered almost completely 1 h after transfer to the weak light of 30~40 ttmol photons · m-2 · s-1. No evident change in the D1 protein content was observed in the leaves after exposure to midday strong light for 3 h. After 3 hours exposure to strong light, the slow-relaxed fluorescence quenching in the leaves treated with streptomycin (SM) increased much more than that in the control leaves, but there was no effect SM on the recovery of Fv/Fm and F0; dithiothretol (DTT) treatment enhanced photoinhibition of photosynthesis and reduced the D1 protein content in the leaves after exposure to midday strong light. These results indicated that under field conditions with no environmental stress other than strong sunlight, photoinhibition of photosynthesis in wheat leaves was not due to the net loss of D1 protein, and it could be attributed mainly by the increased nonradiative energy dissipation.     

Photoinhibition of photosystem II in tobacco plants overexpressing glutathione reductase and poplars overexpressing superoxide dismutase

We studied photoinhibition in two cultivars of tobacco ( Nicotiana tabacum L.) expressing the bacterial gor gene in the cytosol and in four lines of poplar ( Populus tremula × P. alba ) expressing the FeSOD gene of Arabidopsis thaliana in the chloroplast. The respective total activities of glutathione reductase (EC 1.6.4.2) in leaves of gor tobaccos and superoxide dismutase (EC 1.15.1.1) in the FeSOD poplars were 5–8 times higher than in the respective untransformed control plants. Leaves of control and transformed plants were subjected to high-light stress at 20°C, and photoinhibition of photosystem II (PSII) was measured by oxygen evolution and chlorophyll fluorescence. The leaves were illuminated both in the presence and absence of lincomycin, which inhibits chloroplast protein synthesis. In both cases, the time course of loss of PSII activity was identical in plants overproducing superoxide dismutase (SOD) and in the untransformed controls, suggesting that the ability to convert superoxide to hydrogen peroxide is not a limiting factor in protection against photoinhibition, or in the repair of photoinhibitory damage or that the site of O₂⁻ production is not accessible to the transgene product. The rate constant of photoinhibition, measured in lincomycin-treated leaves, was smaller in glutathione reductase (GR) overproducing tobacco cv. Samsun than in the respective wild-type, but this difference was not seen in cv. Bel W3. The steady-state level of PSII activity measured when the PSII repair cycle was allowed to equilibrate with photoinhibitory damage under high light was not higher in the GR overproducing cv. Samsun, suggesting that the repair of photoinhibitory damage was not enhanced in plants overproducing GR in the cytosol.     

High-light stress does not impair biomass accumulation of sun-acclimated tropical tree seedlings (Calophyllum longifolium Willd. and Tectona grandis L. f.)

Studies with seedlings of tropical rainforest trees ( Calophyllum longifolium Willd.; Tectona grandis L. f.) were designed to test whether high-light stress affects photosynthetic performance and growth. Seedlings were cultivated in pots at a field site in Central Panama (9 degrees N) and separated into two groups: (1) plants exposed to full solar radiation; (2) plants subjected to automatic neutral shading (48 %) whenever visible irradiance surpassed 1000, 1200, or 1600 micromol photons m-2 s-1. After 2-4 months, chlorophyll fluorescence (Fv/Fm ratio), photosynthetic net CO2 uptake, pigment composition, alpha-tocopherol content of leaves, and plant biomass accumulation were measured. Fully sun-exposed, compared to periodically shaded plants, experienced substantial high-light stress around midday, indicated by photoinhibition of photosystem II and depressed net CO2 uptake. Higher contents of xanthophyll cycle pigments, lutein, and alpha-tocopherol showed an enhancement of photoprotection in fully sun-exposed plants. However, in all experiments, the maximum capacity of net CO2 uptake and plant dry mass did not differ significantly between the two treatments. Thus, in these experiments, high-light stress did not impair productivity of the seedlings studied. Obviously, the continuously sun-exposed plants were capable of fully compensating for any potential costs associated with photoinhibition and repair of photosystem II, reduced CO2 assimilation, and processes of high-light acclimation.