Leaf discs floated on water are different from intact leaves in photosynthesis and photoinhibition

Photoinhibition has been often evaluated with leaf discs floated on water or placed on wet papers to prevent desiccation. Under these conditions, there is a possibility that CO₂ diffusion is blocked by water, which may lead to reduction in photosynthetic CO₂ assimilation. Using Chenopodium album L. grown at two irradiances, photosynthesis, quantum yield of Photosystem II (ΔF/F _m′), non-photochemical quenching (qN), and photoinhibition were compared between detached leaves and leaf discs. In low-light-grown plants, photoinhibition was greater in leaf discs than in detached leaves, while in high-light-grown plants, there was little difference. Leaf discs showed lower rates of photosynthesis and ΔF/F _m′, and higher qN. The ΔF/F _m′ in leaf discs increased when leaf discs were exposed to high concentration of CO₂, suggesting that CO₂ diffusion to chloroplasts was limited in leaf discs floated on water. This revised version was published online in June 2006 with corrections to the Cover Date.     

Responses of Tobacco Plantlets to Change of Irradiance During Transfer from in vitro to ex vitro Conditions

Chlorophyll a fluorescence kinetics, net photosynthetic rate (P _N), water relations, and photosynthetic pigment contents were studied during acclimation of in vitro grown tobacco to higher irradiance (HL; 700 mol m^–2 s^–1). Plantlets were grown on medium containing sucrose in glass vessels (G-plants) or in Magenta boxes (M-plants) with better CO₂ supply in the latter ones. The effect of HL was studied either (1) in plantlets grown under original in vitro conditions (closed vessels), (2) in in vitro plantlets exposed to ambient CO₂ concentration (covers removed), or (3) in plantlets transplanted to ex vitro into pots with sand and nutrient solution. Higher P _N, and fraction of closed photosystem 2 (PS2) centres (1 – q_P), and lower content of xanthophyll cycle pigments were found in M-plants compared to G-plants. HL treatment caused photoinhibition particularly in plants kept in closed vessels. This was indicated by the decrease in the ratio of F_v/F_m and by the increase in non-photochemical quenching, 1 – qp, and content of xanthophyll cycle pigments. Better CO₂ supply ensured by the removal of closure lead to the moderate reduction of symptoms of photoinhibition, although stomatal conductance (g _s), transpiration rate (E), and P _N were negatively affected. The main reason was the decrease in relative air humidity, which caused similar reduction of P _N, E, and g _s after the transfer of plantlets to ex vitro. Nevertheless, plant response to HL seemed not to be affected by any possible root injury caused by transfer to ex vitro. The differences in contents of xanthophyll cycle pigments, degree of de-epoxidation, P _N, and quenching parameters between M- and G-plantlets were still significant 7 d after ex vitro transfer and HL acclimation.     

Comparison of responses of bean, pea and rape plants to UV-B radiation in darkness and in light

Effect of UV-B radiation on leaves of bean, pea and rape plants was studied. UV-B radiation (11.2 kJ·m⁻²) induced more distinct reduction of the primary photosynthesis activity when applied in darkness than the same UV-B dose, extended in time, and applied with photosynthetic active radiation (PAR). The pea plants were more susceptible to UV-B in darkness, but in the presence of PAR their tolerance was higher. The CO₂ fixation in the bean and rape plants, exposed to UV-B was decreased, but for the pea plants it remained unchanged. The UV-B irradiation caused an increase in the content of ultraviolet-absorbing pigments. Additionally, the bean plants grown at UV-B increased the thickness of leaves, described as SLW.     

Isolate-specific effects of ultraviolet radiation on photosynthesis,growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium <Emphasis Type="Italic">Microcoleus </Emphasis><Emphasis Type="Italic">chthonoplastes</Emphasis>

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12–15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 μmol photon m⁻² s⁻¹) = P; PAR + UV-A (8 W m⁻²) = PA; PAR + UV-A + UV-B (0.4 W m⁻²) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F _v/F _m) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F _v/F _m during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F _v/F _m sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.     

Very high light resistant mutants of Chlamydomonas reinhardtii: Responses of Photosystem II,nonphotochemical quenching and xanthophyll pigments to light and CO2

We have isolated very high light resistant nuclear mutants (VHL ^R) in Chlamydomonas reinhardtii, that grow in 1500–2000 mol photons m^–2 s^–1 (VHL) lethal to wildtype. Four nonallelic mutants have been characterized in terms of Photosystem II (PS II) function, nonphotochemical quenching (NPQ) and xanthophyll pigments in relation to acclimation and survival under light stress. In one class of VHL ^R mutants isolated from wild type (S4 and S9), VHL resistance was accompanied by slower PS II electron transfer, reduced connectivity between PS II centers and decreased PS II efficiency. These lesions in PS II function were already present in the herbicide resistant D1 mutant A251L (L ^*) from which another class of VHL ^R mutants (L4 and L30) were isolated, confirming that optimal PS II function was not critical for survival in very high light. Survival of all four VHL ^R mutants was independent of CO₂ availability, whereas photoprotective processes were not. The de-epoxidation state (DPS) of the xanthophyll cycle pigments in high light (HL, 600 mol photons m^–2 s^–1) was strongly depressed when all genotypes were grown in 5% CO₂. In S4 and S9 grown in air under HL and VHL, high DPS was well correlated with high NPQ. However when the same genotypes were grown in 5% CO₂, high DPS did not result in high NPQ, probably because high photosynthetic rates decreased thylakoid pH. Although high NPQ lowered the reduction state of PS II in air compared to 5% CO₂ at HL in wildtype, S4 and S9, this did not occur during growth of S4 and S9 in VHL. L ^* and VHL ^R mutants L4 and L30, also showed high DPS with low NPQ when grown air or 5% CO₂, possibly because they were unable to maintain sufficiently high pH due to constitutively impaired PS II electron transport. Although dissipation of excess photon energy through NPQ may contribute to VHL resistance, there is little evidence that the different genes conferring the VHL ^R phenotype affect this form of photoprotection. Rather, the decline of chlorophyll per biomass in all VHL ^R mutants grown under VHL suggests these genes may be involved in regulating antenna components and photosystem stoichiometries.This revised version was published online in October 2005 with corrections to the Cover Date.     

Photosynthetic UV-B Response of Beech (Fagus sylvatica L.) Saplings

Cloned saplings of beech (7-y-old) were exposed to enhanced UV-B irradiation (+25 %) continuously over three growing seasons (1999–2001). Analysis of CO₂ assimilation, variable chlorophyll (Chl) a fluorescence, and pigment composition was performed in late summer of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. This influence was responsible for the stimulation of the net assimilation rate (P _N) over a range of irradiances. The increase in P _N was partially connected to increase of the area leaf mass, and thus to the increased leaf thickness. Even a higher degree of UV-B induced stimulation was observed at the level of photosystem 2 (PS2) photochemistry as judged from the irradiance response of electron transport rate and photochemical quenching of Chl a. The remarkably low irradiance-induced non-photochemical quenching of maximum Chl a fluorescence (NPQ) in the UV-B plants over the entire range of applied irradiances was attributed both to the reduced demand on non-radiative dissipation processes and to the considerably reduced contribution of the quenching localised in the inactivated PS2 reaction centres. Neither the content of Chls and total carotenoids expressed per leaf area nor the contents of lutein, neoxanthin, and the pool of xanthophyll cycle pigments (VAZ) were affected under the elevated UV-B. However, the contributions of antheraxanthin (A) and zeaxanthin (Z) to the entire VAZ pool in the dark-adapted UV-B treated plants were 1.61 and 2.14 times higher than in control leaves. Surprisingly, the retained A+Z in UV-B treated plants was not accompanied with long-term down-regulation of the PS2 photochemical efficiency, but it facilitated the non-radiative dissipation of excitation energy within light-harvesting complexes (LHC) of PS2. Thus, in the beech leaves the accumulation of A+Z, induced by other factors than excess irradiance itself, supports the resistance of PS2 against combined effects of high irradiance and elevated UV-B.     

Transient sediment load on blades of Arctic <Emphasis Type="Italic">Saccharina latissima</Emphasis> can mitigate UV radiation effect on photosynthesis

We studied the short-term impact of sediment load on the photosynthetic performance of Saccharina latissima sporophytes exposed to ultraviolet radiation (UVR). The algae were collected from different sediment-influenced environments in Svalbard in August 2007. Initial optimum quantum yield (F _v/F _m) of sediment-covered sporophytes was significantly higher compared to sediment-free sporophytes. Experimental sediment coating on blade discs had a photoprotective function by screening out 92% of the weighted UV-B (UV_ery) treatment. No UVR-induced photoinhibition was observed in sediment-coated blade discs while sediment removal caused a reduction in F _v/F _m not only after 12-h UVR exposure but also after 6-h recovery in low white light compared to the initial value. Thus, sediment coating has a short-term functional significance in mitigating the negative effect of UVR on photosynthesis of an important kelp species and set a baseline for further studies. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Responses of Ribulose-1,5-Bisphosphate Carboxylase,Protein Content,and Stomatal Conductance to Water Deficit in Maize,Tomato, and Bean

We compared responses of maize, tomato, and bean plants to water stress. Maize reached a severe water deficit (leaf water potential –1.90 MPa) in a longer period of time as compared with tomato and bean plants. Maize stomatal conductance (g _s) decreased at mild water deficit. g _s of tomato and bean decreased gradually and did not reach values as low as in maize. The protein content was maintained in maize and decreased at low water potential (_w); in tomato it fluctuated and also decreased at low _w; in bean it gradually decreased. Ribulose-1,5-bisphosphate carboxylase/oxygenase activity remained high at mild and moderate stress in maize and tomato plants; in bean it remained high only at mild stress.     

Inhibition of photosynthetic reactions under water stress: interaction with light level

When the shrub Nerium oleander L., growing under full natural daylight outdoors, was subjected to water stress, stomatal conductance declined, and so did non-stomatal components of photosynthesis, including the CO₂-saturated rate of CO₂ uptake by intact leaves and the activity of electron transport by chloroplasts isolated from stressed plants. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics determined at 77 K (-196°C) for the upper leaf surface and from isolated chloroplasts. The maximum (F _M) and the variable (F _V) fluorescence yield at 692 nm were strongly quenched but there was little effect on the instantaneous (F _O) fluorescence. There was a concomitant quenching of the maximum and variable fluorescence at 734 nm. These results indicate an inactivation of the primary photochemistry associated with photosystem II. The lower, naturally shaded surfaces of the same leaves were much less affected than the upper surfaces and water-stress treatment of plants kept in deep shade had little or no effect on the fluorescence characteristics of either surface, or of chloroplasts isolated from the water-stressed leaves. The effects of subjecting N. oleander plants, growing in full daylight, to water stress are indistinguishable from those resulting when plants, grown under a lower light regime, are exposed to full daylight (photoinhibition). Both kinds of stress evidently cause an inactivation of the primary photochemistry associated with photosystem II. The results indicate that water stress predisposes the leaves to photoinhibition. Recovery from this inhibition, following restoration of favorable water relations, is very slow, indicating that photoinhibition is an important component of the damage to the photosynthetic system that takes place when plants are exposed to water stress in the field. The underlying causes of this water-stress-induced susceptibility to photoinhibition are unknown; stomatal closure or elevated leaf temperature cannot explain the increased susceptibility.Abbreviations and symbols Chl chlorophyll - PFD photon flux area density - PSI, PSII photosystem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - leaf water potential C.I.W.-D.P.B. Publication No. 775     

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.     

Effects of temperature and CO2 enrichment on kinetic properties of NADP+-malate dehydrogenase in two ecotypes of Barnyard grass (Echinochloa crus-galli (L.) Beauv.) from contrasting climates

Summary The apparent energy of activation (E _a), Michaelis-Menten constant (K _mfor oxaloacetate), V _max/K _mratios and specific activities of NADP⁺-malate dehydrogenase (NADP⁺-MDH; EC 1.1.1.82) were analyzed in plants of Barnyard grass from Québec (QUE) and Mississippi (MISS) acclimated to two thermoperiods 28/22°C, 21/15°C, and grown under two CO₂ concentrations, 350 l l^-1 and 675 l l^-1. E _avalues of NADP⁺-MDH extracted from QUE plants were significantly lower than those of MISS plants. K _mvalues and V _max/K _mratios of the enzyme from both ecotypes were similar over the range of 10–30°C but reduced V _max/K _mratios were found for the enzyme of QUE plants at 30 and 40°C assays. MISS plants had higher enzyme activities when measured on a chlorophyll basis but this trend was reversed when activities were expressed per fresh weight leaf or per leaf surface area. Activities were significantly higher in plants of both populations acclimated to 22/28°C. CO₂ enrichment did not modify appreciably the catalytic properties of NADP⁺-MDH and did not have a compensatory effect upon catalysis or enzyme activity under cool acclimatory conditions. NADP⁺-MDH activities were always in excess of the amount required to support observed rates of CO₂ assimilation and these two parameters were significantly correlated. The enhanced photosynthetic performance of QUE plants under cold temperature conditions, as compared to that of MISS plants, cannot be attributed to kinetic differences of NADP⁺-malate dehydrogenase among these ecotypes.     

Photoinhibition in Chilling Stressed Wheat and Maize

At chilling stress, the contents of photosynthetic pigments decreased significantly in maize, but in wheat the contents of chlorophyll (Chl) remained unchanged whereas the contents of total carotenoids (Car) increased. In both species the contents of + carotene and lutein + lutein-5,6-epoxide remained unaffected, but the de-epoxidation state involving the components of the xanthophyll cycle increased. Under chilling stress the photosynthetic electron transport also displayed a general failure in maize but in wheat only photosystem (PS) 2 coupled to the water oxidation complex was inhibited. Moreover, in stressed maize the quinone pool decreased, while the low and high potential forms of cytochrome b ₅₅₉ increased. In wheat only the contents of cytochrome b _559LP decreased. Peroxidation of acyl lipids in the chloroplast lamellae became more distinct in chilling stressed maize but could also be detected in wheat. Thus in chilling stressed maize prevails an impairment of the acceptor site of PS2 while in wheat photodamage is restricted to the electron donation pathway from water to P680 or to the oxygen evolving complex.     

Simultaneous gas exchange and fluorescence measurements indicate differences in the response of sunflower,bean and maize to water stress

Gas exchange and fluorescence measurements of attached leaves of water stressed bean, sunflower and maize plants were carried out at two light intensities (250 mol quanta m^-2s^-1 and 850 mol quanta m^-2s^-1). Besides the restriction of transpiration and CO₂ uptake, the dissipation of excess light energy was clearly reflected in the light and dark reactions of photosynthesis under stress conditions. Bean and maize plants preferentially use non-photochemical quenching for light energy dissipation. In sunflower plants, excess light energy gave rise to photochemical quenching. Autoradiography of leaves after photosynthesis in ¹⁴CO₂ demonstrated the occurrence of leaf patchiness in sunflower and maize but not in bean. The contribution of CO₂ recycling within the leaves to energy dissipation was investigated by studies in 2.5% oxygen to suppress photorespiration. The participation of different energy dissipating mechanisms to quanta comsumption on agriculturally relevant species is discussed.Abbreviations F_o minimal fluorescence - F_m maximal fluorescence - F_p peak fluorescence - g leaf conductance - P_N net CO₂ uptake - q_N coefficient of non-photochemical quenching - q_P coefficient of photochemical quenching     

Dynamic response of UV-absorbing compounds, quantum yield and the xanthophyll cycle to diel changes in UV-B and photosynthetic radiations in an aquatic liverwort

We studied the diel responses of the liverwort Jungermannia exsertifolia subsp. cordifolia to radiation changes under laboratory conditions. The samples were exposed to three radiation regimes: P (only PAR), PA (PAR + UV-A), and PAB (PAR + UV-A + UV-B). The day was divided in four periods: darkness, a first low-PAR period, the high-PAR plus UV period, and a second low-PAR period. After 15 days of culture, we measured photosynthetic pigments, chlorophyll fluorescence and UV-absorbing compounds in the four periods of the day on two consecutive days. With respect to UV-absorbing compounds, we analyzed their global amount (as the bulk UV absorbance of methanolic extracts) and the concentration of seven hydroxycinnamic acid derivatives, both in the soluble (mainly vacuolar) and insoluble (cell wall-bound) fractions of the plant extracts. PAB samples increased the bulk UV absorbance of the soluble and insoluble fractions, and the concentrations of p-coumaroylmalic acid in the soluble fraction and p-coumaric acid in the cell wall. Most of these variables showed significant diel changes and responded within a few hours to radiation changes (more strongly to UV-B), increasing at the end of the period of high-PAR plus UV. F_v/F_m, Φ_PSII, NPQ and the components of the xanthophyll cycle showed significant and quick diel changes in response to high PAR, UV-A and UV-B radiation, indicating dynamic photoinhibition and protection of PSII from excess radiation through the xanthophyll cycle. Thus, the liverwort showed a dynamic protection and acclimation capacity to the irradiance level and spectral characteristics of the radiation received.     

Effects of solar ultraviolet-B radiation,temperature and CO2 on growth and physiology of sunflower and maize seedlings

The effects of solar UV-B radiation, in combination with elevated temperature (4 °C ) and CO₂ (680 L L^-1 concentration, on sunflower and maize seedlings were studied from May to August in 1991 at the research station Quinta de São Pedro in Portugal (38.7°N). The ambient solar radiation of Portugal was reduced to levels of Central European latitudes by using the ozone filter technique. This radiation served as control, while the ambient solar radiation of Portugal was to simulate intense UV-B treatment (+30%). All plants were grown up to 18 days in 4 climate controlled growth chambers simulating a daily course of temperature with T_max=28 °C or 32 °C , resp., and ambient CO₂ concentrations (340 L L^-1); in one chamber the CO₂ concentration was twice as high (680 L L^-1). Under intense UV-B and at 28 °C (T_max) all growth parameters (height, leaf area, fresh and dry weight, stem elongation rate, relative growth rate) of sunflower and maize seedlings were reduced down to 35% as compared to controls. An increase in growing temperature by 4 °C , alone or in combination with doubled CO₂, compensated or even overcompensated the UV-B effect so that the treated plants were comparable to controls. Chlorophyll content, on a leaf area basis, increased under intense UV-B radiation. This increase was compensated by lower leaf areas, resulting in comparable chlorophyll contents. Similar to growth, also the net photosynthetic rates of sunflower and maize seedlings were reduced down to 29% by intense UV-B calculated on a chlorophyll basis. This reduction was compensated by an increased temperature. Doubling of CO₂ concentration had effects only on sunflower seedlings in which the photosynthetic rates were higher than in the controls. Dark respiration rates of the seedlings were not influenced by any experimental condition. Transpiration and water use efficiency (wue) were not influenced by intense UV-B. Higher temperatures led to higher transpiration rates and lower water use efficiencies, resp.. Doubling of CO₂ reduced the transpiration rate drastically while for wue maximum values were recorded.     

Enhanced leaf chlorophyll levels in plants treated with seaweed extract

Application to the soil of an aqueous alkaline extract ofAscophyllum nodosum resulted in higher concentrations of chlorophyll in the leaves of treated plants in comparison to control plants treated with an equivalent volume of water. Positive results were obtained with all species tested (tomato, dwarf French bean, wheat, barley, maize). When the seaweed extract was applied as a foliar spray, similar effects on leaf chlorophyll contents were obtained, except in the case of dwarf French bean plants, for which no significant difference was recorded between test and control plants. When the betaines present in the seaweed extract were applied as a mixture in the same concentrations as those in the diluted seaweed extract (-aminobutyric acid betaine 0.96 mg L^–1, -aminovaleric acid betaine 0.43 mg L^–1, glycinebetaine 0.34 mg L^–1), very similar leaf chlorophyll levels were recorded for the seaweed extract and betaine treated plants. This suggests strongly that the enhanced leaf chlorophyll content of plants treated with seaweed extract is dependent on the betaines present.     

Response of xanthophyll cycle and chloroplastic antioxidant enzymes to chilling stress in tomato over-expressing glycerol-3-phosphate acyltransferase gene

Over-expression of chloroplastic glycerol-3-phosphate acyltransferase gene (LeGPAT) increased unsaturated fatty acid contents in phosphatidylglycerol (PG) of thylakoid membrane in tomato. The effect of this increase on the xanthophyll cycle and chloroplast antioxidant enzymes was examined by comparing wild type (WT) tomato with the transgenic (TG) lines at chilling temperature (4 °C) under low irradiance (100 μmol m⁻² s⁻¹). Net photosynthetic rate and the maximal photochemical efficiency of photosystem (PS) 2 (F_v/F_m) in TG plants decreased more slowly during chilling stress and F_v/F_m recovered faster than that in WT plants under optimal conditions. The oxidizable P700 in both WT and TG plants decreased during chilling stress under low irradiance, but recovered faster in TG plants than in the WT ones. During chilling stress, non-photochemical quenching (NPQ) and the de-epoxidized ratio of xanthophyll cycle in WT plants were lower than those of TG tomatoes. The higher activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in TG plants resulted in the reduction of O₂ ^−· and H₂O₂ contents during chilling stress. Hence the increase in content of unsaturated fatty acids in PG by the over-expression of LeGPAT could alleviate photoinhibition of PS2 and PS1 by improving the de-epoxidized ratio of xanthophyll cycle and activities of SOD and APX in chloroplast.     

The influence of ultraviolet-B radiation on the growth, pigment production and chlorophyll fluorescence of Norway spruce seedlings

Norway spruce (Picea abies (L.)Karst.) from seven seed sources was grown in a greenhouse with 8.3 and 14.7 kJ·m⁻²·d⁻¹ m UV-B_BE (biologically effective UV-B: 280–320 nm) irradiation, and with no supplemental irradiation as control. The seedlings total biomass (dry weight) and shoot growth decreased with high UV-B treatment but spruce from low elevation seed sources were more affected. The seedlings grown at the highest UV-B irradiance (14.7 kJ·m⁻²·d⁻¹) showed from 5 to 38% inhibition of total biomass and 15 to 70 % shoot growth inhibition. Norway spruce populations from higher altitude seed sources manifested greater tolerance to UV-B radiation compared to plants from low altitudes. Changes in phospholipids and protective pigments were also determined. The plants grown at the lower UV-B irradiance (8.3 kJ·m⁻²·d⁻¹) showed greater ability to concentrations UV-B-absorbing pigments then control plants. Chlorophyll a fluorescence parameter R_fd, (R_fd=(F_m-F_s)/F_s) showed a significant decrease in needles of UV-B treated plants and this correlated with the altitude of seed source. Exposure to UV-B affect levels of the ratio of variable to maximum fluorescence (F_v/F_m). Results from this study suggest that the response to increased levels of UV-B radiation is depended upon the ecotypic differentiation of Norway spruce and involved changes in metabolites in plant tissues.     

Loblolly pine grown under elevated CO2 affects early instar pine sawfly performance

Seedlings of loblolly pine Pinus taeda (L.), were grown in open-topped field chambers under three CO₂ regimes: ambient, 150 l l^–1 CO₂ above ambient, and 300 l l^–1 CO₂ above ambient. A fourth, non-chambered ambient treatment was included to assess chamber effects. Needles were used in 96 h feeding trials to determine the performance of young, second instar larvae of loblolly pine's principal leaf herbivore, red-headed pine sawfly, Neodiprion lecontei (Fitch). The relative consumption rate of larvae significantly increased on plants grown under elevated CO₂, and needles grown in the highest CO₂ regime were consumed 21% more rapidly than needles grown in ambient CO₂. Both the significant decline in leaf nitrogen content and the substantial increase in leaf starch content contributed to a significant increase in the starch:nitrogen ratio in plants grown in elevated CO₂. Insect consumption rate was negatively related to leaf nitrogen content and positively related to the starch:nitrogen ratio. Of the four volatile leaf monoterpenes measured, only -pinene exhibited a significant CO₂ effect and declined in plants grown in elevated CO₂. Although consumption changed, the relative growth rates of larvae were not different among CO₂ treatments. Despite lower nitrogen consumption rates by larvae feeding on the plants grown in elevated CO₂, nitrogen accumulation rates were the same for all treatments due to a significant increase in nitrogen utilization efficiency. The ability of this insect to respond at an early, potentially susceptible larval stage to poorer food quality and declining levels of a leaf monoterpene suggest that changes in needle quality within pines in future elevated-CO₂ atmospheres may not especially affect young insects and that tree-feeding sawflies may respond in a manner similar to herb-feeding lepidopterans.