Effects of environmental conditions on isoprene emission from live oak

Live-oak plants (Quercus virginiana Mill.) were subjected to various levels of CO₂, water stress or photosynthetic photon flux density to test the hypothesis that isoprene biosynthesis occurred only under conditions of restricted CO₂ availability. Isoprene emission increases as the ambient CO₂ concentration decreased, independent of the amount of time that plants had photosynthesized at ambient CO₂ levels. When plants were water-stressed over a 4-d period photosynthesis and leaf conductance decreased 98 and 94%, respectively, while isoprene emissions remained constant. Significant isoprene emissions occurred when plants were saturated with CO₂, i.e., below the light compensation level for net photosynthesis (100 mol m^-2 s^-1). Isoprene emission rates increased with photosynthetic photon flux density and at 25 and 50 mol m^-2 s^-1 were 7 and 18 times greater than emissions in the dark. These data indicate that isoprene is a normal plant metabolite and not — as has been suggested — formed exclusively in response to restricted CO₂ or various stresses.Abbreviation PPFD photosynthetic photon flux density     

Photoinhibition of photosynthesis in intact bean leaves: role of light and temperature,and requirement for chloroplast-protein synthesis during recovery

Photoinhibition of photosynthesis was induced in intact leaves of Phaseolus vulgaris L. grown at a photon flux density (PFD; photon fluence rate) of 300 mol·m^-2·s^-1, by exposure to a PFD of 1400 mol·m^-2·s^-1. Subsequent recovery from photoinhibition was followed at temperatures ranging from 5 to 35°C and at a PFD of either 20 or 140 mol·m^-2·s^-1 or in complete darkness. Photoinhibition and recovery were monitored mainly by chlorophyll fluorescence emission at 77K but also by photosynthetic O₂ evolution. The effects of the protein-synthesis inhibitors, cycloheximide and chloramphenicol, on photoinhibition and recovery were also determined. The results demonstrate that recovery was temperature-dependent with rates slow below 15°C and optimal at 30°C. Light was required for maximum recovery but the process was light-saturated at a PFD of 20 mol·m^-2·s^-1. Chloramphenicol, but not cycloheximide, inactivated the repair process, indicating that recovery involved the synthesis of one or more chloroplast-encoded proteins. With chloramphenicol, it was shown that photoinhibition and recovery occurred concomitantly. The temperature-dependency of the photoinhibition process was, therefore, in part determined by the effect of temperature on the recovery process. Consequently, photoinhibition is the net difference between the rate of damage and the rate of repair. The susceptibility of chilling-sensitive plant species to photoinhibition at low temperatures is proposed to result from the low rates of recovery in this temperature range.Abbreviations and symbols Da Dalton - Fo, Fm, Fv instantaneous, maximum, variable fluorescence emission - PFD photon flux density - PSII photosystem II - photon yield C.I.W.-D.P.B. Publication No. 871     

Temperature-dependent feedback inhibition of photosynthesis in peanut

Arachis hypogaea L. is a tropical crop that is slow-growing at temperatures below 25°C. Unadapted CO₂-assimilation rate (A) showed insufficient variation between 15 and 30°C in the short term (hours) to explain this marked reduction in growth. However, at longer periods (12 d), A was depressed as were growth rate and leafproduction rate. To examine the possible relationship between growth, A and sink demand plants were transferred from 30°C, which is near the optimum for growth, to a suboptimal temperature (19°C). In the first 2 d of cooling, A decreased by 50–70%, the stomata stayed open, and the intercellular CO₂ concentration (c_i) rose, i.e. the decrease in A of the cooled plants was the result of non-stomatal factors. Changes in dark respiration did not account for the decline in A.Clear evidence was obtained of sink control of A by independently manipulating the temperature of different leaves on the plant. Cooling (to 19°C) most of the plant (the sink) led to a 70% decline in A of the remaining leaves at 30°C after 3 d, whereas the converse treatments (30°C sink, 19°C source) resulted in small changes (17%). In plants at 19°C which were exposed to low CO₂ concentration to prevent photosynthesis, A was not reduced when measured at normal CO₂ concentrations, indicating that carbohydrate accumulation was responsible for the decline in A. Dry-matter build-up at suboptimal temperature was also consistent with end-product inhibition of photosynthesis.Abbreviations and symbols A (mol·m^-2·s^-1) rate of net CO₂ assimilation - C_i (l·l^-1) substomatal CO₂ concentration - DW (g) dry weight - g (mol·m^-2·s^-1) stomatal conductance to diffusion of water vapour - PFD (mol·m^-2·s^-1) photon flux density     

Stem photosynthesis in Psorothamnus spinosus (smoke tree) in the Sonoran desert of California

Summary Stem photosynthetic responses to environmental parameters were investigated with Psorothamnus spinosus in the Sonoran Desert of California. Light saturation of stem photosynthesis was equal to maximum midday summer irradance (1600–2000 mol·m^-2·s^-1). The optimum temperature for stem photosynthesis was 39°C, and lower stem temperatures (27–35°C) caused significant decreases (up to 50%) in stem photosynthesis. Positive stem photosynthesis was maintained up to 51°C. Stem photosynthesis was relatively insensitive to increasing vpd up to 5 kPa; However, stem conductance decreased by 25% at a vpd of 5 kPa. At vpd greater than 5 kPa stem photosynthesis decreased relatively more than that of stem conductance causing a decrease in water use efficiency and an increase an intercellular carbon dioxide concentration. Maximum stem photosynthetic rates were low (6.2–10.6 mol·m^-2·s^-1) on a stem surface area, but, stem photosynthetic rates of young shoots were substantially higher (19.5–33.3 mol· m^-2·s^-1) on a projected area basis.Dedicated to the memory of Dr. W.H. Muller     

Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply

Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO ₃ ^– content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wild type controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO ₃ ^– content was observed in plants grown in nitrogen replete conditions at low irradiance (170 mol photons·m^–2 ·s^–1). This relationship disappeared at higher irradiance (450 mol photons·m^–2·S^–1) because the maximal extractable NR activity in the leaves of the wild type plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO ₃ ^– accumulated in the leaves of plants grown at 450 mol photons·m^–2·s^–1 than in those grown at 170 mol photons·m^–2·s^–1 in N-replete conditions. The foliar NO ₃ ^– level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO ₃ ^– was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO ₃ ^– in the constitutive NR-expressors. We conclude that the NR protein is specifically targetted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.Abbreviations Chl chlorophyll - N nitrogen - NR NADH-nitrate reductase - WT wild type     

Light and the maintenance of photosynthetic competence in leaves of Populus balsamifera L. during short-term exposures to high concentrations of sulfur dioxide

Leaves of Populus balsamifera grown under full natural sunlight were treated with 0, 1, or 2 l SO₂·1^-1 air under one of four different photon flux densities (PFD). When the SO₂ exposures took place in darkness or at 300 mol photons·m^-2·s^-1, sulfate accumulated to the levels predicted by measurements of stomatal conductance during SO₂ exposure. Under conditions of higher PFD (750 and 1550 mol·m^-2·s^-1), however, the predicted levels of accumulated sulfate were substantially higher than those obtained from anion chromatography of the leaf extracts. Light-and CO₂-saturated capacity as well as the photon yield of photosynthetic O₂ evolution were reduced with increasing concentration of SO₂. At 2 l SO₂·1^-1 air, the greatest reductions in both photosynthetic, capacity and photon yield occurred when the leaves were exposed to SO₂ in the dark, and increasingly smaller reductions in each occurred with increasing PFD during SO₂ exposure. This indicates that the inhibition of photosynthesis resulting from SO₂ exposure was reduced when the exposure occurred under conditions of higher light. The ratio F _v/F _M (variable/maximum fluorescence emission) for photosyntem II (PSII), a measure of the photochemical efficiency of PSII, remained unaffected by exposure of leaves to SO₂ in the dark and exhibited only moderate reductions with increasing PFD during the exposure, indicating that PSII was not a primary site of damage by SO₂. Pretreatment of leaves with SO₂ in the dark, however, increased the susceptibility of PSII to photoinhibition, as such pretreated leaves exhibited much greater reductions inF _V/F _M when transferred to moderate or high light in air than comparable control leaves.Abbreviations and symbols A₁₂₀₀ photosynthetic capacity (CO₂-saturated rate of O₂ evolution at 1200 mol photons·m^-2·s^-1) - F_o instantaneous fluorescence emission - F_M maximum fluorescence emission - F_V variable fluorescence emission - PFD photon flux density (400–700 nm) - PSII photosystem II     

Sensitivity to far-red radiation in stomata of Phaseolus vulgaris L.: Rhythmic effects on conductance and photosynthesis

The influence of far-red (FR; 700–800 nm) radiation on steady-state stomatal conductance and net photosynthesis in P. vulgaris has been studied. Whereas FR radiation alone was relatively ineffective, addition of FR to a background of white light (WL; predominantly 400–700 nm) resulted in increased stomatal conductance. Stomata exhibited a marked diurnal sensitivity to FR. The action maximum for enhancing stomatal conductance was near 714 nm. A combination of FR and infra-red (IR; >800 nm) enhanced net photosynthesis when added to a background of WL. When IR alone was added to WL, there was a net decrease in photosynthesis, indicating that it is the FR waveband which is responsible for the observed photosynthetic effects. Naturally occurring levels of FR radiation (235 mol·m^-2·s^-1) in vegetation-canopy shade enhanced net photosynthetic CO₂ gain by 28% when added to a background of 55 mol·m^-2·s^-1 WL.Abbreviations BL blue - FR far-red - IR infra-red - PAR photosynthetically active radiation - R red - WL white light     

Photosynthesis and apparent affinity for dissolved inorganic carbon by cells and chloroplasts of Chlamydomonas reinhardtii grown at high and low CO2 concentrations

Chloroplasts with high rates of photosynthetic O₂ evolution (up to 120 mol O₂· (mg Chl)^-1·h^-1 compared with 130 mol O₂· (mg Chl)^-1·h^-1 of whole cells) were isolated from Chlamydomonas reinhardtii cells grown in high and low CO₂ concentrations using autolysine-digitonin treatment. At 25° C and pH=7.8, no O₂ uptake could be observed in the dark by high- and low-CO₂ adapted chloroplasts. Light saturation of photosynthetic net oxygen evolution was reached at 800 mol photons·m^-2·s^-1 for high- and low-CO₂ adapted chloroplasts, a value which was almost identical to that observed for whole cells. Dissolved inorganic carbon (DIC) saturation of photosynthesis was reached between 200–300 M for low-CO₂ adapted chloroplasts, whereas high-CO₂ adapted chloroplasts were not saturated even at 700 M DIC. The concentrations of DIC required to reach half-saturated rates of net O₂ evolution (K_m(DIC)) was 31.1 and 156 M DIC for low- and high-CO₂ adapted chloroplasts, respectively. These results demonstrate that the CO₂ concentration provided during growth influenced the photosynthetic characteristics at the whole cell as well as at the chloroplast level.Abbreviations Chl chlorophyll - DIC dissolved inorganic carbon - K_m(DIC) coneentration of dissolved inorganic carbon required for the rate of half maximal net O₂ evolution - PFR photon fluence rate - SPGM silicasol-PVP-gradient medium     

Photosynthesis and growth rates of Laurencia brongniartii J. Agardh (Rhodophyta, Ceramiales) in preparation for cultivation

Laurencia brongniartii is usually found at depths below 4 m, but can be found in shallow subtidal areas in crevices and on the walls of a coral reef in Amami Oshima Island, Kagoshima Prefecture, Japan, where irradiances were significantly lower than those at similar depths in open water. In preparation for the possible cultivation of this species for its antibiotic compounds, the effects of temperature and irradiance on photosynthesis and growth were measured. Photosynthesis and growth rates of L. brongniartii explants were highest at 26 and 28 °C, which closely corresponded to temperatures found during August to late December when it was most abundant. The estimated maximum photosynthesis rate (P _max) was 4.41 mol photon m^–2 s^–1 at 26 °C and 4.07 mol photon m^–2 s^–1 at 28 °C. Saturating irradiance occurred at 95 mol photon m^–2 s^–1 at 26 °C and 65 mol photon m^–2 s^–1 at 28 °C. In contrast, growth experiments at 41.7 mol photon m^–2 s^–1 caused bleaching of explants and the maximum growth rate observed during the study was 3.02 ± 0.75% day^–1 at 28 °C and 25 mol photon m^–2 s^–1. The difference in the saturating irradiance for photosynthesis and the irradiance that caused bleaching in growth experiments suggests that long-term exposure to high irradiance was detrimental and should be addressed before the initiation of large scale cultivation.     

Decreased ribulose-1,5-bisphosphate carboxylase-oxygenase in transgenic tobacco transformed with “antisense” rbcS

Tobacco (Nicotiana tabacum L.) plants transformed with antisense rbcS to decrease the expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants growing at different irradiances. Tobacco plants were grown in controlled-climate chambers under ambient CO₂ at 20°C at 100, 300 and 750 mol·m^–2·s^–1 irradiance, and at 28°C at 100, 300 and 1000 mol·m^–2·s^–1 irradiance. (i) Measurement of photosynthesis under ambient conditions showed that the flux control coefficient of Rubisco (C _infRubisco ^supA ) was very low (0.01–0.03) at low growth irradiance, and still fairly low (0.24–0.27) at higher irradiance. (ii) Short-term changes in the irradiance used to measure photosynthesis showed that C _infRubisco ^supA increases as incident irradiance rises, (iii) When low-light (100 mol·m^–2·s^–1)-grown plants are exposed to high (750–1000 mol·m^–2·s^–1) irradiance, Rubisco is almost totally limiting for photosynthesis in wild types. However, when high-light-grown leaves (750–1000 mol·m^–2·s^–1) are suddenly exposed to high and saturating irradiance (1500–2000 mol·m^–2·s^–1), C _infRubisco ^supA remained relatively low (0.23–0.33), showing that in saturating light Rubisco only exerts partial control over the light-saturated rate of photosynthesis in sun leaves; apparently additional factors are co-limiting photosynthetic performance, (iv) Growth of plants at high irradiance led to a small decrease in the percentage of total protein found in the insoluble (thylakoid fraction), and a decrease of chlorophyll, relative to protein or structural leaf dry weight. As a consequence of this change, high-irradiance-grown leaves illuminated at growth irradiance avoided an inbalance between the light reactions and Rubisco; this was shown by the low value of C _infRubisco ^supA (see above) and by measurements showing that non-photochemical quenching was low, photochemical quenching high, and NADP-malate dehydrogenase activation was low at the growth irradiance. In contrast, when a leaf adapted to low irradiance was illuminated at a higher irradiance, Rubisco exerted more control, non-photochemical quenching was higher, photochemical quenching was lower, and NADP-malate dehydrogenase activation was higher than in a leaf which had grown at that irradiance. We conclude that changes in leaf composition allow the leaf to avoid a one-sided limitation by Rubisco and, hence, overexcitation and overreduction of the thylakoids in high-irradiance growth conditions, (v) Antisense plants with less Rubisco contained a higher content of insoluble (thylakoid) protein and chlorophyll, compared to total protein or structural leaf dry weight. They also showed a higher rate of photosynthesis than the wild type, when measured at an irradiance below that at which the plant had grown. We propose that N-allocation in low light is not optimal in tobacco and that genetic manipulation to decrease Rubisco may, in some circumstances, increase photosynthetic performance in low light.Abbreviations A rate of photosynthesis - C _infRubisco ^supA flux control coefficient of Rubisco for photosynthesis - c_i internal CO₂ concentration - qE energy-dependent quenching of chlorophyll fluorescense - qQ photochemical quenching of chlorophyll fluorescence - NADP-MDH NADP-dependent malate dehydrogenase - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - RuBP ribulose-1,5-bisphosphate This work was supported by the Deutsche Forschungsgemeinschaft (SFB 137).     

Photoinhibition and repair in Dunaliella salina acclimated to different growth irradiances

The light-dependent rate of photosystem-II (PSII) damage and repair was measured in photoautotrophic cultures of Dunaliella salina Teod. grown at different irradiances in the range 50–3000 mol photons · m^–2· s^–1. Rates of cell growth increased in the range of 50–800 mol photons·m^–2·s^–1, remained constant at a maximum in the range of 800–1,500 mol photons·m^–2 ·s^–1, and declined due to photoinhibition in the range of 1500–3000 mol photons·m^–2·s^–1. Western blot analyses, upon addition of lincomycin to the cultures, revealed first-order kinetics for the loss of the PSII reaction-center protein (D1) from the 32-kDa position, occurring as a result of photodamage. The rate constant of this 32-kDa protein loss was a linear function of cell growth irradiance. In the presence of lincomycin, loss of the other PSII reaction-center protein (D2) from the 34-kDa position was also observed, occurring with kinetics similar to those of the 32-kDa form of D1. Increasing rates of photodamage as a function of irradiance were accompanied by an increase in the steady-state level of a higher-molecular-weight protein complex ( 160-kDa) that cross-reacted with D1 antibodies. The steady-state level of the 160-kDa complex in thylakoids was also a linear function of cell growth irradiance. These observations suggest that photodamage to D1 converts stoichiometric amounts of D1 and D2 (i.e., the D1/D2 heterodimer) into a 160-kDa complex. This complex may help to stabilize the reaction-center proteins until degradation and replacement of D1 can occur. The results indicated an intrinsic half-time of about 60 min for the repair of individual PSII units, supporting the idea that degradation of D1 after photodamage is the rate-limiting step in the PSII repair process.Abbreviations Chl chlorophyll - PSI photosystem I - PSII photosystem II - D1 the 32-kDa reaction-center protein of PSII, encoded by the chloroplast psbA gene - D2 the 34-kDa reactioncenter protein of PSII, encoded by the chloroplast psbD gene - Q_A primary electron-accepting plastoquinone of PSII The work was supported by grant 94-37100-7529 from the US Department of Agriculture, National Research Initiative Competitive Grants Program.     

Gas-exchange of ears of cereals in response to carbon dioxide and light

Data for the maximum carboxylation velocity of ribulose-1,5-biosphosphate carboxylase, V_m, and the maximum rate of whole-chain electron transport, J_m, were calculated according to a photosynthesis model from the CO₂ response and the light response of CO₂ uptake measured on ears of wheat (Triticum aestivum L. cv. Arkas), oat (Avena sativa L. cv. Lorenz), and barley (Hordeum vulgare L. cv. Aramir). The ratio J_m/V_m is lower in glumes of oat and awns of barley than it is in the bracts of wheat and in the lemmas and paleae of oat and barley. Light-microscopy studies revealed, in glumes and lemmas of wheat and in the lemmas of oat and barley, a second type of photosynthesizing cell which, in analogy to the Kranz anatomy of C₄ plants, can be designated as a bundle-sheath cell. In wheat ears, the CO₂-compensation point (in the absence of dissimilative respiration) is between those that are typical for C₃ and C₄ plants.A model of the CO₂ uptake in C₃–C₄ intermediate plants proposed by Peisker (1986, Plant Cell Environ. 9, 627–635) is applied to recalculate the initial slopes of the A(p^c) curves (net photosynthesis rate versus intercellular partial pressure of CO₂) under the assumptions that the J_m/V_m ratio for all organs investigated equals the value found in glumes of oat and awns of barley, and that ribulose-1,5-bisphosphate carboxylase is redistributed from mesophyll to bundle-sheath cells. The results closely match the measured values. As a consequence, all bracts of wheat ears and the inner bracts of oat and barley ears are likely to represent a C₃–C₄ intermediate type, while glumes of oat and awns of barley represent the C₃ type.Abbreviations A net photosynthesis rate (mol·m^-2·s^-1) - J_m maximum rate of whole-chain electron transport (mol·e^-·m^-2·s^-1) - p^c (bar) intercellular partial pressure of CO₂ - PEP phosphoenolpyruvate - PPFD photosynthetic photon flux density (mol quanta·m^-2·s^-1) - RuBPCase ribulose bisphosphate carboxylase/oxygenase - RuBP ribulose bisphosphate - V_m maximum carboxylation velocity of RuBPCase (mol·m^-2·s^-1) - T^* CO₂ compensation point in the absence of dissimilative respiration (bar)     

Analysis of oxygen evolution during photosynthetic induction and in multiple-turnover flashes in sunflower leaves

Exchange of CO₂ and O₂ and chlorophyll fluorescence were measured in the presence of 360 1 · 1^–1 CO₂ in nitrogen in Helianthus annuss L. leaves which had been preconditioned in the dark or at a photon flux density (PFD) of 24 mol · m^–2 · s^–1 either in 21 or 0% O₂. An initial light-dependent O₂ outburst of 6 mol · m^–2 was measured after aerobic dark incubation. It was attributed to the reduction of electron carriers, predominantly plastoquinone. The maximum initial rate of O₂ evolution at PFD 8000 mol · m^–2 · s^–1 was 170 mol · m^–2 · s^–2 or about four times the steady CO₂-and light-saturated rate of photosynthesis. Fluorescence measurements showed that the rate was still acceptor-limited. Fast O₂ evolution ceased after electron carriers were reduced in the dark-adapted leaf, but continued for a short time at the lower rate of 62 mol · m^–2 · s^–1 in the light-adapted leaf. The data are interpreted to show that enzymes involved in 3-phosphoglycerate reduction are dark-inhibited, but were fully active in low light. In a dark-adapted leaf, respiratory CO₂ evolution continued under nitrogen; it was partially inhibited by illumination. Prolonged exposure of a leaf to anaerobic conditions caused reducing equivalents to accumulate. This was shown by a slowly increasing chlorophyll fluorescence yield which indicated the reduction of the PSII acceptor Q_A in the dark. When the leaf was illuminated, no O₂ evolution was detected from short light pulses, although transient O₂ production was appreciable during longer light pulses. This indicates that an electron donor (pool size about 2–3 e/PSII reaction center) became reduced in the dark and the first photons were used to oxidise this donor instead of water.Abbreviations Chl chlorophyll - CRC carbon reduction cycle - GAPDH NADP-glyceraldehyde-phosphate dehydrogenase - PFD photon flux density - PGA 3-phosphoglycerate - RuBP ribulose bisphosphate - TCA tricarboxylic acid cycle To whom correspondence should be addressedThis work received support by the Estonian Academy of Sciences, the Gottfried-Wilhelm-Leibniz Program of the Deutsche For-schungsgemeinschaft and the Sonderforschungsbereich 251 of the University of Würzburg.     

Light-dependent enhanced metabolism of chlorotoluron in a substituted urea herbicide-resistant biotype ofLolium rigidum Gaud.

A population ofLolium rigidum Gaud. displays resistance to the herbicide chlorotoluron endowed by enhanced metabolism of this herbicide. The level of resistance in intact plants of this population is light dependent. Resistance is about 4-fold at 110 mol photons·m^–2·s^–1, but increases to 11-fold at 600 mol photons·m^–2·s^–1. For seedlings grown in the dark, the rate of chlorotoluron metabolism is identical between biotypes; however, seedlings of the resistant biotype grown in the light display enhanced chlorotoluron metabolism compared to the susceptible biotype. Specifically, light with blue wavelengths induces chlorotoluron metabolism in the resistant biotype. An analysis of the metabolites produced indicates that two routes of chlorotoluron metabolism occur inL. rigidum. These are characterised by initial reactions leading to ringmethyl hydroxylation orN-demethylation of the herbicide. The ring-methyl hydroxylation pathway is increased greatly in light-grown resistant seedlings compared to susceptible seedlings, whereas theN-demethylation pathway is only slightly increased. The differential induction of these two pathways in resistantL. rigidum by light suggests that enhanced activity of two different enzymes may be involved in chlorotoluron resistance.Abbreviations ABT 1-aminobenzotriazole - LD₅₀ dose giving 50% mortality - LSS liquid scintillation spectroscopy     

Protective systems against active oxygen species in spinach: response to cold acclimation in excess light

Spinach (Spinacia oleracea L.) plants were acclimated to 1° C or maintained at 18° C under the same light regime (260–300 mol photons·m^–2·s^–1). The cold acclimation led to several metabolic and biochemical changes that apparently include improved protection of the photosynthetic apparatus against active oxygen species. In particular, cold-acclimated leaves exhibited a considerably higher ascorbate content and significantly increased activities of superoxide dismutase, ascorbate peroxidase, and monodehydroascorbate reductase in the chloroplasts. The level of dehydroascorbate reductase did not alter. Catalase activity decreased. The photosynthetic pigment composition of cold-acclimated spinach was characterized by increased levels of the xanthophylls lutein + zeaxanthin and violaxanthin. The observed changes are discussed in terms of their possible relevance for plant resistance to photoinhibition at chilling temperatures.Abbreviations DHA dehydroascorbate - GSH reduced glutathione - MDA monodehydroascorbate - SOD superoxide dismutase The authors thank the Deutsche Forschungsgemeinschaft for financial support of this study.     

Light-emitting diodes as a light source for photosynthesis research

Light-emitting diodes (LED) can provide large fluxes of red photons and so could be used to make lightweight, efficient lighting systems for photosynthetic research. We compared photosynthesis, stomatal conductance and isoprene emission (a sensitive indicator of ATP status) from leaves of kudzu (Pueraria lobata (Willd) Ohwi.) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. Stomatal conductance was measured to determine if red LED light could sufficiently open stomata. The LEDs produced an even field of red light (peak emission 656±5 nm) over the range of 0–1500 mol m^-2 s^-1. Under ambient CO₂ the photosynthetic response to red light deviated slightly from the response measured in white light and stomatal conductance followed a similar pattern. Isoprene emission also increased with light similar to photosynthesis in white light and red light. The response of photosynthesis to CO₂ was similar under the LED and xenon arc lamps at equal photosynthetic irradiance of 1000 mol m^-2 s^-1. There was no statistical difference between the white light and red light measurements in high CO₂. Some leaves exhibited feedback inhibition of photosynthesis which was equally evident under irradiation of either lamp type. Photosynthesis research including electron transport, carbon metabolism and trace gas emission studies should benefit greatly from the increased reliability, repeatability and portability of a photosynthesis lamp based on light-emitting diodes.     

The involvement of the photoinhibition of photosystem II and impaired membrane energization in the reduced quantum yield of carbon assimilation in chilled maize

In this study we investigated the basis for the reduction in the quantum yield of carbon assimilation in maize (Zea mays L. cv. LG11) caused by chilling in high light. After chilling attached maize leaves at 5° C for 6 h at high irradiance (1000 mol photons·m^–2·s^–1) chlorophyll fluorescence measurements indicated a serious effect on the efficiency of photochemical conversion by photosystem II (PSII) and measurements of [¹⁴C]atrazine binding showed that the plastoquinone binding site was altered in more than half of the PSII reaction centres. Although there were no direct effects of the chilling treatment on coupling-factor activity, ATP-formation capacity was affected because the photoinhibition of PSII led to a reduced capacity to energize the thylakoid membranes. In contrast to chilling at high irradiance, no photoinhibition of PSII accompanied the 20% decrease in the quantum yield of carbon assimilation when attached maize leaves were chilled in low light (50 mol photons·m^–2·s^–1). Thus it is clear that photoinhibition of PSII is not the sole cause of the light-dependent, chillinduced decrease in the quantum yield of carbon assimilation. During the recovery of photosynthesis from the chilling treatment it was observed that full [¹⁴C]atrazinebinding capacity and membrane-energization capacity recovered significantly more slowly than the quantum yield of carbon assimilation. Thus, not only is photoinhibition of PSII not the sole cause for the decreased quantum yield of carbon assimilation, apparently an appreciable population of photoinhibited PSII centres can be tolerated without any reduction in the quantum yield of carbon assimilation.Abbreviations and Symbols PPFD photosynthetically active photon flux density - PSII photosystem II - F_v/F_m ratio of variable to maximal fluorescence - quantum yield of carbon assimilation This work was supported in part by grants from the UK Agricultural and Food Research Council (AG 84/5) to N.R.B. and from the U.S. Department of Agriculture (Competitive Research Grant 87-CRCR-1-2381) to D.R.O. G.Y.N. was the recipient of a British Council scholarship and N.R.B. received a fellowship from the Organization for Economic Co-operation and Development (Project on Food Production and Preservation).     

Photoinactivation of functional photosystem II and D1-protein synthesis in vivo are independent of the modulation of the photosynthetic apparatus by growth irradiance

To investigate whether the in-vivo photoinhibition of photosystem II (PSII) function by excess light is an intrinsic property of PSII, the maximal photochemical efficiency of PSII (Fv/Fm) and the content of functional PSII (measured by repetitive flash yield of oxygen evolution) were determined in leaves of pea (Pisum sativum L.), grown in 50 (low light), 250 (medium light), and 650 (high light) mol photons·m^–2·s^–1. The modulation of PSII functionality in vivo was induced in 1.1% CO₂ by varying either (i) the duration (0–2 h) of light treatment (fixed at 1800 mol photons· m^–2·s^–1) or (ii) irradiance (0–3200 mol photons·m^–2·s^–1) at a fixed duration (1 h), after infiltration of leaves with water (control), lincomycin (an inhibitor of chloroplast-encoded protein synthesis), or a combination of lincomycin with nigericin (an uncoupler), through the cut petioles of leaves of 22-to 24-d-old plants. The reciprocity law of irradiance and duration of illumination for PSII function in vivo (Park et al. 1995, Planta 196: 401–411) holds in all differently light-grown peas, demonstrating that inactivation of functional PSII depends on photon exposure (mol photons·m^–2), not on the rate of photon absorption. In vivo, PSII acts as an intrinsic photon counter and at higher photon exposures is inactivated following absorption of about 3 × 10⁷ photons. There is a functional heterogeneity of PSII in vivo with 25% less-stable PSIIs that are inactivated at low photon exposure, compared to 75% more-stable PSIIs regardless of modulation of the photosynthetic apparatus. We suggest that the less-stable PSIIs represent monomers located in the nonappressed granal margins, while the more-stable PSIIs are dimers located in the appressed grana membrane cores. The capacity for D1-protein synthesis was the same in all the light-acclimated peas and saturated at low light, indicating that D1-protein repair is also an intrinsic property of PSII. This accounts for the low intensity required for recovery of photoinhibition in sun and shade plants which is independent of light-harvesting antennae size or PSII/PSI stoichiometries.Abbreviations D1-protein psbA gene product - D2 protein psbD gene product - Fo chlorophyll fluorescence corresponding to open PSII reaction centres - Fv, Fm variable and maximum fluorescence after dark incubation, respectively - PS photosystem - Q_B secondary quinone electron acceptor Financial support for this research by the Department of Employment, Education and Training/Australian Research Council International Research Fellowships Program (Korea) is gratefully acknowledged.     

Basal oxygen consumption,ventilatory frequency,and heart rate during the protracted spawning run of the Southern Hemisphere lamprey Geotria australis

Summary Basal oxygen consumption, ventilatory frequency, and heart rate were recorded at four different times during the unusually protracted 15–16-month spawning run of the Southern Hemisphere lamprey Geotria australis. At 15°C, the mean basal oxygen consumption of G. australis caught immediately after they had left the sea and embarked on the spawning run (45 l · g^-1 · h^-1) was less than in young adults about to commence their marine feeding phase (64 l · g^-1 · h^-1), but greater than in large ammocoetes (26.5 l · g^-1 · h^-1). Basal oxygen consumption fell progressively during the spawning-run of to 33 l · g^-1 · h^-1 after 5 months and 25 l · g^-1 · h^-1 after 10 months, before rising to 35 l · g^-1 · h^-1 after 15 months when the animals were approaching sexual maturity. The downwards trend in basal oxygen consumption contrasts with that recorded during the spawning run of Lampetra fluviatilis. Furthermore, these values for spawning-run of G. australis are far lower than those measured at any time during the upstream migration of L. fluviatilis or during the parasitic phase of landlocked Petromyzon marinus. A low and declining metabolic rate during much of the spawning run of G. australis would facilitate the conservation of energy reserves during this very long non-feeding period. Trends shown by ventilatory frequency and heart rate essentially parallel those of basal oxygen consumption. The Q₁₀s for basal oxygen consumption, ventilatory frequency and heart rate over the temperature range 5–25°C were 1.6, 1.6, and 1.7, respectively. The trends shown by basal oxygen consumption during metamorphosis and the upstream migration did not parallel those exhibited by circulating thyroid hormones.     

Long-term chilling of young tomato plants under low light and subsequent recovery

The influence of unfavourable climatic conditions at the onset of the growth period on chilling-sensitive tomato (Lycopersicon esculentum Mill., cv. Abunda) was studied by exposing young plants to combinations of low temperature and low light (60–100 mol quanta · m^–2 · s^–1) for several weeks. When the temperature did not decrease below a critical point (8 ° C) no loss of developmental capacity of the plants was detected. However, while new leaves were readily formed upon return to normal growth conditions (22/18 °C, day/night, in a greenhouse), net accumulation of biomass showed a lag phase of approximately one week. This delay was accompanied by a strong, irreversible inhibition of photosynthesis in the fully expanded leaves which had been exposed to the chilling treatment. When plants were subjected to temperatures below 8 ° C, survival rates decreased after three weeks at 6 ° C and irreversible damage of apical meristematic tissue occurred. Drought-hardening prior to chilling ensured survival at 6 ° C and protected the plants against meristem loss.Abreviation Chl chlorophyll Thanks are due to G.P. Telkamp for technical assistance. This research is financially supported by the Netherlands Technology Foundation (STW, Utrecht, The Netherlands), and is coordinated by the Foundation for Biological Research (BION, 's-Gravenhage, The Netherlands).