The long-wavelength limit of plant photosynthesis

It is a common knowledge that the photosynthesis efficiency drops rapidly under the long-wavelength light excitation above 680 nm. We discovered that in sunflower leaves attached to the plant the initial fall is replaced by an unexpected increase at much longer wavelengths, so that a detectable O(2) evolution is remained till 780 nm. The quantum yield of O(2) evolution at the local maximum at 745 nm reaches almost 20% of the yield at 650 nm. We conclude that extreme long-wavelength chlorophylls may be present in the intact photosystem II antenna system, similarly to photosystem I.     

Effects of extreme high temperature,drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub,Larrea tridentata

The interaction of extreme temperature events with future atmospheric CO₂ concentrations may have strong impacts on physiological performance of desert shrub seedlings, which during the critical establishment phase often endure temperature extremes in conjunction with pronounced drought. To evaluate the interaction of drought and CO₂ on photosynthesis during heat stress, one-year-old Larrea tridentata[DC] Cov. seedlings were exposed to nine days of heat with midday air temperature maxima reaching 53 °C under three atmospheric CO₂ concentrations (360, 550 and 700 mol mol^–1) and two water regimes (well-watered and droughted). Photosynthetic gas exchange, chlorophyll fluorescence and water potential responses were measured prior to, during and one week following the high temperature stress event. Heat stress markedly decreased net photosynthetic rate (A _net), stomatal conductance (g _s), and the photochemical efficiency of photosystem II (F _v/F _m) in all plants except for well-watered L. tridentata grown in 700 mol mol^–1 CO₂. A _net and g _s remained similar to pre-stress levels in these plants. In droughted L. tridentata, A _net was ca. 2× (in 550 mol mol^–1 CO₂) to 3× (in 700 mol mol^–1 CO₂) higher than in ambient-CO₂-grown plants, while g _s and F _v/F _m were similar and low in all CO₂ treatments. Following heat stress, g _s in all well-watered plants rose dramatically, exceeding pre-stress levels by up to 100%. In droughted plants, g _s and A _net rose only in plants grown at elevated CO₂ following release from heat. This recovery response was strongest at 700 mol mol^–1 CO₂, which returned to A _net and g _s values similar to pre-heat following several days of recovery. Extreme heat diminished the photosynthetic down-regulation response to growth at elevated CO₂ under well-watered conditions, similar to the action of drought. Ambient-CO₂-grown L. tridentata did not show significant recovery of photosynthetic capacity (A _\max and CE) after alleviation of temperature stress, especially when exposed to drought, while plants exposed to elevated CO₂ appeared to be unaffected. These findings suggest that elevated CO₂ could promote photosynthetic activity during critical periods of seedling establishment, and enhance the potential for L. tridentata to survive extreme high temperature events.     

Analysing the responses of photosynthetic CO2 assimilation to long-term elevation of atmospheric CO2 concentration

Understanding how photosynthetic capacity acclimatises when plants are grown in an atmosphere of rising CO₂ concentrations will be vital to the development of mechanistic models of the response of plant productivity to global environmental change. A limitation to the study of acclimatisation is the small amount of material that may be destructively harvested from long-term studies of the effects of elevation of CO₂ concentration. Technological developments in the measurement of gas exchange, fluorescence and absorption spectroscopy, coupled with theoretical developments in the interpretation of measured values now allow detailed analyses of limitations to photosynthesisin vivo. The use of leaf chambers with Ulbricht integrating spheres allows separation of change in the maximum efficiency of energy transduction in the assimilation of CO₂ from changes in tissue absorptance. Analysis of the response of CO₂ assimilation to intercellular CO₂ concentration allows quantitative determination of the limitation imposed by stomata, carboxylation efficiency, and the rate of regeneration of ribulose 1:5 bisphosphate. Chlorophyll fluorescence provides a rapid method for detecting photoinhibition in heterogeneously illuminated leaves within canopies in the field. Modulated fluorescence and absorption spectroscopy allow parallel measurements of the efficiency of light utilisation in electron transport through photosystems I and IIin situ.Abbreviations A net rate of CO₂ uptke per unit leaf area (µmol m^–2 s^–1) - A_sat light-saturated A - A₈₂₀ change in absorptance of PSI on removal of illumination (OD) - c CO₂ concentration in air (µmol mol^–1) - c_a c in the bulk air; c_i, c in the intercellular spaces - ce carboxylation efficiency (mol m^–2 s^–1) - E transpiration per unit leaf area (mol m^–2 s^–1) - F fluorescence emission of PSII (relative units) - F_m maximal level of F - F_o minimal level of F upon illumination when PSII is maximally oxidised - F_s the steady-state F following the m peak - F_v the difference between F_m and F_o - F'_m maximal F' generated after the m peak by addition of a saturating light pulse - F'_o the minimal level of F' after the m peak determined by re-oxidising PSII by far-red light - g₁ leaf conductance to CO₂ diffusion in the gas phase (mol m^–2 s^–1) - g'₁ leaf conductance to water vapour diffusion in the gas phase (mol m^–2 s^–1) - k_c and k_o the Michaelis constants for CO₂ and O₂, respectively, (µmol mol^–1); - J_max the maximum rate of regeneration of rubP (µmol m^–2 s^–1) - l stomatal limitation to CO₂ uptake (dimensionless, 0–1) - LCP light compensation point of photosynthesis (µmol m^–2 s^–1) - o_i the intercellular O₂ concentration (mmol mol^–1) - Pi cytosol inorganic phosphate concentration - PSI photosystem I - PSII photosystem II - Q photon flux (µmol m^–2 s^–1) - Q_abs Q absorbed by the leaf - rubisCO ribulose 1:5 bisphosphate carboxylase/oxygenase; rubP, ribulose 1:5 bisphosphate; s, projected surface area of a leaf (m²) - V_c,max is the maximum rate of carboxylation (µmol m^–2 s^–1) - W_c the rubisCO limited rate of carboxylation (µmol m^–2 s¹) - W_j the electron transport limited rate of regeneration of rubP (µmol m^–2 s^–1) - W_p the inorganic phosphate limited rate of regeneration of rubP (µmol m^–2 s^–1) - absorptance of light (dimensionless, 0–1) - _a of standard black absorber ₁, of leaf - _s of integrating sphere walls - , CO₂ compensation point of photosynthesis (µmol mol^–1) - the specificity factor for rubisCO carboxylation (dimensionless) - , convexity of the response of A to Q (dimensionless 0–1) - the quantum yield of photosynthesis on an absorbed light basis (A/Q_abs; dimensionless) - the quantum yield of photosynthesis on an incident light basis (A/Q; dimensionless) - _app the maximum - _m the maximum - _m,app the photochemical efficiency of PSII (dimensionless, 0–1) - _PSII,m the maximum      

Verification of the Z-scheme in Chlamydomonas mutants with Photosystem I deficiency

In conflict with the Z-scheme of photosynthesis, it has recently been reported [Greenbaum et al. Nature (1995) 376: 438–441; Lee et al. Science (1996) 273: 364–367] that Photosystem II can drive ferredoxin reduction and photoautotrophic growth in some mutants of Chlamydomonas lacking detectable Photosystem I reaction centre, P700. Using the same mutants, B4 and F8, here we report that action spectra and parameters of flash yields of different photoreactions show the operation in ferredoxin-dependent H₂ photoproduction and CO₂ fixation of a fraction (at least 5% compared to wild- type) of the only Photosystem I complexes.     

The relationship between the redox state of Q A and photosynthesis in leaves at various carbon-dioxide,oxygen and light regimes

The response of chlorophyll fluorescence elicited by a low-fluence-rate modulated measuring beam to actinic light and to superimposed 1-s pulses from a high-fluence-rate light source was used to measure the redox state of the primary acceptor Q _A of photosystem II in leaves which were photosynthesizing under steady-state conditions. The leaves were exposed to various O₂ and CO₂ concentrations and to different energy fluence rates of actinic light to assess the relationship between rates of photosynthesis and the redox state of Q _A. Both at low and high fluence rates, the redox state of Q _A was little altered when the CO₂ concentration was reduced from saturation to about 600 l·l^-1 although photosynthesis was decreased particularly at high fluence rates. Upon further reduction in CO₂ content the amount of reduced Q _A increased appreciably even at low fluence rates where light limited CO₂ reduction. Both in the presence and in the absence of CO₂, a more reduced Q _A was observed when the O₂ concentration was below 2%. Q _A was almost fully reduced when leaves were exposed to high fluence rates under nitrogen. Even at low fluence rates, Q _A was more reduced in shade leaves of Asarum europaeum and Fagus sylvatica than in leaves of Helianthus annuus and Fagus sylvatica grown under high light. Also, in shade leaves the redox state of Q _A changed more during a transition from air containing 350 l·l^-1 CO₂ to CO₂-free air than in sun leaves. The results are discussed with respect to the energy status and the CO₂-fixation rate of the leaves.Abbreviations and symbols L 1,2 first and second actinic light beam - Q _A primary acceptor of photosystem II - q _Q Q-quenching     

Adaptation of photosynthesis under iron deficiency in maize

This paper explores the effects of high light stress on Fe-deficient plants. Maize (Zea mays) plants were grown under conditions of Fe deficiency and complete nutrition. Attached, intact leaves of Fe-deficient and control plants were used for gas exchange experiments under suboptimal, optimal and photoinhibitory illumination. Isolated chloroplasts were used to study photosynthetic electron transport system, compromised by the induction of Fe deficiency. The reaction centers of PS II (measured as reduction of Q, the primary electron acceptor of P 680) and PS I (measured as oxidation of P 700) were estimated from the amplitude of light induced absorbance change at 320 and 700 nm, respectively. Plants were subjected to photoinhibitory treatment for different time periods and isolated chloroplasts from these plants were used for electron transport studies. Carbon dioxide fixation in control as well as in Fe-deficient plants decreased in response to high light intensities. Total chlorophyll, P 700 and Q content in Fe-deficient chloroplasts decreased, while Chl a/b ratio and Q/P 700 ratio increased. However, electron transport through PS II suffered more after photoinhibitory treatment as compared to electron transport through PS I or whole chain. Electron transfer through PS I+PS II, excluding the water oxidation complex showed a decrease in Fe-deficient plants. However, electron transport through this part of the chain did not suffer much as a result of photoinhibition, suggesting a defect in the oxidising side of PS II.     

Metabolic regulation of carbon flux during C4 photosynthesis

The potential for glycolate and glycine metabolism and the mechanism of refixation of photorespiratory CO₂ in leaves of C₄ plants were studied by parallel inhibitor experiments with thin leaf slices, different leaf cell types and isolated mitochondria of C₃ and C₄ Panicum species. CO₂ evolution by leaf slices of P. bisulcatum, a C₃ species, fed glycolate or glycine was light-independent and O₂-sensitive. The C₄ P. maximum and P. miliaceum leaf slices fed glycolate or glycine evolved CO₂ in the dark but not in the light. In C₄ species, dark CO₂ evolution was abolished by the addition of phosphoenolpyruvate (PEP)⁴. The addition of maleate, a PEP carboxylase inhibitor, resulted in photorespiratory CO₂ efflux by C₄ leaf slices in the light also. However, PEP and maleate had no effect on either glycolate-dependent O₂ uptake by the C₄ leaf slices or on glycolate and glycine metabolism in C₃ leaf slices. The rate of photorespiratory CO₂ evolution in the C₃ Panicum species was 3 times higher than that observed with the C₄ species. The ratio of glycolate-dependent CO₂ evolution to O₂ uptake in both groups was 1:2. Isolated C₄ mesophyll protoplasts or their mitochondria did not metabolize glycolate or glycine. However, both C₃ mesophyll protoplasts and C₄ bundle sheath strands readily metabolized glycolate and glycine in a light-independent, O₂-sensitive manner, and the addition of PEP or maleate had no effect. C₄ bundle sheath- and C₃-mitochondria were capable of oxidizing glycine. This oxidation was linked to the mitochondrial electron transport chain, was coupled to three phosphorylation sites and was sensitive to electron transport inhibitors. C₄ bundle sheath- and C₃-mitochondrial glycine decarboxylation was stimulated by oxaloacetate and NAD had no effect. In marked contrast, mitochondria isolated from C₄ mesophyll cells were incapable of oxidizing or decarboxylating added glycine. The results suggest that in leaves of C₄ plants bundle sheath cells are the primary site of O₂-sensitive photorespiratory CO₂ evolution and the PEP carboxylase present in the mesophyll cells has the Potential for efficiently refixing CO₂ before it escapes out of the leaf. The relative role of the PEP carboxylase mediated CO₂ pump and reassimilation of photorespiratory CO₂ are discussed in relation to the apparent lack of photorespiration in leaves of C₄ species.Abbreviations BSA bovine serum albumin - Chl chlorophyll - PEP phosphoenolpyruvate - Rbu-P ₂ ribulose 1,5-bisphosphate - Rib-5-P ribose-5-phosphate - Ru-5-P ribuluse-5-phosphate - FCCP carbonyl cyanide p-trifluoromethoxyphenylhydrazone Journal Series Paper, New Jersey Agricultural Experiment Station     

Microgravity effects on thylakoid,single leaf,and whole canopy photosynthesis of dwarf wheat

The concept of using higher plants to maintain a sustainable life support system for humans during long-duration space missions is dependent upon photosynthesis. The effects of extended exposure to microgravity on the development and functioning of photosynthesis at the leaf and stand levels were examined onboard the International Space Station (ISS). The PESTO (Photosynthesis Experiment Systems Testing and Operations) experiment was the first long-term replicated test to obtain direct measurements of canopy photosynthesis from space under well-controlled conditions. The PESTO experiment consisted of a series of 21–24 day growth cycles of Triticum aestivum L. cv. USU Apogee onboard ISS. Single leaf measurements showed no differences in photosynthetic activity at the moderate (up to 600 μmol m⁻² s⁻¹) light levels, but reductions in whole chain electron transport, PSII, and PSI activities were measured under saturating light (>2,000 μmol m⁻² s⁻¹) and CO₂ (4000 μmol mol⁻¹) conditions in the microgravity-grown plants. Canopy level photosynthetic rates of plants developing in microgravity at ∼280 μmol m⁻² s⁻¹ were not different from ground controls. The wheat canopy had apparently adapted to the microgravity environment since the CO₂ compensation (121 vs. 118 μmol mol⁻¹) and PPF compensation (85 vs. 81 μmol m⁻² s⁻¹) of the flight and ground treatments were similar. The reduction in whole chain electron transport (13%), PSII (13%), and PSI (16%) activities observed under saturating light conditions suggests that microgravity-induced responses at the canopy level may occur at higher PPF intensity.     

Lateral diffusion of CO<Subscript>2</Subscript> in leaves of the crassulacean acid metabolism plant<Emphasis Type="Italic"> Kalanchoë daigremontiana</Emphasis> Hamet et Perrier

Dynamic patchiness of photosystem II (PSII) activity in leaves of the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perrier, which was independent of stomatal control and was observed during both the day/night cycle and circadian endogenous oscillations of CAM, was previously explained by lateral CO₂ diffusion and CO₂ signalling in the leaves [Rascher et al. (2001) Proc Natl Acad Sci USA 98:11801–11805; Rascher and Lüttge (2002) Plant Biol 4:671–681]. The aim here was to actually demonstrate the importance of lateral CO₂ diffusion and its effects on localized PSII activity. Covering small sections of entire leaves with silicone grease was used for local exclusion of a contribution of atmospheric CO₂ to internal CO₂ via transport through stomata. A setup for combined measurement of gas exchange and chlorophyll fluorescence imaging was used for recording photosynthetic activity with a spatiotemporal resolution. When remobilization of malic acid from vacuolar storage and its decarboxylation in the CAM cycle caused increasing internal CO₂ concentrations sustaining high PSII activity behind closed stomata, PSII activity was also increased in adjacent leaf sections where vacuolar malic acid accumulation was minimal as a result of preventing external CO₂ supply due to leaf-surface greasing, and where therefore CO₂ could only be supplied by diffusion from the neighbouring malic acid-remobilizing leaf tissue. This demonstrates lateral CO₂ diffusion and its effect on local photosynthetic activity.     

Glyoxylate decreases the oxygen sensitivity of nitrogenase activity and photosynthesis in the cyanobacterium Anabaena cylindrica

Raising the pO₂ reduced nitrogenase activity (C₂H₂ reduction) of Anabaena cylindrica for both glyoxylate-treated (5 mM) and untreated cells. The stimulation caused by glyoxylate, however, increased with increases of pO₂ from 2 to 99 kPa. As the pO₂ increased the net CO₂ fixation was lowered (Warburg effect) while the CO₂ compensation point increased. Glyoxylate partly relieved this sensitivity of net photosynthesis to oxygen and reduced the compensation point considerably. The cells used were preincubated in the dark to exhaust photosynthetic pools. A more pronounced reduction in sensitivity of nitrogenase to oxygen for glyoxylate-treated cells was evident when a preincubation in air with reduced pCO₂ (13 l l^-1) was used. This was, however, not evident until after a 10-h incubation in air. Before this point 2 kPa O₂ sustained the highest nitrogenase activity. Addition of 0.5 and 5 mM of HCO ₃ ^- to Anabaena cultures preincubated at low CO₂ levels (29 l l^-1) abolished the stimulatory effect of glyoxylate on the nitrogenase. Thus, the results sustain the suggestion that glyoxylate may act as an inhibitor of photorespiratory activities in cyanobacteria and can be used as a means of increasing their nitrogen and CO₂ fixation capacities.Abbreviation RuBP ribulose 1,5-bisphosphate     

Solar light effects on growth,net photosynthesis,and leaf morphology of in vitro kiwifruit (Actinidia deliciosa) CV hayward

Summary Proliferating axillary shoots of kiwifruit (Actinidia deliciosa A. Chev., C. F. Liang and A. R. Ferguson), var.deliciosa, cv. ‘Hayward’ were grown under solar (SL), white (WL), and blue (BL) light regimens to determine the accumulation of fresh and dry weight, proliferation rate, shoot growth (length), and the net leaf photosynthetic capacity at the CO₂ concentration ranges of 200 to 350, 400 to 600, and 1200 to 1500 ppm. An histologic study determined the effects of light source on leaf stomatal density and tissue morphology. Dry and fresh matter accumulation was greatest, but callus development most limited under the SL regimen. Shoot proliferation was highest under WL and length under BL. Net photosynthetic capacity was highest for leaves grown under SL and lowest for those under BL; the leaves exposed to the latter regimen were also thinner and exhibited a less compact arrangement of palisade cells than those under WL and SL. Leaf stomata density was highest under the BL source.     

Effects of high temperatures on the photosynthetic systems in spinach: Oxygen-evolving activities,fluorescence characteristics and the denaturation process

Activities of oxygen evolution, fluorescence F_v (a variable part of chlorophyll fluorescence) values, and amounts of the 33 kDa protein remaining bound to the thylakoids in intact spinach chloroplasts were measured during and after high-temperature treatment. The following results were obtained. (1) Both the F_v value and the flash-induced oxygen evolution measured by an oxygen electrode were decreased at high temperatures, but they showed partial recovery when the samples were cooled down and incubated at 25°C for 5 min after high-temperature treatment. (2) Oxygen evolution was more sensitive to high temperatures than the F_v value, and the decrease in the F_v/F_m ratio at high temperatures rather corresponded to that in the oxygen evolution measured at 25°C after high-temperature treatment. (3) Photoinactivation of PS II was very rapid at high temperatures, and this seems to be a cause of the difference between the F_v values and the oxygen-evolving activities at high temperatures. (4) At around 40°C, the manganese-stabilizing 33 kDa protein of PS II was supposed to be released from the PS II core complexes during heat treatment and to rebind to the complexes when the samples were cooled down to 25°C. (5) At higher temperatures, the charge separation reaction of PS II was inactivated, and the PS II complexes became less fluorescent, which was recovered partially at 25°C. (6) Increases in the F_v value due to a large decrease in the electron flow from Q_A to Q_B became prominent after high-temperature treatment at around 50°C. This was the main cause of the discrepancy between the F_v values and the oxygen-evolving activities measured at 25°C. Relationship between the process of heat inactivation of PS II reaction center complexes and the fluorescence levels is discussed.     

Fluorescence decay kinetics of mutants of corn deficient in photosystem I and photosystem II

The fast fluorescence decay kinetics of two photosynthetic mutants of corn (Zea mays) have been compared with those of normal corn. The fluorescence of normal corn can be resolved into three exponential decay components of lifetime 900–1500 ps (slow), 300–500 ps (middle) and 50–120 ps (fast), the yields of which are affected by light intensity and Mg²⁺ levels. The Photosystem II-(PS II)-defective mutant hcf-3 has similar decay lifetimes (approx. 1200, 450 and 100 ps) but is not affected by light intensity, reflecting the absence of PS II charge recombination. However, yields do respond to Mg²⁺ in a fashion typical of normal corn, which may be correlated with the presence of normal levels of light-harvesting chlorophyll a + b complex (LHCP). The PS I mutant hcf-50 also shows three-component decay kinetics. In conjunction with the results on the LHCP-deficient mutant of barley presented in a recent paper (Karukstis, K.K. and Sauer, K. (1984) Biochim. Biophys. Acta 766, 148–155), these data suggest that the slow component of normal chloroplasts is kinetically controlled by the decay processes of the LHCP and that the energy comes from one of two sources: (a) charge recombination in the reaction centre or (b) energy transferred within or between LHCP units only. The fast component appears to originate from both PS I and PS II. The complex response of the middle component to cations and light intensity, and its presence in all of the mutants, suggests that it also may have multiple origins.     

Feedback inhibition of photosynthesis in rice measured by O2 dependent transients

The kinetic properties of photosynthesis (both transient and steady-state) were monitored using three non-invasive techniques to evaluate limitations on triose-phosphate (triose-P) conversion to carbohydrate in rice. These included analyzing the O₂ sensitivity of CO₂ fixation and the assimilatory charge (AC) using gas exchange (estimate of the ribulose 1,5- bisphosphate pool) and measuring Photosystem II activity by chlorophyll fluorescence analysis under varying light, temperature and CO₂ partial pressures. Photosynthesis was inhibited transiently upon switching from 20 to 2 kPa O₂ (reversed O₂ sensitivity), the degree of which was correlated with a terminal, steady-state suppression of low O₂ enhancement of photosynthesis. Under current ambient levels of CO₂ and moderate to high light, the transient pattern was more obvious at 18 °C than at 26 °C while at 34 °C no tra nsient response was observed. The transient inhibition at 18 °C ranged from 15% to 31% depending on the pre-measurement temperature. This pattern, symptomatic of feedback, was observed with increasing light and CO₂ partial pressures with the degree of feedback decreasing from moderate (18 °C) up to high temperature (34 °C). Under feedback conditions, the rate of assimilation is shifted from being photorespiration limited to being triose-P utilization limited. Transitory changes in CO₂ assimilation rates (A) under low O₂ indicative of feedback coincided with a transitory drop in assimilatory charge (AC) and inhibition of electron transport. In contrast to previous studies with many C₃ species, our studies indicate that rice shows susceptibility to feedback inhibition under moderate temperatures and current atmospheric levels of CO₂.     

Influence of enhanced CO2 on growth and photosynthesis of the red algaeGracilaria sp. andG. chilensis

The influence of elevated CO₂ concentrations on growth and photosynthesis ofGracilaria sp. andG. chilensis was investigated in order to procure information on the effective utilization of CO₂. Growth of both was enhanced by CO₂ enrichment (air + 650 ppm CO₂, air + 1250 ppm CO₂, the enhancement being greater inGracilaria sp. Both species increased uptake of NO₃ ^– with CO₂ enrichment. Photosynthetic inorganic carbon uptake was depressed inG. chilensis by pre-culture (15 days) with CO₂ enrichment, but little affected inGracilaria sp. Mass spectrometric analysis showed that O₂ uptake was higher in the light than in the dark for both species and in both cases was higher inGracilaria sp. The higher growth enhancement inGracilaria sp. was attributed to greater depression of photorespiration by the enrichment of CO₂ in culture.     

Interactions between carbon dioxide and oxygen in the photosynthesis of three species of marine red macroalgae

Summary

Red algae have the highest known selectivity factor (S_rel) for CO₂ over O₂ of ribulose bisphosphate carboxylase-oxygenase (RUBISCO). This allows the prediction that a red alga relying on diffusive supply of CO₂ to RUBISCO from air-equilibrated solution should have less O₂ inhibition of photosynthesis than would an otherwise similar non-red alga with a lower S_rel of RUBISCO. Furthermore, RUBISCO shows an increased S_rel values at low temperatures. The prediction that O ₂inhibition of photosynthesis should be small for marine red algae relying on diffusive CO₂ entry growing in the North Sea with an annual temperature range of 4–16°C was tested in O₂ electrode experiments at 12°C. Phycodrys rubens and Plocamium cartilagineum, which rely on diffusive CO₂ entry showed, as predicted, only a small inhibition at lower inorganic C concentrations. Palmaria palmata, which has a CO₂ concentrating mechanism, had the expected negligible O ₂ inhibition of photosynthesis at any inorganic C concentration except (non-significantly) for saturating inorganic C.     

The effects of chilling in the dark and in the light on photosynthesis of tomato: electron transfer reactions

Exposure of tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) to chilling temperatures in the dark for as little as 12 h resulted in a sizable inhibition in the rate of light- and CO₂-saturated photosynthesis. However, when photosynthesis was measured at low light intensity, the inhibition disappeared and the quantum yield of CO₂ reduction was diminished only slightly. Chilling the tomato plants under strong illumination caused an even more rapid and severe decline in the rate of light- and CO₂-saturated photosynthesis, accompanied by a large decline in the quantum efficiency. Sizeable inhibition of photosystem II activity was observed only after dark exposures to low temperature of grater than 16 h. No inhibition of photosystem I electron transfer capacity was observed even after 40 h of dark chilling. Chilling under high light resulted in a rapid decline in both photosystem I and photosystem II electron transfer capacity as well as in significant reaction center inactivation.Regardless of whether the chilling exposure was in the presence or absence of illumination and regardless of its duration, the electron transfer capacity of thylakoid membranes isolated from the treated plants was always in excess of that necessary to support light- and CO₂-saturated photosynthesis. Thus, in neither case of chilling inhibition of photosynthesis does it appear that impaired electron transfer capacity represents a significant rate limitation to whole plant photosynthesis.Abbreviations BSA bovine serum albumin - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-Dichlorophenyl)-1,1-dimethylurea - DHQ duroquinol - EDTA ethylene-diamine-tetraacetic acid - HEPES N-2-hydroxylpiperazine-N-2-ethanesulfonic acid - MES 2-(N-Morpholino)ethanesulfonic acid - MV methylviologen - PS I & II photosystems I and II - PD_OX p-phenylenediimine (oxidized) - TMPD N,N,N,N-tetramethyl-p-phenylenediamine     

Engineering Rubisco activase from thermophilic cyanobacteria into high-temperature sensitive plants

In the past decade, various strategies to improve photosynthesis and crop yield, such as leaf morphology, light interception and use efficiency, biochemistry of light reactions, stomatal conductance, carboxylation efficiency, and source to sink regulation, have been discussed at length. Leaf morphology and physiology are tightly coupled to light capturing efficiency, gas exchange capacity, and temperature regulation. However, apart from the photoprotective mechanism of photosystem-II (PSII), i.e. non-photochemical quenching, very low genetic variation in the components of light reactions has been observed in plants. In the last decade, biochemistry-based enhancement of carboxylation efficiency that improves photosynthesis in plants was one of the potential strategies for improving plant biomass production. Enhancement of activation of the ubiquitous enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) by Rubisco activase may be another potential strategy for improving a photosynthesis-driven increase in crop yield. Rubisco activase modifies the conformation of the active center in Rubisco by removing tightly bound inhibitors, thereby contributing to enzyme activation and rapid carboxylation. Thermophilic cyanobacteria are oxygenic photosynthetic bacteria that thrive in high-temperature environments. This critical review discusses the prospects for and the potential of engineering Rubisco activase from thermophilic cyanobacteria into temperature-sensitive plants, to increase the threshold temperature and survival of these plants in arid regions.