The growth of soybean under free air [CO<Subscript>2</Subscript>] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity

Down-regulation of light-saturated photosynthesis (A_sat) at elevated atmospheric CO₂ concentration, [CO₂], has been demonstrated for many C₃ species and is often associated with inability to utilize additional photosynthate and/or nitrogen limitation. In soybean, a nitrogen-fixing species, both limitations are less likely than in crops lacking an N-fixing symbiont. Prior studies have used controlled environment or field enclosures where the artificial environment can modify responses to [CO₂]. A soybean free air [CO₂] enrichment (FACE) facility has provided the first opportunity to analyze the effects of elevated [CO₂] on photosynthesis under fully open-air conditions. Potential ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (V_c,max) and electron transport through photosystem II (J_max) were determined from the responses of A_sat to intercellular [CO₂] (C_i) throughout two growing seasons. Mesophyll conductance to CO₂ (g_m) was determined from the responses of A_sat and whole chain electron transport (J) to light. Elevated [CO₂] increased A_sat by 15–20% even though there was a small, statistically significant, decrease in V_c,max. This differs from previous studies in that V_c,max/J_max decreased, inferring a shift in resource investment away from Rubisco. This raised the C_i at which the transition from Rubisco-limited to ribulose-1,5-bisphosphate regeneration-limited photosynthesis occurred. The decrease in V_c,max was not the result of a change in g_m, which was unchanged by elevated [CO₂]. This first analysis of limitations to soybean photosynthesis under fully open-air conditions reveals important differences to prior studies that have used enclosures to elevate [CO₂], most significantly a smaller response of A_sat and an apparent shift in resources away from Rubisco relative to capacity for electron transport.Abbreviations FACE Free air [CO₂] enrichment - Rubisco Ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP Ribulose-1,5-bisphosphate - SoyFACE Soybean free air [CO₂] enrichment - VPD Vapor pressure deficit     

Growth,N2 fixation and photosynthesis in a cyanobacterium,Trichodesmium sp., under Fe stress

Trichodesmium sp., isolated from the Great Barrier Reef lagoon, was cultured in artificial seawater media containing a range of Fe concentration. Fe additions stimulated growth, N₂ fixation, cellular chlorophyll a content, light-saturated chlorophyll a-specific gross photosynthetic capacity (P_m ^chla) and the dark respiration rate (R_d ^chla). Cell yields only doubled for 9 nM Fe relative to zero added Fe, whereas N₂ fixation increased 11-fold considerably for 450 nM Fe. The results suggest that N₂ fixation of Trichodesmium is more sensitive to Fe limitation than are the cell yields.     

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.     

Characterisation of carbon dioxide and bicarbonate transport during steady-state photosynthesis in the marine cyanobacterium Synechococcus strain PCC7002

Net O₂ evolution, gross CO₂ uptake and net HCO _inf3 ^su– uptake during steady-state photosynthesis were investigated by a recently developed mass-spectrometric technique for disequilibrium flux analysis with cells of the marine cyanobacterium Synechococcus PCC7002 grown at different CO₂ concentrations. Regardless of the CO₂ concentration during growth, all cells had the capacity to transport both CO₂ and HCO _inf3 ^su– ; however, the activity of HCO _inf3 ^su– transport was more than twofold higher than CO₂ transport even in cyanobacteria grown at high concentration of inorganic carbon (C_i = CO₂ + HCO _inf3 ^su– ). In low-C_i cells, the affinities of CO₂ and HCO _inf3 ^su– transport for their substrates were about 5 (CO₂ uptake) and 10 (HCO _inf3 ^su– uptake) times higher than in high-C_i cells, while air-grown cells formed an intermediate state. For the same cells, the intracellular accumulated C_i pool reached 18, 32 and 55 mM in high-C_i, air-grown and low-C_i cells, respectively, when measured at 1 mM external C_i. Photosynthetic O₂ evolution, maximal CO₂ and HCO _inf3 ^su– transport activities, and consequently their relative contribution to photosynthesis, were largely unaffected by the CO₂ provided during growth. When the cells were adapted to freshwater medium, results similar to those for artificial seawater were obtained for all CO₂ concentrations. Transport studies with high-C_i cells revealed that CO₂ and HCO _inf3 ^su– uptake were equally inhibited when CO₂ fixation was reduced by the addition of glycolaldehyde. In contrast, in low-C_i cells steady-state CO₂ transport was preferably reduced by the same inhibitor. The inhibitor of carbonic anhydrase ethoxyzolamide inhibited both CO₂ and HCO _inf3 ^su– uptake as well as O₂ evolution in both cell types. In high-C_i cells, the degree of inhibition was similar for HCO _inf3 ^su– transport and O₂ evolution with 50% inhibition occurring at around 1 mM ethoxyzolamide. However, the uptake of CO₂ was much more sensitive to the inhibitor than HCO _inf3 ^su– transport, with an apparent I₅₀ value of around 250 M ethoxyzolamide for CO₂ uptake. The implications of our results are discussed with respect to C_i utilisation in the marine Synechococcus strain.Abbreviations Chl chlorophyll - C_i inorganic carbon (CO₂ + HCO _inf3 ^su– ) - CA carbonic anhydrase - CCM CO₂-concentrating mechanism - EZA ethoxyzolamide - GA glycolaldehyde - K_1/2 concentration required for half-maximal response - Rubisco ribulose-1,5,-bisphosphate carboxylase-oxygenase D.S. is a recipient of a research fellowship from the Deutsche Forschungsgemeinschaft (D.F.G.). In addition, we are grateful to Donald A. Bryant, Department of Molecular and Cell Biology and Center of Biomolecular Structure Function, Pennsylvania State University, USA, for sending us the wild-type strain of Synechococcus PCC7002.     

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     

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     

Photoprotection, photosynthesis and growth of tropical tree seedlings under near-ambient and strongly reduced solar ultraviolet-B radiation

Seedlings of two late-successional tropical rainforest tree species, Tetragastris panamensis (Engler) O. Kuntze and Calophyllum longifolium (Willd.), were field grown for 3-4 months at an open site near Panama City (9 degrees N), Panama, under plastic films that either transmitted or excluded most solar UV-B radiation. Experiments were designed to test whether leaves developing under bright sunlight with strongly reduced UV-B are capable of acclimating to near-ambient UV-B conditions. Leaves of T. panamensis that developed under near-ambient UV-B contained higher amounts of UV-absorbing substances than leaves of seedlings grown under reduced UV-B. Photosynthetic pigment composition, content of alpha-tocopherol, CO(2) assimilation, potential photosystem II (PSII) efficiency (evaluated by F(v)/F(m) ratios) and growth of T. panamensis and C. longifolium did not differ between seedlings developed under near-ambient and reduced solar UV-B. When seedlings were transferred from the reduced UV-B treatment to the near-ambient UV-B treatment, a pronounced inhibition of photosynthetic capacity was observed initially in both species. UV-B-mediated inhibition of photosynthetic capacity nearly fully recovered within 1 week of the transfer in C. longifolium, whereas in T. panamensis an about 35% reduced capacity of CO(2) uptake was maintained. A marked increase in UV-absorbing substances was observed in foliage of transferred T. panamensis seedlings. Both species exhibited enhanced mid-day photoinhibition of PSII immediately after being transferred from the reduced UV-B to the near-ambient UV-B treatment. This effect was fully reversible within 1d in T. panamensis and within a few days in C. longifolium. The data show that leaves of these tropical tree seedlings, when developing in full-spectrum sunlight, are effectively protected against high solar UV-B radiation. In contrast, leaves developing under conditions of low UV-B lacked sufficient UV protection. They experienced a decline in photosynthetic competence when suddenly exposed to near-ambient UV-B levels, but exhibited pronounced acclimative responses.     

Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO₂ is evaluated from the results of over 40 studies focusing on the effect of long-term CO₂ enrichment on the short-term response of photosynthesis to intercellular CO₂ (the A/C_i response). The effect of CO₂ enrichment on the A/C_i response was dependent on growth conditions, with plants grown in small pots (< 5 L) or low nutrients usually exhibiting a reduction of A at a given C_i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C_i following a doubling or tripling of atmospheric CO₂ during growth. Using theoretical interpretations of A/C_i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C_i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO₂ enrichment usually had little effect or increased the value of A at all C_i. However, a minority of species grown at elevated CO₂ exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO₂ (the C_i/C_a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C_i/C_a exhibited no consistent change in a variety of C₃ species, indicating no stomatal acclimation. Under drought or humidity stress, C_i/C_a declined in high-CO₂ grown plants, indicating stomata will become more conservative during stress episodes in future high CO₂ environments.Abbreviations A net CO₂ assimilation rate - C_i (C_a) intercellular (ambient) partial pressure of CO₂ - operational C_i intercellular partial pressure of CO₂ at a given ambient partial pressure of CO₂ - g_s stomatal conductance - normal CO₂ current atmospheric mole fraction of CO₂ (330 to 355 mol mol^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

Novel characteristics of cassava,Manihot esculenta Crantz,a reputed C3-C4 intermediate photosynthesis species

The cassava plant, Manihot esculenta, grows exceptionally well in low fertility and drought prone environments, but the mechanisms that allow this growth are unknown. Earlier, and sometimes contradictory, work speculated about the presence of a C₄-type photosynthesis in cassava leaves. In the present work we found no evidence for a C₄ metabolism in mature attached cassava leaves as indicated i) by the low, 2 to 8%, incorporation of ¹⁴CO₂ into C₄ organic acids in short time periods, 10 s, and the lack of ¹⁴C transfer from C₄ acids to other compounds in ¹²CO₂, ii) by the lack of C₄ enzyme activity changes during leaf development and the inability to detect C₄ acid decarboxylases, and iii) by leaf CO₂ compensation values between 49 and 65 l of CO₂ 1^–1 and by other infrared gas exchange photosynthetic measurements. It is concluded that the leaf biochemistry of cassava follows the C₃ pathway of photosynthesis with no indication of a C₃-C₄ mechanism.However, cassava leaves exhibit several novel characteristics. Attached leaves have the ability to effectively partition carbon into sucrose with nearly 45% of the label in sucrose in about one min of ¹⁴CO₂ photosynthesis, contrasting with 34% in soybean (C₃) and 25% in pigweed (C₄). Cassava leaves displayed a strong preference for the synthesis of sucrose versus starch. Field grown cassava leaves exhibited high rates of photosynthesis and curvilinear responses to increasing sunlight irradiances with a tendency to saturate only at high irradiances, above 1500 mol m^–2 s^–1. Morphologically, the cassava leaf has papillose epidermal cells on its lower mesophyll surface that form fence-like arrangements encircling guard cells. It is proposed that the active synthesis of sugars has osmotic functions in the cassava plant and that the papillose epidermal cells function to maintain a healthy leaf water status in various environments.Abbreviations ADP adenosine diphosphate - Asp aspartate - BSA bovine serum albumin - CoA coenzyme A - DTT dithiothreitol - EDTA ethylenediaminetetraacetic acid - FBP fructose-1,6-biphosphate - Gly glycine - HEPES N-2-hydroxyethylpiperazine-N-2-ethansulfonic acid - Mal malate - NAD nicotinamide adenine dinucleotide (oxidized form) - NADH nicotinamide adenine dinucleotide (reduced form) - NADP nicotinamide adenine dinucleotide phosphate (oxidized form) - PAR photosynthetic active radiation (400–700 nm) - PEP phosphenolpyruvate carboxylase - p-FBPase plastid fructose-1,6-biphosphatase - PGA 3-phosphoglyceric acid - PMSF phenylmethylsulfonyl fluoride - PVP polyvinylpyrrolidone - Rubisco ribulose-1,5-biphosphate carboxylase/oxygenase - RuBP ribulose-1,5-biphosphate - Ser serine - sugar-P sugar-phosphates     

Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment

Legumes, with the ability to fix atmospheric nitrogen (N), may help alleviate the N limitations thought to constrain plant community response to elevated concentrations of atmospheric carbon dioxide (CO₂). To address this issue we assessed: (1) the effects of the presence of the perennial grassland N₂ fixer, Lupinus perennis, on biomass accumulation and plant N concentrations of nine-species plots of differing plant composition; (2) leaf-level physiology of co-occurring non-fixing species (Achillea millefolium, Agropyron repens, Koeleria cristata) in these assemblages with and without Lupinus; (3) the effects of elevated CO₂ on Lupinus growth and symbiotic N₂ fixation in both monoculture and the nine-species assemblages; and (4) whether assemblages containing Lupinus exhibit larger physiological and growth responses to elevated CO₂ than those without. This study was part of a long-term grassland field experiment (BioCON) that controls atmospheric CO₂ at current ambient and elevated (560 µmol mol^–1) concentrations using free-air CO₂ enrichment. Nine-species plots with Lupinus had 32% higher whole plot plant N concentrations and 26% higher total plant N pools than those without Lupinus, based on both above and belowground measurements. Co-occurring non-fixer leaf N concentrations increased 22% and mass-based net photosynthetic rates increased 41% in plots containing Lupinus compared to those without. With CO₂ enrichment, Lupinus monocultures accumulated 32% more biomass and increased the proportion of N derived from fixation from 44% to 57%. In nine-species assemblages, Lupinus N derived from fixation increased similarly from 43% to 54%. Although Lupinus presence enhanced photosynthetic rates and leaf N concentrations of co-occurring non-fixers, and increased overall plant N pools, Lupinus presence did not facilitate stronger photosynthetic responses of non-fixing species or larger growth responses of overall plant communities to elevated CO₂. Non-fixer leaf N concentrations declined similarly in response to elevated CO₂ with and without Lupinus present and the relationship between net photosynthesis and leaf N was not affected by Lupinus presence. Regardless of the presence or absence of Lupinus, CO₂ enrichment resulted in reduced leaf N concentrations and rates of net photosynthesis.     

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     

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.     

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.     

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 C₄Panicum species. CO₂ evolution by leaf slices of P. bisulcatum, a C₃ species, fed glycolate or glycine was light-independent and O₂-sensitive. The C₄P. 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 C₃Panicum 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-P₂ ribulose 1,5-bisphosphate - Rib-5-P ribose-5-phosphate - Ru-5-P ribuluse-5-phosphate - FCCP carbonyl cyanide p-trifluoromethoxyphenylhydrazoneJournal Series Paper, New Jersey Agricultural Experiment Station     

Characterization of photosynthesis of Populus euphratica grown in the arid region

Net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), transpiration rate (E), water use efficiency (WUE), and stomatal limitation (L_s) of Populus euphratica grown at different groundwater depths in the arid region were measured. g _s of the trees with groundwater depth at 4.74 m (D₄) and 5.82 m (D₅) were lower and a little higher than that at 3.82 m (D₃), respectively. Compared with C _i and L_s of the D₃ trees, C _i decreased and L_s increased at 4.74 m, however, Ci increased and L_s decreased at D₅. Hence photosynthetic reduction of P. euphratica was attributed to either stomatal closure or non-stomatal factors depending on the groundwater depths in the plant locations. P _N of the D₃ trees was significantly higher than those at D₄ or D₅. The trees of D₄ and D₅ did not show a significant difference in their P _N, indicating that there are mechanisms of P. euphratica tolerance to mild and moderate drought stress.     

Photoinhibition of photosynthesis in willow leaves under field conditions

Chlorophyll fluorescence of leaves of a willow (Salix sp.) stand grown in the field in northern Sweden was measured on several occasions during the growing season of 1987. For leaves that received mostly full daylight, the F _V/F _P ratio declined roughtly 15% in the afternoon on cloudless days in July (F _P is the fluorescence at the peak of the induction curve obtained at the prevailing air temperature after 45 min of dark adaptation, and F _V is variable fluoresence, F _V=F _P-F _O, where F _O is minimal fluorescence). There was no decrease in the F _V/F _P ratio on cloudy days, while the effect was intermediate on changeable days. In view of this light dependence, together with the fact that the decline in the F _V/F _P ratio was paralleled with an equal decline in the corresponding fluorescence ratio F _V/F _M at 77K, and a similar decline in the maximum quantum yield of O₂ evolution, it is suggested that the decline in the F _V/F _P ratio represents a damage in photosyntem II attributable to photoinhibition. Recovery of the F _V/F _P ratio in dim light following a decline on a cloudless day took 7–16 h to go to completion; the F _V/F _P ratio was fully restored the following morning. When all active leaves of a peripheral shoot were compared, the F _V/F _P ratio in the afternoon of a day of bright light varied greatly from leaf to leaf, though the majority of leaves showed a decline. This variation was matched by a pronounced variation in intercepted photon flux density. When leaves developed in the shade were exposed to full sunlight by trimming of the stand an increased sensitivity to photoinhibition was observed as compared to peripheral leaves. The present study indicates that peripheral willow shoots experienced in the order of 10–20% photoinhibition during an appreciable part of their life. This occurred even though the environmental conditions were within the optimal range of photosynthesis and growth.Abbreviations and symbols F _O minimum fluorescence - F _P fluorescence at the peak of the induction curve obtained at normal ambient temperatures - F _V variable fluorescence - F _M maximum fluorescence obtained at 77K - PPFD photosynthetic photon flux density     

Respiration of blue-green algae in the light

The CO₂ evolution in the light of Anabaena as well as several other blue-green algae is below 10% of the dark control. Addition of DCMU restores CO₂ evolution in the light almost to the dark level. Furthermore, by adding unlabeled NaHCO₃, a ¹⁴CO₂ release is observed with prelabeled algal cells attaining 15 to 100% of dark control. Analysis by double-reciprocal plots exhibits a competitive relationship between added and endogenously released carbon dioxide. We conclude that CO₂ evolved by respiration is immediately refixed in the light without being liberated.The degree of ¹⁴CO₂ release induced by unlabeled bicarbonate in the light allows to determine true photoinhibition of respiration. Anabaena variabilis Kütz. exhibits almost no inhibition while in eight other species respiration is light-inhibited between 50 and 85% of the dark control.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - TCA trichloroacetic acid