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)     

Relationship of photosynthetic acclimation to changes of Rubisco activity in field-grown winter wheat and barley during growth in elevated carbon dioxide

The responses of photosynthesis, Rubisco activity, Rubisco protein, leaf carbohydrates and total soluble protein to three carbon dioxide treatments were studied in winter wheat [Triticum aestivum (L.)] and barley [Hordeum vulgare (L.)]. Barley and wheat plants were grown in small field plots during 1995 and 1996 in clear, acrylic chambers (1.2–2.4 m²) and were provided with continuous carbon dioxide fertilization at concentrations of 350, 525 and 700 mol mol^–1. Photosynthetic rates of barley penultimate leaves and wheat flag leaves measured at growth carbon dioxide concentrations decreased with leaf age in all three CO₂ treatments during 1995 and 1996. Photosynthetic acclimation to elevated CO₂ was observed on seven of eight measurement dates for barley and ten of eleven measurement dates for wheat over both years. Initial Rubisco activity, total soluble protein and Rubisco protein in barley penultimate leaves and wheat flag leaves also decreased with leaf age. Total Rubisco activity was not used because of enzyme degradation. There was a significant CO₂ treatment effect on initial Rubisco activity, total soluble protein and Rubisco protein for wheat in 1995 and 1996 and for barley in 1995. Responses of barley penultimate leaf Rubisco activity and leaf protein concentrations to elevated carbon dioxide were nonsignificant in 1996. A significant CO₂ treatment effect also was detected when means of Rubisco activity, soluble protein and Rubisco protein for wheat flag leaves were combined over harvests and years. These three flag leaf parameters were not significantly different in the 350 and 525 mol mol^–1 CO₂ treatments but were decreased during growth in 700 mol mol^–1 CO₂ relative to the other two CO₂ treatments. Ratios of photosynthesis at 700 and 350 mol mol^–1 were compared to ratios of Rubisco activity at 700 and 350 mol mol^–1 using wheat flag leaf data from 1995 and 1996. Regression analysis of these data were linear [y = 0.586 + 1.103t x (r² = 0.432)] and were significant at P 0.05. This result indicated that photosynthetic acclimation was positively correlated with changes of initial Rubisco activity in wheat flag leaves in response to CO₂ enrichment. Effects of elevated CO₂ on wheat leaf proteins during 1995 and 1996 and on barley during 1995 were consistent with an acceleration of senescence.     

Effects of temperature on the regulation of photosynthetic carbon assimilation in leaves of maize and barley

The aim of this work was to examine the effect of temperature in the range 5 to 30 ° C upon the regulation of photosynthetic carbon assimilation in leaves of the C₄ plant maize (Zea mays L.) and the C₃ plant barley (Hordeum vulgare L.). Measurements of the CO₂-assimilation rate in relation to the temperature were made at high (735 bar) and low (143 bar) intercellular CO₂ pressure in barley and in air in maize. The results show that, as the temperature was decreased, (i) in barley, pools of phosphorylated metabolites, particularly hexose-phosphate, ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate, increased in high and low CO₂; (ii) in maize, pools of glycerate 3-phosphate, triose-phosphate, pyruvate and phosphoenolpyruvate decreased, reflecting their role in, and dependence on, intercellular transport processes, while pools of hexose-phosphate, ribulose 1,5-bis phosphate and fructose 1,6-bisphosphate remained approximately constant; (iii) the redox state of the primary electron acceptor of photosystem II (Q_A) increased slightly in barley, but rose abruptly below 12° C in maize. Non-photochemical quenching of chlorophyll fluorescence increased slightly in barley and increased to high values below 20 ° C in maize. The data from barley are consistent with the development of a limitation by phosphate status at low temperatures in high CO₂, and indicate an increasing regulatory importance for regeneration of ribulose 1,5-bisphosphate within the Calvin cycle at low temperatures in low CO₂. The data from maize do not show that any steps of the C₄ cycle are particularly cold-sensitive, but do indicate that a restriction in electron transport occurs at low temperature. In both plants the data indicate that regulation of product synthesis results in the maintenance of pools of Calvin-cycle intermediates at low temperatures.Abbreviations Glc6P glucose-6-phosphate - Fru6P fructase-6-phosphate - Frul,6bisP fructose-1,6-bisphosphate - PGA glycerate-3-phosphate - p _i intercellular partial pressure of CO₂ - RuBP ribulose-1,5-bisphosphate - triose-P sum of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate We thank the Agricultural and Food Research Council, UK (Research grant PG50/67) and the Science and Engineering Research Council, UK for financial support. C.A.L. was supported by the British Council, by the Conselho Nacional de Desenvolvimento Cientiflco e Tecnologico (CNPq), Brazil and by an Overseas Research Student Award. We also thank Mark Stitt (Bayreuth, FRG) and Debbie Rees for helpful discussions.     

Calculation of leaf photosynthetic parameters from light-response curves for ecophysiological applications

Measurement of the light response of photosynthetic CO₂ uptake is often used as an implement in ecophysiological studies. A method is described to calculate photosynthetic parameters, such as the maximum rate of whole electron transport and dissimilative respiration in the light, from the light response of CO₂ uptake. Examples of the light-response curves of flag leaves and ears of wheat (Triticum aestivum cv. ARKAS) are shown.Abbreviations and symbols A net photosynthesis rate - D ¹ rate of dissimilative respiration occurring in the light - f loss factor - I incident PPFD - I effective absorbed PPFD - J rate of whole electron transport - J _m maximum rate of whole electron transport - p ^c intercellular CO₂ partial pressure - PPFD photosynthetic photon flux density - q effectivity factor for the use of light (electrons/quanta) - absorption coefficient - I ^* CO₂ compensation point in the absence of dissimilative respiration (bar) - II conversion factor for calculation of CO₂ uptake from the rate of whole electron transport - convexity factor Gas-exchange rates relate to the projective area and are given in mol·m^-2·s^-1. Electron-transport rates are given in mol electrons·m^-2·s^-1; PPFD is given in mol quanta·m^-2·s^-1.     

Lack of C4 photosynthetic metabolism in ears of C3 cereals

There is continuing controversy over whether a degree of C₄ photosynthetic metabolism exists in ears of C₃ cereals. In this context, CO₂ exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat (Triticum durum Desf.) and two six-rowed barley (Hordeum vulgare L.) cultivars. Three weeks after anthesis, the CO₂ compensation concentration at 210 mmol mol^?1 O₂ in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O₂ dependence of the CO₂ compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C₃ photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with ¹⁴CO₂ during a 10s pulse, and the initial products of fixation were studied in various parts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade. All tissues assimilated CO₂ mainly by the Calvin (C₃) cycle, with little fixation of ¹⁴CO₂ into the C₄ acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C₃ cereals C₄ photosynthetic metabolism is nil.     

Light and dark CO2 fixation in Clusia uvitana and the effects of plant water status and CO2 availability

Summary In well-watered plants of Clusia uvitana, a species capable of carbon fixation by crassulacean acid metabolism (CAM), recently expanded leaves gained 5 to 13-fold more carbon during 12 h light than during 12 h dark periods. When water was withheld from the plants, daytime net CO₂ uptake strongly decreased over a period of several days, whereas there was a marked increase in nocturnal carbon gain. Photosynthetic rates in the chloroplasts were hardly affected by the water stress treatment, as demonstrated by measurements of chlorophyll a fluorescence of intact leaves, indicating efficient decarboxylation of organic acids and refixation of carbon in the light. Within a few days after rewatering, plants reverted to the original gas exchange pattern with net CO₂ uptake predominantly occurring during daytime. The reversible increase in dark CO₂ fixation was paralleled by a reversible increase in the content of phosphoenolpyruvate (PEP) carboxylase protein. In wellwatered plants, short-term changes in the degree of dark CO₂ fixation were induced by alterations in CO₂ partial pressure during light periods: a decrease from 350 to 170 bar CO₂ caused nocturnal carbon gain, measured in normal air (350 bar), to increase, whereas an increase to 700 bar CO₂, during the day, caused net dark CO₂ fixation to cease. The increased CAM activity in response to water shortage may, at least to some extent, be directly related to the reduced carbon gain during daytime.     

Temperature effects on the photosynthetic response of C3 plants to long-term CO2 enrichment

To assess the long-term effect of increased CO₂ and temperature on plants possessing the C₃ photosynthetic pathway, Chenopodium album plants were grown at one of three treatment conditions: (1) 23 °C mean day temperature and a mean ambient partial pressure of CO₂ equal to 350 bar; (2) 34 °C and 350 bar CO₂; and (3) 34 °C and 750 bar CO₂. No effect of the growth treatments was observed on the CO₂ reponse of photosynthesis, the temperature response of photosynthesis, the content of Ribulose-1,5-bisphosphate carboxylase (Rubisco), or the activity of whole chain electron transport when measurements were made under identical conditions. This indicated a lack of photosynthetic acclimation in C. album to the range of temperature and CO₂ used in the growth treatments. Plants from every treatment exhibited similar interactions between temperature and CO₂ on photosynthetic activity. At low CO₂ (< 300 bar), an increase in temperature from 25 to 35 °C was inhibitory for photosynthesis, while at elevated CO₂ (> 400 bar), the same increase in temperature enhanced photosynthesis by up to 40%. In turn, the stimulation of photosynthesis by CO₂ enrichment increased as temperature increased. Rubisco capacity was the primary limitation on photosynthetic activity at low CO₂ (195 bar). As a consequence, the temperature response of A was relatively flat, reflecting a low temperature response of Rubisco at CO₂ levels below its k_m for CO₂. At elevated CO₂ (750 bar), the temperature response of electron transport appeared to control the temperature dependency of photosynthesis above 18 °C. These results indicate that increasing CO₂ and temperature could substantially enhance the carbon gain potential in tropical and subtropical habitats, unless feedbacks at the whole plant or ecosystem level limit the long-term response of photosynthesis to an increase in CO₂ and temperature.Abbreviations A net CO₂ assimilation rate - C _a ambient partial pressure of CO₂ - C _i intercellular partial pressure of CO₂ - Rubisco Ribulose-1,5-bisphosphate carboxylase - VPD vapor pressure difference between leaf and air     

Elevated atmospheric partial pressure of carbon dioxide and dry matter production of konjak (Amorphophallus konjac K. Koch)

Konjak (Amorphophallus konjac K. Koch) was grown under normal (350 bar) or enriched (700 bar) CO₂ partial pressure in glasshouses kept at 33/26 °C. Doubling the CO₂ partial pressure resulted in twice the yield of corm because the net CO₂ assimilation rate doubled and, due to the simple source-sink relationship, the increased production was partitioned to the corm. The response to CO₂ of assimilation by konjak is discussed in relation to its original habitat in the tropics.     

The kinetics of ribulose-1,5-bisphosphate carboxylase/oxygenase in vivo inferred from measurements of photosynthesis in leaves of transgenic tobacco

Transgenic tobacco (Nicotiana tabacum L. cv. W38) with an antisense gene directed against the mRNA of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit was used to determine the kinetic properties of Rubisco in vivo. The leaves of these plants contained only 34% as much Rubisco as those of the wild type, but other photosynthetic components were not significantly affected. Consequently, the rate of CO₂ assimilation by the antisense plants was limited by Rubisco activity over a wide range of CO₂ partial pressures. Unlike in the wild-type leaves, where the rate of regeneration of ribulose bisphosphate limited CO₂ assimilation at intercellular partial pressures above 400 ubar, photosynthesis in the leaves of the antisense plants responded hyperbolically to CO₂, allowing the kinetic parameters of Rubisco in vivo to be inferred. We calculated a maximal catalytic turnover rate, k_cat, of 3.5+0.2 mol CO₂·(mol sites)^–1·s^–1 at 25° C in vivo. By comparison, we measured a value of 2.9 mol CO₂·(mol sites)^–1·^–1 in vitro with leaf extracts. To estimate the Michaelis-Menten constants for CO₂ and O₂, the rate of CO₂ assimilation was measured at 25° C at different intercellular partial pressures of CO₂ and O₂. These measurements were combined with carbon-isotope analysis (¹³C/¹²C) of CO₂ in the air passing over the leaf to estimate the conductance for transfer of CO₂ from the substomatal cavities to the sites of carboxylation (0.3 mol·m^–2·s^–1·bar^–1) and thus the partial pressure of CO₂ at the sites of carboxylation. The calculated Michaelis-Menten constants for CO₂ and O₂ were 259 ±57 bar (8.6±1.9M) and 179 mbar (226 M), respectively, and the effective Michaelis-Menten constant for CO₂ in 200 mbar O₂ was 549 bar (18.3 M). From measurements of the photocompensation point (_* = 38.6 ubar) we estimated Rubisco's relative specificity for CO₂, as opposed to O₂ to be 97.5 in vivo. These values were dependent on the size of the estimated CO₂-transfer conductance.Abbreviations and Symbols A CO₂-assimilation rate - g_w conductance for CO₂ transfer from the substomatal cavities to the sites of carboxylation - K_c, K_o Michaelis-Menten constants for carboxylation, oxygenation of Rubisco - k_cat V_cmax/[active site] - O partial pressure of O₂ at the site of carboxylation - p_c partial pressure of CO₂ at the site of carboxylation - p_i intercellular CO₂ partial pressure - R_d day respiration (non-photorespiratory CO₂ evolution) - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - S_c/o relative specificity factor for Rubisco - SSu small subunit of Rubisco - V_cmax, V_omax maximum rates of Rubisco carboxylation, oxygenation - _* partial pressure of CO₂ in the chloroplast at which photorespiratory CO₂ evolution equals the rate of carboxylation     

Influence of differences in leaf anatomy on net photosynthetic rates of some cultivars of Cassava

Gas exchange measurements and leaf anatomy of 10 cassava cultivars were conducted to study the interrelationship between the relatively high photosynthetic rates and the factors limiting internal CO₂ diffusion. The internal mesophyll surface area per unit leaf surface area (A^mes/A) and the intracellular components of CO₂ diffusion and fixation resistance (R^cellCO₂) were determined. Among the group of cultivars tested net CO₂ exchange rates were 26±2.5 mol CO₂ m^–2 s^–1 in normal air and intense light and A^mes/A ranged from 14 to 38. Estimated R^cellCO₂ ranged from 4300 to 13,000 s m^–1. The combined and compensating effects of A^mes/A and R^cellCO₂ accounted for both the high net photosynthetic rates (P_n) and the lack of large differences in P_n among cultivars.     

Production and diurnal utilization of assimilates in leaves of spinach (Spinacia oleracea L.) and barley (Hordeum vulgare L.)

Rates of CO₂ fixation during the light period and the rates of CO₂ release during the night period were measured using mature leaves from 39- to 49-d-old spinach (Spinacia oleracea L., US Hybrid 424; grown in 9 h light, 15 h darkness, daily) and mature leaves from 21-d-old barley (Hordeum vulgare L., cv. Apex; grown in 14 h light, 10 h darkness, daily). At certain times during the light and dark periods leaves were harvested for assay of their contents of soluble carbohydrates, starch, malate and the various amino acids. Evaluation of the results of these measurements shows that in spinach and barley leaves 46% and 26%, respectively, of the carbon assimilated during the light period is deposited in the leaves for export during the night period. Taking into account the carbon consumption in the source leaves by dark respiration, it is evaluated that rates of assimilate export during the light period from spinach and barley leaves [38 and 42 atom C · (mg Chl)^–1 · h^–1] are reduced in the dark period to 16 atom C · (mg Chl)^–1 · h^–1 in both species. The calculated C/N ratios of the photoassimilates exported during the dark period were 0.029 and 0.015 for spinach and barley leaves, respectively.This work was supported by the Deutsche Forschungsgemeinschaft. We thank Dr. Dieter Heineke for stimulating discussions and Mrs. Petra Hoferichter and Mrs. Marita Feldkämper for their technical assistance.     

Photosynthesis in enzymatically isolated leaf cells from the CAM plant Sedum telephium L.

A technique has been developed for the enzymatic isolation of leaf cells from the Crassulacean acid-metabolism plant Sedum telephium. The cells exhibited high activity in both ¹⁴CO₂ incorporation (30–70 mol CO₂ mg^-1 chlorophyll h^-1) and O₂ evolution in the presence of bicarbonate (60–110 mol O₂ mg^-1 chlorophyll h^-1). Half-maximum saturation of ¹⁴CO₂ incorporation occurred at a bicarbonate concentration of ca. 2 mM (20 M CO₂) at pH 8.4 and 30°C. Two types of light-dependent O₂ evolution are reported: O₂ evolution in the absence of exogenously supplied bicarbonate (endogenous O₂ evolution), and bicarbonate-stimulated O₂ evolution. Oxygen evolution in the presence of approximately ambient concentrations of CO₂ appeared to be a combination of the endogenous O₂ evolution and O₂ evolution from fixation of the exogenously supplied CO₂.Abbreviations CAM Crassulacean acid metabolism - cirlo chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PEP phosphoenolpyruvate - RuDP ribulose-1,5-diphosphate     

Electrostatic surface properties of plasmalemma vesicles from oat and wheat roots. Ion binding and screening investigated by 9-aminoacridine fluorescence

Right-side-out and sealed plasmalemma vesicles were isolated from roots of spring wheat (Triticum aestivum L. cv. Drabant) and oat (Avena sativa L. cv. Brighton) by two-phase partition in a medium containing sucrose (0.25 mol l^-1). Oat root plasmalemma vesicles were discovered to contain a strongly fluorescent compound with an emission maximum at 418 nm. The surface potential of the membranes was monitored by 9-aminoacridine fluorescence and the effect of protein concentration, mannitol versus sucrose, absence of osmoticum, concentrations of salt, and titrations with chelators investigated. It is concluded that i) protein concentrations of less than 50 g ml^-1 for oat and 100 g ml^-1 for wheat plasmalemma vesicles should be used to avoid serious problems with non-linearity of response of 9-aminoacridine fluorescence, ii) mannitol can be used instead of sucrose as the osmoticum, iii) the vesicles were ruptured in the absence of osmoticum allowing us to monitor both sides of the membranes, iv) plasmalemma vesicles from oat roots are more negative than vesicles from wheat roots, and v) oat and wheat root plasmalemma vesicles are isolated with about the same amounts of bound Ca²⁺ and Mg²⁺. These bound divalent cations may not, however, reflect the in-vivo conditions since the tissues were homogenised in the presence of ethylenediaminetetraacetic acid.Abbreviations EDTA ethylenediaminetetraacetic acid - EGTA ethylene glycol-bis(-aminoethyl ether)-N,N,N,N-tetraacetic acid - c1/2 value concentration at which half of the maximum effect is observed - Mops 3-(N-morpholino)propanesulfonic acid     

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.     

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

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

Carbon isotope ratios in ear parts of triticale : influence of grain filling

Four triticale (×Triticosecale Wittmack) genotypes were grown under rainfed conditions with limited irrigation support in Lleida in northeast Spain. For each variety, samples consisting of 10 tillers with half-sterilized spikes were taken three times from anthesis to maturity. Carbon isotope ratios (δ¹³C) were then determined in water extracts from ear bracts (glumes, paleas, and lemmas), awns and flag leaves, and in powdered kernels. For the half-sterilized spikes, carbon isotope analysis was carried out separately in bracts and awns from fertile and nonfertile spikelets. The δ¹³C in the water-soluble fraction of awns, glumes, and glumells from fruitless spikelets was significantly higher than that from fertile spikelets sampled at mid-grain filling. Differences in δ¹³C among sterile and fertile spikelets were not significant in samples taken a few days after anthesis or at maturity. These results are in accordance with some degree of refixation by awns and ear bracts of the CO₂ respired by grains during grain filling. There was progressively higher δ¹³C from flag leaf blades to awns, glumes, and glumells. This variation in δ¹³C along plant parts may be caused by differences in the ratio of assimilation rate to CO₂-diffusive conductance. Values of δ¹³C of mature kernels were between the values at anthesis and mid-grain filling for the water-soluble fraction of flag leaves and inner bracts and were fairly similar to those of glumes and awns.     

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     

Ear photosynthesis, carbon isotope discrimination and the contribution of respiratory CO2 to differences in grain mass in durum wheat

The role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidum var durum L.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO₂ diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (R_d) was greater than the net photosynthesis rate (P_n) by a factor of 1.74 in the spike, whereas R_d was much smaller, only 22.1, 65.7 and 24.8% of P_n in blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO₂ exchange. Results of ¹³C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high R_d values of ears as well as their low diffusive conductance suggest that CO₂ from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO₂ as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear-darkened plants, probably because of some degree of refixation (recycling) of respired CO₂ in the grains.     

A light-enhanced metabolism of sulfite in cells of Cucumis sativus L. cotyledons

The effects of light and several photosynthetic inhibitors on the rate of sulfite metabolism in cells obtained from Cucumis sativus L. cotyledons was studied. The cells were treated with 200 M Na₂SO₃ and the disappearance of sulfite was monitored using either dithiobisnitrobenzoic acid or fuchsin. The rate of sulfite disappearance in light was double the dark rate. Disalicylidene propanediamine at 1 mM increased this light-enhanced metabolism approx. 50%; neither 1 M 3,4-dichlorophenyl-N,N-dimethylurea nor 0.1 mM cyanazine, which completely inhibited CO₂-dependent oxygen evolution, affected the rate of sulfite metabolism. Addition of 200 M Na₂SO₃ to the cells partially inhibited ¹⁴CO₂ fixation. The rate of sulfite consumption by the cells did not affect this inhibition. We conclude that light-dependent sulfite metabolism is cucumber cells may utilize reduced ferredoxin generated as a result of photosynthetic electron transport. An injurious interaction between CO₂ fixation and sulfite appears to occur independently of the sulfite-metabolism process.Abbreviations DCMU 3,4-dichlorophenyl-N,N-dimethylurea - DSPD disalicylidene propanediamine - DTNB 5,5-dithiobis-(2-nitrobenzoic acid)     

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