Rates Of Photorespiration Research Articles

Adaptation of tobacco plants to elevated CO2: influence of leaf age on changes in physiology, redox states and NADP-malate dehydrogenase activity

Transgenic tobacco plants (Nicotiana tabacum L. cv. Xanthi) with altered chloroplast NADP-malate dehydrogenase (NADP-MDH) content were grown under ambient or under doubled atmospheric CO2 in order to analyse the effect of elevated CO2 on the redox state of the chloroplasts. Since large differences exist between the individual leaves of tobacco plants, gas exchange characteristics, enzyme capacities and metabolite contents were measured separately for each leaf of the plants. Large variations between leaves of different age were found in nearly every parameter analysed, and the differences between younger and older leaves were, in most cases, larger than the differences between comparable leaves at ambient or elevated CO2. For all parameters (chlorophyll fluorescence, P700 reduction, NADP-MDH activation) that are indicative for the redox situation in the electron transport chains and in the chloroplast stroma, more oxidized values were determined under elevated CO2. The increased redox state of ferredoxin, observed at ambient conditions in the NADP-MDH-under-expressing plants, disappeared under elevated CO2. It was concluded that the reduced rate of photorespiration under elevated CO2 decreases the amount of excess electrons. Interestingly, this lowered not only the activation state of NADP-MDH, but also the expression of the enzyme in the wild-type plants. The results are discussed with respect to a possible interaction between stromal reduction state and gene expression.

A Mutant of Chlamydomonas reinhardtii with Reduced Rate of Photorespiration

Photorespiration rates under air-equilibrated condi- tions (0.04% CO2 and 21% O2) were measured in Chla- mydomonas reinhardtii wild-type 2137, a phosphoglyco- late-phosphatase-deficient (pgpl) mutant and a suppressor double mutant (7FR2N) derived from the pgpl mutant. In both cells grown under 5% CO2 and adapted air for 24 h in the suppressor double mutant, the maximal rate of pho- torespiration (phosphoglycolate synthesis) was only about half of that in either the wild type or the pgpl mutant (18-7F) cells. In the progeny, the reduced rate of pho- torespiration was accompanied by increased photosynthet- ic affinity for inorganic carbon and the capacity for growth under air whether accompanied by the pgpl background or not. Tetrad analyses suggested that these three character- istics all resulted from a nuclear single-gene mutation at a site unlinked to the pgpl mutation. The decrease in pho- torespiration was, however, not due to an increase in the CO2/O2 relative specificity of ribulose-l,5- bisphosphate car- boxylase/oxygenase of 7FR2N or of any other suppressor double mutants tested. The relationship between the de- crease in the rate of photorespirat ion and the CO2-con- centrating mechanism is discussed.

Uptake and utilization of atmospheric ammonia in three native Poaceae species: leaf conductances, composition of apoplastic solution and interactions with root nitrogen supply

The physiological potential for acquisition of atmospheric ammonia (NH3) was investigated in three European meadow grasses (Arrhenatherum elatius, Bromus erectus and Brachypodium pinnatum) competing in chalk grasslands. Experiments were carried out with plants cultivated for about three months on a soil–sand mixture at high root nitrogen supply or in nutrient solutions at low root nitrogen supply. Two different root nitrogen regimes were applied to the solution‐grown plants: 130 μmol NO3 plant−1 wk−1 (approx. 50 kg nitrogen ha−1 in three months); or 130 μmol NO3− plus 130 μmol NH4+ plant−1 wk−1. Each regime was combined with two levels of NH3 fumigation (0 and 70 nmol mol−1 air for 24 d). Uptake of gaseous NH3 in the shoots was investigated under controlled environmental conditions including NH3 concentrations ranging from 0 to 30 nmol mol−1 air. Concurrently, photosynthesis, glutamine synthetase activity, nitrogen allocation, biomass allocation and apoplastic cation composition were measured. For A. elatius, the influence of photorespiration on NH3 acquisition was also assessed. Independently of plant nitrogen status, ammonia compensation points in A. elatius and B. erectus plants were <0.5 nmol mol−1. The total leaf conductance to NH3 absorption remained constant at increasing NH3 concentrations, showing that the capacity for assimilation was unaltered. Whereas internal factors in the leaves did not cause differences in the potential for NH3 acquisition between the species, other factors of NH3 acquisition were quite different: B. erectus had higher stomatal conductance and, thus, higher NH3 uptake rates per unit leaf area compared to A. elatius and B. pinnatum; higher stomatal conductances of B. erectus were to a large extent offset by a lower leaf area per plant, resulting from a lower growth rate and thicker leaves than in the two other species. The rate of photorespiration in Arrhenatherum constituted at least 15% of the net photosynthetic rate. Surprisingly, suppression of photorespiration indicated that NH3 uptake was supported by photorespiration. Bromus responded to fumigation with 70 nmol NH3 mol−1 air for 24 d by lowering the root∶shoot ratio and increasing the nitrogen concentration in the stem dry matter. The total leaf conductance to NH3 uptake decreased in all three species upon exposure to NH3, while the stomatal conductance was unaffected. The NH3 exposure caused lower apoplastic concentrations of H+, Mg2+ and Ca2+ in A. elatius and B. erectus.

Estimation of photorespiratory carbon dioxide recycling during photosynthesis

Photorespiration rate can be estimated in vivo but the actual amount of photorespiratory CO2 emitted or recycled by the leaf is largely unknown. We exploited the insensitivity of infrared gas analy-zers to 13CO2 to detect the photorespiratory CO2 emission of leaves exposed to air containing only 13CO2. Photorespiratory CO2 emission was calculated by subtracting the 12CO2 emitted under non-photorespiratory (2% O2) conditions from that emitted under photorespiratory (21% O2) conditions. Illuminated leaves of herbaceous and tree species emitted a constant amount of 12CO2 within 30–60 s after switching to 13CO2. Photorespiratory CO2 emission was less than 20% of the photorespiratory rate simultaneously calculated by combining fluorescence and gas exchange. Thus, the leaves recycled more than 80% of photorespiratory CO2. The highest recycling was associated with high rates of photosynthesis in tomato and spinach. In the dark, mitochondrial respiration measured by the emission of 12CO2 in air with 13CO2 was similar to that measured by conventional gas exchange in ambient air, thus confirming the accuracy of our method. In the light, mitochondrial respiration estimated from the emission of 12CO2 after complete labeling of photorespiration was lower than in the dark, suggesting either light-inhibition or recycling of respiratory carbon.

Light use in relation to carbon gain in the mangrove, Avicennia marina, under hypersaline conditions

Photosynthesis was studied in relation to light use in the mangrove, Avicennia marina (Forsk.) Vierh. var. australasica (Walp.) Moldenke, growing under soil salinities equivalent to one and two times seawater (i.e. 35 and 60‰). Midday CO2 assimilation rates averaged 7.6 0.7 and 4.3 0.3 µmol m–2 s–1 at the seawater and hypersaline sites, respectively. Despite this difference, xanthophyll pool sizes per Chl and epoxidation states were similar at both sites. Non-photochemical quenching also indicated comparable energy dissipation from pigment beds. Electron transport rates calculated from fluorescence characteristics were also similar and exceeded the requirements to sustain measured assimilation rates. However, cell wall conductance was low in seawater plants (75 mmol m2 s–1 ) and declined to 40 mmol m–2 s–1 in hypersaline plants. This would cause CO2 concentrations in chloroplasts (Cc ) to be lower than expected from measurements of intercellular CO2 concentrations (Ci ). In seawater plants, Cc was estimated to be 144 µmol mol–1 when Ci was 245 mmol mol–1, while values for Cc and Ci in hypersaline plants were 78 and 212 mmol mol–1, respectively. Reductions in Cc would enhance rates of photorespiration relative to assimilation, with the higher photorespiratory rates being sufficient to account for apparent excess electron transport rates.

Expression of tobacco carbonic anhydrase in the C4 dicot flaveria bidentis leads to increased leakiness of the bundle sheath and a defective CO2-concentrating mechanism

Flaveria bidentis (L.) Kuntze, a C4 dicot, was genetically transformed with a construct encoding the mature form of tobacco (Nicotiana tabacum L.) carbonic anhydrase (CA) under the control of a strong constitutive promoter. Expression of the tobacco CA was detected in transformant whole-leaf and bundle-sheath cell (bsc) extracts by immunoblot analysis. Whole-leaf extracts from two CA-transformed lines demonstrated 10% to 50% more CA activity on a ribulose-1,5-bisphosphate carboxylase/oxygenase-site basis than the extracts from transformed, nonexpressing control plants, whereas 3 to 5 times more activity was measured in CA transformant bsc extracts. This increased CA activity resulted in plants with moderately reduced rates of CO2 assimilation (A) and an appreciable increase in C isotope discrimination compared with the controls. With increasing O2 concentrations up to 40% (v/v), a greater inhibition of A was found for transformants than for wild-type plants; however, the quantum yield of photosystem II did not differ appreciably between these two groups over the O2 levels tested. The quantum yield of photosystem II-to-A ratio suggested that at higher O2 concentrations, the transformants had increased rates of photorespiration. Thus, the expression of active tobacco CA in the cytosol of F. bidentis bsc and mesophyll cells perturbed the C4 CO2-concentrating mechanism by increasing the permeability of the bsc to inorganic C and, thereby, decreasing the availability of CO2 for photosynthetic assimilation by ribulose-1,5-bisphosphate carboxylase/oxygenase.

Enhanced carbon dioxide leads to a modified diurnal rhythm of nitrate reductase activity in older plants, and a large stimulation of nitrate reductase activity and higher levels of amino acids in young tobacco plants

Higher rates of nitrate assimilation are required to support faster growth in enhanced carbon dioxide. To investigate how this is achieved, tobacco plants were grown on high nitrate and high light in ambient and enhanced (700 μmol mol–1) carbon dioxide. Surprisingly, enhanced carbon dioxide did not increase leaf nitrate reductase (NR) activity in the middle of the photoperiod. Possible reasons for this anomalous result were investigated. (a) Measurements of biomass, nitrate, amino acids and glutamine in plants fertilized once and twice daily with 12 mol m–3 nitrate showed that enhanced carbon dioxide did not lead to a nitrate limitation in these plants. (b) Enhanced carbon dioxide modified the diurnal regulation of NR activity in source leaves. The transcript for nia declined during the light period in a similar manner in ambient and enhanced carbon dioxide. The decline of the transcript correlated with a decrease of nitrate in the leaf, and was temporarily reversed after re‐irrigating with nitrate in the second part of the photoperiod. The decline of the transcript was not correlated with changes of sugars or glutamine. NR activity and protein decline in the second part of the photoperiod, and NR is inactivated in the dark in ambient carbon dioxide. The decline of NR activity was smaller and dark inactivation was partially reversed in enhanced carbon dioxide, indicating that post‐transcriptional or post‐translational regulation of NR has been modified. The increased activation and stability of NR in enhanced carbon dioxide was correlated with higher sugars and lower glutamine in the leaves. (c) Enhanced carbon dioxide led to increased levels of the minor amino acids in leaves. (d) Enhanced carbon dioxide led to a large decrease of glycine and a small decrease of serine in leaves of mature plants. The glycine:serine ratio decreased in source leaves of older plants and seedlings. The consequences of a lower rate of photorespiration for the levels of glutamine and the regulation of nitrogen metabolism are discussed. (e) Enhanced carbon dioxide also modified the diurnal regulation of NR in roots. The nia transcript increased after nitrate fertilization in the early and the second part of the photoperiod. The response of the transcript was not accentuated in enhanced carbon dioxide. NR activity declined slightly during the photoperiod in ambient carbon dioxide, whereas it increased 2‐fold in enhanced carbon dioxide. The increase of root NR activity in enhanced carbon dioxide was preceded by a transient increase of sugars, and was followed by a decline of sugars, a faster decrease of nitrate than in ambient carbon dioxide, and an increase of nitrite in the roots. (f) To interpret the physiological significance of these changes in nitrate metabolism, they were compared with the current growth rate of the plants. (g) In 4–5‐week‐old plants, the current rate of growth was similar in ambient and enhanced carbon dioxide (≈ 0·4 g–1 d–1). Enhanced carbon dioxide only led to small changes of NR activity, nitrate decreased, and overall amino acids were not significantly increased. (h) Young seedlings had a high growth rate (0·5 g–1 d–1) in ambient carbon dioxide, that was increased by another 20% in enhanced carbon dioxide. Enhanced carbon dioxide led to larger increases of NR activity and NR activation, a 2–3‐fold increase of glutamine, a 50% increase of glutamate, and a 2–3‐fold increase in minor amino acids. It also led to a higher nitrate level. It is argued that enhanced carbon dioxide leads to a very effective stimulation of nitrate uptake, nitrate assimilation and amino acid synthesis in seedlings. This will play an important role in allowing faster growth rates in enhanced carbon dioxide at this stage.

Photosynthetic and respiratory characteristics of Cortaderia pilosa (D’Urv.) Hack., the dominant component of falkland islands

Summary Cortaderia pilosa (whitegrass) is a slow growing grass adapted to the cool and moist climate of the Falkland Islands. In well drained, sheltered areas it tends to produce tussock forms whereas on level and poorly drained soils it produces a lax growth habit. Both morphotypes have a similar N-nutrition requirement as well as toleration to waterlogging and anaerobiosis. However, the tussock morphotype appears to be inherently larger and more vigorous than the lax morphotype. The photosynthetic and respiratory characteristics in vivo were measured in leaves of both morphotypes: Fv/Fm = an indicator of maximal quantum yield of PSII photochemistry, net photosynthesis (Pn), photorespiration (PR), dark respiration (DR), the CO 2 compensation concentration(Γ), and content of chlorophylls and carotenoids. Both morphotypes had similar, low values of Fv/Fm in the range 0.64–0.65. The rates of Pn and PR and content of photosynthetic pigments were markedly higher in the tussock than the lax morphotype whereas rates of DR were by about 30% higher in the lax than the tussock morphotype. The ratio of DR/Pn in the tussock and lax morphotypes was 0.14 and 0.27, respectively. Increased DR in the lax morphotype may be a protective mechanism against stress by which respiratory-derived ATP and reductant are provided to continue cell metabolism at a reduced level of photosynthesis. It is concluded that the better growth of tussock than lax morphotype of Cortaderia pilosa can be attributed to its higher rates of net photosynthesis, higher content of photosynthetic pigments and to lower rates of dark respiration.

Enhanced ozone-tolerance in wheat grown at an elevated CO2 concentration: ozone exclusion and detoxification.

Elevated [CO2 ] has been shown to protect photosynthesis and growth of wheat against moderately elevated [O3 ]. To investigate the role of ozone exclusion and detoxification in this protection, spring wheat (Triticum aestivum L. ev. Wembley) was grown from seed, in controlled-environment chambers, under reciprocal combinations of [CO2 ] at 350 or 700 μmol mol-1 and [O3 ] peaking at < 5 or 60 nmol mol-1 , respectively. Cumulative ozone dose to the mesophyll and antioxidant status were determined throughout flag leaf development. Catalase activity correlated with rates of photorespiration and declined in response to elevated [CO2 ] and/or [O3 ]. Superoxide dismutase activity was not significantly affected by either condition. Neither ascorbate nor glutathione content was enhanced by elevated [CO2 ]. In wheat, at moderately elevated [O3 ], our results show that stomatal exclusion plays a major role in the protective effect of elevated [CO2 ] against O3 damage.

Photosynthesis and fluorescence quenching, and the mRNA levels of plastidic glutamine synthetase or of mitochondrial serine hydroxymethyltransferase (SHMT) in the leaves of the wild-type and of the SHMT-deficient stm mutant of Arabidopsis thaliana in relation to the rate of photorespiration.

The regulation by photorespiration of the transcript level corresponding to plastidic glutamine synthetase (GS-2) was investigated in the leaves of Arabidopsis thaliana (L.) Heynh.. Photorespiration was suppressed by growing the plants in an atmosphere containing 300 Pa CO2. Suppression of photorespiration was demonstrated by the ability of the conditionally lethal serine hydroxymethyltransferase (SHMT)-deficient stm mutant of A. thaliana to grown normally under these conditions. In contrast to previous studies with bean or pea that were performed at very high CO2 partial pressure (2-4 kPa; Edwards and Coruzzi, 1989, Plant Cell 1:241-248; Cock et al., 1991, Plant Mol Biol 17: 761-771), suppression of photorespiration during growth of A. thaliana in an atmosphere with 300 Pa CO2 had no effect of the leaf GS-2 transcript level. In the short term, neither suppression of photorespiration induced by the transfer of air-grown A. thaliana plants into a CO2-enriched atmosphere, nor an increase in the rate of photorespiration achieved by the transfer of high-CO2-grown A. thaliana plants into air resulted in a change in the GS-2 mRNA level. The absence of photorespiratory ammonium release in leaves of the stm mutant had no effect on the GS-2 transcript level. Overall, our data argue against a control by photorespiration of the A. thaliana leaf GS-2 mRNA pool. In contrast, regulation of the leaf SHMT mRNA level may involve a negative feedback effect of at least one metabolite derived from the glycine/serine conversion during photorespiration, as indicated by the overexpression of SHMT transcripts in the leaves of the stm mutant.

Control of photosynthesis in barley plants with reduced activities of glycine decarboxylase

A mutant (LaPr 87/30) of barley (Hordeum vulgare L.) deficient in glycine decarboxylase (GDC; EC 2.1.2.10) was crossed with wild-type plants to generate heterozygous plants with reduced GDC activities. Plants of the F2 generation were grown in air and analysed for reductions in GDC proteins and GDC activity. The leaves of heterozygous plants contained reduced amounts of H-protein, and when the content of H-protein was lower than 60% of the wild-type, the P-protein was also reduced. The contents of the other two proteins of the GDC complex, T-protein and L-protein were not affected. Glycine decarboxylase activities, measured as the decarboxylation of [1-14C]glycine by intact mitochondria released from protoplasts, were between 47% and 63% of the wild-type activity in heterozygous plants and between 86% and 100% in plants with normal contents of H-protein. The enzyme activity was linearly correlated with the relative content of H-protein. Plants with reduced GDC activities developed normally and did not show major pleiotropic effects. In air, the reduction in GDC activity had no effect on the leaf metabolite content or photosynthesis, but under conditions of enhanced photorespiration (low CO2 and high light), glycine accumulated and the rates of photosynthesis decreased compared to the wild-type. The accumulation of glycine did not lead to a depletion of amino donors or to the accumulation of glyoxylate. The lower rates of photosynthesis were probably caused by an impaired recycling of carbon in the photorespiratory pathway. It is concluded that GDC has no control over CO2 assimilation under normal growth conditions, but appreciable control by GDC becomes apparent under conditions leading to higher rates of photorespiration.

A study of photorespiratory ammonia production in the C4 plant Amaranthus edulis, using mutants with altered photosynthetic capacities

Previous studies have indicated that the rate of photorespiration in C4 plants is low or negligible. In this study, wild‐type and mutant leaves of the C4 plant Amaranthus edulis were treated with the glutamine synthetase inhibitor, phosphinothricin and the glycine decarboxylase inhibitor, aminoacetonitrile, at different concentrations of CO2. The time course of ammonia accumulation in leaves of the wild type was compared with a mutant lacking phosphoenolpyruvate carboxylase activity (EC 4.1.1.31), and with three different mutants that accumulated glycine. The increase in the concentration of ammonia in the leaves, stimulated by the treatments was used as a measurement of the rate of photorespiration in C4 plants.The application of glutamine and glycine maintained the rate of photorespiratory ammonia production for a longer period in the wild type, and increased the rate in a mutant lacking phosphoenolpyruvate carboxylase suggesting that there was a lack of amino donors in these plants.The calculated rate of photorespiration in Amaranthus edulis wild‐type leaves when the supply of amino donors was enough to maintain the photorespiratory nitrogen flow, accounted for approximately 6% of the total net photosynthetic CO2 assimilation rate. In a mutant lacking phosphoenolpyruvate carboxylase, however, this rate increased to 48%, when glutamine was fed to the leaf, a value higher than that found in some C3 plants. In mutants of Amaranthus edulis that accumulated glycine, the rate of photorespiration was reduced to 3% of the total net CO2 assimilation rate. The rate of ammonia produced during photorespiration was 60% of the total produced by all metabolic reactions in the leaves. The data suggests that photorespiration is an active process in C4 plants, which can play an important role in photosynthetic metabolism in these plants.

Distribution of Inorganic Carbon Among its Component Species in Cyanobacteria: Do Cyanobacteria in fact Actively Accumulate Inorganic Carbon?

A mathematical model is presented, which describes the distribution of inorganic carbon (Ci) between the species CaCO3, CaHCO3+, CO32−, HCO3−and CO2in the cytosol of a high CO2- requiring mutant of the cyanobacteriumSynechocystisPCC 6803, which lacks carboxysomes. The model assumes that entry and efflux of Ci occurs via a CO32−transport and diffusion of CO2. HCO3−is considered as impermeable. Intracellular Ci is distributed among the species according to the cytoplasmic pH and the calcium concentration. Former models considered entry of HCO3−, and not CO32−, and the intracellular interactions of calcium with CO32−and their implications for the Ci accumulating mechanism were not considered.The model predicts that in the presence of 10−3–10−2M calcium, at low and moderate external inorganic carbon concentrations, CaCO3is the main cytoplasmic Ci species, whereas in its absence HCO3−is the main Ci species. Due to sequestration of CO32−by calcium and low permeability of the cells to CO2, the CO2, concentration is low and no net photosynthesis is observed. Photosynthesis is predicted to occur only at external Ci concentrations higher than 1 mM, where the CO32−transport is saturated and the main source of Ci is diffusion of CO2. This converts the mutant to a CO2user with a high photosynthetic Km(CO2), unlike the wild type which utilizes external CO32−as a substrate for photosynthesis at low external concentration.The sequestration of CO32−by Ca2+efficiently reduced the Co32−and the CO2accumulation ratio in comparison with that of total Ci resulting in an inward CO2gradient and reduced CO32−gradient over a wide range of external Ci concentrations. This raises doubts as to whether the transport of Ci as such is an active transport.The simulation predicts that in cases where the affinity of the uptake mechanism for CO32−is higher than that measured for this mutant and the accumulation ratio for Ci is consequently higher, the predicted accumulation ratio for the transported species CO32−remains low. Taking the permeability of the cells to gases to be as low as previously measured, the simulation predicts that O2may be accumulated in the cytoplasm up to saturation at CO2saturated photosynthesis rates. This results in a high rate of photorespiration and competitive inhibition of CO2fixation.Entry of CO32−, accumulation of charged Ci species, Ci utilization and efflux of carbon stimulates ions movements required to compensate the charges of CO32−, HCO3−and CaHCO3+.The model predicts accumulation of cations up to 50 mEq × 1−1or exclusion of the same amount of anions required to electroneutralize the accumulation of charged Ci species in the absence of calcium and up to 28 mEq × liter−1in the presence of 20 mM calcium. The steady-state CO32−dependent flux of electroneutralizing ions was found to be equal to the steady-state rate of Ci utilization and efflux (photosynthesis, photorespiratory CO2evolution and CO2leakage) multiplied by the valency of CO32−. The interconversion of Ci species resulting from Ci utilization and dissipation of Ci involves H+liberation in the hydration reaction and the dissociation of HCO3−to CO32−or H+consumption in the conversion of CO32−to HCO3−and dehydration of the latter. The H+production and the CO32−dependent transport of positive ions are interrelated and are non-monotonously altered in anti-parallel fashion as a function of the external Ci concentration. The presence of calcium reduces the magnitude of both these fluxes.

セイヨウナシ葉の諸形質の変異および炭酸ガス拡散抵抗要因と光合成活性との関係

Factors related to variations of leaf photosynthetic activities in many pear cultivars were studied by using a rapid measuring system for leaf characteristics, leaf diffusive resistances, leaf apparent photosynthetic rates (Pn), and the respiratory factors.1. Vegetative shoots were sampled in early summer, dipped in water and light-saturated under an artificial source. The Pn values and diffusive resistance factors of the leaves attached to the middle of a shoot were measured by a portable measuring apparatus (Li-Cor Model 6200 and its modefication) for photosynthesis, transpiration, and the dark respiratin (Rd) and photorespiration rates (R1). Furthermore, the leaf area, leaf width, leaf specific weight, facial density of leaf chlorophyll, chromatic value of leaf surface, leaf optical properties, percent of leaf vein area and the 3 phases (liquid, solid, and gas) of leaf tissue, were measured rapidly by several portable apparatus which we bought or built.2. Higher coefficients of variation were found in chromatic values (a* and b*) of leaf adaxial face, transmittance and reflectance of PPFD of leaf tissue and leaf area, whereas lower coefficients were found in relative values of facial density of leaf chlorophyll (SPAD), leaf width, and percent of liquid phase in leaf tissue within the 94 cultivars. The principal component analysis using the 60 cultivars suggested that leaf color or amount of substance absorbing PPFD in leaf tissue, physical properties of leaf tissue, reflectance of adaxial face and leaf moisture was I, II, III and IV principal component, respectively.3. Coefficients of regression of the Pn values from multiple regression analysis using the 18 cultivars was about 90% when the diffusive resistance in the mesophyll (Rm) is included as one of the variables and about 60% when excluded.4. Our simple correlations, the results from the analysis using polynominal fitted lines for Pn using a simple variable and the results from the multiple regression analysis indicated that a higher activity of leaf photosynthesis appears in leaves with a dull somewhat rugose surface, a dark green color and watery tissue.

Seasonal changes in sugar beet photosynthesis as affected by exogenous cytokinin N&lt;sup&gt;6&lt;/sup&gt;-(m- hydroxybenzyl)adenosine

Foliar sprays with N 6-(m-hydroxybenzytyadenosine, (mOH)-[9R]BAP, one of the synthetic cytokinins, were applied to field-grown sugar beet twice during the vegetation period: before full canopy closing (I), 6 weeks before harvest (II) or both. Application of (mOH)[9R]BAP retained high cytokinin content that usually declines prior to harvest. The total content of isoprenoid cytokinins at harvest was 2.6-fold higher in (mOH)[9R]BAP-treated plants as compared to the controls. Treatment I had no significant effect on contents of chlorophylls (Chl)a andb and carotenoids, nor on the rates of net photosynthesis (PN) or photorespiration (Rl), or on CO2 compensation concentration (Γ) measured during the whole vegetation period on detached leaves under optimum environmental conditions. Increased values in PN, RL and Chla andb contents were found in variantsII and I+II linked with a delay in leaf senescence before harvest. Transpiration rate and stomatal conductances of adaxial and abaxial epidermes were not significantly affected by any treatment.

Photosynthetic characteristics of leaves of male‐sterile and hermaphrodite sex types of Plantago lanceolata grown under conditions of contrasting nitrogen and light availabilities

Plantago lanceolata is a gynodioecious species: In natural populations male steriles (MS) coexist with hermaphrodites (H). Since male steriles have a reproductive disadvantage, without any compensation for their loss in male function by an increase in female function, they are expected to disappear from the population. In this study we investigated the possibility that differences in ecologically important photosynthetic characteristics, between MS and H lines of P. lanceolata. play a role in maintaining gynodioecy. One MS line and two H lines were grown under conditions of high N and light availability, as well as under either N limitation or light limitation, to investigate whether the sex types respond differently to environmental constraints. Photosynthetic light‐response and CO2‐response curves were made, together with leaf organic N and chlorophyll determinations. There were only few small differences between the lines and since the MS line did not differ in any of the determined photosynthetic characteristics from either H line, it is unlikely that these differences are involved in maintaining male sterility in populations of P. lanceolata. The low‐light‐grown plants showed a high degree of acclimation as shown by a two‐fold higher leaf area to leaf weight ratio (SLA), a two‐fold higher investment of N in light harvesting, and higher net photosynthetic rates under low‐light conditions, as compared to the high‐light‐grown plants. The low‐N‐grown plants used their organic N more efficiently in photosynthesis compared to plants grown at an optimal N supply. This was mainly due to the N‐limited plants having leaves with a lower organic N content and thus lower photosynthetic capacities. To a lesser extent it was due to the higher value for the curvature factor of the light‐response curves of the N‐limited plants, to their decreased rates of photorespiration and possibly to their relatively higher allocation of organic N to photosynthetic functions.

Evidence for the Contribution of the Mehler-Peroxidase Reaction in Dissipating Excess Electrons in Drought-Stressed Wheat.

Gross O2 evolution and uptake by attached, drought-stressed leaves of wheat (Triticum aestivum) were measured using a 16O2/ 18O2 isotope technique and mass spectrometry. The activity of photosystem II, determined from the rate of 16O2 evolution, is only slightly affected under drought conditions. During drought stress, net CO2 uptake decreases due to stomatal closure, whereas the uptake of 18O2 is stimulated. The main O2-consuming reactions in the light are the Mehler-peroxidase (MP) reaction and the photorespiratory pathway. From measurements of the rate of carbon flux through the photorespiratory pathway, estimated by the analysis of the specific radioactivities of glycolate, we conclude that the rate of photorespiration is decreased with drought stress. Therefore, the O2 taken up in the light appears to be preferentially used by the MP reaction. In stressed leaves, 29.1% of the photosynthetic electrons are consumed in the MP reaction and 18.4% drive the photorespiratory pathway. Thus, overreduction of the electron transport chain is avoided preferably by the MP reaction when drought stress restricts CO2 reduction.

Estimation of diffusive resistance of bundle sheath cells to CO2 from modeling of C4 photosynthesis

Bundle sheath resistance to diffusion of CO2 (rc) is a critical component of C4 photosynthesis which allows accumulation of inorganic carbon in bundle sheath cells of C4 plants. Several analyses were made to evaluate the magnitude of rc in C4 plants. Experimental data on the O2 inhibition of photosynthesis (Dai et al. (1993) Plant Physiol 103: 83-90; (1995) Plant Physiol 107: 815-825) and rates of photorespiration (de Veau and Burris (1989) Plant Physiol 90: 500-511) in Z. mays at different stages of development were analyzed using mathematical models of C4 photosynthesis. In young and senescing leaves modeled values of rc and the CO2 partial pressure in bundle sheath cells (Cbs) were lower and fractional leakage of CO2 from bundle sheath cells (fL) was higher than in mature leaves. Diffusive resistance of bundle sheath cells of C4 plants was also evaluated by analyzing the response of photosynthetic rates to varying CO2 in Amaranthus edulis in which the C4 cycle was dysfunctional by chemical mutagenesis (Dever et al. (1995) J Exp Bot 46: 1363-1376) and in Sorghum bicolor, Panicum maximum and Panicum miliaceum in which the C4 cycle was chemically inhibited (Brown and Byrd (1993) Plant Physiol 103: 1183-1188). These analyses indicate that in mature leaves of C4 plants the values of rc are substantially lower (ca. 50-200 m(2) s mol(-1)) than previous suggested (ca. 500-1500 m(2) s mol(-1)) for C4 photosynthesis and that there is considerable leakage of CO2 from bundle sheath cells. Nevertheless, rc and Cbs values are sufficiently high in mature leaves to minimize photorespiration in C4 plants under normal levels of CO2.

Oxygen-dependent electron transport and protection from photoinhibition in leaves of tropical tree species.

The roles of photorespiration and the Mehlerperoxidase pathway in sustaining electron transport and protection from photoinhibition were studied in outer canopy leaves of two species of tropical trees: the drought-deciduous Pseudobombax septenatum (Jacq.) Dug. and the evergreen Ficus insipida Willd. Ficus had a higher photosynthetic capacity than Pseudobombax and also a greater capacity for light-dependent electron transport under photorespiratory conditions (in the absence of CO2). As a consequence, in the absence of CO2, Ficus was able to maintain a largely oxidized electron-transport chain at higher photon flux densities than Pseudobombax. Under the same light conditions, photoinhibition (reduction in Fv/Fm) was always greater in Pseudobombax than Ficus, was increased when leaves were exposed to 2% O2 in nitrogen compared to 21% O2 in CO2-free air, but was not increased by the absence of CO2. Rates of electron transport due to the Mehler-peroxidase pathway (assessed in 2% O2 in nitrogen) ranged between 16-40 μmol · m-2·s-1 in both species. As the dry season approached and Pseudobombax neared leaf senescence there was a decline in the capacity for photorespiratory flux to maintain electron transport in Pseudobombax, but not in Ficus. Ratios of light-dependent electron transport to net CO2 fixation for Pseudobombax, Ficus and two other species in the field, Luehea seemannii Tr. & Planch, and Didymopanax morototoni (Aubl.) Dec. & Planch., ranged from 6.2 (Ficus) to 16.7 (Pseudobombax). High in-situ rates of photorespiration combined with the decreased capacity of Pseudobombax for photorespiratory flux as the dry season approached indicates a decreased capacity to protect against photooxidative damage. This may contribute to the promotion of leaf senescence in Pseudobombax during the transition from wet to dry season.

Oxygen sensitivity of photosynthesis and photorespiration in different photosynthetic types in the genus Flaveria.

Two major indicators were used to access the degree of photorespiration in various photosynthetic types of Flaveria species (C3, C3-C4, C4-like, and C4): the O2 inhibition of photosynthesis measured above the O2 partial pressure which gives a maximum rate, and O2- and light-dependent whole-chain electron flow measured at the CO2 compensation point (Γ). The optimum level of O2 for maximum photosynthetic rates under atmospheric levels of CO2 (34 Pa) was lower in C3 and C3-C4 species (ca. 2 kPa) than in C4-like and C4 species (ca. 9 kPa). Increasing O2 partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of photosynthesis which was more severe under lower CO2. This inhibition was calculated as the O2 inhibition index (ΘA, the percentage inhibition of photosynthesis per kPa increase in O2). From measurements of 18 Flaveria species at atmospheric CO2, the ΘA values decreased from C3 (1.9-2.1) to C3-C4 (1.2-1.6), C4-like (0.6-0.8) and C4 species (0.3-0.4), indicating a progressive decrease in apparent photorespiration in this series. With increasing irradiance at Γ under atmospheric levels of O2, and increasing O2 partial pressure at 300 μmol quanta·m-2·s-1, there was a similar increase in the rate of O2 evolution associated with whole-chain electron flow (Jo2, calculated from chlorophyll fluorescence analysis) in the C3 and C3-C4 species compared to a much lower rate in the C4-like and C4 species. The results indicate that there is substantial O2-dependent electron flow in C3 and C3-C4 species, reflecting a high level of photorespiration compared to that in C4-like and C4 species. Consistent with these results, there was a significant decrease in Γ from C3 (6-6.2 Pa) to C3-C4 (1.0-3.0 Pa), to C4-like and C4 species (0.3-0.8 Pa), indicating a progressive decrease in apparent photorespiration. However, C3 and C3-C4 species examined had high intrinsic levels of photorespiration with the latter maintaining low apparent rates of photorespiration and lower Γ values, primarily by refixing photorespired CO2. The C4-like and C4 Flaveria species had low, but measurable, levels of photorespiration via selective localization of ribulose-1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO2 pump via the C4 pathway.