Stimulation of photosynthesis by 2% oxygen at low temperatures is restored by phosphate

The effect of phosphate feeding on the influence of low (2%) oxygen on photosynthetic carbon assimilation has been investigated in leaf discs of spinach (Spinacia oleracea L.) at 12°C. The following observations were made. First, after the transition from 20% O₂ to 2% O₂, the rate of CO₂ uptake was inhibited at CO₂ concentrations between about 250 and about 800 l CO₂·l^-1. Second, phosphate feeding stimulated the rate of CO₂ uptake in 20% O₂ at higher concentrations of CO₂ (500–900 l·l^-1). Third, phosphate feeding stimulated the rate of CO₂ uptake in 2% O₂ at all but the highest (900 l·l^-1) and lowest 74 (l·l^-1) concentrations of CO₂ employed. Phosphate thereby restored the stimulation of photosynthesis by 2% O₂ and it did so over a wide range of lower temperatures. Fourth, oscillatory behaviour, however generated, was dampened by phosphate feeding, even at very low concentrations of CO₂. Contents of leaf metabolites were measured during the transition to 2% O₂ in control and phosphate-fed leaf discs. During this period the ratio glycerate-3-phosphate/triose phosphate rose steeply, but fell again only in the phosphate-treated leaf discs. These data, taken together with measured ATP/ADP ratios, showed that assimilatory power, the ratio [ATP]·[NAD(P)H]/[ADP]·[Pi]·[NAD(P)], decreased when leaves were exposed to 2% O₂, but that this decrease was minimised by previous feeding of phosphate. The mechanism of phosphate limitation is discussed in the light of the results.Abbreviations C_i intercellular concentration of CO₂ - RuBP ribulose-1,5-bisphosphate     

Phosphate sequestration by glycerol and its effects on photosynthetic carbon assimilation by leaves

Glycerol induced a limitation on photosynthetic carbon assimilation by phosphate when supplied to leaves of barley (Hordeum vulgare L.) and spinach (Spinacia oleracea L.). This limitation by phosphate was evidenced by (i) reversibility of the inhibition of photosynthesis by glycerol by feeding orthophosphate (ii) a decrease in light-saturated rates of photosynthesis and saturation at a lower irradiance, (iii) the promotion of oscillations in photosynthetic CO₂ assimilation and in chlorophyll fluorescence, (iv) decreases in the pools of hexose monophosphates and triose phosphates and increases in the ratio of glycerate-3-phosphate to triose phosphate, (v) decreased photochemical quenching of chlorophyll fluorescence, and increased non-photochemical quenching, specifically of the component which relaxed rapidly, indicating that thylakoid energisation had increased. In barley there was a massive accumulation of glycerol-3-phosphate and an increase in the period of the oscillations, but in spinach the accumulation of glycerol-3-phosphate was comparatively slight. The mechanism(s) by which glycerol feeding affects photosynthetic carbon assimilation are discussed in the light of these results.Abbreviations Chl chlorophyll - C _i intercellular concentration of CO₂ - P phosphate - PGA glycerate-3-phosphate - Pi orthophosphate - triose-P sum of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate     

Ca2+ transport by opercular epithelium of the fresh water adapted euryhaline teleost,Fundulus heteroclitus

We examined transepithelial transport of Ca²⁺ across the isolated opercular epithelium of the euryhaline killifish adapted to fresh water. The opercular epithelium, mounted in vitro with saline on the serosal side and fresh water (0.1 mmol·l^–1 Ca²⁺) bathing the mucosal side, actively transported Ca²⁺ in the uptake direction; net flux averaged 20–30 nmol·cm^–2·h^–1. The rate of Ca²⁺ uptake varied linearly with the density of mitochondria-rich cells in the preparations. Ca²⁺ uptake was saturable, apparent K _1/2 of 0.348 mmol·l^–1, indicative of a multistep transcellular pathway. Ca²⁺ uptake was inhibited partially by apically added 0.1 mmol·l^–1 La³⁺ and 1.0 mmol·l^–1 Mg²⁺. Addition of dibutyryl-cyclic adenosine monophosphate (0.5 mmol·l^–1)+0.1 mmol·l^–1 3-isobutyl-l-methylxanthine inhibited Ca²⁺ uptake by 54%, but epinephrine, clonidine and isoproterenol were without effect. Agents that increase intracellular Ca²⁺, thapsigargin (1.0 mol·l^–1, serosal side), ionomycin (1.0 mol·l^–1, serosal side) and the calmodulin blocker trifluoperazine (50 mol·l^–1, mucosal side) all partially inhibited Ca²⁺ uptake. In contrast, apically added ionomycin increased mucosal to serosal unidirectional Ca²⁺ flux, indicating Ca²⁺ entry across the apical membrane is rate limiting in the transport. Verapamil (10–100 mol·l^–1, mucosal side), a Ca²⁺ channel blocker, had no effect. Results are consistent with a model of Ca²⁺ uptake by mitochondria rich cells that involves passive Ca²⁺ entry across the apical membrane via verapamil-insensitive Ca²⁺ channels, intracellular complexing of Ca²⁺ by calmodulin and basolateral exit via an active transport process. Increases in intracellular Ca²⁺ invoke a downregulation of transcellular Ca²⁺ transport, implicating Ca²⁺ as a homeostatic mediator of its own transport.Abbreviations DASPEI 2-(4-dimethylaminostyryl)-N-ethylpyridinium iodide - db-cAMP dibutyryl-cyclic adenosine monophosphate - FW fresh water - G _t transepithelial conductance - I _sc short-circuit current - IBMX 3-isobutyl-1-methylxanthine - SW sea water - TFP trifluoperazine - V _t transepithelial potential     

In vivo photosynthetic electron transport does not limit photosynthetic capacity in phosphate-deficient sunflower and maize leaves

The effects of extreme phosphate (Pi) deficiency during growth on the contents of adenylates and pyridine nucleotides and the in vivo photochemical activity of photosystem II (PSII) were determined in leaves of Helianthus annuus and Zea mays grown under controlled environmental conditions. Phosphate deficiency decreased the amounts of ATP and ADP per unit leaf area and the adenylate energy charge of leaves. The amounts of oxidized pyridine nucleotides per unit leaf area decreased with Pi deficiency, but not those of reduced pyridine nucleotides. This resulted in an increase in the ratio of reduced to oxidized pyridine nucleotides in Pi-deficient leaves. Analysis of chlorophyll a fluorescence at room temperature showed that Pi deficiency decreased the efficiency of excitation capture by open PSII reaction centres (φ_e), the in vivo quantum yield of PSII photochemistry (φ_PSII) and the photochemical quenching co-efficient (q_P), and increased the non-photochemical quenching co-efficient (q_N) indicating possible photoinhibitory damage to PSII. Supplying Pi to Pi-deficient sunflower leaves reversed the long-term effects of Pi-deficiency on PSII photochemistry. Feeding Pi-sufficient sunflower leaves with mannose or FCCP rapidly produced effects on chlorophyll a fluorescence similar to long-term Pi-deficiency. Our results suggest a direct role of Pi and photophosphorylation on PSII photochemistry in both long-and short-term responses of photosynthetic machinery to Pi deficiency. The relationship between φ_PSII and the apparent quantum yield of CO₂ assimilation determined at varying light intensity and 21 kPa O₂ and 35 Pa CO₂ partial pressures in the ambient air was linear in Pi-sufficient and Pi-deficient leaves of sunflower and maize. Calculations show that there was relatively more PSII activity per mole of CO₂ assimilated by the Pi-deficient leaves. This indicates that in these leaves a greater proportion of photosynthetic electrons transported across PSII was used for processes other than CO₂ reduction. Therefore, we conclude that in vivo photosynthetic electron transport through PSII did not limit photosynthesis in Pi-deficient leaves of sunflower and maize and that the decreased CO₂ assimilation was a consequence of a smaller ATP content and lower energy charge which restricted production of ribulose, 1-5, bisphosphate, the acceptor for CO₂.     

Regulation of mesophyll photosynthesis in intact wheat leaves by cytoplasmic phosphate concentrations

The effect of D-(+)-mannose, inorganic phosphate (Pi) and mannose-6-phosphate on net mesophyll CO₂ assimilation rate (A) and stomatal conductance (g_s) of wheat (Triticum aestivum L.) leaves was studied. The compounds were supplied through the transpiration stream of detached leaves from plants grown in sand in growth cabinets or glasshouses, with different concentrations of Pi (0.25, 1.0 and 4.0 mM) supplied during growth. In all cases, 10 mM D-(+)mannose caused 40–60% reduction of A within 30 min, though the time courses differed for flag leaves and the sixth leaf on the mainstem of glasshouse- and cabinet-grown plants. D-(+)Mannose had a similar effect on A in leaves having a fourfold range in total phosphate content. Effects of D-(+)mannose in reducing g_s were always slower than on A. When the CO₂ concentration in the leaf chamber was adjusted to maintain intercellular CO₂ concentration (C_i) constant as A declined after mannose supply, g_s still declined indicating that stomatal closure was not caused by changing C_i. Supplying mannose-6-phosphate at 10 and 1 mM and Pi at 5 and 10 mM concentrations caused rapid reductions in g_s and also direct reductions in A. The observed effects of mannose and Pi on assimilation are consistent with the proposed regulatory role of cytoplasmic Pi in determining mesophyll carbon assimilation that has been derived previously using leaf discs, protoplasts and chloroplasts.Abbreviations and symbols A net mesophyll CO₂-assimilation rate - C_a, C_i external (assimilation-chamber) and intercellular CO₂ concentration, respectively - g_s stomatal conductance - Man6P mannose-6-phosphate - Pi orthophosphate     

The roles of malate and aspartate in C4 photosynthetic metabolism of Flaveria bidentis (L.)

In C₄ grasses belonging to the NADP-malic enzyme-type subgroup, malate is considered to be the predominant C₄ acid metabolized during C₄ photosynthesis, and the bundle sheath cell chloroplasts contain very little photosystem-II (PSII) activity. The present studies showed that Flaveria bidentis (L.), an NADP-malic enzyme-type C₄ dicotyledon, had substantial PSII activity in bundle sheath cells and that malate and aspartate apparently contributed about equally to the transfer of CO₂ to bundle sheath cells. Preparations of bundle sheath cells and chloroplasts isolated from these cells evolved O₂ at rates between 1.5 and 2 mol · min^–1 · mg^–1 chlorophyll (Chl) in the light in response to adding either 3-phosphoglycerate plus HCO ₃ ^– or aspartate plus 2-oxoglutarate. Rates of more than 2 mol O₂ · min^–1 · mg^–1 Chl were recorded for cells provided with both sets of these substrates. With bundle sheath cell preparations the maximum rates of light-dependent CO₂ fixation and malate decarboxylation to pyruvate recorded were about 1.7 mol · min^–1 · mg^–1 Chl. Compared with NADP-malic enzyme-type grass species, F. bidentis bundle sheath cells contained much higher activities of NADP-malate dehydrogenase and of aspartate and alanine aminotransferases. Time-course and pulse-chase studies following the kinetics of radiolabelling of the C-4 carboxyl of C₄ acids from ¹⁴CO₂ indicated that the photosynthetically active pool of malate was about twice the size of the aspartate pool. However, there was strong evidence for a rapid flux of carbon through both these pools. Possible routes of aspartate metabolism and the relationship between this metabolism and PSII activity in bundle sheath cells are considered.Abbreviations DHAP dihydroxyacetone phosphate - NADP-ME(-type) NADP-malic enzyme (type) - NADP-MDH NADP-malate dehydrogenase - OAA oxaloacetic acid - 2-OG 2-oxoglutarate - PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - Pi orthophosphate - Ru5P ribulose 5-phosphate     

Phosphite accelerates programmed cell death in phosphate-starved oilseed rape (<Emphasis Type="Italic">Brassica napus</Emphasis>) suspension cell cultures

Phosphite (H₂PO₃^–, Phi) prevents the acclimation of plants and yeast to orthophosphate (Pi, HPO₄^2–) deprivation by specifically obstructing the derepression of genes encoding proteins characteristic of their Pi-starvation response. In this study, we report that prolonged (i.e., 3–4 weeks) culture of Brassica napus L. suspension cells in Pi-deficient (–Pi) media leads to programmed cell death (PCD). However, when the B. napus cells were subcultured into –Pi media containing 2 mM Phi, they initiated PCD within 5 days, with 95% cell death observed by day 9. Dying cells exhibited several morphological and biochemical features characteristic of PCD, including protoplast shrinkage, chromatin condensation, and fragmentation of nuclear DNA. Immunoblotting indicated that B. napus cells undergoing PCD upregulated a 30-kDa cysteine endoprotease that is induced during PCD in the inner integument cells of developing B. napus seeds. It is concluded that PCD in B. napus suspension cells is triggered by extended Pi starvation, and that Phi treatment greatly accelerates this process. Our results also infer that the adaptive value of acclimating at the molecular level to Pi-stress is to extend the viability of –Pi B. napus cell cultures by about 3 weeks.Abbreviations APase acid phosphatase (EC 3.1.3.2) - BnCysP B. napus cysteine proteinase - DAPI 4,6-diamidino-2-phenylindole - FDA fluorescein diacetate - PCD programmed cell death - Phi phosphite - +Pi and –Pi Pi-sufficient and -deficient, respectively - PI propidium iodide - PSI Pi-starvation inducible     

Enzyme activities in an artificial stroma medium

When spinach leaf tissue was subjected to evaporative dehydration, photosynthetic capacity at very high (5%) CO₂ concentration and saturating irradiance (300 W·m^-2), decreased in parallel to the relative water content (RWC). A 50% inhibition was observed at 60–40% RWC. In order to examine whether the inhibition was caused by increased solute concentrations in chloroplasts or cytoplasm, an artificial stroma medium (ASM) was set up containing all major osmotically relevant solutes measured in isolated intact spinach chloroplasts. Subsequently, the response of enzyme activities to normal and to increased concentrations of ASM was examined. Inhibition of enzymes by a concerted increase of all solutes was well correlated to the in-vivo response of photosynthesis to dehydration (60% inhibition at double-strength ASM). Inhibitory solutes were mainly divalent inorganic anions, such as sulfate and phosphate. Inhibition of ribulose-1,5-bisphosphate carboxylase by these ions as studied in more detail. Inhibition of the enzyme by sulfate and phosphate was competitive with respect to ribulose-1,5-bisphosphate, but not with respect to CO₂. The K_I for sulfate was 2.1 mmol·l^-1 and for phosphate 0.57 mmol·l^-1. Sugars and amino acids at the concentrations found in spinach chloroplasts did not prevent inhibition of enzymes by anions. The results indicate that increased anion concentrations in cells and organelles are responsible for primary, quickly reversible effects of moderate dehydration on plant tissues.Abbreviations ASM artificial stroma medium - RuBP ribulose 1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate-carboxylase/oxygenase - RWC relative water content     

Induction of a high-capacity nitrate-uptake mechanism in barley roots prompted by nitrate uptake through a constitutive low-capacity mechanism

Roots of nitrate-starved and nitrate-pretreated seedlings of Hordeum vulgare were used to investigate the induction of a high-capacity uptake mechanism for nitrate. When exposed to 0.2 mmol·l^-1KNO₃, nitrate-starved roots took up nitrate at a rate of approx. 1 mol·(g FW)^-1·h^-1; K⁺ was absorbed at a rate ten-times higher. Nitrate uptake accelerated after a lag of about 1 h, until it matched the rate of K⁺ uptake about 4 h later. p-Fluorophenylalanine (FPA), which prevents the synthesis of functioning proteins, suppressed the development of the high-capacity mechanism. Pretreatment of the roots with 0.2 mmol·l^-1 Ca(NO₃)₂ for 24 h established the high-capacity mechanism. Pretreated roots were able to absorb nitrate at high rates immediately upon exposure to 0.2 mmol·l^-1KNO₃, in the absence or presence of FPA. The high-capacity mechanism, once established, appeared to have a protein turnover as slow as that of the low-capacity mechanism or that of the mechanism involved in the uptake of K⁺. In contrast, the mechanisms for the transport of nitrate and K⁺ into the xylem vessels were completely blocked by FPA within 1 h of application, confirming earlier evidence for a rapid turnover of the transport proteins in the xylem parenchyma.Nitrate reduction proceeded at rates which were roughly one-tenth as large as the rates of the respective nitrate-uptake processes, indicating that nitrate-reductase activity was determined by the rate of nitrate uptake and not vice versa.We conclude that the formation of a high-capacity nitrate-uptake mechanism in barley roots occurs in response to nitrate uptake through a constitutive mechanism of low capacity which appears to function as a sensing mechanism for nitrate in the environment of the roots.Abbreviation FPA p-fluorophenylalanine     

Growth,nutrient dynamics,and efficiency responses to carbon dioxide and phosphorus nutrition in soybean

Plant mineral nutrients such as phosphorus may exert major control on crop responses to the rising atmospheric carbon dioxide (CO₂) concentrations. To evaluate the growth, nutrient dynamics, and efficiency responses to CO₂ and phosphorus nutrition, soybean (Glycine max (L.) Merr.) was grown in controlled environment growth chambers with sufficient (0.50 mM) and deficient (0.10 and 0.01 mM) phosphate (Pi) supply under ambient and elevated CO₂ (aCO₂, 400 and eCO₂, 800 µmol mol^?1, respectively). The CO₂ × Pi interaction was detected for leaf area, leaf and stem dry weight, and total plant biomass. The severe decrease in plant biomass in Pi-deficient plants (10–76%) was associated with reduced leaf area and photosynthesis (P_net). The degree of growth stimulation (0–55% total biomass) by eCO₂ was dependent upon the severity of Pi deficiency and was closely associated with the increased phosphorus utilization efficiency. With the exception of leaf and root biomass, Pi deficiency decreased the biomass partitioning to other plant organs with the maximum decrease observed in seed weight (8–42%) across CO₂ levels. The increased tissue nitrogen (N) concentration in Pi-deficient plants was accredited to the lower biomass and increased nutrient uptake due to the larger root to shoot ratio. The tissue P and N concentration tended to be lower at eCO₂ versus aCO₂ and did not appear to be the main cause of the lack of CO₂ response of growth and P_net under severe Pi deficiency. The leaf N/P ratio of >16 was detrimental to soybean growth. The tissue P concentration needed to attain the maximum productivity for biomass and seed yield tended to be higher at eCO₂ versus aCO₂. Therefore, the eCO₂ is likely to increase the leaf critical P concentration for maximum biomass productivity and yield in soybean.     

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.     

Effects of soil phosphorus and Glomus intraradices on growth,nonstructural carbohydrates,and photosynthetic activity of Citrus aurantium

Sour orange (Citrus aurantium L.) grown in low-P (9–12 ppm) and high-P (420 ppm) soil inoculated with or without Glomus intraradices (G.i.), were evaluated for biomass, carbohydrates, ribulose bisphosphate carboxylase (RuBPCase), phosphoenolpyruvate carboxylase (PEPCase) activity, leaf ¹⁴CO₂ incorporation, and other physiological parameters. Growth of plants in the low-P, noninoculated soil was lowest, with total dry biomass reduced up to half of the low-P, inoculum treatment. Total nonstructural carbohydrates were 40% lower in leaves of plants in the low-P, noninoculated soil, compared with the other treatments. Inoculation of the low-P soil enhanced leaf ¹⁴CO₂ incorporation by 67%, total chlorophyll content by 28%, and RuBPCase activity by 42%, compared with low-P, noninoculated treatment. Improved P-use efficiency by G.i. in low-P soil was comparable to high-P nutrition in improving leaf ¹⁴CO₂ incorporation and concentration of major leaf photosynthetic products that include starch and sucrose. Leaf PEPCase activity in the low-P, noninoculated treatment, however, was at least threefold higher than the other treatments, suggesting a possible alteration in organic acid metabolism in sour orange leaves as a result of P deficiency.     

Effect of pH on transepithelial electrical parameters in seawater-adapted eel intestine

The sensitivity to external pH of Cl^- absorption was studied in isolated stripped intestinal mucosa of the eel, Anguilla anguilla, mounted in Ussing chambers. Short-circuit current, transepithelial potential difference and conductance were measured in bathing solutions containing various combinations of HCO₃ ^--concentration (0–25 mmol·l^-1), partial pressure of CO₂ (0–76 mm Hg) and pH (6.9–7.9). A linear relationship was found between pH and short-circuit current in the range of pH studied both in HCO₃ ^-/CO₂ Ringer and in Hepes Ringer. The pH effect was almost completely reversible. It was not affected by the presence of mucosal Ba²⁺ (10^-3 mol·l^-1) but it was inhibited by the presence of luminal (10^-5 mol·l^-1) or serosal (10^-4 mol·l^-1) bumetanide. The results obtained suggest that the Cl^- absorption in the European eel intestine is pH sensitive. The data do not indicate whether the pH affects directly the Na⁺–K⁺–Cl^- cotransport and/or the basolateral Cl^- conductance or other mechanisms indirectly linked to Cl^- absorption.Abbreviations g _t transepithelial conductance - Hepes N-2-Hydroxyethylpiperazine-N'-2-ethanesulfonic acid - I _sc short circuit current - R _t transepithelial resistence - V _t transepithelial potential difference     

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

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

Development of dependence on aerial respiration in Polypterus senegalus (Cuvier)

The respiratory behaviour and partitioning of O₂ uptake between air and water were investigated in Polypterus genegalus using continuous-flow and two-phase respirometers and lung gas replacement techniques P. senegalus rarely resorts to aerial respiration under normal conditions. Partitioning of O₂ consumption depends on the activity and age of fish and the availability of aquatic oxygen. Immature fish (12–22 g) cannot utilize aerial O₂ but older fish exhibit age-dependent reliance on aerial respiration in hypoxic and hypercarbic waters. Pulmonary respiration accounts for 50% of the total requirement at aquatic O₂ concentrations of about 3.5 mg · l^–1 (or CO₂ of about 5%) and fish rely exclusively on aerial respiration at O₂ concentrations of less than 2.5 mg · l^–1. Branchial respiration is initially stimulated by hypercarbia (CO₂: 0.5–0.8%) but increased hypercarbia (CO₂ – 1%) greatly depresses (by over 90%) brancial respiration and initiates (CO₂: 0.5%) and sustains pulmonary respiration.     

Decomposition of leaf and root tissue of three perennial grass species grown at two levels of atmospheric CO2 and N supply

Leaf and root tissue of Lolium perenne L., Agrostis capillaris L. and Festuca ovina L. grown under ambient (350 μl l^-1 CO₂) and elevated (700 μl l^-1) CO₂ in a continuously ¹⁴C-labelled atmosphere and at two soil N levels, were incubated at 14°C for 222 days. Decomposition of leaf and root tissue grown in the low N treatment was not affected by elevated [CO₂], whereas decomposition in the high N treatment was significantly reduced by 7% after 222 days. Despite the increased C/N ratio (g g^-1) of tissue cultivated at elevated [CO₂] when compared with the corresponding ambient tissue, there was no significant correlation between initial C/N ratio and ¹⁴C respired. This finding suggests that the CO₂-induced changes in decomposition rates do not occur via CO₂-induced changes in C/N ratios of plant materials. We combined the decomposition data with data on ¹⁴C uptake and allocation for the same plants, and give evidence that elevated [CO₂] has the potential to increase soil C stores in grassland via increasing C uptake and shifting C allocation towards the roots, with an inherent slower decomposition rate than the leaves. An overall increase of 15% in ¹⁴C remaining after 222 days was estimated for the combined tissues, i.e., the whole plants; the leaves made a much smaller contribution to the C remaining (+6%) than the roots (+26%). This shows the importance of clarifying the contribution of roots and leaves with respect to the question whether grassland soils act as a sink or source for atmospheric CO₂. This revised version was published online in June 2006 with corrections to the Cover Date.     

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)     

Intracellular and luminal ion concentrations in sea turtle salt glands determined by x-ray microanalysis

Summary The lachrymal salt glands of hatchlings of the green sea turtle (Chelonia mydas) secrete a hyperosmotic (up to 2000 mosmol·kg^–1) NaCl solution. X-ray microanalysis of frozen-hydrated glands showed that during secretion intracellular Na⁺ concentration in the principal cells increased from 13 to 34 mmol·l^–1 of cell water, whilst Cl^– and K⁺ concentrations remained unchanged at 81 mmol·l^–1 and 160–174 mmol·l^–1, respectively. The high Cl^– concentration and the change in Na⁺ concentration are consistent with the prevailing paradigm for secretion by the structurally and functionally similar elasmobranch rectal gland. Concentrations of Na⁺, Cl^– and K⁺ in the lumina of secretory tubules of secreting (Na⁺ 122, Cl^– 167, K⁺ 38 mmol·l^–1) and non-secreting (Na⁺ 114, Cl^–1 174, K⁺ 44 mmol·l^–1) glands were similar and the fluid was calculated to be approximately isosmotic with blood. In the central canals Na⁺ and Cl^– concentrations were similar but K⁺ concentration was lower (11–15 mmol·l^–1). It is concluded that either a high transepithelial NaCl gradient in secretory tubules and central canals is very rapidly dissipated during the short time between gland excision and freezing, or that ductal modification of an initial isosmotic secretion occurs.     

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.     

Does the threshold of transcutaneous partial pressure of carbon dioxide represent the respiratory compensation point or anaerobic threshold?

On reaching the respiratory compensation point (RCP) during rapidly increasing incremental exercise, the ratio of minute ventilation (V_E) to CO₂ output (VCO₂) rises, which coincides with changes of arterial partial pressure of carbon dioxide (P _aCO₂). Since P _aCO₂ changes can be monitored by transcutaneous partial pressure of carbon dioxide (PCO_2,tc) RCP may be estimated by PCO_2,tc measurement. Few available studies, however, have dealt with comparisons between PCO_2,tc threshold (T _AT) and lactic, ventilatory or gas exchange threshold (V _AT), and the results have been conflicting. This study was designed to examine whether this threshold represents RCP rather than V _AT. A group of 11 male athletes performed incremental excercise (25 W · min^–1) on a cycle ergometer. The PCO_2,tc at (44°C) was continuously measured. Gas exchange was computed breath-by-breath, and hyperaemized capillary blood for lactate concentration ([la^–]_b) and P _aCO₂ measurements was sampled each 2 min. The T _AT was determined at the deflection point of PCO_2,tc curve where PCO_2,tc began to decrease continuously. The V _AT and RCP were evaluated with VCO₂ compared with oxygen uptake (VO₂) and V_E compared with the VCO₂ method, respectively. The PCO_2,tc correlated with P _aCO₂ and end-tidal PCO₂. At T _AT, power output [P, 294 (SD 40) W], VO₂ [4.18 (SD 0.57)l · min^–1] and [la^–] [4.40 (SD 0.64) mmol · l^–1] were significantly higher than those at V _AT[P 242 (SD 26) W, VO₂ 3.56 (SD 0.53) l · min^–1 and [la^–]_b 3.52 (SD 0.75), mmol · l^–1 respectively], but close to those at RCP [P 289 (SD 37) W; VO₂ 3.97 (SD 0.43) l · min^– and [la^–]_b 4.19 (SD 0.62) mmol · l^–1, respectively]. Accordingly, linear correlation and regression analyses showed that P, VO₂ and [la^–]_b at T _AT were closer to those at RCP than at V _AT. In conclusion, the T _AT reflected the RCP rather than V _AT during rapidly increasing incremental exercise.