Nitrogen stress reduces the efficiency of the C4 CO2 concentrating system, and therefore quantum yield, in Saccharum (sugarcane) species

Nitrogen deficiency reduces the photosynthetic capacity of both C3 and C4 plants. The regulation of photosynthetic gas exchange in eight clones of the C4 grass, sugarcane (Saccharum spp.), grown at three levels of N availability was studied to determine whether N stress diminishes the efficiency of the C4 CO2 concentrating system in addition to reducing overall rates of photosynthesis. The quantum yield for CO2 uptake decreased linearly with decreasing leaf N content. Genetic variation in quantum yield at a given level of N supply was also observed. Leaf tissue carbon isotope discrimination () increased linearly with decreasing quantum yield. Concurrent determinations of the prevailing ratio of intercellular to ambient partial pressure of CO 2 (pi/pa) during leaf gas exchange suggested that the observed variation in was almost entirely attributable to variation in bundle sheath leakiness to CO2 () rather than pi/pa. Taken together, these results point to substantial environmental and genetic variation in the efficiency of the CO2 concentrating system in sugarcane. Reduced partitioning of carboxylase activity to ribulose-1,5-bisphosphate carboxylase relative to phosphoenolpyruvate carboxylase in N-deficient plants suggested that the associated increase in and decline in quantum yield may have been attributable largely to a decline in C3 cycle activity in the bundle sheath relative to C4 cycle activity in the mesophyll. Quantum yield and intrinsic water use efficiency (WUE) were negatively correlated. In contrast with the trade-off between intrinsic light- and water use efficiency, photosynthetic nitrogen-use efficiency and intrinsic WUE were positively correlated.     

Short communication. A steep Ca2[IMAGE]-dependence of a K[IMAGE] channel in a unicellular green alga

An increase of cytosolic Ca² in the unicellular green alga Eremosphaera viridis activities Ca²-dependent K channels causing a hyperpolarization of the plasma membrane. Data from parallel calcium, and potential measurements were combined with I/V relationships. This yielded a steep Ca²-dependence of K channels with a co-operativity of 4 and an affinity of 300 nM.Key words: Eremosphaera viridis, plasma membrane, Ca²-dependent K channel, co-operative binding.      

Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community

Periods of limited soil water availability are a feature of many temperate pasture systems and these have the potential to modify pasture plant and community responses to elevated atmospheric CO2. Using large pasture turves, previously exposed to elevated CO2 concentrations of 350 or 700 mol mol^-1 for 324 d under well-watered conditions the morphological and physiological responses of pasture species growing at these CO2 concentrations were compared when subjected to a soil moisture deficit-and to recovery from the deficit-with those that continued to be well watered.Net leaf photosynthesis of Trifolium repens (C3 legume), Plantago lanceolata (C3) and Paspalum dilatatum (C4) was increased by exposure to elevated CO2, but there was no consistent effect of CO2 on stomatal conductance. At low soil moistures, net photosynthesis declined and stomatal conductance increased in these three species. There was a strong CO2 x water interaction in respect of net photosynthesis; in Trifolium repens, for example, elevated CO2 increased net photosynthesis by approximately 50% under well-watered conditions and this increased to over 300% when soil moisture levels reached their minimum values. Similar values were recorded for both Paspalum dilatatum and Plantago lanceolata. Potential water use efficiency (net photosynthesis/stomatal conductance) was increased by both exposure to elevated CO2 and drought.Leaf water status was measured in three species: Trifolium repens, Paspalum dilatatum and Holcus lanatus (C3). Total leaf water potential (t) and osmotic potential () were decreased by drought, but CO2 concentration had no consistent effect. t and were highest in the C4 species Paspalum dilatatum and lowest in the legume Trifolium repens.In the wet turves, rates of leaf extension of the C3 grasses Holcus lanatus and Lolium perenne at elevated CO2 were frequently higher than those at ambient CO2, but there was no effect of CO2 concentration on the rate recorded in the C4 grass Paspalum dilatatum or the rate of leaf appearance in the legume Trifolium repens. Drought reduced leaf extension rate irrespective of CO2 in all species, but in Holcus lanatus the reduction was less severe at elevated CO2. Immediately after the dry turves were rewatered the leaf extension rate on tillers of Holcus lanatus and Lolium perenne were higher than on tillers in the wet turves, but only at ambient CO2. Consequently, despite the greater leaf extension rate during the soil moisture deficit at elevated CO2, because of the overcompensation after rewatering at ambient CO2, total leaf extension over both the drying and rewetting period did not differ between CO2 concentrations for these C3 grass species. Further investigation of this difference in response between CO2 treatments is warranted given the frequent drying and wetting cycles experienced by many temperate grasslands.     

Mechanism of Inorganic Carbon Uptake in Chlorella saccharophila: The Lack of Involvement of Carbonic Anhydrase

The acid-tolerant green alga Chlorella saccharophila maintainedphotosynthesis and accumulated intracellular pools of inorganiccarbon over a a range of external pH from 4.0 to 7.5. This accumulationwas unaffected by treatment of cells with 10 mol m^–3 acetazolamide(AZA). Cells grown at alkaline pH had extracellular carbonicanhydrase (CA), but CA activity was repressed when cells weregrown at pH 5.0. Acid-grown cells retained a high affinity forCO₂, both at acid and alkaline pH, and the ability to accumulateinorganic carbon. Rates of photosynthesis of acid-grown cellsand alkaline-grown AZA-treated cells at pH 8.0 were 2.5-foldhigher than the rate of CO₂ supply from the uncatalysed dehydrationof , indicating that the cells can take up as a source of substrate for photosynthesis. Isotopic disequilibrium experiments with acid-grown cells maintainingsteady-state photosynthesis at pH 7.5 demonstrate that ¹⁴C from¹⁴CO₂ was taken up more rapidly than from H¹⁴. This uptake takes place against a concentration gradient. Theseresults demonstrate that C. saccharophila cells have activetransport systems for the uptake of both CO₂ and and both operate without the mediation of CA. Key words: Bicarbonate transport, carbon dioxide, carbonic anhydrase, Chlorella saccharophila, inorganic carbon accumulation     

Evidence for Bicarbonate Transport in Species of Red and Brown Macrophytic Marine Algae

Cook, C. M., Lanaras, T. and Colman, B. 1986. Evidence for bicarbonatetransport in species of red and brown macrophytic marine algae.—J.exp. Bot. 37: 977–984. The capacity of 17 species of marine macrophytes to take up has been examined by comparing the rate of photosynthetic O₂-evolution with the photosyntheticrate which could be supported solely by CO₂ arising from theuncatalysed dehydration of . No external carbonic anhydrase was detected by potentiometricassay in any of the species used. At pH 8?0, the rates of photosyntheticO₂-evalution exceeded the CO₂ supply rate 6 to 24-fold in 15species of red algae, and 7 to 11-fold in 2 species of brownalgae. The ratio of photosynthetic O₂-evalution to the CO₂ supplyrate was even higher (19 to 101: 1) at pH 9?0. It is evidentfrom this data that the rate of CO₂ supply from the spontaneousbreakdown of cannot support the observed rates of photosynthesis in these algae. Thus, thedata provide substantive evidence that is taken up by these marine macrophytes as a sourceof substrate for photosynthesis. Key words: Macrophytic marine algae, bicarbonate transport     

Calcification in the Green Alga Halimeda: III. THE SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS AND CALCIFICATION AND A MODEL OF THE MECHANISM OF CALCIFICATION

Calcification and photosynthetic rates in Halimeda tuna weremeasured by the ¹⁴C method under conditions of differing pHand total inorganic carbon (CO₂) concentrations. The effectsof pH and CO₂ on photosynthesis and respiration were also monitoredwith a polarographic O₂ electrode. The results obtained indicatethat the intercellular pH and CO₂ differ from those of the externalmedium. Experiments carried out over a range of pH values show thatHalimeda can use for photosynthesis. Photosynthesis appears to stimulate calcification by removing CO₂ from theintercellular spaces. As these spaces are isolated from theexternal sea water by the layer of cell wall of the adpressedperipheral utricles, the removal of CO₂ results in a rise in[] and a rise in pH. This results in an increased rate of CaCO₃ precipitation. Respiratory CO₂ evolution has aninhibitory effect on calcification by decreasing the pH and[]. A model for calcification in Halimeda is proposed based on theresults of this and previous papers. Calcification in Halimedais seen to be a result of the anatomy of the thallus in whichthe sites of calcification are within a semi-isolated chamberwhere removal or addition of CO₂ due to photosynthesis or respirationcan effectively change [CO] thereby resulting in precipitation of CaCO₃. In the Appendix to this paper theoreticalcalculations illustrate the effects of CO₂, , and removal or addition in a closed system on the relative concentrations of the other inorganic carbonspecies.     

Quantitative effects of the genes Lf, Sn, E, and Hr on time to flowering in pea (Pisum sativum L.)

Flowering time in pea (Pisum sativum L.) is determined by genetically controlled responses to photoperiod and temperature. To investigate these responses, 11 lines homozygous for the flowering genes Lf, Sn, E, and Hr were grown under contrasting semi-controlled photothermal environments and the durations (d) from sowing to first flower (f) were recorded. The effects of the four genes were quantified using a two-plane photothermal model which linearly relates the rate of progress from sowing to flowering (1/f) with the mean pre-flowering values of temperature (T) and/or photo-period (P), based on 1/fa + bT (when P is longer than the critical photoperiod, Pc) and 1/fa + bT + cP (when P<Pc). The main effect of Lf alleles was on temperature sensitivity (b) when P>Pc, which increased in the sequence Lf^d<Lf< lf<lf^a. Gene Hr, when together with Sn, increased photoperiod sensitivity (c) and reduced the intercept (a) when P<Pc. Allele sn determined a single plane response to temperature alone (i.e. a day-neutral response). Gene E, when present with lf Sn, increased 1/f in both the thermal (P<Pc) and photothermal (PPc) domains, mainly by increasing a and b, respectively. Variations in the coefficients of the thermal and photothermal responses determined that the critical photoperiod varied with temperature in all photoperiod-sensitive genotypes. A common base temperature of 0.2C was determined amongst Day-Neutral Class genotypes (sn) and thermal time from sowing to flowering increased in the sequence lf^a<lf< <:f<Lf^d. Intra-Class variations attributed to the Lf alleles were also detected in the Late (Sn hr) and Late High Response (Sn Hr) Classes. The linear photothermal model provided a sound basis for studying the quantitative effects of flowering genes in pea.     

Photosynthetic activity of the calyx, green shoulder, pericarp, and locular parenchyma of tomato fruit

Photosynthesis of tomato fruit was studied using green fruit from six heritage cultivars of Lycopersicon esculentum Mill. and one of Lycopersicon pimpinellifolium. Chlorophyll concentrations in the green shoulder, pericarp and locular parenchyma of the fruit were determined and the apparent photosynthetic electron transport activity (ETR) and chlorophyll fluorescence quenching characteristics of these tissues and the calyx were compared. In all cultivars, green shoulder formation, apparent as intense pigmentation of the proximal pericarp shoulder, was positively related to the degree of shading of the fruit during development. Appearing as a photosynthetic adaptive trait for increasing the photoautotrophic capacity of fruit grown under low light, the green shoulder contained 17-57% of the total pericarp chlorophyll content. The pericarp below the green shoulder had lower chlorophyll a+b. At a photon flux density (PFD) of 1200 mol m^-2 s^-1, different fruit tissues were found to have different levels of ETR. In 'Yellow Pear', the upper surface of the calyx had an ETR of 154 mol m^-2 s^-1, while the lower surface had an ETR of 88 mol m^-2 s^-1. On the green shoulder, ETR was 203 mol m^-2 s^-1, whereas in the pericarp distal to the green shoulder, ETR was 97 mol m^-2 s^-1. In the locular parenchyma, ETR was 66 mol m^-2 s^-1. This trend towards a lower ETR in distal and internal fruit tissues appeared to indicate a shift towards a more shade-type photosynthesis. Concomitant with this shift were changes in chlorophyll fluorescence quenching characteristics. Generally when tissues displayed reduced levels of ETR they also displayed a faster decrease in the photochemical quenching coefficient qp and a more rapid diversion of absorbed photon energy to non-photosynthetic activity found in the calyx, green shoulder, pericarp, and locular parenchyma suggest that all of these tissues have significant roles in CO2 scavenging and the provision of carbon assimilates. The potential role of fruit photosynthesis in influencing the fruit acid to sugar ratio and hence fruit quality is discussed.     

Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata

Plantago lanceolata with or without the mycorrhizal fungus Glomus mosseae were grown over a 100 d period under ambient (38050 mol mol^-1) and elevated (600150 mol mol^-1) atmospheric CO2 conditions. To achieve similar growth, non-mycorrhizal plants received phosphorus in solution whereas mycorrhizal plants were supplied with bonemeal. Measures of plant growth, photosynthesis and carbon input to the soil were obtained. Elevated CO2 stimulated plant growth to the same extent in mycorrhizal and non0mycorrhizal plants, but had no effect on the partitioning of carbon between shoots and roots or on shoot tissue phosphorus concentration. Mycorrhizal colonization was low, but unaffected by CO2 treatment. Net photosynthesis was stimulated both by mycorrhizal colonization and elevated CO2, and there was a more than additive effect of the two treatments on net photosynthesis. Colonization by mycorrhizal fungi inhibited acclimation, in terms of net carbon assimilation, or plants to elevated CO2. ¹³C natural abundance techniques were used to measure carbon input into the soil, although the results were not conclusive. Direct measurements of below-ground root biomass showed that elevated CO2 did stimulate carbon flow below-ground and this was higher in mycorrhizal than non-mycorrhizal plants. For the four treatment combinations, the observed relative differences in amount of below-ground carbon were compared with those expected from the differences in net photosynthesis. A considerable amount of the extra carbon fixed both as a result of mycorrhizal colonization and growth in elevated CO2 did not reveal itself as increased plant biomass. As there was no evidence for a substantial increase in soil organic matter, most of this extra carbon must have been respired by the mycorrhizal fungus and the roots or by the plants as dark-respiration. The need for detailed studies in this area is emphasized.     

Salt-Stimulated Bicarbonate-Dependent Photosynthesis in the Marine Angiosperm Zostera muelleri

Millhouse, J. and Strother, S. 1986. Salt-stimulated bicarbonate-dependentphotosynthesis in the marine angiosperm Zostera muelleri.—J.exp. Bot. 37: 965–976. Photosynthetic oxygen evolution in the seagrass Zostera muelleriIrmisch ex Aschers. was inhibited in iso-osmotic sucrose. Theapparent affinity of the leaves for CO₂ in seawater increasesfrom pH 8?2 to 8?9 indicating that as well as CO₂ may act as a substrate for photosynthesis. Theaffinity for CO₂ was lower in iso-osmotic sucrose and was notaffected by pH. Under these conditions was not a substrate for photosynthesis. The differencebetween the photosynthetic rate in seawater and iso-osmoticsucrose at the same concentration of CO₂ was used to estimate assimilation. The Briggs-Maskell equation, which allows for an unstirred layer around the tissuewas more appropriate than the Michaelis-Menten theory for calculatingthe apparent affinity of the leaf slices for CO₂. The apparentK_m CO₂ was calculated as 116 mmol m^–3 at pH 8?2 by Michaelis-Mentenkinetics but only 8?10 mmol m^–3 by the Briggs-Maskellequation. The stimulation by various ions in Seawater of use was investigated. The cations,in decreasing order of effectiveness were Ca²⁺, Mg²⁺, K⁺ andNa⁺ Anions were ineffective. No single cation at its concentrationin seawater was capable of supporting use at the rate observed in seawater. Acetazolamide,an inhibitor of carbonic anhydrase, inhibited the use of for photosynthesis but had littleeffect on CO₂ photosynthesis. Thus, carbonic anhydrase activityis required for -dependent photosynthesis. Key words: Zostera muelleri, photosynthesis, salinity     

The effect of rhizosphere dissolved inorganic carbon on gas exchange characteristics and growth rates of tomato seedlings

The possibility that an enhanced supply of dissolved inorganic carbon (DIC=CO2+HCO3^-) to the root solution could increase the growth of Lycopersicon esculentum (L.) Mill. cv. F144 was investigated under both saline and non-saline root medium conditions. Tomato seedlings were grown in hydroponic culture with and without NaCl and the root solution was aerated with CO2 concentrations in the range between 0 and 5000 mol mol^-1. The biomass of both control and salinity-stressed plants grown at high temperatures (daily maximum of 37C) and an irradiance of 1500 mol m^-2 s^-1 was increased by up to 200% by enriched rhizosphere DIC. The growth rates of plants grown with irradiances of less than 100 mol m^-2 s^-1 were increased by elevated rhizosphere DIC concentrations only when grown at high shoot temperatures (35C) or with salinity 28°C). At high light intensities, the photosynthetic rate, the CO2 and light-saturated photosynthetic rate (jmax) and the stomatal conductance of plants grown at high light intensity were lower in plants supplied with enriched compared to ambient DIC. This was interpreted as 'down-regulation' of the photosynthetic system in plants supplied with elevated DIC. Labelled organic carbon in the xylem sap derived from root DI¹⁴C incorporation was found to be sufficient to deliver carbon to the shoot at rates equivalent to 1% and 10% of the photosynthetic rate of the plants supplied with ambient- and enriched-DIC, respectively. It was concluded that organic carbon derived from DIC incorporation and translocated in the xylem from the root to the shoot may provide a source of carbon for the shoots, especially under conditions where low stomatal conductance may be advantageous, such as salinity stress, high shoot temperatures and high light intensities.     

Optimization theory of stomatal behaviour II. Stomatal responses of several tree species of north Australia to changes in light, soil and atomospheric water content and temperature

In a companion paper several methods of calculating the marginal unit water cost of plant carbon gain (E/A) were tested to determine whether stomata were behaving optimally in relation to regulating leaf gas exchange. In this paper one method is applied to several tropical tree species when leaf-to-air vapour pressure difference (D), photosynthetic photon flux density, leaf temperature, and atmospheric soil water availability were manipulated. The response of leaves that had expanded during the dry season were also compared to that of leaves that had expanded in the wet season. Few differences in absolute value of E/A, or the form of the relationship, were observed between species or between seasons. In the majority of species, E/A increased significantly as either leaf-to-air vapour pressure difference increased, at a leaf temperature of either 33C or 38C, or as in photosynthetic photon flux density increased. In contrast, as leaf temperature increased at constant D, E/A was generally constant. As pre-dawn water potential declined, E/A declined. The relationship between E/A and D did not differ whether internal or ambient carbon dioxide concentration were kept constant. It is concluded that stomata are only behaving optimally over a very small range of D. If a larger range of D is used, to incorporate values that more closely reflect those experienced by tropical trees in a savanna environment optimization is incomplete.Key words: Stomatal optimization theory, marginal unit water cost.      

The influence of predawn leaf water potential on stomatal responses to atmospheric water content at constant Ci and on stem hydraulic conductance and foliar ABA concentrations

Stomata and photosynthetic responses to increasing leaf-to-air vapour pressure difference (V) were investigated in watered and droughted Eucalyptus tetrodonta when either ambient CO2 (Ca) or internal CO2 concentration (Ci) were constant. Stem hydraulic conductance and xylem and foliar abscisic acid levels were measured periodically during the drought period.As V increased, stomatal conductance (gs) declined. Maintaining Ci did not affect the response of Gs to V or predawn leaf water potential (pd). In fully watered plants the decline in gs was insufficient to prevent increased transpiration rates (Et) with increasing V. In contrast, in droughted plants, stomatal closure was sufficient to prevent increasing Et. Stomatal sensitivity to increasing V was increased by drought. As drought developed, the three phases of stomatal responses to V progressively collapsed to one where feedforward mechanisms result in decreasing Et with increasing V. Thus as drought developed the feedforward response of stomata to V entirely dominated.Net photosynthesis (Pn) and gs responded in unison to changes in V and pd. The decline in Pn as V increased, despite Ci being maintained constant, was observed in fully watered plants and in severely droughted plants. Pn was most dependent on gs at large values of V and in droughted plants.As drought progressed and pd declined, stem hydraulic conductance decreased and foliar ABA concentrations increased. The decline in maximum gs was correlated with foliar ABA levels. It is concluded here that hydraulic signals, be they atmospheric water or soil water in origin, and possibly also chemical signals regulate gs, which in turn can limit assimilation rates in seasonally dry savannas.Key words: Eucalyptus tetrodonta, stomatal sensitivity, transpiration, hydraulic conductance, ABA.      

Carbon Metabolism in Seagrasses: I. THE UTILIZATION OF EXOGENOUS INORGANIC CARBON SPECIES IN PHOTOSYNTHESIS

Four species of seagrasses, Halophila stipulacea, Thalassodendronciliatum, Halodule uninervis, and Syringodium isoetifolium,were investigated for their ability to utilize and CO₂ as exogenous carbon sources for photosynthesis. Ratesof photosynthesis were measured as rates of O₂ evolution ina closed system in which the pH was continuously controlled.A computer program was written to calculate the concentrationsof different carbon species as a function of pH and other specifiedexperimental conditions. Bicarbonate as well as CO₂ were readily assimilated by all fourseagrass species. Saturating concentrations of , at saturating light intensities, were 0.5–1.8 mM dependingon the species. Rates of photosynthesis under such conditionswere 0.1–0.55 µmol O₂ min^–1 mg^–1 chlorophyll.At saturating CO₂ concentrations, i.e. 0.5–1.3 mM, ratesof photosynthesis were 0.22–1.4 µmol CO₂ min^–1mg^–1 chlorophyll. Photosynthetic rates in each specieswere considerably higher when CO₂ rather than was supplied at saturating concentrations. The concentration of in natural seawater was found to be saturating, and that of CO₂ insufficient forconsiderable photosynthetic rates in these plants under thegiven conditions It was thus concluded that is the major carbon source for photosynthesis in seagrasses.     

Expression of Characteristics of Ammonium Nutrition as Affected by pH of the Root Medium

To study the effect of root-zone pH on characteristic responsesof -fed plants, soybeans (Glycine max {L.}Merr. cv. Ransom) were grown in flowing solution culture for21 d on four sources of N (1.0 mol m^–3 , 0.67 mol m^–3 plus 0.33 mol m^–3, 0.33 mol m^–3 plus 0.67 mol m^–3 , and 1.0 mol m^–3) with nutrient solutions maintained at pH 6.0, 5.5, 5.0, and 4.5. Amino acid concentration increased inplants grown with as the sole source of N at all pH levels. Total amino acid concentration in the rootsof -fed plants was 8 to 10 times higher than in -fed plants, with asparagine accounting for more than 70% of the total in the roots of these plants.The concentration of soluble carbohydrates in the leaves of-fed plants was greater than that of -fed plants, but was lower in roots of -fed plants, regardless of pH. Starch concentration was only slightlyaffected by N source or root-zone pH. At all levels of pH tested,organic acid concentration in leaves was much lower when was the sole N source than when all or part of theN was supplied as . Plants grown with mixed plus N sources were generally intermediate between - and -fed plants. Thus, changes in tissue compositioncharacteristic of nutrition when root-zone pH was maintained at 4.5 and growth was reduced, still occurredwhen pH was maintained at 5.0 or above, where growth was notaffected. The changes were slightly greater at pH 4.5 than athigher pH levels. Key words: Ammonium, nitrogen nutrition, root-zone pH, soybean, tissue composition     

Photorespiratory glycine enhances glutathione accumulation in both the chloroplastic and cytosolic compartments

Transformed poplars overexpressing -glutamylcysteine synthetase (-ECS) in the chloroplast (Lggs) were used to investigate chloroplastic biosynthesis of glutathione (GSH). In Lggs leaves, GSH contents were enhanced by up to 3.7 fold. In general, the highest GSH contents were observed in lines with highest -glutamylcysteine (-EC) contents. These lines had relatively low glycine. In darkness, foliar GSH decreased and -EC increased. Illumination of pre-darkened Lggs in air resulted in a 5-fold decrease in the -EC : GSH ratio. This light-induced decrease was largely abolished if leaves were illuminated at high CO2. Consequently, the -EC : GSH ratio of illuminated leaves was much higher at high CO2 than in air. At high CO2 total foliar amino acids were higher, but glycine and serine were lower, than in air. These results suggest that photorespiratory glycine is used in chloroplastic GSH synthesis. Despite this net CO2 fixation was similar in Lggs to untransformed poplars. Pre-illuminated leaf discs from Lggs, and poplars overexpression -ECS in the cytosol (ggs), were incubated in darkness with a range of metabolites. After 15 h, discs for both types of transformant incubated on water had accumulated high levels of -EC and showed marked increases in the -EC : GSH ratio. Feeding glycine, serine, glycollate or phosphoserine, attenuated the dark-induced changes in the -EC : GSH ratio, whereas 3-phosphoglycerate (PGA), phosphoenolpyruvate, glycerate, and hydroxypyruvate did not. Glycine produced from glycollate was therefore required for maximal GSH accumulation in both the chloroplastic and cytosolic compartment. Production of glycine from PGA failed to meet the demand of increased GSH synthetic capacity.     

Effect of high CO2 level on the titres of {gamma}-aminobutyric acid, total polyamines and some pathogenesis-related proteins in cherimoya fruit stored at low temperature

In this study the effect of treatment with 20% CO2 plus 20% O2 for 3 d on -aminobutyric acid (GABA) was analysed and total polyamine titres and the production of some pathogenesis-related proteins (PR-proteins) in cherimoya (Annona cherimola Mill.) fruit stored at low temperature. In immunoassays with anti-PR-Q and PR-2 protein sera, high CO2 levels were found to provoke the co-ordinated accumulation of a chitinase-like protein and 1,3--glucanase. Chitinase activity was higher in treated than in untreated fruit. At the end of CO2 treatment, total polyamine and -aminobutyric acid content and uptake of O2 were observed to be higher in treated compared to untreated fruit, but the accumulation of these compounds decreased when the fruit was transferred to air. Since this treatment effectively retains the fruit quality (Escribano et al., 1997), high CO2 levels may have a direct effect on the activation of the above-mentioned specific responses that enables cherimoya fruit to overcome chilling temperature. The relationship between the activation of the defence system and the capacity to regulate cytoplasmic pH in CO2-treated fruits was also addressed.     

Effect of elevated CO2 concentration on acclimation of tobacco plantlets to ex vitro conditions

Nicotiana tabacum L. plants grown in vitro were transferred to ex vitro conditions and grown for 28 d in a greenhouse under normal CO2 concentration (C, 330 mol mol^-1) or elevated CO2 concentration (E, 1000 mol mol^-1). Stomatal conductances of abaxial and adaxial epidermes measured under optimal conditions were not significantly affected by growth under E, but the stomatal regulation of gas exchange was better. Leaf photosynthetic rate (A) of elevated CO2 plants was similar to that of control plants when both were measured under normal CO2, but higher when both were measured under elevated CO2. The A of elevated CO2 plants was much higher than the A of control plants when measured under their respective growth CO2 concentration, which resulted in their higher growth rate. Chlorophyll a and b contents, and activities of whole electron transport chain and of photosystem (PS) II were not markedly affected by growth under E, and the maximum efficiency of PSII measured as the ratio of variable to maximum fluorescence was even slightly increased. Hence no down-regulation of photosynthesis occurred in transplanted plants grown for 4 weeks under E. The contents of -carotene and of xanthophyll cycle pigments (violaxanthin + antheraxanthin + zeaxanthin) were lower in E plants. The degree of de-epoxidation of xanthophyll cycle pigments was not changed or was even lower after transfer to ex vitro conditions, which indicated that no photoinhibition occurred. Therefore, CO2 enrichment can improve acclimation of in vitro-grown plantlets to ex vitro conditions.Keywords: Carotenoids, chlorophyll content and fluorescence, Nicotiana tabacum stomatal conductance      

Influence of Inorganic Phosphate on Photosynthesis of Wheat Chloroplasts: I. PHOTOSYNTHESIS AND ASSIMILATE EXPORT AT 5 ?C AND 25 ?C

Chloroplasts were isolated from 10 d old wheat seedlings andilluminated at 5 ?C or 25 ?C in various concentrations of . Photosynthetic oxygen evolution, ATP content, andexport of triose phosphates and 3-phosphoglycerate were measured.Incorporation of ¹⁴C from NaH¹⁴CO₃ into pentose monophosphates,fructose monophosphate, and glucose monophosphates was determined. The ATP content in illuminated chloroplasts decreased when the concentration in the medium was low. The ATP content increased when the concentration was increased. A higher . concentration in the medium was needed to increase the ATP at 5 ?C than at 25?C. This would suggest that deficiency occurs more readily at low than at high temperatures. More ¹⁴C wasincorporated into photosynthetic metabolites within the chloroplastsat 5 ?C than at 25 ?C, indicating decreased assimilate exportwhen the temperature was low. Dihydroxyacetone phosphate waspreferentially exported when the concentration enabled a high rate of photosynthesis at 25 ?C. However, underconditions of deficiency, either due to low concentration in the medium or due to low temperature, 3-phosphoglycerate was preferred for export. The results suggest that the relatively high photosyntheticrates at low temperature are due to increased concentrationsof photosynthetic metabolites. The assimilate export at lowtemperature seems to be decreased due to decreased concentrationsof dihydroxyacetone phosphate in the stroma. Preferential exportof 3-phosphoglycerate at low temperature or at low concentration in the medium may be a consequence of high stromalconcentrations of this metabolite. On the other hand, it couldalso be due to decreased stromal pH. Key words: Chloroplast, Photosynthesis, Phosphate, Temperature, Translocation, ATP, Wheat     

The Effect of Hydroxymethanesulphonate on Photosynthesis in Chlorella pyrenoidosa

Photosynthesis under conditions known to favour glycollate excretionby algae did not result in glycollate excretion in a strainof Chlorella pyrenoidosa unless an inhibitor of glycollate oxidase,-hydroxypyridin-2yl-methane sulphonate (-HPMS), was present.This inhibitor increased the total amount of glycollate presentin the supernatant from the cells during photosynthetic carbondioxide fixation and gave accumulation of ¹⁴C in glycollateduring ¹⁴CO₂ fixation under conditions favouring glycollatesynthesis. At pH 8.3 -HPMS did not stimulate photosynthetic¹⁴CO₂ fixation in C. pyrenoidosa as occurs with some algae.Photoassimilation of acetate was inhibited by -HPMS, and thiswas shown to result from acetyl-CoA synthetase inhibition by-HPMS.