Sensitivity Of Photosynthesis Research Articles

Differential physiological behavior of sugarcane genotypes in response to sparingly soluble phosphorus‐sources

AbstractAims: This study was initiated to evaluate different responses of sugarcane (Saccharum officinarum L.) genotypes to sparingly soluble phosphorus (P) treatments and to identify useful plant traits contributing to P efficiency for the selection of efficient and responsive genotypes.Methods: Plant growth, root morphological parameters, photosynthetic parameters, and P efficiency were evaluated under soluble P [KH2PO4 (K‐P)] and sparingly soluble P [Ca3(PO4)2 (Ca‐P), AlPO4 (Al‐P), FePO4 · 4 H2O (Fe‐P), and sodium phytate C6H6O24P6Na12 (Org‐P)] treatments using sand cultures.Results: Sugarcane growth was inhibited under sparingly soluble P or low P treatments, by an average of 22.9% or 71.3% reduction in shoot and root biomass, respectively. Al‐P and Org‐P had higher bioavailability than Fe‐P and Ca‐P for the tested sugarcane genotypes, which showed lower rate decreases for biomass, root morphological and photosynthetic parameters under Al‐P and Org‐P treatment than those under Fe‐P and Ca‐P. In addition, the P content in shoot decreased by an average of 29.2% and 39.2% under Al‐P and Org‐P treatment, respectively, while 55.7% and 62.9% reduction was detected under Fe‐P and Ca‐P treatment, respectively, compared with K‐P treatment. BC2‐32 (hybrid progeny of sugarcane and E. arundinaceus) exhibiting greater P absorption efficiency than other two genotypes under P‐starvation treatments; and the root length, root surface areas, root volume, P accumulation, net photosynthetic rate, and shoot and root dry matter of BC2‐32 were greater than those of other genotypes under sparingly soluble P forms. Significantly correlations were found between root morphological parameters, photosynthetic parameters and P efficiency under sparingly soluble P and low‐P treatment, indicating that stronger root system and less sensitivity of photosynthesis may contribute greatly to activate and take up more P from sparingly soluble P sources.Conclusions: Therefore, BC2‐32 would be the ideal material for breeding good varieties and exhibited better growth behavior to P‐deficient soils, and root morphological parameters and photosynthetic rate would be the useful plant traits in combination to screen these sugarcane varieties.

Experimental warming enhances the carbon gain but does not affect the yield of maize (Zea mays L.) in the North China Plain

Temperature response and sensitivity of photosynthesis and respiration are critical for projection of changes in the carbon exchange and net primary production of terrestrial ecosystems under global warming. Understanding the mechanisms and processes of photosynthetic and respiratory acclimation in response to warming may shed further lights on the change of crop yield in agricultural ecosystems in a warmer climate regime. We examined the temperature responses and sensitivity of net photosynthetic rate (An) and dark respiration (Rd) for exploring the mechanisms of thermal acclimation associated with physiological and biochemical processes affecting maize yield in the North China Plain with a field manipulative warming experiment. We found that warming substantially enhanced the carbon gain of maize plants through facilitating CO2 diffusion from ambient air to chloroplasts by altering stomatal structure and spatial distribution pattern, and benefitting CO2 assimilation efficiency with smaller vascular bundles and bigger chloroplasts. Moreover, we also found that acclimation of An to temperature (T), evidenced by the upward shift of An–T, was determined by the maximum velocity of Rubisco carboxylation (Vcmax), the maximum rate of electron transport (Jmax), and the stomatal-regulated CO2 diffusion process, whereas the balance between respiration and gross photosynthetic rate (Rd/Ag), and/or regeneration of RuBP and the Rubisco carboxylation (Jmax/Vcmax) made little contribution to the thermal acclimation of An in maize plants. In addition, temperature response and sensitivity of Rd was closely associated with the changes in foliar N concentration induced by warming. As a result, experimental warming barely affected the yield and biomass of maize plants. These results suggest that the impacts of future climate warming on maize production may be mitigated or even offset by the leaf-level thermal acclimation of photosynthesis and respiration. Our findings may have important implications for improving the accuracy of process-based ecosystem models and advancing the understanding on the interactions between ecosystem functions and climate warming.

Leaf-age effects on temperature responses of photosynthesis and respiration of an alpine oak, Quercus aquifolioides, in southwestern China.

Temperature responses and sensitivity of photosynthesis (A(n_)T) and respiration for leaves at different ages are crucial to modeling ecosystem carbon (C) cycles and productivity of evergreen forests. Understanding the mechanisms and processes of temperature sensitivity may further shed lights on temperature acclimation of photosynthesis and respiration with leaf aging. The current study examined temperature responses of photosynthesis and respiration of young leaves (YLs) (fully expanded in current growth season) and old leaves (OLs) (fully expanded in last growth season) of Quercus aquifolioides Rehder and E.H. Wilson in an alpine oak forest, southwestern China. Temperature responses of dark respiration (R(dark)), net assimilation (A(n)), maximal velocity of carboxylation (V(cmax)) and maximum rate of electron transport (J(max)) were significantly different between the two leaf ages. Those differences implied different temperature response parameters should be used for leaves of different ages in modeling vegetation productivity and ecosystem C cycles in Q. aquifolioides forests and other evergreen forests. We found that RuBP carboxylation determined the downward shift of A(n_)T in OLs, while RuBP regeneration and the balance between Rubisco carboxylation and RuBP regeneration made little contribution. Sensitivity of stomatal conductance to vapor pressure deficit changed in OLs and compensated part of the downward shift. We also found that OLs of Q. aquifolioides had lower An due to lower stomatal conductance, higher stomatal conductance limitation and deactivation of the biochemical processes. In addition, the balance between R(dark) and A(n) changed between OLs and YLs, which was represented by a higher R(dark)/A(n) ratio for OLs.

Identification and characterization of a null-activity mutant containing a cryptic pre-mRNA splice site for cytosolic fructose-1,6-bisphosphatase in Flaveria linearis

Cytosolic fructose-1,6-bisphosphatase (cytFBPase) (E.C. 3.1.3.11) catalyzes the first irreversible reaction of daytime sucrose synthesis. A Flaveria linearis (F. linearis) mutant (line 84-9) previously shown to have ~10% wildtype cytFBPase activity contains no cytFBPase activity based on enzymatic and immunoprecipitation analysis. Genetic segregation and Southern analysis of an F2 population shows one gene copy of cytFBPase in F. linearis and linkage of null cytFBPase activity to the cytFBPase structural gene. A point mutation is present in the structural gene coding for cytFBPase in the mutant, causing a cryptic pre-mRNA splice site and a corresponding 24 amino acid deletion spanning the active site of the enzyme. Collectively, these data support the identification of a null-activity mutant for cytFBPase in F. linearis. This is the first report of a null mutant in the daytime sucrose synthesis pathway confirmed by both enzymatic and molecular analysis. Null cytFBPase in F. linearis does not predispose all lines to high starch accumulation due to an epistatic gene interaction; low starch accumulation in null cytFBPase lines segregates with elevated pyrophosphate-dependent phosphofructokinase (PFP) activity when grown in a 16 h photoperiod. Surprisingly, growth of parental lines and F2 progeny having null cytFBPase in continuous light rescued the wildtype growth phenotype. All null cytFBPase lines showed CO(2)-insensitivity/reversed sensitivity of photosynthesis, indicating that null cytFBPase causes a reduced total capacity for both photosynthesis and end-product synthesis regardless of starch and PFP phenotype. Collectively, the data indicate that F. linearis, a C3-C4 photosynthetic intermediate, has alternative cytFBPase-independent pathways for daytime sucrose synthesis.

Simulating Canopy Transpiration and Photosynthesis of Corn Plants under Contrasting Water Regimes Using a Coupled Model

A process-based corn simulation model (MaizSim) was coupled with a two-dimensional soil simulator (2DSOIL) to simulate transpiration and photosynthesis of corn under contrasting water regimes. To improve the simulation of stomatal reaction to drought stress, a hydraulic stomatal control algorithm was implemented in the coupled photosynthesis-stomatal conductance module. Corn plants were grown in sunlit growth chambers and irrigated at three different frequencies. Simulated soil water content, transpiration, and photosynthesis rates were compared with those measured in the chambers. Comparison among simulations and measurements indicated that the coupled model was able to simulate the changes in soil water contents as well as the changes in transpiration and photosynthesis rates of corn plants under varying water status over the growing season. It was also found that the simulated photosynthesis rates were not as sensitive to reduction in stomatal conductance as the simulated transpiration rates. Reasons for this difference are discussed from the modeling viewpoint. This result agreed with the differences in sensitivity of photosynthesis and transpiration to changes in stomatal closure that have been reported in the literature. These results suggest that the coupled model not only is a valuable tool in studying corn transpiration and photosynthesis under drought stress, but it also provides a platform to implement and evaluate algorithms in studies of corn crop water dynamics and CO2 assimilation.

Effects of chronic elevated ozone exposure on gas exchange responses of adult beech trees ( Fagus sylvatica) as related to the within-canopy light gradient

The effects of elevated O 3 on photosynthetic properties in adult beech trees ( Fagus sylvatica) were investigated in relation to leaf mass per area as a measure of the gradually changing, within-canopy light availability. Leaves under elevated O 3 showed decreased stomatal conductance at unchanged carboxylation capacity of Rubisco, which was consistent with enhanced δ 13C of leaf organic matter, regardless of the light environment during growth. In parallel, increased energy demand for O 3 detoxification and repair was suggested under elevated O 3 owing to enhanced dark respiration. Only in shade-grown leaves, light-limited photosynthesis was reduced under elevated O 3, this effect being accompanied by lowered F v/ F m. These results suggest that chronic O 3 exposure primarily caused stomatal closure to adult beech trees in the field regardless of the within-canopy light gradient. However, light limitation apparently raised the O 3 sensitivity of photosynthesis and accelerated senescence in shade leaves.

Sensitivity of photosynthesis in a C4 plant, maize, to heat stress.

Our objective was to determine the sensitivity of components of the photosynthetic apparatus of maize (Zea mays), a C4 plant, to high temperature stress. Net photosynthesis (Pn) was inhibited at leaf temperatures above 38 degrees C, and the inhibition was much more severe when the temperature was increased rapidly rather than gradually. Transpiration rate increased progressively with leaf temperature, indicating that inhibition was not associated with stomatal closure. Nonphotochemical fluorescence quenching (qN) increased at leaf temperatures above 30 degrees C, indicating increased thylakoid energization even at temperatures that did not inhibit Pn. Compared with CO(2) assimilation, the maximum quantum yield of photosystem II (F(v)/F(m)) was relatively insensitive to leaf temperatures up to 45 degrees C. The activation state of phosphoenolpyruvate carboxylase decreased marginally at leaf temperatures above 40 degrees C, and the activity of pyruvate phosphate dikinase was insensitive to temperature up to 45 degrees C. The activation state of Rubisco decreased at temperatures exceeding 32.5 degrees C, with nearly complete inactivation at 45 degrees C. Levels of 3-phosphoglyceric acid and ribulose-1,5-bisphosphate decreased and increased, respectively, as leaf temperature increased, consistent with the decrease in Rubisco activation. When leaf temperature was increased gradually, Rubisco activation acclimated in a similar manner as Pn, and acclimation was associated with the expression of a new activase polypeptide. Rates of Pn calculated solely from the kinetics of Rubisco were remarkably similar to measured rates if the calculation included adjustment for temperature effects on Rubisco activation. We conclude that inactivation of Rubisco was the primary constraint on the rate of Pn of maize leaves as leaf temperature increased above 30 degrees C.

Roles of acidic lipids in photosynthesis

We produced mutants of Synechocystis sp. PCC6803 defective in phosphatidylglycerol (PG) or sulfoquinovosyl diacylglycerol (SQDG) synthesis to investigate the roles of these acidic lipids in photosynthesis. The mutant as to PG, CDP-diacylglycerol synthase gene (cdsA) of which was disrupted by homologous recombination, required PG supplementation for its growth. The cdsA mutant, shifted from PG-supplemented to PG-free medium, showed a decrease in cellular content of PG with the repressed growth. The decrease in PG content was accompanied with a reduction in cellular content of Chl, which was accounted for largely by a reduction of content of PSI complex. Simultaneously, p-benzoquinone-dependent oxygen evolution, higher than photosynthesis before the shift, became lower than photosynthesis, indicating some damage in PSII. On the other hand, the mutant as to SQDG, sqdB gene of which was disrupted, required SQDG supplementation for its growth. The sqdB mutant showed a decrease in cellular content of SQDG with the repressed growth after the shift from SQDG-supplemented to SQDG-free medium. Simultaneously, the sqdB mutant, in contrast to cdsA mutant, showed no decrease in Chl content. However, with the decrease in SQDG content, sensitivity of photosynthesis to DCMU was elevated, indicating that PSII was impaired. These results indicate that, for Synechocystis sp. PCC6803, PG and SQDG are respectively essential and responsible for normal functioning of PSII. However, accumulation of Chl-protein complexes in the thylakoid membranes are contributed by PG, but not by SQDG.

Daily and seasonal CO 2 exchange in Scots pine grown under elevated O 3 and CO 2 : experiment and simulation

Starting in early spring of 1994, naturally regenerated, 30-year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers and exposed in situ to doubled ambient O3,doubled ambient CO2 and a combination of O3 and CO2 from 15 April to 15 September. To investigate daily and seasonal responses of CO2 exchange to elevated O3 and CO2, the CO2 exchange of shoots was measured continuously by an automatic system for measuring gas exchange during the course of one year (from 1 Januray to 31 December 1996). A process-based model of shoot photosynthesis was constructed to quantify modifications in the intrinsic capacity of photosynthesis and stomatal conductance by simulating the daily CO2 exchange data from the field. Results showed that on most days of the year the model simulated well the daily course of shoot photosynthesis. Elevated O3 significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O3 also led to a delay in onset of photosynthetic recovery in early spring and an increase in the sensitivity of photosynthesis to environmental stress conditions. The combination of elevated O3 and CO2 had an effect on photosynthesis and stomatal conductance similar to that of elevated O3 alone, but significantly reduced the O3-induced depression of photosynthesis. Elevated CO2 significantly increased the photosynthetic capacity of Scots pine during the main growing season but slightly decreased it in early spring and late autumn. The model calculation showed that, compared to the control treatment, elevated O3 alone and the combination of elevated O3 and CO2 decreased the annual total of net photosynthesis per unit leaf area by 55% and 38%, respectively. Elevated CO2 increased the annual total of net photosynthesis by 13%.

Leaf growth, photosynthesis and tissue water relations of greenhouse-grown Eucalyptus marginata seedlings in response to water deficits.

Leaf growth, rate of leaf photosynthesis and tissue water relations of shoots of Eucalyptus marginata Donn ex Sm. (jarrah) seedlings were studied during a soil drying and rewatering cycle in a greenhouse experiment. Rates of leaf growth and photosynthesis were sensitive to water deficits. The rate of leaf growth decreased linearly with predawn leaf water potential to reach zero at -1.5 MPa. Rate of leaf growth did not recover completely within the first three days after rewatering. Midday photosynthetic rates declined to 40% of those of well-watered seedlings at a predawn leaf water potential of -1.0 MPa and reached zero at -2.2 MPa. Photosynthetic rate recovered rapidly following rewatering and almost fully recovered by the second day after rewatering. All tissue water relations parameters, except the bulk modulus of elasticity, changed significantly as the soil dried and recovered completely by the third day after rewatering. Changes in osmotic pressure at full turgor of 0.4 MPa indicated considerable capacity for osmotic adjustment. However, because there was little osmotic adjustment until predawn leaf water potential fell below -1.5 MPa, this capacity would not have enhanced seedling growth, although it may have increased seedling survival. The sensitivity of photosynthesis and relative water content to water deficits suggests that greenhouse-grown E. marginata seedlings behave like mesophytic plants, even though E. marginata seedlings naturally grow in a drought-prone environment.

Effect of light and temperature on the photosynthetic activity of the cyanobacterium Spirulina platensis

The response of the photosynthetic activity of Spirulina platensis M2 to temperature and light stress was studied. The optimal temperature for photosynthesis was 35°C, while dark respiration was highest at 45 C. At temperature extremes outside those optimal for growth, both respiratory and photosynthetic activity declined. However, the sensitivity of respiration to such extremes was significantly greater than the sensitivity of photosynthesis under the same conditions. Under conditions where respiration was completely inhibited, photosynthetic oxygen evolution was maintained at about 30% of the optimum value. Exposing Spirulina cells to high photon flux densities results in a significant reduction in all the photosynthetic parameters (i.e. initial slope, light-saturated rate and the convexity of the photosynthesis-light response curve). The photoinhibitory stress resulted in much larger decreases in the quantum yield than in the light-saturated rate of photosynthetic oxygen evolution. The extent of photoinhibition was much higher when applied at temperatures over or below the optimum for photosynthesis. The implications of these findings are discussed with respect to the outdoor cultivation of Spirulina.

Estimation of Mesophyll Conductance to CO2 Flux by Three Different Methods

The resistance to diffusion of CO(2) from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, (13)C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO(2) diffusion determined in unstressed plants at high light. The relatively low CO(2) partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O(2) sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO(2) partial pressure upon switching to low O(2). We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO(2) and O(2) partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO(2)-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO(2) level.

Mineral nutrient deficiency increases the sensitivity of photosynthesis to sulphur dioxide in needles of a coniferous tree, Abies nordmanniana.

Abies nordmanniana (Stev.) Spach was cultivated in rooting media either rich in nutrients (control) or low in magnesium (low Mg) or low in magnesium and nitrogen (low Mg-N), respectively. Intact, attached needles were exposed, in the light (460 μmol photons m-2 s-1), to an atmosphere containing 1 ppm SO2 for 5 h. Measurements of light- and CO2-saturated rates of photosynthetic O2 evolution, A max, were performed before and after SO2 treatments. In needles from well fertilized plants, A max was high (about 50 μmol m-2 s-1) and was not affected by SO2. Needles from low-Mg and low-Mg-N plants had lower photosynthetic rates and showed a marked decline in A max in response to the SO2 treatment. Stomatal conductance was similar in the three groups of plants during SO2 treatments.

Sensitivity of photosynthesis by spinach chloroplast membranes to osmotic stress in vitro: Rapid inhibition of O2 evolution in presence of magnesium

Thylakoids prepared from spinach (Spinacea oleracea L.) chloroplasts were exposed to osmotic stress in vitro in the presence or absence of different inorganic salts. By an hour after incubation in 1.0 M sorbitol and 10 mM (or more) MgCl2, the thylakoids lost approximately 80% of their photosystem (PS) II activity, but not PS I. The inhibition occurred only in presence of magnesium as indicated by the combinations of several cations/anions. The PS II activity was relatively insensitive to osmotic stress in the presence of diphenyl carbazide. We therefore conclude that under conditions of water stress in the presence of 10 mM or higher Mg(2+), the oxygen evolving system in chloroplasts is rapidly inactivated.

The effect of air pollutants on physiological processes in plants

Abstract. Important physiological processes, photosynthesis, respiration, carbon allocation and stomatal function are known to be affected by air pollutants. A wide range in sensitivity of photosynthesis both within and between species is evident from the literature for the pollutants sulphur dioxide, ozone, nitrogen oxide and hydrogen fluoride. Some of this variation is clearly due to genetic factors, but much is in response to differences in environmental conditions both prior to and during fumigation. Exposure of plants to mixtures of pollutants generally reduced the threshold at which effects were first detected and increased the level of inhibitory responses. In the majority of studies on stomatal responses to air pollutants, opening occurs at low concentrations, below the threshold for effects on photosynthesis, and closure occurs at injurious concentrations; this latter response often following the inhibition of photosynthesis. Effects on carbon allocation have been reported in response to air pollutants. Changes usually favour leaf development over root growth, which can compensate for a decline in net assimilation rate up to a certain point but may limit water uptake from soils with low moisture content. Future research into physiological effects of air pollutants should incorporate an integrated approach in which both key physiological parameters and growth parameters are measured together with estimates of the effective dose of pollutant. In this way, the underlying mechanisms to changes in growth and development will be more fully understood.

Photosynthetic characteristics of Prochloron sp./ascidian symbioses

The prokaryotic green alga Prochloron sp. (Prochlorophyta) is found in symbiotic association with colonial didemnid ascidians that inhabit warm tropical waters in a broad range of light environments. We sought to determine the light-adaptation features of this alga in relation to the natural light environments in which the symbioses are found, and to characterize the temperature sensitivity of photosynthesis and respiration of Prochloron sp. in order to assess its physiological role in the productivity and distribution of the symbiosis. Colonies of the host ascidian Lissoclinum patella were collected from exposed and shaded habitats in a shallow lagoon in Palau, West Caroline Islands, during February and March, 1983. Some colonies from the two light habitats were maintained under conditions of high light (2 200 μE m−2 s−1) and low light (400 μE m−2 s−1) in running seawater tanks. The environments were characterized in terms of daily light quantum fluxes, daily periods of light-saturated photosynthesis (Hsat), and photon flux density levels. Prochloron sp. cells were isolated from the hosts and examined for their photosynthesis vs irradiance relationships, respiration, pigment content and photosynthetic unit features. In addition, daily P:R ratios, photosynthetic quotients, carbon balances and photosynthetic carbon release were also characterized. It was found that Prochloron sp. cells from low-light colonies possessed lower chlorophyll a/b ratios, larger photosynthetic units sizes based on both reaction I and reaction II, similar numbers of reaction center I and reaction center II per cell, lower respiration levels, and lower Pmax values than cells from high-light colonies. Cells isolated from low-light colonies showed photoinhibition of Pmax at photon flux densities above 800 μE m−2 s−1. However, because the host tissue attenuates about 60 to 80% of the incident irradiance, it is unlikely that these cells are normally photoinhibited in hospite. Collectively, the light-adaptation features of Prochloron sp. were more similar to those of eukaryotic algae and vascular plant chloroplasts than to those of cyanobacteria, and the responses were more sensitive to the daily flux of photosynthetic quantum than to photon flux density per se. Calculation of daily minimum carbon balances indicated that, though high-light cells had daily P:R ratios of 1.0 compared to 4.6 for low-light cells, the cells from the two different light environments showed nearly identical daily carbon gains. Cells isolated from high-light colonies released between 15 and 20% of their photosynthetically-fixed carbon, levels sufficient to be important in the nutrition of the host. Q10 responses of photosynthesis and respiration in Prochloron sp. cells exposed briefly (15–45 min) to temperatures between 15° and 45°C revealed a discontinuity in the photosynthetic response at the ambient growth temperatures. The photosynthetic rates were found to be more than twice as sensitive to temperatures below ambient (Q10=3.47) than to temperatures above ambient (Q10=1.47). The Q10 for respiration was constant (Q10=1.66) over the temperature range examined. It appears that the photosynthetic temperature sensitivity of Prochloron sp. may restrict its distribution to warmer tropical waters. The ecological implications of these findings are discussed in relation to published data on other symbiotic systems and free-living algae.

Reactions in chloroplasts, cytoplasm and mitochondria of leaf slices under osmotic stress

The effect of osmotic dehydration on metabolic reactions in three different subcellular compartments (chloroplast, cytoplasm and mitochondria) was studied in vacuum-infiltrated thin leaf slices from various plants, in the absence of stomatal control. The reactions tested were CO2 fixation in the light (chloroplast), CO2 fixation in the dark (cytoplasm), and O2 uptake in the dark (mitochondria). In most plants, the sensitivity of dark CO2 fixation to dehydration was similar to the sensitivity of photosynthesis. In leaf slices from a plant with Crassulacean acid metabolism (Kalanchoe pinnata), dark CO2 fixation (which reached similar rates as light fixation) was slightly more sensitive to osmotic stress than photosynthesis. Dark respiration (measured as O2 uptake) was significantly more resistant to hypertonic stress than both types of CO2 fixation. In crude leaf extracts from spinach, the response of soluble enzymes from the three different subcellular compartments to high concentrations of various electrolytes and neutral compounds was examined and compared with the in-vivo data.

Respiratory Activities of Plant Mitochondria Under Osmotic Stress

Summary In thin slices of leaves or potato tubers dehydrated in hypertonic sorbitol solutions, photosynthesis or dark-fixation of CO 2 was almost fully inhibited at an osmotic potential of 50 bar, while respiratory oxygen uptake was less than 50% reduced. At the same osmotic potential, coupled oxidation of succinate and exogenous NADH by isolated leaf or tuber mitochondria suspended in media containing sorbitol or sucrose was between 20% and 60% inhibited. As in situ , oxidation was more sensitive to osmotic stress in tuber than in leaf mitochondrial preparations. Coupled oxidation of glycine or malate by leaf mitochondria was only insignificantly reduced when sorbitol was used as osmoticant, while strong inhibition was observed with sucrose or raffinose. The difference appeared to be due to penetration of sorbitol which caused less osmotic shrinkage of mitochondria thus preventing internal solutes from reaching injurious levels. Inhibition of substrate oxidation during osmotic dehydration of mitochondria decreased respiratory control and, less significantly, ADP/O ratios. In contrast to osmotic inhibition of photosynthesis in isolated chloroplasts (Kaiser et al., 1981 a), osmotic inhibition of respiratory activity was found to be reversible on transfer of mitochondria from a hypertonic to an isotonic medium. The low sensitivity of respiration to water stress in vivo compared to the higher sensitivity of photosynthesis is ascribed to different effects. In contrast to chloroplasts illuminated with high light intensities, mitochondria do not work at full capacity in situ . Inhibition of excess work capacity by water stress does not necessarily find expression in performance which reflects ATP turnover and slippage (basal electron transfer). In addition, entry of compatible neutral compounds such as monosaccharides into mitochondria may prevent excessive shrinkage during dehydration. In vitro , this reduces inhibition of respiratory reactions. Leaf mitochondria are more permeable to monosaccharides than are chloroplasts.

The effect of nickel on the growth, photosynthesis, and nitrogenase activity of Anabaena inaequalis.

Anabaena inaequalis was sensitive to nickel ion in the order of decreasing sensitivity of growth, photosynthesis, and acetylene reduction. At a culture density of 9 x 10(4) cells per millilitre, growth after 12 days was completely inhibited by 0.125 ppm (microgram/mL) Ni2+. Nickel caused the increase of both the lag phase of growth and the culture doubling time, and caused the retardation phase to be sooner. Photosynthesis and acetylene reduction were completely inhibited by 10 and 20 ppm Ni2+, respectively, at a cell concentration of 1.3 x 10(6) cells per millilitre. Preincubation for 24 h in the presence of nickel ion significantly increased the sensitivity of photosynthesis and acetylene reduction. Under these conditions acetylene reduction was more sensitive than photosynthesis. Nickel ion reduced culture growth by 35% at a level of 0.05 ppm and inhibited that culture's acetylene-reducing ability by 29% while leaving photosynthesis unaffected. Nickel caused some damage to filament apical cells and induced pigment bleaching in aged cultures. Nickel toxicity was proposed to be due to poisoning of intracellular enzyme systems by nickel ions.

Phosphoenolpyruvate carboxylase reduces photorespiration in Panicum milioides, a C 3C 4 intermediate species

Panicum milioides represents the first well-documented example of a higher plant species with reduced photorespiration and O 2 inhibition of photosynthesis. We have investigated the biochemical mechanism(s) involved in reducing O 2 sensitivity of photosynthesis in this species by parallel enzyme inhibitor experiments with thin leaf slices of P. milioides and C 3 and C 4 Panicum species. The reduced O 2 sensitivity of net photosynthesis in P. milioides gradually increased with increasing concentrations of the phosphoenolpyruvate carboxylase (EC 4.1.1.31) inhibitors, maleate and malonate. At saturating levels of inhibitor, photosynthesis in 2% O 2 was decreased by about 18%, and the inhibitory effects of both 21% O 2 and 49% O 2 were identical to those observed with a C 3 Panicum species in the absence or presence of inhibitor. A significant potential for C 4 photosynthesis in P. milioides, compared to its complete absence in a C 3 Panicum species, was demonstrated on the basis of: (a) a coupling of leaf slice CO 2 fixation by phosphoenolpyruvate carboxylase with the C 3 cycle; (b) NAD-malic enzyme (EC 1.1.1.39)-dependent aspartate and malate decarboxylation in leaf slices; (c) a full complement of C 4 cycle enzymes in leaf extracts, including pyruvate, P i dikinase (EC 2.7.9.1) and NAD-malic enzyme; and (d) Kranz-like leaf anatomy with numerous plasmodesmata traversing the mesophyll-bundle sheath interfacial cell wall. These data indicate that the reduced photorespiration and O 2 inhibition of photosynthesis in P. milioides is due to phosphoenolpyruvate carboxylase participation, possibly by creating a limited C 4-like CO 2 pump, rather than an altered ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39).