Reduced osmotic potential effects on photosynthesis : identification of stromal acidification as a mediating factor

Addition of sorbitol, which facilitated reductions in reaction medium osmotic potential from standard (0.33 molar sorbitol, −10 bars) isotonic conditions to a stress level of 0.67 molar sorbitol (−20 bars), inhibited the photosynthetic capacity of isolated spinach (Spinacia oleracea) chloroplasts. This inhibition, which ranged from 64 to 74% under otherwise standard reaction conditions, was dependent on reaction medium inorganic phosphate concentration, with the phosphate optimum for photosynthesis reduced to 0.05 millimolar at the low osmotic potential stress treatment from a value of 0.25 millimolar under control conditions.

Stromal alkalating agents such as NH₄Cl (0.75 millimolar) and KCl (35 millimolar) were also found to affect the degree of low osmotic potential inhibition of photosynthesis. Both agents doubled the rate of NaHCO₃-supported O₂ evolution under the stress treatment, while hardly affecting the control rate at optimal concentrations. These agents also reduced the length of the lag phase of photosynthetic O₂ evolution under the stress treatment to a much greater degree. The rate-enhancement effect of these agents under the stress treatment was reversed by sodium acetate, which is known to facilitate stromal acidification.

The reaction medium pH optimum for photosynthesis under the stress treatment was higher than under control conditions. In the presence of optimal NH₄Cl, this shift was no longer evident.

Internal pH measurements indicated that the stress treatment caused a 0.43 and 0.24 unit reduction in the stromal and intrathylakoid pH, respectively, under illumination. This osmotically induced acidification was not evident in the dark. The presence of 0.75 millimolar NH₄Cl partially reversed the osmotically induced reduction in the illuminated stromal pH. It was concluded that stromal acidification is a mediating mechanism of the most severe site of low osmotic potential inhibition of the photosynthetic process.

     

Effect of osmotic stress on photosynthesis studied with the isolated spinach chloroplast : site-specific inhibition of the photosynthetic carbon reduction cycle

The effects of reduced osmotic potential on the photosynthetic carbon reduction cycle were investigated by monitoring photosynthetic processes of spinach (Spinacia oleracea L. var. Long Standing Bloomsdale) chloroplasts exposed to increased assay medium sorbitol concentrations. CO₂ assimilation was found to be inhibited at 0.67 molar sorbitol by about 60% from control rates at 0.33 molar sorbitol. This level of stress inhibition was greater than that affecting the reductive phase of the cycle; glycerate 3-phosphate reduction was inhibited at 0.67 molar by 27 to 40%. Sorbitol (0.67 molar) inhibited the rate of O₂ evolution at saturating and limiting concentrations of NaHCO₃, and extended the lag phase of O₂ evolution. This indicated that factors which are rate-limiting to the photosynthetic process are adversely affected by reduced osmotic potential.

Analysis of photosynthetic products following CO₂ fixation in 0.33 molar sorbitol and 0.67 molar sorbitol indicated that reduced osmotic potential facilitated increases in the levels of fructose 1,6-bisphosphate and triose phosphates with reductions in glucose 6-phosphate and fructose 6-phosphate, implicating fructose 1,6-bisphosphatase as a site of osmotic stress. Osmotic inhibition of the reductive portion (glycerate 3-phosphate to triose phosphate) of the photosynthetic carbon reduction cycle was partially attributed to feedback inhibition by the product, triose phosphate, on glycerate 3-phosphate reduction. A saturating concentration of ribose 5-phosphate partially overcame osmotic inhibition of CO₂-supported O₂ evolution, indicating another but apparently less severe site of stress inhibition in the sequence of ribose 5-phosphate to glycerate 3-phosphate.

     

Stromal acidification mediates in vivo water stress inhibition of nonstomatal-controlled photosynthesis

Stromal acidification has been reported to mediate reduced osmotic potential (ψ_π) effects on photosynthesis in the isolated spinach chloroplast (Berkowitz, Gibbs 1983 Plant Physiol 72: 1100-1109). To determine if stromal acidification mediates osmotic dehydration inhibition of photosynthesis in vivo, the effects of a weak base (NH₄Cl), which raises stromal pH, on CO₂ fixation of vacuum-infiltrated spinach leaf slices, Chlamydomonas reinhardii cells and Aphanocapsa 6308 cells under isotonic and dehydrating conditions were investigated. Five millimolar NH₄Cl stimulated spinach leaf slice CO₂ fixation by 43% under stress (0.67 molar sorbitol) conditions, and had little effect on fixation under isotonic (0.33 molar sorbitol) conditions. Chlamydomonas cells were found to be more sensitive to reduced ψ_π than spinach leaf slices. CO₂ fixation in the cells of the green alga Chlamydomonas reinhardii was 99 and 17 micromoles per milligram chlorophyll per hour, respectively, at 0.1 molar mannitol and 0.28 molar mannitol. Five millimolar NH₄Cl stimulated CO₂ fixation of Chlamydomonas cells by 147% under stress (0.28 molar mannitol) conditions. Aphanocapsa 6308 cells (blue-green alga) were also found to be sensitive to reduced ψ_π, and inhibitions in photosynthesis were partially reversed by NH₄Cl. These data indicate that in vivo water stress inhibition of photosynthesis is facilitated by stromal acidification, and that this inhibition can be at least partially reversed in situ.     

Chloroplast osmotic adjustment and water stress effects on photosynthesis

Previous studies have suggested that chloroplast stromal volume reduction may mediate the inhibition of photosynthesis under water stress. In this study, the effects of spinach (Spinacia oleracea, var `Winter Bloomsdale') plant water deficits on chloroplast photosynthetic capacity, solute concentrations in chloroplasts, and chloroplast volume were studied. In situ (gas exchange) and in vitro measurements indicated that chloroplast photosynthetic capacity was maintained during initial leaf water potential (Ψ_w) and relative water content (RWC) decline. During the latter part of the stress period, photosynthesis dropped precipitously. Chloroplast stromal volume apparently remained constant during the initial period of decline in RWC, but as leaf Ψ_w reached −1.2 megapascals, stromal volume began to decline. The apparent maintenance of stromal volume over the initial RWC decline during a stress cycle suggested that chloroplasts are capable of osmotic adjustment in response to leaf water deficits. This hypothesis was confirmed by measuring chloroplast solute levels, which increased during stress. The results of these experiments suggest that stromal volume reduction in situ may be associated with loss of photosynthetic capacity and that one mechanism of photosynthetic acclimation to low Ψ_w may involve stromal volume maintenance.     

Leaky vessels as a potential source of stromal acidification in tumours

Malignant tumours are characterised by higher rates of acid production and a lower extracellular pH than normal tissues. Previous mathematical modelling has indicated that the tumour-derived production of acid leads to a gradient of low pH in the interior of the tumour extending to a normal pH in the peritumoural tissue. This paper uses mathematical modelling to examine the potential of leaky vessels as an additional source of stromal acidification in tumours. We explore whether and to what extent increasing vascular permeability in vessels can lead to the breakdown of the acid gradient from the core of the tumour to the normal tissue, and a progressive acidification of the peritumoural stroma. We compare our mathematical simulations to experimental results found in vivo with a tumour implanted in the mammary fat pad of a mouse in a window chamber construct. We find that leaky vasculature can cause a net acidification of the normal tissue away from the tumour boundary, though not a progressive acidification over time as seen in the experiments. Only through progressively increasing the leakiness can the model qualitatively reproduce the experimental results. Furthermore, the extent of the acidification predicted by the mathematical model is less than as seen in the window chamber, indicating that although vessel leakiness might be acting as a source of acid, it is not the only factor contributing to this phenomenon. Nevertheless, tumour destruction of vasculature could result in enhanced stromal acidification and invasion, hence current therapies aimed at buffering tumour pH should also examine the possibility of preventing vessel disruption.     

Activation of Na+/H+ exchange in lymphocytes by osmotically induced volume changes and by cytoplasmic acidification

After swelling in hypotonic solutions, peripheral blood mononuclear cells (PBM) shrink toward their original volumes. Upon restoration of isotonicity, the cells initially shrink but then regain near-normal size again. This regulatory volume increase (RVI) is abolished by removal of Na+o or Cl-o or by addition of amiloride. RVI is unaffected by removal of K+o or by ouabain and is only partially inhibited by 1 mM furosemide. As a result of increased influx, the cells gain both Na+ and K+ during reswelling. In contrast, only Na+ content increases in the presence of ouabain. Amiloride largely eliminates the changes in the content of both cations. Using diS-C3-(5), no significant membrane potential changes were detected during RVI, which suggests that the fluxes are electroneutral. The cytoplasmic pH of volume-static cells was measured with 5,6-dicarboxyfluorescein. After acid loading, the addition of extracellular Na+ induced an amiloride-inhibitable alkalinization, which is consistent with Na+/H+ exchange. Cytoplasmic pH was not affected by cell shrinkage itself, but an internal alkalinization, which was also amiloride sensitive and Na+ dependent, developed during reswelling. In isotonic lightly buffered solutions without HCO-3, an amiloride-sensitive acidification of the medium was measurable when Na+ was added to shrunken PBM. K+ was unable to mimic this effect. The observations are compatible with the model proposed by Cala (J. Gen. Physiol. 1980. 76:683-708), whereby an electroneutral Na+o/H+i exchange is activated by osmotic shrinking. Cellular volume gain occurs as Cl-o simultaneously exchanges for either HCO-3i or OH-i. Na+i is secondarily replaced by K+ through the pump, but this step is not essential for RVI.     

Asymmetry of canine tracheal epithelium: osmotically induced changes

The symmetry of osmotic conductivity of the canine tracheal epithelial cells was examined in vitro. When an osmotic load of 100 mosM sucrose was added to the serosal bathing solution, no change in the transepithelial potential difference was observed in 15 tissue preparations. In contrast, when the same osmotic load was added to the mucosal bathing solution, there was a rapid decrease in the transepithelial potential difference of 3.9 +/- 0.5 mV (n = 23); ouabain (10(-4) M) eliminated this change. Tissues that had been exposed to the osmotic load added to either the mucosal or serosal side were compared with the control using light and electron microscopy. When the osmotic load was added to the mucosal fluid, there was no change in the nuclear-to-cytoplasmic area ratio of the cell types examined. However, when the same osmotic load was added to the serosal fluid, a marked increase in the nuclear-to-cytoplasmic area ratio of the ciliated cells was observed. This finding indicated cell shrinkage. Dilution potentials measured by substituting NaCl with mannitol also showed asymmetry. The morphological features are probably caused by differences in the osmotic conductivity (Lp) of the basolateral and apical cell membranes, with the Lp of the apical membrane being less than that of the basolateral membrane. The basis for osmotically induced potentials remained undetermined.     

Microenvironmental changes support evidence of photosynthesis and calcification inhibition in Halimeda under ocean acidification and warming

The effects of elevated CO₂ and temperature on photosynthesis and calcification of two important calcifying reef algae (Halimeda macroloba and Halimeda cylindracea) were investigated with O₂ microsensors and chlorophyll a fluorometry through a combination of two pCO₂ (400 and 1,200 μatm) and two temperature treatments (28 and 32 °C) equivalent to the present and predicted conditions during the 2100 austral summer. Combined exposure to pCO₂ and elevated temperature impaired calcification and photosynthesis in the two Halimeda species due to changes in the microenvironment around the algal segments and a reduction in physiological performance. There were no significant changes in controls over the 5-week experiment, but there was a 50–70 % decrease in photochemical efficiency (maximum quantum yield), a 70–80 % decrease in O₂ production and a threefold reduction in calcification rate in the elevated CO₂ and high temperature treatment. Calcification in these species is closely coupled with photosynthesis, such that a decrease in photosynthetic efficiency leads to a decrease in calcification. Although pH seems to be the main factor affecting Halimeda species, heat stress also has an impact on their photosystem II photochemical efficiency. There was a strong combined effect of elevated CO₂ and temperature in both species, where exposure to elevated CO₂ or temperature alone decreased photosynthesis and calcification, but exposure to both elevated CO₂ and temperature caused a greater decline in photosynthesis and calcification than in each stress individually. Our study shows that ocean acidification and ocean warming are drivers of calcification and photosynthesis inhibition in Halimeda. Predicted climate change scenarios for 2100 would therefore severely affect the fitness of Halimeda, which can result in a strongly reduced production of carbonate sediments on coral reefs under such changed climate conditions.     

Characteristics of the osmotically induced membrane rupture

The phenomenon of reciprocating mechanical oscillations of electrofused erythrocytes was used to investigate the mechanical characteristics of ruptures induced in erythrocyte membranes by colloid osmotic pressure. The rupture characteristics follow an exponentially decaying time function. Time constants determined for opening times of ruptures decreased from 5.5 ms at 10 degrees C to 3.8 ms and 2.0 ms at 40 degrees C for the first and the last observable rupture, respectively. Evidence is given that the diameter of the membrane rupture exceeds the size of a haemoglobin molecule. With repetitive membrane rupturing, the ability of the membrane bilayer and associated structures to heal decreases, owing to the reduced ability to withstand pressure gradients. This change allows oscillating doublets to be classified according to one of three groups: group A showing no development in response to swell times, group B showing a continuous decrease in response to swell times, and group C showing a spontaneous decrease in response to swell times. These results suggest that oscillations cease as a result of defects of membrane healing. Calculations of respective temperature ranges are in agreement with temperature ranges for spectrin denaturation. Thus, conclusions obtained from this study suggest that the spectrin network plays a key role in membrane healing processes after mechanical membrane rupture.     

Rapid heating of intact leaves reveals initial effects of stromal oxidation on photosynthesis

Heat stress in leaves under natural conditions is characterized by rapid fluctuations in temperature. These fluctuations can be on the order of 10 degrees C in 7 s. By using a specially modified gas-exchange chamber, these conditions were mimicked in the laboratory to analyse the biochemical response to heat spikes. The decline in ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) activity during prolonged heat stress is generally associated with an increase in ribulose 1,5-bisphosphate (RuBP) levels. However, rapid heating caused an initial decline in RuBP which was subsequently followed by a small decline in Rubisco carbamylation. The ratio of RuBP to Rubisco sites declined from a saturating concentration to a sub-saturating concentration, providing a possible mechanism for the decarbamylation of Rubisco. If RuBP is saturating (>1.8 RuBP Rubisco site(-1)), it acts as a cap on the catalytic site and keeps Rubisco activated. Measurements of triose-phosphate levels and NADP-malate dehydrogenase activation (a stromal redox proxy) indicated that the regeneration of RuBP by the Calvin cycle was limited by the availability of redox power.     

The osmotic activation of transporter ProP is tuned by both its C-terminal coiled-coil and osmotically induced changes in phospholipid composition

Transporter ProP of Escherichia coli (ProPEc) senses extracellular osmolality and mediates osmoprotectant uptake when it is rising or high. A replica of the ProPEc C terminus (Asp468-Arg497) forms an intermolecular alpha-helical coiled-coil. This structure is implicated in the osmoregulation of intact ProPEc, in vivo. Like that from Corynebacterium glutamicum (ProPCg), the ProP orthologue from Agrobacterium tumefaciens (ProPAt) sensed and responded to extracellular osmolality after expression in E. coli. The osmotic activation profiles of all three orthologues depended on the osmolality of the bacterial growth medium, the osmolality required for activation rising as the growth osmolality approached 0.7 mol/kg. Thus, each could undergo osmotic adaptation. The proportion of cardiolipin in a polar lipid extract from E. coli increased with extracellular osmolality so that the osmolality activating ProPEc was a direct function of membrane cardiolipin content. Group A ProP orthologues (ProPEc, ProPAt) share the C-terminal coiled-coil domain and were activated at low osmolalities. Like variant ProPEc-R488I, in which the C-terminal coiled-coil is disrupted, ProPEc derivatives that lack the coiled-coil and Group B orthologue ProPCg required a higher osmolality to activate. The amplitude of ProPEc activation was reduced 10-fold in its deletion derivatives. The coiled-coil structure is not essential for osmotic activation of ProP per se. However, it tunes Group A orthologues to osmoregulate over a low osmolality range. Coiled-coil lesions may impair both coiled-coil formation and interaction of ProPEc with amplifier protein ProQ. Cardiolipin may contribute to ProP adaptation by altering bulk membrane properties or by acting as a ProP ligand.     

Oxygen inhibition of photosynthesis

The effect of leaf temperature, O₂ and calculated O₂/CO₂ solubility ratio in the leaf on the quantum yield of photosynthesis was studied for the C₄ species, Zea mays L., and the C₃ species, Triticum aestivum L. Over a range of leaf temperatures of 16 to 35° C, the quantum yield of Z. mays was relatively constant and was similar under 1.5 and 21% O₂, being ca. 0.059 mol CO₂ mol^-1 quanta absorbed. Under 1.5% O₂ and atmospheric levels of CO₂, the quantum yield of T. aestivum was relatively constant (0.083 mol CO₂ mol^-1 quanta absorbed) at leaf temperatures from 15 to 35° C. Atmospheric levels of O₂ (21%) reduced the quantum yield of photosynthesis in T. aestivum and as leaf temperature increased, the quantum yield decreased from 0.062 at 15°C to 0.046 mol CO₂ mol^-1 quanta absorbed at 35°C. Increasing temperature decreases the solubility of CO₂ relatively more than the solubility of O₂, resulting in an increased solubility ratio of O₂/CO₂. Experimentally manipulating the atmospheric levels of O₂ or CO₂ to maintain a near-constant solubility ratio of O₂/CO₂ at varying leaf temperatures largely prevented the temperature-dependent decrease in quantum yield in t. aestivum. Thus, the decrease in quantum yield with increasing leaf temperature in C₃ species may be largely caused by a temperaturedependent change in the solubility ratio of O₂/CO₂.J and II=Ku and Edwards, 1977a, b     

Characterization of osmotically induced filaments of Salmonella enterica

Salmonella enterica forms aseptate filaments with multiple nucleoids when cultured in hyperosmotic conditions. These osmotic-induced filaments are viable and form single colonies on agar plates even though they contain multiple genomes and have the potential to divide into multiple daughter cells. Introducing filaments that are formed during osmotic stress into culture conditions without additional humectants results in the formation of septa and their division into individual cells, which could present challenges to retrospective analyses of infectious dose and risk assessments. We sought to characterize the underlying mechanisms of osmotic-induced filament formation. The concentration of proteins and chromosomal DNA in filaments and control cells was similar when standardized by biomass. Furthermore, penicillin-binding proteins in the membrane of salmonellae were active in vitro. The activity of penicillin-binding protein 2 was greater in filaments than in control cells, suggesting that it may have a role in osmotic-induced filament formation. Filaments contained more ATP than did control cells in standardized cell suspensions, though the levels of two F(0)F(1)-ATP synthase subunits were reduced. Furthermore, filaments could septate and divide within 8 h in 0.2 × Luria-Bertani broth at 23°C, while nonfilamentous control cells did not replicate. Based upon the ability of filaments to septate and divide in this diluted broth, a method was developed to enumerate by plate count the number of individual, viable cells within a population of filaments. This method could aid in retrospective analyses of infectious dose of filamented salmonellae.     

The inhibition of photosynthesis and photorespiration in isolated mesophyll cells of Phaseolus and Lycopersicon by reduced osmotic potentials

Mesophyll cells isolated from Phaseolus vulgaris and Lycopersicon esculentum show decreasing photosynthetic rates when suspended in media containing increasing concentrations of osmoticum. The photosynthetic activity was sensitive to small changes in osmotic potential over a range of sorbitol concentrations from 0.44 M (−1.08 MPa) to 0.77 M (−1.88 MPa). Photorespiration assayed by ¹⁴CO₂ release in CO₂-free air and by ¹⁴CO₂ release from the oxidation of [1–¹⁴C] glycolate also decreased as the osmotic potential of the incubation medium was reduced. The CO₂ compensation points of the cells increased with increasing concentration of osmoticum from approximately 60 μ I⁻¹1 at −1.08 MPa to 130 μl 1⁻¹ for cells stressed at −1.88 MPa. Changes in photosynthetic and photorespiratory activities occurred at moderate osmotic potentials in these cells suggesting that in whole leaves during a reduction in water potential, non- stomatal inhibition of CO₂ assimilation and glycolate pathway metabolism occurs simultaneously with stomatal closure.     

Modulation of osmotic stress effects on photosynthesis and respiration by temperature in mesophyll protoplast of pea

Exposure of mesophyll protoplast of pea to osmotic stress decreases the rate of photosynthesis while stimulating marginally the respiratory rate of mesophyll protoplasts. The interaction of osmotic and temperature stress during the modulation of photosynthetic and respiratory rates of pea (Pisum sativum var Azad P1) mesophyll protoplasts was investigated. The protoplasts were exposed to either iso-osmotic (0.4 M) or hyper-osmotic (1.0 M) concentration of sorbitol at 15 degrees and 25 degrees C. The rates of photosynthesis and respiration were studied. At optimum temperature of 25 degrees C, there was a decrease in photosynthesis (< 10%) at hyper-osmoticum (osmotic effect), whereas respiration increased marginally (by about 15%). Low temperature (15 degrees C) aggravated the sensitivity of both respiration and photosynthesis to osmotic stress. At 15 degrees C, the decrease in photosynthesis due to osmotic stress was > 35%, while the respiratory rate was stimulated by 30%. The relative proportion of cytochrome pathway decreased by about 50% at both 15 degrees C and 25 degrees C while that of alternative pathway increased, more so, at 15 degrees C, when the mesophyll protoplasts were subjected to hyper-osmoticum stress. The titration experiments showed that extent of engagement of alternative pathway was higher, the slope value was slightly higher for 15 degrees C compared to 25 degrees C. Low temperature modulates the effect of hyper-osmoticum stress on photosynthesis and respiration, and results in increased participation of alternative pathway.     

The membrane potential of Arabidopsis thaliana guard cells; depolarizations induced by apoplastic acidification

The apoplastic pH of guard cells probably acidifies in response to light, since light induces proton extrusion by both guard cells and epidermal leaf cells. From the data presented here, it is concluded that these apoplastic pH changes will affect K⁺ fluxes in guard cells of Arabidopsis thaliana (L.) Heynh. Guard cells of this species were impaled with double-barrelled microelectrodes, to measure the membrane potential (E_m) and the plasma-membrane conductance. Guard cells were found to exhibit two states with respect to their E_m, a depolarized and a hyperpolarized state. Apoplastic acidification depolarized E_m in both states, though the origin of the depolarization differed for each state. In the depolarized state, the change in E_m was the result of a combined pH effect on instantaneously activating conductances and on the slow outward rectifying K⁺ channel (s-ORC). At a more acidic apoplastic pH, the current through instantaneously activated conductances became more inwardly directed, while the maximum conductance of s-ORC decreased. The effect on s-ORC was accompanied by an acceleration of activation and deactivation of the channel. Experiments with acid loading of guard cells indicated that the effect on s-ORC was due to a lowered intracellular pH, caused by apoplastic acidification. In the hyperpolarized state, the pH-induced depolarization was due to a direct effect of the apoplastic pH on the inward rectifying K⁺ channel. Acidification shifted the threshold potential of the channel to more positive values. This effect was accompanied by a decrease in activation times and an increase of deactivation times, of the channel. From the changes in E_m and membrane conductance, the expected effect of acidification on K⁺ fluxes was calculated. It was concluded that apoplastic acidification will increase the K⁺-efflux in the depolarized state and reduce the K⁺-influx in the hyperpolarized state. Received: 28 April 1997 / Accepted: 10 November 1997     

Ocean acidification at high latitudes: potential effects on functioning of the Antarctic bivalve Laternula elliptica

Ocean acidification is a well recognised threat to marine ecosystems. High latitude regions are predicted to be particularly affected due to cold waters and naturally low carbonate saturation levels. This is of concern for organisms utilising calcium carbonate (CaCO₃) to generate shells or skeletons. Studies of potential effects of future levels of pCO₂ on high latitude calcifiers are at present limited, and there is little understanding of their potential to acclimate to these changes. We describe a laboratory experiment to compare physiological and metabolic responses of a key benthic bivalve, Laternula elliptica, at pCO₂ levels of their natural environment (430 µatm, pH 7.99; based on field measurements) with those predicted for 2100 (735 µatm, pH 7.78) and glacial levels (187 µatm, pH 8.32). Adult L. elliptica basal metabolism (oxygen consumption rates) and heat shock protein HSP70 gene expression levels increased in response both to lowering and elevation of pH. Expression of chitin synthase (CHS), a key enzyme involved in synthesis of bivalve shells, was significantly up-regulated in individuals at pH 7.78, indicating L. elliptica were working harder to calcify in seawater undersaturated in aragonite (Ω_Ar = 0.71), the CaCO₃ polymorph of which their shells are comprised. The different response variables were influenced by pH in differing ways, highlighting the importance of assessing a variety of factors to determine the likely impact of pH change. In combination, the results indicate a negative effect of ocean acidification on whole-organism functioning of L. elliptica over relatively short terms (weeks-months) that may be energetically difficult to maintain over longer time periods. Importantly, however, the observed changes in L. elliptica CHS gene expression provides evidence for biological control over the shell formation process, which may enable some degree of adaptation or acclimation to future ocean acidification scenarios.     

Temperature and leaf osmotic potential as factors in the acclimation of photosynthesis to high temperature in desert plants

Seasonal changes in the high temperature limit for photosynthesis of desert winter annuals growing under natural conditions in Death Valley, California were studied using an assay based upon chlorophyll fluorescence. All species of this group were 6 to 9°C more tolerant of high temperature at the end of the growing season (May) than at its beginning (February). Over this same time period, the mean daily maximum air temperatures increased by 12°C. Laboratory studies have demonstrated that increases in thermal tolerance could be induced by increasing growth temperature alone. For plants growing under field conditions there was also a good correlation between the thermal tolerance of leaves and the osmotic potential of leaf water, indicating that increases in the concentrations of some small molecules might also confer increased thermal tolerance. Isolated chloroplast thylakoids subjected to increasing concentrations of sorbitol could be demonstrated to have increased thermal tolerance.