Salt-stimulated Phosphoenolpyruvate Carboxylase in Cakile maritima

The effects of NaCl and other salts, in vivo and in vitro, on the activity of phosphoenolpyruvate carboxylase from the coastal C₃ halophyte Cakile maritima Scop, were investigated. Plants grown with 100 mM NaCl in their growth medium yielded some 30% higher rates of phosphoenolpyruvate carboxylase activity than did salt-depleted plants. Activity of the enzyme was stimulated when NaCl was added to the reaction mixture in concentrations of up to 200 mM. The magnitude of this in vitro stimulation was similar for plants grown in the presence or absence of NaCl. The effect seems to be caused by chloride rather than by sodium ions.     

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.     

Carbon Metabolism in Seagrasses: II. PATTERNS OF PHOTOS YNTHETIC CO2 INCORPORATION

Patterns of initial photosynthetic CO₂ incorporation were determinedfor some seagrasses and were related to activities of primarycarbon fixing enzymes, carbonic anhydrase activities, and ¹³Cvalues. According to the incorporation patterns, Cymodocea nodosa wasa C4 species while Thalassia hemprichli and Thalassodendronciliatum were C₃ plants. Halophila stipulacea showed an unusualincorporation pattern which could be viewed as intermediatebetween typical C₃ and C₄ pathways. The activity ratios of ribulose-l,5-bisphosphate carboxylase (RUBPcase) to phosphoenolpyruvatecarboxylase (PEPcase) were about 3 for Thalassodendron ciliatumand 1 for Cymodocea nodosa and Halophila stipulacea. The lattervalue, which is intermediate to ratios found in terrestrialC₃ and C₄ plants, may correlate with the incorporation patternsfound for Halophila stipulacea. Since the C₄ seagrass lackedthe Kranz anatomy, it may, in addition, point to a flexibleincorporation potential for these plants. The high ¹³C values found in these and other seagrasses didnot correlate with their photosynthetic pathways as in terrestrialplants. This discrepancy is probably due to a ‘closedsystem’ type of photosynthesis in which CO₂ is efficientlyutilized. The C₃ species which utilize CO₂ enzymatically must convertexogenous HCO-₃ to CO₂ internally. Even though carbonic anhydraseactivities were very low, conversion rates seemed to be sufficientfor high rates of photosynthesis. Since enzymatic fixation ratesapproached photosynthetic rates even at CO₂ saturation, thelimitation for these seagrasses to express their high photosyntheticpotential is most probably the HCO^–₃ uptake system.     

Carbon Metabolism in Seagrasses: III. ACTIVITIES OF CARBON-FIXING ENZYMES IN RELATION TO INTERNAL SALT CONCENTRATIONS

The internal salt content and distribution in photosynthetictissues as well as the effect of NaCl on photosynthetic carbonfixation enzymes was investigated in two seagrass species fromthe Red Sea. Concentrations of both Na⁺ and Cl^– were lower in the chloroplast-richepidermis than in underlying cell layers in Halophila stipulacea.In Halodule uninervis, the concentration of Na⁺ was lower inthe epidermis than in the underlying cells, while K⁺ was evenlydistributed between cell layers. The epidermal concentrationsof Na+ were estimated to be 0.17 and 0.10 M for Halophila stipulaceaand Halodule uninervis, respectively, which were about to the average leaf concentrations. Epidermal Cl^– concentrationof Halophila stipulacea was estimated to be 0.08 M, a valueonly about of the overall leaf concentration. Phosphoenolpyruvate carboxylase (PEPcase) extracted from leavesof these seagrasses showed increased activity at 0.05–0.3M NaCl in vitro. Ribulose-l, 5-bisphosphate carboxylase (RuBPcase)activity, on the other hand, was inhibited by NaCl at all testedconcentrations. At epidermal NaCl concentrations, PEPcase activitywas thus stimulated while RuBPcase was inhibited. The reducedRuBPcase activity at such concentrations compared to salt-freeconditions was still sufficient to account for observed photosyntheticrates. We conclude that these seagrasses have adapted to a saline environmentboth by maintaining relatively low ion concentrations in theepidermis where photosynthesis occurs and by having carbon-fixingenzymes capable of functioning in the presence of salt.     

The Effect of Sodium Chloride on the Balance between the C3- and C4-Carbon Fixation Pathways

Young leaves of salt-depleted Aeluropus litoralis Parl. plants show CO₂ fixation by the C₃-carbon fixation pathway. No detectable activity of phosphoenol pyruvate (PEP) carboxylase was found. When A. litoralis plants were exposed to a NaCl solution, the leaves showed a high activity of PEP carboxylase as well as a significant CO₂ fixation by the C₄-pathway. — Also in Zea mays L. and Chloris gayana Kunth., the presence of NaCl in the medium influences the balance between phosphoenol pyruvate carboxylase and ribulose-1,5-diphosphate carboxylase.     

Distribution of Sodium and Chloride in Leaves of Suaeda monoica

A modified electron probe was used for the determination of the distribution of Na and Cl in cross sections of Suaeda monoica leaves. The results were corrected for differences in local mass values along the analysis line and expressed as percentage of the ions in the dried tissue. Sodium was found lo be selectively accumulated in the outer layer of chlorenchyma, whereas Cl was concentrated mostly in the cells of the inner chlorenchyma. Considerable pan of the osmotic potential of these cells is thus ascribed to the presence of those ions.     

Ecophysiology of Salt Excretion in Aeluropus litoralis (Graminae)

Various aspects of salt excretion from leaves of Aeluropus were investigated. Salt excretion exhibited an optimum-type of curve when measured against external salt concentration, while sodium content of the leaves increased linearly. The ‘relative excretion’, i.e. rate of excreted ions: change in leaf ion content, was maximal in the low salt concentration range, and decreased when external sodium chloride concentration increased. Concentration of the excreted droplets was higher than the external concentration when the leaves were exposed to low salt concentrations in the medium, but the reverse occurred when the external salt concentrations were high. The excretion process was sensitive to water-stress conditions, caused either by high external salt concentrations or by exposure to dry atmosphere. A considerable fraction of the leaf sodium content in salt-treated leaves was only slightly available for excretion. Salt excretion in Aeluropus was enhanced by light. Such enhancement was indirect and is attributed to the increase of salt transport via transpiration stream. Selectivity of the salt-excretion mechanism is in favour of sodium and against potassium. On the other hand, potassium has a high affinity for the accumulation systems within the leaves. The ecological significance of the results is discussed.