Bibliothek

Notizen: We present a mechanism of regulation of growth and activity of legume root nodules which is consistent with published experimental observations. The concentration of reduced nitrogen compounds, probably amino acids, flowing into the nodules from the phloem, is sensed by the nodules; growth and activity of the nodules is adjusted accordingly. In many legumes this response may involve changes in the oxygen diffusion resistance of the nodule cortex. A straightforward feedback mechanism in which nodule activity is lowered when reduced N in the phloem is high and increased when it is low is envisaged. Almost all import into nodules is via the phloem sap originating in the lower leaves. As a plant develops, these mature leaves no longer utilize nitrogen delivered in the xylem and so export it in the phloem. In plants with an adequate nitrogen supply (from nodules or combined nitrogen in soil), a high concentration of nitrogen containing compounds in the phloem from the lower leaves may inhibit nodule growth as well as activity. This suggestion is an alternative to the hypotheses of carbohydrate deprivation or nitrate inhibition which are commonly used to explain the effects of combined nitrogen on nodule growth and activity.

Notizen: Nitrogen stable isotope (15N, 14N) natural abundance has been much less used than carbon isotopes (13C, 12C) in plant physiology and ecology. Analytical problems, the lower fractional abundance of 15N than of 13C in the biosphere, the greater complexity of the N cycle relative to the C cycle, and smaller expressed discriminations in nature, are contributing factors. The major N pools, globally, have different isotope signatures: atmospheric N2 is 15N-depleted relative to organic N (including sedimentary N), a situation resulting from a greater expressed discrimination in the organic N to N2 (via denitrification) reaction than of diazotrophy during accumulation of the reduced N. Essentially all of the enzymes except nitrogenase which transform N compounds show discrimination against 15N, although for glutamine synthetase, and the amination of 2-oxoglutarate and pyruvate, this is only seen in terms of NH4+ rather than the true substrate, NH3. Discrimination is expressed in various N interconversions within plants, leading to substantial differences in δ15N (up to 12‰) among N compounds and macroscopic plant parts. N isotope fractionation during assimilation of exogenous combined N is often much lower than that expected from studies of isolated enzymes due to processes which show very little discrimination, such as limitation by transport through aqueous solution and membranes. Application of 15N/14N discrimination studies to plant ecology have concentrated largely on distinguishing diazotrophy from N supplied from combined N, based on the lower 15N/14N in diazotrophs due to the higher 15N/14N of combined N sources not being offset by fractionation during uptake. While potentially very useful, a number of pitfalls are discussed in its ecological use in both terrestrial and aquatic systems. N isotope discrimination is also useful in tracking N through food webs, and hence, back to combined N sources for plants.

Notizen: The use of stable isotope natural abundance measurements in plant ecophysiological research is discussed in the context of studies of 13C/12C ratios in marine plants, with emphasis on the uniqueness of the information given by natural abundance measurements and of the importance of complementary data obtained by other techniques in making full use of the natural abundance data. (1) Inorganic C acquisition and assimilation in marine plants can involve diffusive entry of CO2, or the occurrence of a CO2-concentrating mechanism frequently involving active HCO3− influx. For diffusive CO2 entry, the δ13C measurements can give unique information on the fractional limitation of photosynthesis by CO2 transport which, with photosynthetic rate measurements, can be used to compute transport conductances. For active HCO3−, influx, the δ13C values uniquely permit computation of the ratio of the bidirection fluxes (influx/efflux) which, with photon yield data, can be used to given information on the mechanism of the efflux. The analyses are absolutely dependent on external (non-δ13C) data distinguishing between diffusive CO2 entry and the occurrence of a CO2 concentrating mechanism. (2) δ13C measurements on marine photolithotrophs and on members of other trophic levels collected from the sea can give unique data on food webs, with measurements of δ values for other isotopes and compositional data adding precision to the interpretations. (3) Measurements of in situδ13C values for extant marine photolithotrophs, compared with δ13C values for ancient atmospheric CO2, can give unique information on the mechanism of atmospheric CO2 draw-down at the start of glacials; other information permits more concrete conclusions to be drawn.

Notizen: Abstract. Photosynthesis by many marine phytoplankton algae is saturated by the inorganic C concentration in air-equilibrated sea water. These organisms appear to use an active inorganic C transport process (CO2-concentrating mechanism) which increases the CO2 concentration around rubisco and saturates this enzyme with CO2 and suppresses its oxygenase activity. A minority of marine phytoplankton algae have photosynthetic characteristics more suggestive of diffusive CO2 entry; the inorganic C concentration present in sea water does not saturate photosynthesis by these organisms. Theoretical considerations, tested when possible against observation, suggest that the organisms with a CO2-concentrating mechanism could have a lower cost of photons, nitrogen, iron, manganese and molybdenum to achieve a given rate of carbon accumulation by the cells than is the case for the organisms with diffusive CO2 entry. Zinc and selenium costs may show the reverse effect. The increased sea-surface inorganic C, and CO2 concentrations which will result from anthropogenic increases in atmospheric CO2 content are predicted to increase the rate of photosynthesis, and of growth when other resources are abundant, and to reduce, or reverse, the higher resource (photons, nitrogen, iron, manganese and molybdenum) cost of a given rate of CO2 assimilation in organisms with CO2 diffusion relative to those which have CO2 concentrating mechanisms and do not repress them at higher inorganic C concentrations. These effects may well alter species composition, and overall resource cost of growth, of phytoplankton; any influence that these effects may have on CO2 removal from the atmosphere are severely constrained by other trophic levels and, especially, oceanic circulation patterns. Changed sea-surface temperatures are unlikely to qualitatively alter these conclusions.

Notizen: Abstract. pH is an all-pervasive variable in the environment of phototrophs. Phototrophs as a whole, and even the single genus Dunaliella, can grow over essentially the whole range of pH values found in nature. Such a large range of pH values, combined with other chemical variations in the environment, impose a range of constraints on plant behaviour related to intracellular pH regulation, nutrient acquisition, and avoidance of toxic effects. No single genotype can grow well over the whole pH range compatible with growth of phototrophs as a whole, although some deliberately alter surface pH so as to create a 5–6 unit pH gradient over the surface related to nutrient acquisition and avoidance of toxic influences. The regulation of these various processes does not, on current evidence, involve pH-sensing by any extracellular sensor which is not part of the catalytic or regulatory mechanism of a membrane protein such as a porter.

Notizen: Abstract Rates of uptake of 14C-labelled inorganic carbon were measured for whole Chara hispida plants, detached parts of the shoot and isolated (split-chamber technique) apices, lateral branchlets and rhizoid—node complexes.The rates of inorganic carbon uptake by the rhizoid—node complex expressed per gram fresh weight whole plant were three to four orders of magnitude less than the uptake for the whole plant. Up to 70% of the carbon taken up by the rhizoid—node complex was translocated to the shoot. After 12 h exposure to 14C-labelled inorganic carbon the concentration of 14C was greater in apices than in uppermost or central internodal cells and in all lateral branchlets, regardless of whether label was supplied to the whole plant or isolated rhizoid—node complexes. Measurement of inorganic carbon uptake by detached internodal cells and detached and isolated apices and lateral branchlets showed that lateral branchlets had the greatest rates of inorganic carbon uptake. During 12 h exposure to 14C, isolated lateral branchlets translocated to the attached shoot 55% of the labelled carbon taken up; for isolated apices this value was only 13%.It is concluded that it is highly unlikely that the rhizoid of Chara hispida could acquire a significant fraction of the whole plant requirement for inorganic carbon and that apices are sink regions for photosynthate while lateral branchlets are source regions.

Notizen: Abstract The acid-base balance during ammonium (used to mean NH 4+ and/or NH3) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m−3 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H+ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H+ accumulate in the medium per organic N synthesized. Granted a permeability (PNH3) of some 10−3 cm s−1, and a finite [NH3] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH 4+. This means that most of the H + appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H+ accumulates in the medium equivalent to any NH3 entry by requilibration from exogenous NH 4+. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO3+grown cells, suggesting that there is no action of a ‘biochemical pH stat’ during longterm assimilation of NO3+in H. africanum.Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO3+-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 μmol dm−3) the net influx was all attributable to NH4+influx via a uniporter, probably partly short-circuited by a passive NH3 efflux due to intrinsic membrane permeability to NH3. The net ammonium influx was in all cases associated with H+ accumulation in the medium. (1.3-1.7 H + per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated.Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a ‘pH-regulating’ change in H+ fluxes.

Notizen: A number of non-green plant tissues have high rates of HCO3−-consuming reactions in the cytosol, i.e. C4 dicarboxylic acid production preceding organic acid anion transport into dicarboxylate consuming compartments in N2-fixing root nodules, in lipogenic tissues, and in thermogenic aroid spadices and, in the case of lipogenic tissues, in acetyl CoA incorporation into lipid in plastid stroma. Since inorganic C supply to the cytosol or stroma by decarboxylation reactions, and by transmembrane fluxes, involves only CO2, the HCO3− consumed in the rapid metabolic processes must originate from hydration (hydroxylation) of CO2. Computations based on the first-order rate constant for uncatalysed conversion of CO2 to HCO3− and the most likely in vivo CO2 concentration show that the uncatalysed reaction is possibly adequate to supply the observed HCO3− requirement in the HCO3−-consuming compartments. However, carbonic anhydrase activity is well established in legume root nodules, and also appears to occur in aroid spadices. In addition to coping with any heterogeneities in HCO3−, consumption in the cytosol, the root nodule activity may be involved in optimizing haemoglobin function. Further work is needed on carbonic anhydrase expression is tissues with rapid HCO3− consumption, especially in view of reports of negligible carbonic anhydrase activity in some non-green plant tissues. Other possible roles of carbonic anhydrase in non-green plant tissues are briefly discussed.

Notizen: Levels of carbonic anhydrase activity were determined on a total (60 EUmg−1 protein), external (7.36 EU), internal (50.14 EU) and protoplast (15.63 EU) basis for Ranunculus penicillatus (Dumort.) Bab ssp. pseudofluitans (Syme) S. Webster, a freshwater aquatic macrophyte, by conventional electrometric methods. The site of activity of ‘external’ carbonic anhydrase (CA) has been visualized using 5-Dimethylaminonapthalene-1- sulphonamide (DNSA)-CA fluorescent complex formation, and is postulated to be closely associated with the epidermal cell wall. The photosynthetic rate of R. penicillatus ssp. pseudofluitans at pH 9.0 is in excess of the uncatalysed rate of production of CO2 from HCO−3, and this plant is therefore using HCO−3 for photosynthesis. The possible contribution of CA activity to inorganic carbon assimilation, and specifically to transport of HCO−3, in submerged aquatic plants is discussed.

Notizen: Abstract. Field measurements of the growth rate of the red freshwater macroalga Lemanca mamillosa Kutz, in the Dighty Burn, together with measurements of water velocity, [CO2], [NO3], [NH3+ NH4+] and [phosphate], have been made between February and July. This period covers the growth of the erect gametophyte and later of the carposporophyte inside the gametophyte. Hydrodynamic studies in the laboratory on benzoic acid models of the gametophyte suggest an average in situ unstirred layer some 12 μm thick. For growth of the gametophyte, this estimated boundary layer thickness, together with the measured inorganic C transport pathway within the plant, suggest that growth is not significantly restricted by CO2 transport from the bulk phase to the plastids. δ13C measurements on source CO2 and on plant organic C bear this out. Habitat choice (low temperatures: CO2 enrichment from ground-water input: rapid water flow), plant morphology and anatomy (turbulence-generating ‘knobbles’ on the nodes; plastids close to the outside of the plant), and plant biochemistry (high CO2 affinity of the RUBISCO carboxylase; quite high carbonic anhydrase activity) are responsible for this lack of limitation by inorganic C transport in the growing gametophyte which lacks HCO3 transport and a CO2 concentrating mechanism. Transport through the boundary layer does not significantly restrict acquisition by the plant of N (probably as NH4+, despite the preponderance of NO3 in the environment) or of P (as orthophosphate) in the field. The membrane transporters, which have high substrate affinities (K½'s about 2 mmol NH4+ m-3 and 〈 2 mmol inorganic phosphate m−3), probably impose the major limitation. The development of the carposporophyte later in the season, and an increase in the thickness of the cortex of the gametophyte, result in an increased (less negative) δ13C, suggesting a significant diffusion limitation to CO2 transport. This conclusion is reinforced by consideration of the opposing effect on Δδ13 C of the decreased demand for products of phosphoenolpyruvate carboxylase activity as the N/C ratio decreases late in the growing season.