Showing papers in "Physiologia Plantarum in 1988"

Photoinhibition of photosynthesis. An evaluation of damaging and protective mechanisms

Abstract: Inhibition of photosynthesis by excess excitation energy is initiated in the reaction center of photosystem II The primary site of photoinhibition in the reaction center (components of primary charge separation or secondary electron acceptor QB) is still disputed Photoinhibition is characterized by quenching of variable chlorophyll flurescence (Fv), resulting from increased thermal dissipation of excitation energy Varying responses of initial fluorescence (F0), however, seem to indicate involvement of different mechanisms As far as photoinhibition is reversible within minutes to hours, it can be viewed as a controlled protective mechanism that serves to dissipate excessive energy, Supposedly, another dissipative mechanism, distinguished by its faster kinetics (response within seconds), is related to the energy-dependent fluorescence quenching

Estimating the rate of photorespiration in leaves

Abstract: Photorespiration is the light-dependent evolution of CO2, which accompanies photosynthesis in C3plants. The four best known methods of measuring the rate of photorespiration have theoretical or technical problems, which make the results unreliable. However, the rate of photorespiration can be calculated from the rate of net CO2assimilation and the partial pressures of CO2and O2. Estimates of rates of photorespiration in the past and future can be made. The rate of photorespiration as a proportion of the rate of photosynthesis will fall to one half the current rate when the CO2level in the atmosphere doubles.

Toxic oxygen species and protective systems of the chloroplast

Abstract: Salin, M. L. 1988. Toxic oxygen species and protective systems of the chloroplast. -Physiol. Plant. 72: 681–689. As a consequence of living in an environment enriched in oxygen, which they themselves at least partially generate, photosynthetic organisms are exposed to large fluxes of oxyradicals and reactive oxygen species. Among these are superoxide, hydrogen peroxide, hydroxyl radical and singlet oxygen. These highly reactive intermediates pose the threat of toxicity unless neutralized by scavenger substrates or enzymes. The production of oxyradicals and intermediates by chloroplasts as well as the means of protection are discussed in this review.

Effects of excessive ammonium deposition on the nutritional status and condition of pine needles

Abstract: In the southeastern part of the Netherlands many Scots pine (Pinus sylvestris L.) trees show signs of yellowing. To investigate whether there is a relation between this phenomenon and the high ammonium deposition, needle and soil samples were analyzed. Soil samples from the discoloured forests contained more extractable nitrogen than samples from healthy stands, whereas differences in pH values were small. Needles from yellow trees had higher levels of total nitrogen than needles from green trees as well as severe imbalances of Mg, K+ and P relative to N. The amount of leaf pigments was substantially lower in needles of the diseased trees, but they contained much higher quantities of free arginine, which accounted for a major part of total nitrogen. This may be an indication of a severe nitrogen overload. The linkage between this excessive nitrogen nutrition and the observed process of yellowing is discussed.

Polyphenol oxidase: The chloroplast oxidase with no established function

Abstract: Vaughn, K. C, Lax, A. R. and Duke, S. O. 1988. Polyphenol oxidase: The chloroplast oxidase with no established function. - Physiol. Plant. 72: 659–665. Polyphenol oxidase (PPO) is an enzyme localized on the thylakoids of chloroplasts and in vesicles or other bodies in non-green plastid types. Although virtually all plastids contain PPO, little or no detectable activity is associated with guard cell and bundle sheath cell chloroplasts. Despite this nearly ubiquitous occurrence, no function for this enzyme has been established. The enzyme is nuclear-encoded and, unlike most chloroplast proteins is not encoded as a larger M, precursor molecule. This lack of a transit peptide sequence may be related to a unique mechanism of uptake, apparently involving inner envelope-derived vesicles. The M, range of most of the PPO forms is 36–45 kDa. PPO is apparently not involved in phenolic biosynthesis but is probably involved with the production of o-quinones during pathogen invasion. A role for PPO as an “oxygen buffer” is postulated, but little concrete data have been collected on any other functional role for this enzyme.

A portable, microprocessor operated instrument for measuring chlorophyll fluorescence kinetics in stress physiology

Abstract: A PORTABLE, MICROPROCESSOR OPERATED INSTRUMENT FOR MEASURING CHLOROPHYLL FLUORESCENCE KINETICS IN STRESS PHYSIOLOGY

Measurement of the activity and capacity of the alternative pathway in intact plant tissues: Identification of problems and possible solutions

Abstract: Moller, I. M., Berczi, A., van der Plas, L. H. W. and Lambers, H. 1988. Measurement of the activity and capacity of the alternative pathway in intact plant tissues: Identification of problems and possible solutions. - Physiol. Plant. 72: 642–649. The cyanide-insensitive, benzhydroxamic acid-sensitive (e.g. salicylhydroxamic acid, SHAM) alternative pathway is located in the inner membrane of plant mitochondria and electron flow through it is not coupled to H+ pumping and ATP synthesis. When estimating the activity and capacity of the alternative pathway in intact plant tissues three main problems arise: 1) There is almost always a substantial (10–50%) KCN-insensitive, SHAM-insensitive residual respiration, which may be due to peroxisomal a-oxidation of fatty acids, and which must be subtracted from all data in the further analyses. 2) There is a (KCN-sensitive) peroxidase in many tissues that is stimulated by low SHAM concentrations (1–10 mAf), but inhibited at higher concentrations (15–50 mM). 3) High concentrations of SHAM may inhibit the cytochrome pathway. Means of identifying and alleviating these problems are presented. Provided experimental conditions are chosen such as to minimize the three problems for each new plant organ or species or each new growth condition, SHAM can be used to estimate the size of the alternatively pathway in vivo.

Respiratory energy costs for the maintenance of biomass, for growth and for ion uptake in roots of Carex diandra and Carex acutiformis

Abstract: van der Werf, A., Kooijman, A., Welschen, R. and Lambers, H. 1988. Respiratory energy costs for the maintenance of biomass, for growth and for ion uptake in roots of Carex diandra and Carex acutiformis. - Physiol. Plant. 72: 483–491. The respiratory characteristics of the roots of Carex diandra Schrank and Carex acutiformis Ehrh. were investigated. The aims were, firstly to determine the respiratory energy costs for the maintenance of root biomass, for root growth and for ion uptake, and secondly to explain the higher rate of root respiration and ATP production in C. diandra. The three respiratory energy components were derived from a multiple regression analysis, using the relative growth rate and the net rate of nitrate uptake as independent variables and the rate of ATP production as a dependent variable. Although the rate of root respiration and ATP production was significantly higher in C. diandra than in C. acutiformis, the two species showed no significant difference in their rate of ATP production for the maintenance of biomass, in the respiratory energy coefficient for growth (the amount of ATP production per unit of biomass produced) and the respiratory energy coefficient for ion uptake (amount of ATP production per unit of ions absorbed). It is concluded that the higher rate of root respiration of C. diandra is caused by a higher rate of nitrate uptake. At relatively high rates of growth and nitrate uptake, the contribution of the rate of ATP production for ion uptake to the total rate of ATP production amounted to 38 and 25% for C. diandra and C. acutiformis, respectively. At this growth rate, the respiratory energy production for growth contributed 37 and 50%, respectively, to the total rate of ATP production. The relative contribution of the rate of ATP production for the maintenance of biomass increased from 25 to 70% with increasing plant age for both species. The results suggest that ion uptake is one of the major sinks for respiratory energy in roots. These experimentally derived values for the rate of ATP production for the maintenance of biomass, the respiratory energy coefficient for growth and the respiratory energy coefficient for ion uptake are discussed in relation to other experimentally and theoretically derived values.

Identification of potentially embryogenic microspores in Brassica napus

Abstract: Studies were undertaken with Brassica napus L. cv. Topas to identify buds containing microspores predisposed to embryogenesis in vitro and to investigate bud and microspore development in relation to this process. No significant correlation was found between the final embryo number and bud components. There appears to be a developmental window of less than 8 h duration during which microspores are very likely to form embryos: over 70% of the microspores can undergo division and up to 70% of these can form embryos. Embryos were mainly obtained from late uninuucleate to early binucleate microspores: the former contained mainly a G2 or M phase nucleus located at the microspore periphery and the latter a generative nucleus (associated with the intine) and a vegetative nucleus. Observations indicated that only the vegetative nucleus contributed to embryo formation. The first embryogenic division occurred between 8 and 16 h for uninucleate- and between 8 and 48 h for binucleate-derived embryos.

Nutrient uptake and allocation at steady‐state nutrition

Abstract: Ingestad, T. and Agren, G. I. 1988. Nutrient uptake and allocation at steady-state nutrition. - Physiol. Plant. 72: 450–459. Net nutrient uptake and translocation rates are discussed for conditions of steady-state nutrition and growth. Under these conditions, the relative uptake rate is equal to the relative growth rate, for whole plants as well as for plant parts, since the root/shoot ratio and internal concentrations remain stable. The nutrient productivity and the minimum internal concentration are parameters characteristic for the plant and the nutrient. A conceptual, mathematical model, based on these two fundamental parameters is used for calculation and prediction of the net nutrient uptake rate, which is required to maintain steady-state nutrition at a specified internal nutrient concentration or relative growth rate. When uptake rate is expressed on the basis of the root growth rate, there is, up to optimum, a strong linear relationship between uptake rate and the internal concentration of the limiting nutrient. More complicated and less consistent relationships are obtained when uptake rate is related to root biomass. The limiting factor for suboptimum uptake is the amount of nutrients becoming available at the root surface. When replenishment is efficient, e.g. with vigorous stirring, the concentration requirement at the root surface appears to be extremely low, even at optimum. In the suboptimum range of nutrition, the effect of nutrient status on root growth rate is a critical factor with a strong feed-back on nutrition, growth and allocation. At supraoptimum conditions, the uptake mechanism is interpreted as a protection against too high uptake rates and internal concentrations at high external concentration. In birch (Betula pendula Roth.), the allocation of nitrogen to the shoots is high compared to that of potassium and also to that of phosphorus at low nitrogen or phosphorus status. With decreasing stress, phosphorus allocation becomes more and more similar to nitrogen allocation. The formulation of a mathematical model for calculation of allocation of biomass and nutrients requires more exact information on the quantitative dependence of the growth-regulating processes on nutrition.

Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake

Abstract: Soybean [Glycine max (L.) Merr.] plants were grown in pot cultures and inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus GIomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or provided with P fertilizer (non-VAM plants). After an initial growth period (21 days), plants were exposed to cycles of severe, moderate or no drought stress over a subsequent 28-day period by rewatering at soil water potentials of -1.0, -0.3 or -0.05 MPa. Dry weights of VAM plants were greater at severe stress and smaller at no stress than those of non-VAM plants. Phosphorus fertilization was applied to produce VAM and non-VAM plants of the same size at moderate stress. Root and leaf P concentrations were higher in non-VAM plants at all stress levels. All plants were stressed to permanent wilting prior to harvest. VAM plants had lower soil moisture content at harvest than non-VAM plants. Colonization of roots by G. mosseae did not vary with stress, but the biomass and length of the extraradical mycelium was greater in severely stressed than in non-stressed plants. Growth enhancement of VAM plants relative to P-fertilized non-VAM plants under severe stress was attributed to increased uptake of water as well as to more efficient P uptake. The ability of VAM plants to deplete soil water to a greater extent than non-VAM plants suggests lower permanent wilting potentials for the former.

Homoglutathione: isolation, quantification and occurrence in legumes

Abstract: Homoglutathione (hGSH: γ-glutamyl-eysteinyl-β-alanine) was purified from seeds of Phaseolus coccineus L. cv. Preisgewinner, using anion-exchange chromatography and Cu2O precipitation. Quantitative and specific determination of this thiol is possible by high-performance liquid chromatography (HPLC) after monobromobimane derivatization. The enzymatic recycling assay based on yeast glutathione reductase (EC 1.6.4.2) can also be applied, but only to samples containing either hGSH or glutathione (GSH), since enzyme reaction with hGSH is 2.7 times faster than with GSH. Using the very sensitive HPLC method, the thiol content of leaves, roots and seeds of several legumes was investigated. Although GSH and hGSH were found in all plants analysed, the GSH/hGSH ratio varied greatly within the different tribes as well as within the different organs of plants of one species. In seeds and leaves of Vicieae, only traces of hGSH were found beside the main thiol GSH, whereas in roots the hGSH content exceeded the GSH content. The Trifolieae contained both tripeptides and in the tribe Phaseoleae, hGSH predominated by far.

Competitive inhibition of the auxin‐induced elongation by α‐D‐oligogalacturonides in pea stem segments

Abstract: Branca, C, De Lorenzo, G. and Cervone, F. 1988. Competitive inhibition of the auxin-induced elongation by α-D-oligogalacturonides in pea stem segments. - Physiol. Plant. 72: 499–504. α-D-galacturonide oligomers (OG) were prepared by partial hydrolysis of sodium polypectate with an homogeneous Aspergillus niger endopolygalacturonase (EC 3.2.1.15). OG, obtained after digestion for 10, 20, 30, 60, 120 min and 24 h, were assayed for their ability to interfere with the IAA-induced elongation of pea (Pisum sativum L. cv. Alaska) stems. Maximum inhibiting activity was exhibited by oligomers with an approximate degree of polymerization higher than 8. Inhibition by longer OG was much lower, and the products of the 24 h digestion and the unhydrolysed polypectate were ineffective. The addition of OG to pea stems caused a parallel shift to the right of the IAA dose-effect curve. The shift depended on the amount of OG used, showing that oligogalacturonides behave as competitive antagonists of IAA. The presence of OG caused the disappearance of the second maximum of the elongation rate and reduced the first maximum. OG were also tested for their ability to inhibit IAA-induced ethylene evolution of pea stem segments. Maximal inhibition was obtained with OG of the same size as those that interfered with IAA-induced elongation. Inhibition of the auxin action seemed to be specific as OG did not interfere with the activity of gibberellic acid (GA3) or kinetin. It was concluded that oligogalacturonides strongly interfere with the activity of IAA, although they are by themselves incapable to influence the elongation of pea stem segments directly.

Somatic embryogenesis and plant regeneration from suspension cultures of Picea glauca (White spruce)

Abstract: Hakman, I. and von Arnold, S. 1988. Somatic embryogenesis and plant regeneration from suspension cultures of Picea glauca (White spruce). - Physiol. Plant. 72: 579–587. Plantlets were regenerated from long-term embryogenic cultures of Picea glauca (Moench) Voss. (White spruce). Embryogenic calli, initiated from immature zygotic embryos and maintained by monthly subculture for 16 months, were used to establish suspension cultures. Small somatic embryos were continuously produced in liquid culture medium containing auxin and cytokinin and the cultures showed a sustained regeneration capacity for >6 months. Somatic embryos propagated in the suspension cultures developed further into embryos bearing cotyledons, about 1 month after transfer to solidified medium containing abscisic acid. Electron microscopic examination revealed that storage nutrients, lipids, proteins and carbohydrates, accumulated in the somatic embryos during this treatment with abscisic acid (ABA). Upon subculture to medium lacking plant growth regulators such embryos could develop into small green plantlets.

Nitrate reductase and its role in nitrate assimilation in plants

Abstract: Nitrate reductase (EC 1.6.6.1) is an enzyme found in most higher plants and appears to be a key regulator of nitrate assimilation as a result of enzyme induction by nitrate. The biochemistry of nitrate reductase has been elucidated to a great extent and the role that nitrate reductase plays in regulation of nitrate assimilation is becoming understood.

Does O2 photoreduction occur within chloroplasts in vivo

Abstract: Robinson, J. M. 1988. Does O2 photoreduction occur within chloroplasts in vivo? -Physiol. Plant. 72: 666–680. This discussion reviews evidence supporting the hypothesis that within intact chloroplasts in vivo, molecular O2 may serve as an alternative Hill oxidant (electron acceptor) on the reducing side of Photosystem I. Depending upon the availability of Hill oxidants such as NADP+ and NO−2, there is the potential within intact plastids in vivo, for photolytically derived reducing equivalents to reduce O2 to O−2 and H2O2 (the Mehler reaction). In chloroplasts of healthy tissues, the products of photosyn-thetic O2 reduction O−2 and H2O2) are rapidly removed by superoxide dismutase (EC 1.15.1.1) and L-ascorbate peroxidase (EC 1.11.1.11) to prevent toxicity. The presence of these two enzymes within chloroplasts in vivo reflects the potential for linear (non-cyclic) photosynthetic electron transport systems to draw upon molecular O2 as a terminal oxidant. In the intact plastid, O2 may act as an electron acceptor in the place of any other physiological Hill oxidant, e.g., NADP+, NO−2, and, presumably, oxidized thioredoxin. Under aerobic, physiological conditions, photo reduced ferre-doxin (Fdred), and/or reduced flavoprotein enzymes, e.g., ferredoxin:NADP+ oxidoreductase (EC 1.18.1.2), can donate electrons to O2; this reductive reaction appears to be non-enzymatic, but it is rapid. Stated from another viewpoint, O2 may serve as a Hill oxidant to support some linear electron flow when reductant supplies are in excess of reductant demands. For example, there are nitrogen assimilatory sites in the chloroplast, i.e., ferredoxin-nitrite reductase (NiR; EC 1.7.7.1) and glutamate synthase (ferredoxin) (GOGAT; EC 1.4.7.1), to which Fdred is allocated as reductant. Because NADH:nitrate reductase (NR; EC 1.6.6.1) is the rate limiting step of nitrogen assimilation, and, because NiR and GOGAT activities are in excess of NR activities by a factor of 2 or more, then an excess of unreacted Fdred could accumulate. Alternatively, the allocated Fdred would reduce the excess NiR and GOGAT sites, but the excess of reduced enzymes would not have substrates (e.g., NO−2, glutamine, and α-ketoglutarate) with which to react. Therefore, if ‘excess’ NiR and GOGAT binding sites were not employed, the available excess Fdred, and/or the reduced NiR and GOGAT proteins, would be susceptible to oxidation by O2. The resulting O2 photoreduction could account for nearly all of the observed in vivo Mehler type reactions. In vivo, apparent foliar O2 photoreduction occurs simultaneously with maximal CO2 photoassimilation, and, in high light, average rates have been determined by direct measurement to range from 10 to 40 μmol O2 consumed (mg Chl)−1 h−1. Therefore O2 reduction would support a low rate of linear (non-cyclic) electron flow which, in turn, could maintain a low, but significant rate of ATP production. However, there is not total agreement among researchers that the physiological role of O2 is that of serving as an alternative Hill oxidant in order to recycle unutilized Fdred or other photoreduced proteins. Also, there continues to be considerable controversy on whether or not O2 reduction supports significant photosynthetic phosphorylation. The total process of O2 photoreduction, and its physiological role(s), requires much more study before absolute functions can be assigned to O2 terminated, linear electron transport. Summary Molecular O2 possesses the physico-chemical properties that permit this molecule to serve as an alternative Hill oxidant within chloroplasts in vivo. Additionally, the physical and physiological properties within the chloroplast in vivo favor the potential for O2 to serve as an electron acceptor on the reducing side of Photosystem I. This may reflect an important ‘fail-safe mechanism’ which prevents over-reduction of linear photosynthetic electron transport chain proteins. This review has focused on the possibility that unutilized Fdred and/or other non-utilized, reduced plastid enzymes (e.g., NiR) may be electron donors to O2. It is hypothesized that this oxidation ultimately would be reflected as an in vivo Mehler reaction. However, it remains for future studies to establish without doubt, that in vivo, photoreduced chloroplast enzyme proteins can utilize O2 as a terminal electron acceptor. Further, that O2 photoreduction supports a significant level of photophosphorylation in vivo remains to be firmly established. Certainly, considerable evidence, gained with experiments utilizing isolates of intact chlo-roplasts and reconstituted chloroplast systems, supports the hypothesis that O2-terminated linear electron transport has the potential to support high rates of ATP production. However, in vivo studies e.g., with intact leaf tissues, which actually quantitate the relationship between O2 photoreduction and associated ATP production have not been conducted. These will be difficult experiments to perform, because, in vivo, it will be difficult to separate photosynthetic ATP production mediated by O2 from ATP production mediated by those other, more predominant Hill oxidants (e.g., NADP+, NO−2). Also, it continues to be a possibility that it is cyclic, and not pseudocyclic photophosphorylation that provides additional ATP to support photosynthetic cell metabolism. To establish beyond doubt that an in vivo role of the Mehler reaction is that of supplying ‘additional ATP’, remains for considerable future study.

Cell number, cell size and hormone levels in semi-isogenic mutants of Lycopersicon pimpinellifolium differing in fruit size

Abstract: Tomato fruit growth parameters, cell number and cell size, and hormone levels [IAA, abscisic acid (ABA), zeatin (Z)/zeatin riboside (ZR), isopentenyladenosine (i-Ado)/isopentenlyadenine (i-Ade)], in the wild-type (Lycopersicon pimpinellifolium Mill.) and a semi-isogenic mutant (mutant III) differing in fruit size were investigated during fruit development. An image-processing system was used for the determination of cell number and single cell size per fruit and hormone levels were measured by radioimmuno-assay (RIA). The bigger fruits of mutant III showed higher cell numbers throughout fruit development and cells enlarged faster than in wild-type fruits. During the first 10 days of fruit growth, the main cell division period after fertilization, high concentrations of cytokinins were found, these being correlated with high cell division activity. There were only slight differences in IAA and ABA levels in the different sized fruits. The results emphasized the importance of the cell number per fruit at anthesis as a determining factor of final fruit size in tomatoes. A possible relationship between cytokinins and subsequent fruit development is discussed.

Exodermal Casparian bands: their significance for ion uptake by roots

Abstract: The roots of many angiosperm species possess two Casparian bands, one in the endodermis and one in the outermost layer of the cortex. Over most of the root surface in these species, ions are taken up by the epidermis and may be transported symplastically to the xylem.

Nutrition and growth of birch seedlings at varied relative phosphorus addition rates

Abstract: Birch (Betula pendula Roth.) was investigated under steady state nutrition and growth at different relative addition rates of phosphorus (Rp). Phosphorus deficiency symptoms appeared on the leaves when the internal phosphorus concentration decreased, but disappeared again under steady state nutrition, independent of the stress level. The increased root/shoot ratio and the exploratory type of root systems developed during the adjustment stage remained under steady state conditions. At nonoptimum and close to optimum relative addition rates, independent of the rate, the phosphorus concentration of the culture solution did not exceed 2 μmol dm−3 and was generally < 1 μmol dm−3 immediately after phosphorus additions. The phosphorus concentration just before additions was generally < 0.5 μmol dm−3. The nutrition/growth relationships were similar to those for nitrogen, with relative growth rate (Rg) closely related to the Rp applied and with a strong linear relationship between internal phosphorus concentration and Rg. Regression was much steeper than that for nitrogen. The slope of the optimum nutrition was attained at a lower phosphorus weight proportion to nitrogen (8–10 P: 100 N) than previously estimated (= 13 P: 100 N), but a higher relative phosphorus requirement was observed under stress conditions. Birch seedlings had a strong tendency to consume phosphorus in excess of immediate requirements with a small effect on growth above optimum. This resulted in rapidly decreasing phosphorus productivity (Pp, growth rate per unit of phosphorus) with increasing internal phosphorus concentrations above optimum.

Zinc‐dependent changes in ESR signals, NADPH oxidase and plasma membrane permeability in cotton roots

Abstract: The effect of Zn2+ on the plasma membrane permeability and superoxide radical (O2-) formation in roots was studied with cotton (Gossypium hirsutum L. cv. Delta-pine 15/21) plants grown in nutrient solution with different Zn2+ supply. Compared to Zn-sufficient plants, the plasma membrane permeability of Zn-deficient plants was increased as indicated by a 3-, 5- and 2.5-fold increase in root cell leakage of K+, NO3- and organic carbon compounds, respectively. Resupply of Zn2+ to Zn-deficient plants for 12 h substantially decreased this leakage. The effects of Zn2+ on membrane permeability were closely correlated with the levels of O2- measured by electron spin resonance (ESR) spectroscopy in the microsomal membrane fraction and in the cytosol fraction of root cells. The amplitudes of the O2- -derived Tiron ESR signal also coincided with a O2- -generating oxidase activity which was strongly dependent on the presence of NADPH and FAD. The results suggest that Zn2+ directly affects the integrity of the plasma membrane, at least in part, by interfering with O2- generation by a membrane-bound NADPH oxidase.

Related mannose‐specific lectins from different species of the family Amaryllidaceae

Abstract: Bulbs from three species of the plant family Amaryllidaceae (Narcissus pseudonurcissus L., Leucojum aestivum L. and Leucojum vernum L.) were found to contain mannose-specific lectins. These lectins were serologically identical to a previously reported Amaryllidaceae lectin from Galanthus nivalis L. bulbs, but had a different molecular structure. The lectins described in this paper are dimeric proteins composed of subunits of 13 kDa, which are not held together by disulphide bridges. In hapten-inhibition assays Amaryllidaceae lectins exhibited exclusive specificity towards mannose. Furthermore, they all had a high specific agglutination activity with trypsin-treated rabbit erythrocytes, whereas human red blood cells were not agglutinated.

Cell enlargement and growth-induced water potentials

Abstract: Understanding the origin and role of growth-induced water potentials helps to explain how cell enlargement is inhibited without a decrease in turgor when water is depleted in the soil or more rapidly lost by transpiration. In many cases, changes in growth-induced water potentials probably cause the inhibition initially, but eventually the cell walls lose extensibility and specific proteins accumulate so that metabolic changes in the walls probably limit later growth.

Oxygen limitation of N2 fixation in stem‐girdled and nitrate‐treated soybean

Abstract: The effects of increasing rhizosphere pO2on nitrogenase activity and nodule resistance to O2diffusion were investigated in soybean plants [Glycine max (L.) Merr. cv. Harosoy 63] in which nitrogenase (EC 1.7.99.2) activities were inhibited by (a) removal of the phloem tissue at the base of the stem (stem girdling), (b) exposure of roots to 10 mM NO3over 5 days (NO3-treated), or (c) partial inactivation of nitrogenase activity by an exposure of nodulated roots to 100 kPa O2(O2-inhibitcd). In control plants and in plants which had been treated with 100 kPa O2, increasing rhizosphere O2concentrations in 10 kPa increments from 20 to 70 kPa did not alter the steady-state nitrogenase activity. In contrast, in plants in which nitrogenase activities were depressed by stem girdling or by exposure to NO3, increasing rhizosphere pO2resulted in a recovery of 57 or 67%, respectively, of the initial, depressed rates of nitrogenase activity. This suggests that the nitrogenase activity of stem-girdled and NO3-treated soybeans was O2-limited. For each treatment, theoretical resistance values for O2diffusion into nodules were estimated from measured rates of CO2exchange, assuming a respiratory quotient of 1.1 and 0 kPa of O2in the infected cells. At an external partial pressure of 20 kPa O2, the stem-girdled and NO3--treated plants displayed resistance values which were 4 to 8.6 times higher than those in the nodules of the control plants. In control and O2-inhibited plants, increases in pO2from 20 to 70 kPa in 10 kPa increments resulted in a 2.5- to 3.9-fold increase in diffusion resistance to O2, and had little effect on either respiration or nitrogenase activity. In contrast, in stem-girdled and NO3--treated plants, increases in external pO2had little effect on diffusion resistance to O2, but resulted in a 2.3- to 3.2-fold increase in nodule respiration and nitrogenase activity. These results are consistent with stem-girdling and NO3--inhibition treatments limiting phloem supply to nodules causing an increase in diffusion resistance to O2at 20 kPa and an apparent insensitivity of diffusion resistance to increases in external pO2.

Vulnerability of xylem to embolism in a mangrove vs an inland species of Rhizophoraceae

Abstract: Vulnerability of xylem conduits to cavitation and embolism was compared in two species of Rhizophoraceae, the mangrove Rhizophora mangle L. and the tropical moist-forest Cassipourea elliptica (Sw.) Poir. Cavitation (water column breakage preceeding embolism) was monitored by ultrasonic detection; embolism was quantified by its reduction of xylem hydraulic conductivity. Acoustic data were not predictive of loss in hydraulic conductivity, probably because signals from cavitating vessels were swamped by more numerous ones from cavitating fibers. Rhizophora mangle was the less vulnerable to embolism of the two species, losing 80% of its hydraulic conductivity between – 6.0 and – 7.0 MPa. Cassipourea elliptica lost conductivity in linear proportion to decreasing xylem pressure from – 0.5 to – 7.0 MPa. Species vulnerability correlated closely with physiological demands of habitat; the mangrove Rhizophora mangle had field xylem pressures between – 2.5 and – 4.0 MPa. whereas the minimum for Cassipourea elliptica was – 1.6 MPa. Differences in vulnerability between species could be accounted for by differences in the measured air permeability of intervessel pit membranes. According to this explanation, embolism occurs when air enters a water-filled vessel from a neighboring air-filled one via pores in shared pit membranes.

The effect of an elevated atmospheric CO2 concentration on growth, photosynthesis and respiration of Plantago major

Abstract: The effect of an elevated atmospheric CO2 concentration on growth, photosynthesis and root respiration of Plantago major L. ssp. major L. was investigated. Plants were grown in a nutrient solution in growth chambers at 350 and 700 μl I−1 CO2 during 7 weeks. The total dry weight of the Co2‐enriched plants at the end of this period was 50% higher than that of control plants. However, the relative growth rate (RGR) was stimulated only during the first half of the growing period. The transient nature of the stimulation of the RGR was not likely to be due to end‐product inhibition of photosynthesis. It is suggested that in P. major , a rosette plant, self‐shading causes a decline in photosynthesis and results in an increase in the shoot: root ratio and a decrease in RGR. CO2‐enriched plants grow faster and cosequently suffer more from self‐shading. Corrected for this ontogenetic drift, high CO2 concentrations stimulated the RGR of P. major throughout the entire experiment.

Modulation of plasma membrane H+-ATPase from oat roots by lysophosphatidylcholine, free fatty acids and phospholipase A2

Abstract: Plasma membrane vesicles were purified from 8-day-old oat (Avena sativa L. cv. Brighton) roots in an aqueous polymer two-phase system. The plasma membranes possessed high specific ATPase activity [ca 4 μmol P1 (mg protein)−1 min−1 at 37°C]. Addition of lysophosphatidylcholine (lyso-PC) produced a 2–3 fold activation of the plasma membrane ATPase, an effect due both to exposure of latent ATP binding sites and to a true activation of the enzyme. Lipid activation increased the affinity for ATP and caused a shift of the pH optimum of the H+ -ATPase activity to 6.75 as compared to pH 6.45 for the negative H+-ATPase. Activation was dependent on the chain length of the acyl group of the lyso-PC, with maximal activition obtained by palmitoyl lyso-PC. Free fatty acids also activated the membrane-bound H+-ATPase. This activation was also dependent on chain length and to the degree of unsaturation, with linolenic and arachidonic acid as the most efficient fatty acids. Exogenously added PC was hydrolyzed to lyso-PC and free fatty acids by an enzyme in the plasma membrane preparation, presumably of the phospholipase A type. Both lyso-PC and free fatty acids are products of phospholipase A2 (EC 3.1.1.4) action, and addition of phospholipase A2 from animal sources increased the H+-ATPase activity within seconds. Interaction with lipids and fatty acids could thus be part of the regulatory system for H+-ATPase activity in vivo, and the endogenous phospholipase may be involved in the regulation of the H+-ATPase activity in the plasma membranne.

Cadmium‐induced changes in the composition and structure of the light‐harvesting chlorophyll a/b protein complex II in radish cotyledons

Abstract: Five-day-old etiolated radish (Raphanux salivux L. cv. Saxa) seedlings exposed to white continuous light in the presence of Cd2+ (0.2 mM) showed characteristic changes in their light-harvesting chlorophyll a/b protein complex II after 48 h of greening. The content of its oligomeric supramolecular form was greatly diminished with a concomitant increase in the level of the monomer. The isolation of highly purified light-harvesting chlorophyll a/b protein complex II from control and Cd2+ treated radish cotyledons and a detailed analysis of its structure and composition revealed that first of all, Cd2+ altered the content of the specific phosphatidylglycerol fatty acid -trans-Δ3-hexadecenoic acid, widely accepted as a component responsible for the oligomerization of this chlorophyll-protein complex. This fatty acid in the thylakoid membrane phosphatidylglycerol pool seems to be very sensitive to different environmental stresses lowering its content, which indicates the vital significance of this component for the supramolecular organization and proper functioning of the light-harvesting chlorophyll a/b protein complex II.

A reassessment of glycolysis and gluconeogenesis in higher plants

Abstract: Sung, S.-J. S., Xu, D.-P., Galloway, C. M. and Black, C. C., Jr. 1988. A reassessment of glycolysis and gluconeogenesis in higher plants. - Physiol. Plant. 72: 650–654. Sucrose is the starting point of glycolysis and end point of gluconeogenesis in higher plants. During both glycolysis and gluconeogenesis alternative enzymes are present at various steps to carry out parallel pathways; alternatives are available for utilizing nucleotide triphosphates and pyrophosphate; fructose 2,6-bisphosphate serves as a strong internal regulator; and plants use these cytoplasmic alternatives as they develop and as their environments change.

Salt tolerance of the reed plant Phragmites communis

Abstract: Reed plants (Phragmites communis Trinius) were grown at NaCl concentrations up to 500 mM and their growth, mineral contents and leaf blade osmotic potential were determined. Addition of NaCl up to 300 mM did not affect growth significantly. Sucrose, Cl-and Na+ concentrations in the shoots increased with the salinity of the medium and the shoot water content decreased. K+ always contributed most to the leaf osmotic potential. Even in the presence of 250 mM NaCl in the rooting medium, the leaf blade contained only 50 mM Na+, suggesting that the plants have an efficient mechanism for Na+ exclusion. 22Na+ uptake experiments suggested that the retranslo-cation of absorbed Na+ from shoots to the rooting medium lowered the uptake of Na+.

Short-term pH regulation in plants

Abstract: Cellular pH regulation consists of two features: (i) Long-term pH homeostasis, which ensures that all H+ or OH− produced in excess is ultimately removed from the cell and which requires metabolic energy; (ii) short-term reactions of the cell(s) to sudden shifts in intracellular pH, in order to prevent acute disturbances of metabolism. Recent progress in measuring and understanding of mainly short-term cellular regulation is summarized, including cellular responses to pH loads that arise from different sources such as external pH, weak acids/bases, protonophores, metabolic inhibitors, H+/cotransport, light and phytohormones. Whereas the plasma membrane H+ pump and metabolic adjustments may serve both long- and short-term pH control, physico-chemical buffering and the translocation of H+ from and to cellular compartments render only time-limited capacity for the neutralization of pH loads and seem exhausted within minutes. In spite of the widespread opinion that, because of tight regulation, intracellular pH does not vary with time, there is good evidence for long-lasting pH changes in plant cells, i.e. after hormonal stimulation, light/dark changes or carboxylation during crassulacean acid metabolism (CAM). This emphasizes that cytoplasmic pH, besides being well regulated, is essential not only for the regulation of membrane transport but also as a cellular messenger.