Abscission of citrus leaf explants: no correlation with naphthaleneacetic Acid conjugation in the abscission zone

The role of α-naphthaleneacetic acid (NAA) in the control of abscission in Citrus (Citrus sinensis L. Osbeck) leaf explants and its conjugation were studied in non-aged and 24-hour-aged explants. Dipping non-aged explants in 1.5 micromolar NAA for 15 minutes immediately after excision did not delay abscission whereas 150 micromolar NAA effectively delayed it. As incubation time was prolonged up to 24 hours after excision, the delaying effect of both concentrations gradually increased. In general, both concentrations did not delay abscission when applied to 24-hour-aged explants held for an additional period of up to 24 hours. The uptake and conjugation of ¹⁴C-NAA to glucose and aspartic acid were similar in petiole, abscission zone, and leaf blade of non-aged and aged tissues, for all NAA concentrations. No correlation was established between the kinetics of abscission and the rate of conjugation in the abscission zone.     

Ethylene Biosynthesis and Cadmium Toxicity in Leaf Tissue of Beans (Phaseolus vulgaris L.)

Stress ethylene production in bean (Phaseolus vulgaris L., cv. Taylor's Horticultural) leaf tissue was stimulated by Cd²⁺ at concentrations above 1 micromolar. Cd²⁺-induced ethylene biosynthesis was dependent upon synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase. Activity of ACC synthase and ethylene production rate peaked at 8 h of treatment. The subsequent decline in enzyme activity was most likely due to inactivation of the enzyme by Cd²⁺, which inhibited ACC synthase activity in vitro at concentrations as low as 0.1 micromolar. Decrease in ethylene production rate was accompanied by leakage of solutes and increasing inhibition of ACC-dependent ethylene production. Ca²⁺, present during a 2-hour preincubation, reduced the effect of Cd²⁺ on leakage and ACC conversion. This suggests that Cd²⁺ exerts its toxicity through membrane damage and inactivation of enzymes. The possibility of an indirect stimulation of ethylene biosynthesis through a wound signal from injured cells is discussed.     

Reversal of Abscisic Acid-Induced Stomatal Closure by trans-Cinnamic and p-Coumaric Acid

Abscisic acid (ABA)-induced increase in stomatal diffusive resistance (SDR) in excised leaves of bean (Phaseolus vulgaris L. cv Pencil Pod) and maize (Zea mays L. cv Golden Bantam) is inhibited by low concentrations of trans-cinnamic acid (TCA) (1 micromolar) and p-coumaric acid (PCA) (10 micromolar) when given together with ABA (10 micromolar) in the transpiration stream through the cut end of the petiole or leaf blade. A concentration effect is observed both in the ABA action and its reversal by phenolic acids. Leaves having attained a high diffusive resistance in ABA solution recover rapidly when transferred to water. ABA (10 micromolar) induced closure of the stomata in onion, Allium cepa L. and Vicia faba epidermal peels. This is associated with loss of K⁺ from guard cells. In the presence of TCA (10 micromolar) and PCA (10 micromolar) K⁺ is retained in the guard cells with open stomata. The dark closure of stomata is also inhibited by TCA and PCA. It is suggested that these phenolic acids may inhibit the ABA effect by competing with or acting on some ABA-specific site, probably located on the plasma membrane, regulating flux of K⁺ ions. A weak association of ABA with the plasma membrane is envisaged because of the rapid recovery obtained upon transferral of the leaves to water.     

Uptake and Release of Abscisic Acid by Isolated Photoautotrophic Mesophyll Cells, Depending on pH Gradients

Uptake and release of abscisic acid (AbA) by isolated mesophyll cells of Papaver somniferum is characterized by the following observations: (a) Uptake rate is a linear function of the external AbA concentration in the range from 10⁻⁶ to 5 × 10⁻⁵ molar, and decreases with increasing pH. At any pH, uptake rate is linearly related to the concentration of undissociated abscisic acid, calculated from the pK = 4.7 according to the Henderson-Hasselbalch equation. At low external pH (5.0), AbA accumulation in the cells is about 10-fold. (b) Uptake of AbA is completely inhibited by salts such as KNO₂ or sodium acetate, which decrease the pH gradient between medium and cells. KCN or m-chlorocarbonylcyanide phenylhydrazone inhibits AbA uptake only after longer incubation periods (20-40 minutes). (c) Uptake rate as well as equilibrium concentration is significantly higher in light than in darkness. (d) At low external pH, release of AbA from preloaded cells is strongly stimulated by KNO₂. It is concluded that AbA is distributed between leaf cells and free space according to pH gradients, with the undissociated abscisic acid being the main penetrating species. Uptake and release occur via diffusion, without participation of a carrier.     

A novel interaction of magnesium translocation with the supply of phosphorus to roots of grapevine (Vitis vinifera L.)*

Abstract. The role of phosphorus (P) in leaf magnesium (Mg) concentrations and photosynthesis was investigated in field and glasshouse experiments with grapevine (Vitis vinifera L., cvs. Chenin blane. Chardonnay, and Carignane). In the field, leaves of vines growing on soil with low available P exhibited symptoms of Mg deficiency and had low P and Mg concentrations. The rate of photosynthesis for leaves of untreated control vines was approximately 0.7 nmol CO₂ cm ² s ¹. When P fertilizer was applied to the soil, Mg deficiency symptoms were eliminated, and leaf P and Mg concentrations increased to above critical levels. When Mg was applied as a foliar spray, leaf Mg increased to above critical levels, but leaf P did not change significantly. In both experiments, the rate of photosynthesis increased to greater than 1.0 nmol CO₂ cm ² s ¹ after nutrient applications. Thus, under low soil P conditions, leaf photosynthesis was limited by leaf Mg concentrations. In glasshouse experiments in which vines were grown with and without P for three seasons, Mg accumulated in large roots of - P vines to approximately twice the concentration found in roots of + P vines. Analysis of the xylem exudate from detopped plants showed that Mg concentration in xylem sap of + P vines was twice as great as that in - P vines. When P was supplied to - P vines, the concentration of Mg increased to the concentration of + P vines within 2 days. The results show that the translocation of Mg from roots to shoots of grapevine is dependent upon P supply to the roots and suggest that Mg translocation is more sensitive than uptake to P supply.     

The development of photophosphorylation and photosynthesis in greening bean leaves

Photophosphorylation and oxygen evolution were measured in 8-day-old dark-grown bean leaves (Phaseolus vulgaris) after various times of greening in far red light and in white light. The sequence of development was the same for both greening regimes, but the processes were much more rapid in white light. The capacity for photophosphorylation, as assayed by the firefly luciferase assay, appeared after 12 hours in far red light. At this stage and for times up to 24 hours, photophosphorylation was not inhibited by 10⁻⁵m 3-(3,4-dichlorophenyl)-1,1-dimethylurea. At 24 hours, the capacity for oxygen evolution appeared and photophosphorylation became partially inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea at concentrations which inhibited oxygen evolution. In white light photophosphorylation appeared after 15 minutes, and oxygen evolution at one hour. Photophosphorylation became partially sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea when oxygen evolution appeared. Carbonylcyanide m-chlorophenyl-hydrazone inhibited photophosphorylation and photosynthesis at low concentrations, 10⁻⁵m, with immature leaves, but the leaves developed resistance to carbonylcyanide m-chlorophenyl-hydrazone as they greened.     

Inhibition of photosynthesis by ethylene-a stomatal effect

Ethylene at hormonally significant levels inhibited net photosynthesis of the cultivated peanut (Arachis hypogaea L.) as measured by gas analysis. Upon the removal of ethylene, the inhibition was naturally overcome at the concentration-exposure duration combinations tested. Increased length of exposure of 1 microliter of ethylene per liter of air up to 6 hours increased the degree of net photosynthesis inhibition (68% reduction after 6-hour exposure). Significantly greater inhibition of photosynthesis by ethylene was detected on peanut genotypes having higher photosynthetic efficiency. In contrast to peanut, hormonal concentrations of ethylene only moderately inhibited sweet potato, Jerusalem artichoke, and sunflower photosynthesis and was without effect on beans, peas, Irish potato, Mimosa pudica, and white clover. No inhibition could be found by ethylene on ribulose 1,5-biphosphate carboxylase activity in vitro. Photosynthesis was lowered at all CO₂ concentrations below ambient at an O₂ concentration of 1.5%, indicating that the action of ethylene was not affected by low O₂; concomitantly, an increase in the CO₂ compensation point occurred. Diffusion resistance measurements of leaf water vapor loss made on ethylene-treated peanut leaves showed a measurable decrease in leaf conductance which correlated with net photosynthesis decrease. Ethylene influenced the conductance of abaxial stomata more so than adaxial.     

Salicylhydroxamic Acid (SHAM) Inhibition of the Dissolved Inorganic Carbon Concentrating Process in Unicellular Green Algae

Rates of photosynthetic O₂ evolution, for measuring K_0.5(CO₂ + HCO₃⁻) at pH 7, upon addition of 50 micromolar HCO₃⁻ to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K₁(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO₂ uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O₂ evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO₃), and the rate of ¹⁴CO₂ fixation with 100 micromolar [¹⁴C] HCO₃⁻. Salicylhydroxamic acid inhibition of O₂ evolution and ¹⁴CO₂-fixation was reversed by higher levels of NaHCO₃. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO₂ accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related.     

Influence of Translocation of Photosynthetic Efficiency of Phaseolus vulgaris L

Measurements of net photosynthesis show that in Phaseolus vulgaris L. the cultivar Michelite-62 exceeds the cultivar Red Kidney in net CO₂ uptake by 23 to 31%. Data on translocation of pulse label indicate that export of a pulse of photosynthetically assimilated ¹⁴C from the source leaf of either M-62 or Red Kidney follows an exponential pattern and shows an initial rapid phase followed by a second slower phase. The steeper slope for both phases in M-62 suggests its rate of translocation of pulse label is higher than that of Red Kidney. Furthermore, only 38% of the ¹⁴C remains in the leaf of M-62 after 8 hours, while Red Kidney retains up to 60% of the label. Leaf autoradiographs obtained after pulse labeling demonstrate a much faster rate of vein loading in M-62 and are considered evidence for the higher translocation efficiency of M-62. These results provide evidence for a positive correlation between photosynthetic efficiency and translocation efficiency in M-62 and Red Kidney and give support to our hypothesis that translocation is one of the important physiological factors controlling the varietal differences in photosynthetic efficiency in Phaseolus vulgaris.     

Inorganic Carbon Uptake by Chlamydomonas reinhardtii

The rates of CO₂-dependent O₂ evolution by Chlamydomonas reinhardtii, grown with either air levels of CO₂ or air with 5% CO₂, were measured at varying external pH. Over a pH range of 4.5 to 8.5, the external concentration of CO₂ required for half-maximal rates of photosynthesis was constant, averaging 25 micromolar for cells grown with 5% CO₂. This is consistent with the hypothesis that these cells take up CO₂ but not HCO₃⁻ from the medium and that their CO₂ requirement for photosynthesis reflects the K_m(CO₂) of ribulose bisphosphate carboxylase. Over a pH range of 4.5 to 9.5, cells grown with air required an external CO₂ concentration of only 0.4 to 3 micromolar for half-maximal rates of photosynthesis, consistent with a mechanism to accumulate external inorganic carbon in these cells. Air-grown cells can utilize external inorganic carbon efficiently even at pH 4.5 where the HCO₃⁻ concentration is very low (40 nanomolar). However, at high external pH, where HCO₃⁻ predominates, these cells cannot accumulate inorganic carbon as efficiently and require higher concentrations of NaHCO₃ to maintain their photosynthetic activity. These results imply that, at the plasma membrane, CO₂ is the permeant inorganic carbon species in air-grown cells as well as in cells grown on 5% CO₂. If active HCO₃⁻ accumulation is a step in CO₂ concentration by air-grown Chlamydomonas, it probably takes place in internal compartments of the cell and not at the plasmalemma.     

Involvement of phaseolotoxin in halo blight of beans: transport and conversion to functional toxin

Phaseolotoxin ([N^δ-phosphosulfamyl]ornithylalanylhomoarginine) is produced by Pseudomonas phaseolicola (Burkh.) Dows. in liquid culture. When phaseolotoxin was applied to leaves of bean (Phaseolus vulgaris L.) at 0.1 to 1 nmoles/g fresh weight of leaf by a prick-assay procedure, the characteristic “halo” symptom of bean halo blight disease developed after 24 to 48 hours. At higher concentrations (10-100 nmoles/g fresh weight) the systemic symptoms, which are commonly a feature of diseased plants, also developed after 24 to 48 hours.     

Glyceraldehyde 3-Phosphate:NADP Reductase of Spinach Leaves : Steady State Kinetics and Effect of Inhibitors

The steady state kinetics of glyceraldehyde 3-phosphate:NADP⁺ oxidoreductase (GNR) (EC 1.2.1.9) have been investigated. The enzyme exhibits hyperbolic behavior over a wide range of substrate concentrations. Double-reciprocal plots are nearly parallel or distantly convergent with limiting K_m values of 2 to 5 micromolar for NADP⁺ and 20 to 40 micromolar for D-glyceraldehyde 3-phosphate (G3P). The velocity response to NADP⁺ as the varied substrate is however sigmoidal if G3P concentration exceeds 10 micromolar, whereas the response to G3P may show inhibition above this concentration. This `G3P-inhibited state' is alleviated by saturating amounts of NADP⁺ or NADPH. Product inhibition patterns indicate NADPH as a potent competitive inhibitor to NADP⁺ (K_i 30 micromolar) and mixed inhibitor towards G3P, and 3-phosphoglycerate (3PGA) as mixed inhibitor to both NADP⁺ and G3P (K_i 10 millimolar). The data, and those obtained with dead-end inhibitors, are consistent with a nonrapid equilibrium random mechanism with two alternative kinetic pathways. Of these, a rapid kinetic sequence (probably ordered with NADP⁺ binding first and G3P binding as second substrate) is dominant in the range of hyperbolic responses. A reverse reaction with 3PGA and NADPH as substrates is unlikely, and was not detected. Of a number of compounds tested, erythrose 4-phosphate (K_i 7 micromolar) and Pi (K_i 2.4 millimolar) act as competitive inhibitors to G3P (uncompetitive towards NADP⁺) and are likely to affect the in vivo activity. Ribose 5-phosphate, phosphoenolpyruvate, ATP, and ADP are also somewhat inhibitory. Full GNR activity in the leaf seems to be allowed only under high photosynthesis conditions, when levels of several inhibitors are low and substrate is high. We suggest that a main function of leaf GNR is to supply NADPH required for photorespiration, the reaction product 3PGA being cycled back to chloroplasts.     

The Relationship between Ribulose Bisphosphate Concentration, Dissolved Inorganic Carbon (DIC) Transport and DIC-Limited Photosynthesis in the Cyanobacterium Synechococcus leopoliensis Grown at Different Concentrations of Inorganic Carbon

To examine the factors which limit photosynthesis and their role in photosynthetic adaptation to growth at low dissolved inorganic carbon (DIC), Synechococcus leopoliensis was grown at three concentrations (as signified by brackets) of DIC, high (1000-1800 micromolar), intermediate (200-300 micromolar), and low (10-20 micromolar). In all cell types photosynthesis varied from being ribulose bisphosphate (RuBP)-saturated at low external [DIC] to RuBP-limited at high external [DIC]. The maximum rate of photosynthesis (P_max) was achieved when the internal concentration of RuBP fell below the active site density of RuBP carboxylase/oxygenase (Rubisco). At rates of photosynthesis below P_max, photosynthetic capacity was limited by the ability of the cell to transport inorganic carbon and to supply CO₂ to Rubisco. Adaptation to low DIC was reflected by a decrease in the [DIC] required to half-saturate photosynthesis. Simultaneous mass-spectrometric measurement of rates of photosynthesis and DIC transport showed that the initial slope of the photosynthesis versus [DIC] curve is identical to the initial slope of the DIC transport versus [DIC] curve. This provided evidence that the enhanced capacity for DIC transport which occurs upon adaptation to low [DIC] was responsible for the increase in the initial slope of the photosynthesis versus [DIC] curve and therefore the decrease in the half saturation constant of photosynthesis with respect to DIC. Levels of RuBP and in vitro Rubisco activity varied only slightly between high and intermediate [DIC] grown cells but fell significantly (65-70%) in low [DIC] grown cells. Maximum rates of photosynthesis followed a similar pattern with P_max only slightly lower in intermediate [DIC] grown cells than in high [DIC] grown cells, but much lower in low [DIC] grown cells. The changing response of photosynthesis to [DIC] during adaptation to low DIC, may be explained by the interaction between DIC-transport limited and [RuBP]-limited photosynthesis.     

Salinity effects on photosynthesis in isolated mesophyll cells of cowpea leaves

Mesophyll cells from leaves of cowpea (Vigna unquiculata [L.] Walp.) plants grown under saline conditions were isolated and used for the determination of photosynthetic CO₂ fixation. Maximal CO₂ fixation rate was obtained when the osmotic potential of both cell isolation and CO₂ fixation assay media were close to leaf osmotic potential, yielding a zero turgor pressure. Hypotonic and hypertonic media decreased the rate of photosynthesis regardless of the salinity level during plant growth. No decrease in photosynthesis was obtained for NaCl concentrations up to 87 moles per cubic meter in the plant growing media and only a 30% decrease was found at 130 moles per cubic meter when the osmotic potential of cell isolation and CO₂ fixation media were optimal. The inhibition was reversible when stress was relieved. At 173 moles per cubic meter NaCl, photosynthesis was severely and irreversibly inhibited. This inhibition was attributed to toxic effects caused by high Cl⁻ and Na⁺ accumulation in the leaves. Uptake of sorbitol by intact cells was insignificant, and therefore not associated with cell volume changes. The light response curve of cells from low salinity grown plants was similar to the controls. Cells from plants grown at 173 moles per cubic meter NaCl were light saturated at a lower radiant flux density than were cells from lower salinity levels.     

Suppression of Plant Growth by Nitrogen Dioxide

Nicotiana glutinosa and pinto bean seedlings (Phaseolus vulgaris) were exposed for short periods (3 days or less) to high concentrations of NO₂ (4.11-20.53 mg/m³ to compare the resulting leaf lesions with ozone damage produced at concentrations of 0.43 to 0.86 mg/m³. Although the same physiological age leaf tissue was damaged by both toxicants, damage caused by NO₂ was unlike that caused by ozone.

Pinto bean (Phaseolus vulgaris) and Pearson improved tomato (Lycopersicon esculentum) seedlings were continuously exposed for 10 to 22 days, to low concentrations of NO₂ (less than 1.03 mg/m³). These exposures caused significant growth suppression, increase in green color (total chlorophyll content), and distortion of leaves.

     

Deferral of senescence and abscission by chemical inhibition of ethylene synthesis and action in bean explants

Three compounds known to inhibit ethylene synthesis and/or action were compared for their ability to delay senescence and abscission of bean explants (Phaseolus vulgaris L. cv Contender). Aminoethoxyvinyl-glycine (AVG), AgNO₃, and sodium benzoate were infiltrated into the petiole explants. Their effect on abscission was monitored by measuring the force required to break the abscission zone, and their effect on senescence was followed by measuring chlorophyll and soluble protein in the distal (pulvinus) sections. AVG at concentrations between 1 and 100 micromolar inhibited ethylene synthesis by about 80 to 90% compared to the control during sampling periods of 24 and 48 hours after treatment. This compound also delayed the development of abscission and senescence. Treatment with AgNO₃ at concentrations between 1 and 100 micromolar progressively reduced ethylene production, but to a lesser extent than AVG. The effects of AgNO₃ on senescence and abscission were quite similar to those of AVG. Sodium benzoate at 50 micromolar to 5 millimolar did not inhibit ethylene synthesis during the first 24 hours, but appreciably inhibited ethylene synthesis 48 hours after treatment. It also delayed the development of abscission and senescence. The effects of AVG, Ag⁺, and sodium benzoate suggest that ethylene could play a major role in both the senescence induction phase and the separation phase in bean explants.     

Metabolic Activity of Isolated Leaf Cells of Phaseolus vulgaris in Relation to Leaf Development

Mesophyll cells were isolated from primary leaves of 5- to 21-day Phaseolus vulgaris plants. The rate of photosynthesis and respiration, and RNA, protein, and lipid synthesis was determined for these cells. Appropriate ¹⁴C substrates and product purification procedures were used for each process prior to liquid scintillation counting. The size of the leaves increased about 5-fold between days 5 and 11, and then remained relatively constant. The greatest increase in size occurred between days 5 and 6. The age of the leaf from which the cells were isolated had a pronounced effect on the rate of all of these processes. The largest changes occurred during the period of leaf expansion (days 5-11). Initially the rate of RNA, protein, and lipid synthesis increased rapidly, maintained a maximum rate for only 1 day (day 6 or day 7), and then declined. The rate of photosynthesis increased more slowly reaching a maximum at day 9, remained relatively constant until day 15, and then declined. The rate of respiration decreased during the first 4 days to low level which was maintained throughout the experiment. The time course patterns of these biochemical processes in isolated cells were similar to those which have been reported for intact leaves. It seems that isolation of leaf cells does not modify their metabolic activity.     

AUR1, a novel gene conferring aureobasidin resistance onSaccharomyces cerevisiae: a study of defective morphologies in Aur1p-depleted cells

Aureobasidin A (AbA), a cyclic depsipeptide produced byAureobasidium pullulans R106, is highly toxic to fungi includingSaccharomyces cerevisiae. We isolated several dominant mutants ofS. cerevisiae which are resistant to more than 25 µg/ml of AbA. From a genomic library of one suchAUR1 mutant, theAUR1 ^R (foraureobasidinresistant) mutant gene was isolated as a gene that confers resistance to AbA on wild-type cells. Its nucleotide sequence showed that the predicted polypeptide is a hydrophobic protein composed of 401 amino acids, which contains several possible transmembrane domains and at least one predicted N-linked glycosylation site. Comparison of the mutant gene with the wild-typeaur1 ⁺ gene revealed that the substitution of Phe at position 158 by Tyr is responsible for acquisition of AbA resistance. We suggest that the gene product of the wild-typeaur1 ⁺ is a target for AbA on the basis of following results. Firstly, cells that overexpress the wild-typeaur1 ⁺ gene become resistant to AbA, just as cells with anAUR1 ^R mutation do. Secondly, disruption of theaur1 ⁺ gene demonstrated that it is essential for growth. Thirdly, in the cells with a disruptedaur1 locus, pleiotropic morphological changes including disappearance of microtubules, degradation of tubulin and abnormal deposition of chitin were observed. Some of these abnormalities are also observed when wild-type cells are treated with AbA. The abnormality in microtubules suggests that the Aur1 protein is involved in microtubule organization and stabilization.     

Induction of delta-Aminolevulinic Acid Synthase Activity and Inhibition of Heme Synthesis in Euglena gracilis by N-Methyl Mesoporphyrin IX

N-Methyl mesoporphyrin IX, an inhibitor of heme synthesis, increases extractable δ-aminolevulinic acid (ALA) synthase activity when administered to growing cultures of Euglena gracilis Klebs strain Z Pringsheim in micromolar concentrations. Wild-type light-grown green cells and white aplastidic cells exhibited 2.8-fold and 1.8-fold increases, respectively, in ALA synthase activity within five to six hours after incubation with 4 × 10⁻⁶ molar N-methyl mesoporphyrin IX. Protoheme levels were decreased and ⁵⁹Fe incorporation into heme was inhibited by N-methyl mesoporphyrin IX, indicating that, as in animal cells, N-methyl mesoporphyrin IX acts specifically to block iron insertion into protoporphyrin IX. Chlorophyll synthesis in wild-type cells was not affected within the first 6 hours after administration of N-methyl mesoporphyrin IX.     

Relationships between Cadmium, Zinc, Cd-Peptide, and Organic Acid in Tobacco Suspension Cells

Responses of tobacco (Nicotiana tabacum) suspension cells to Cd and Zn were studied in the presence and absence of ligand of Cd-peptide in order to understand the role of this peptide versus other mechanisms in Cd and Zn accumulation and accommodation in plants. With 45 micromolar Cd and 300 micromolar Zn (non-growth-inhibiting levels), metals appeared rapidly within cells, and intracellular Cd and Zn reached medium concentrations after 6 to 10 hours. Cd-peptide was observed in response to Cd after 2 hours, but this form only accounted for ~30% of soluble Cd after 24 hours. Peptide was not observed in cells exposed to 300 micromolar Zn for up to 7 days. Organic acid-to-metal stoichiometry indicated that endogenous organic acid content of cells was more than sufficient to complex absorbed metals and no evidence was found for stimulation of organic acid biosynthesis by Cd or Zn. Metal-complexing potential of organic acids for Cd and Zn versus endogenous cations is discussed as is vacuolar-extravacuolar distribution of metals. The absence of Cd-peptide does not limit Cd-accumulation in the system studied. Results suggest that tobacco suspension cells accommodate the presence of non-growth-inhibiting and growth-inhibiting levels of Cd and Zn by sequestration in the vacuole as complexes with endogenous organic acids and that this may be a principal means for accommodation of Cd as well as Zn in the presence and absence of Cd-peptide.