Hypobaric Control of Ethylene-Induced Leaf Senescence in Intact Plants of Phaseolus vulgaris L

A controlled atmospheric-environment system (CAES) designed to sustain normal or hypobaric ambient growing conditions was developed, described, and evaluated for its effectiveness as a research tool capable of controlling ethylene-induced leaf senescence in intact plants of Phaseolus vulgaris L.

Senescence was prematurely-induced in primary leaves by treatment with 30 parts per million ethephon. Ethephon-derived endogenous ethylene reached peak levels within 6 hours at 26°C. Total endogenous ethylene levels then temporarily stabilized at approximately 1.75 microliters per liter from 6 to 24 hours. Thereafter, a progressive rise in ethylene resulted from leaf tissue metabolism and release. Throughout the study, the endogenous ethylene content of ethephon-treated leaves was greater than that of nontreated leaves.

Subjecting ethephon-treated leaves to atmospheres of 200 millibars, with O₂ and CO₂ compositions set to approximate normal atmospheric partial pressures, prevented chlorophyll loss. Alternately, subjecting ethephon-treated plants to 200 millibars of air only partially prevented chlorophyll loss. Hypobaric conditions (200 millibars), with O₂ and CO₂ at normal atmospheric availability, could be delayed until 48 hours after ethephon treatment and still prevent most leaf senescence. In conclusion, hypobaric conditions established and maintained within the CAES prevented ethylene-induced senescence (chlorosis) in intact plants, provided O₂ and CO₂ partial pressures were maintained at levels approximating normal ambient availability.

An unexpected increase in endogenous ethylene was detected within nontreated control leaves 48 hours subsequent to relocation from winter greenhouse conditions (latitude, 42°00″ N) to the CAES operating at normal ambient pressure. The longer photoperiod and/or higher temperature utilized within the CAES are hypothesized to influence ethylene metabolism directly and growth-promotive processes (e.g. response thresholds) indirectly.

     

The effect of soil moisture on the tolerance of Lupinus pilosus genotypes to a calcareous soil

Commercial narrow-leafed lupins (Lupinus angustifolius L.) grown on calcareous soils commonly display chlorotic symptoms resembling Fe deficiency. The severity of chlorosis increases with concurrent increases in soil moisture content. Our research has indicated that the rough-seeded lupin species, Lupinus pilosus Murr., has a range of adaptation to calcareous soils, from tolerant to intolerant. A pot experiment was conducted comparing a tolerant, a moderately tolerant and a moderately intolerant genotype of L. pilosus. Plants were grown for 35 days in a calcareous soil (50% CaCO₃) at three moisture contents (80%, 100% and 120% of field capacity); the growth was compared with that on a fertile black cracking clay control soil at 70% of field capacity. Visual chlorosis score, chlorophyll meter readings, number of leaves and shoot dry weights were recorded at 14, 21, 28 and 35 days after sowing. Concentrations of chlorophyll, active Fe and nutrients in the youngest fully expanded leaves were also measured. Results showed that increased soil moisture increased the severity of chlorotic symptoms (increased chlorosis score) in all genotypes. The tolerant genotype showed significantly less symptoms than other genotypes at all moisture contents. All genotypes were able to recover from chlorosis symptoms at 80% moisture in the calcareous soil. Chlorosis score negatively correlated with chlorophyll meter readings, chlorophyll concentration and foliar active and total Fe, and Mn concentrations. Visual chlorosis score appeared to be a cost effective, accurate and efficient method enabling classification of the tolerance of genotypes. The chlorotic symptoms were likely to be due to HCO₃ ^- induced nutrient deficiencies or a direct effect of HCO₃ ^- on chlorophyll synthesis. This study indicates that the most probable mechanism of tolerance is related to an ability to prevent uptake of HCO₃ ^- or efficiently sequester it once inside the root which prevents increases in internal pH and transport to the shoots.     

Manganese Toxicity in Sugarcane Plantlets Grown on Acidic Soils of Southern China

Ratoon sugarcane plantlets in southern China have suffered a serious chlorosis problem in recent years. To reveal the causes of chlorosis, plant nutrition in chlorotic sugarcane plantlets and the role of manganese (Mn) in this condition were investigated. The study results showed that the pH of soils growing chlorotic plantlets ranged from 3.74 to 4.84. The symptoms of chlorosis were similar to those of iron (Fe) deficiency while the chlorotic and non-chlorotic plantlets contained similar amount of Fe. Chlorotic plantlets had 6.4-times more Mn in their leaf tissues compared to the control plants. There was a significantly positive correlation between Mn concentration in the leaves and the exchangeable Mn concentration in the soils. Moreover, leaf Mn concentration was related to both seasonal changes in leaf chlorophyll concentration and to the occurrence of chlorosis. Basal stalks of mature sugarcanes contained up to 564.36 mg·kg^-1 DW Mn. Excess Mn in the parent stalks resulted in a depress of chlorophyll concentration in the leaves of sugarcanes as indicated by lower chlorophyll concentration in the leaves of plantlets emerged from basal stalks. Ratoon sugarcane plantlets were susceptible to chlorosis due to high Mn accumulation in their leaves (456.90–1626.95 mg·kg^-1 DW), while in planted canes chlorosis did not occur because of low Mn accumulation (94.64–313.41mg·kg^-1 DW). On the other hand, active Fe content in chlorotic plantlets (3.39 mg kg^-1 FW) was only equivalent to 28.2% of the concentration found in the control. These results indicate that chlorosis in ratoon sugarcane plantlets results from excessive Mn accumulated in parent stalks of planted cane sugarcanes grown on excessive Mn acidic soils, while active Fe deficiency in plantlets may play a secondary role in the chlorosis.     

The acceptor side of photosystem II is damaged more severely than the donor side of photosystem II in ‘Honeycrisp’ apple leaves with zonal chlorosis

To investigate the photoinhibition of photosynthesis in ‘Honeycrisp’ apple (Malus domestica Borkh. cv. Gala) leaves with zonal chlorosis, we compared pigments, CO₂ assimilation and chlorophyll (Chl) a fluorescence (OJIP) transient between chlorotic leaves and normal ones. Chl and carotenoids (Car) contents, Chl a/b ratio, and absorptance were lower in chlorotic leaves than in normal ones, whereas Car/Chl ratio was higher in the former. Although CO₂ assimilation and stomatal conductance were lower in chlorotic leaves, intercellular CO₂ concentration did not differ significantly between the two leaf types. Compared with normal leaves, chlorotic ones had increased deactivation of oxygen-evolving complexes (OEC), minimum fluorescence (F _o), dissipated energy, relative variable fluorescence at L-, W-, J- and I-steps, and decreased maximum fluorescence (F _m), maximum quantum yield for primary photochemistry (F _v /F _m or TR_o/ABS), quantum yield for electron transport (ET_o/ABS), quantum yield for the reduction of end acceptors of photosystem I (PSI) (φ_Ro and RE_o/ABS), maximum amplitude of IP phase, amount of active photosystem II (PSII) reaction centers (RCs) per cross section (CS) and total performance index (PI_tot,abs). In conclusion, photoinhibition occurs at both the donor (i.e., the OEC) and the acceptor sides of PSII in chlorotic leaves. The acceptor side is damaged more severely than the donor side, which possibly is the consequence of over-reduction of PSII due to the slowdown of Calvin cycle. In addition to decreasing light absorptance by lowering Chl level, energy dissipation is enhanced to protect chlorotic leaves from photo-oxidative damage.     

Photosynthetic carbon fixation in iron-chlorotic and recovered green sugarcane leaves

Summary Sugarcane var. Co 740 is grown in various parts of Maharashtra (India) and is susceptible to chlorosis due to physiological non-utilization of iron. The physiological disorder is seen over a large area and it results in poor yield. Low sucrose yield can be recovered by foliar sprays of ferrous sulphate. The nonchlorotic and chlorotic leaves were used for the photosynthetic studies. The leaves after ferrous sulphate treatment show an increase in total chlorophyll contents and at the same time show an improved chlorophylla to chlorophyllb ratio which is affected in the chlorotic ones. The recovered green leaves have higher uptake of nitrogen, phosphorus, potassium and iron.¹⁴CO₂ fixation studies for short- and long-term experiments reveal that recovered green leaves can synthesize malate more efficiently and also utilize it for sucrose synthesis more rapidly than in the chlorotic ones. On the contrary more amino acids, reducing sugars and sugar phosphates are synthesized in the chlorotic leaves. There is also an accumulation of citrate, glutamate, and tartrate in the chlorotic leaves. Our results indicate that sucrose synthesis is disturbed in the chlorotic leaves and can be corected byfolia: sprays of ferrous sulphate.     

Complex, localized changes in CO2 assimilation and starch content associated with the susceptible interaction between cucumber mosaic virus and a cucurbit host

Infection of the cotyledons of Cucurbita pepo L. with cucumber mosaic virus (CMV) results in the formation of chlorotic, starch-containing lesions. To characterize the physiological changes occurring within lesions, the distribution of the virus was examined by immunolocalization and correlated with starch accumulation, 14_CO2 assimilation and chlorophyll a fluorescence quenching. These techniques resolved the lesion into a complex and reproducible arrangement of cell types of diverse physiology. The region of infected cells extended beyond specific circular zones of cells which variously showed enhanced rates of CO₂ assimilation, enhanced chlorophyll a fluorescence quenching, starch accumulation, starch degradation and chlorosis. This indicates a series of physiological changes occurring over several days following viral replication within a cell. Starch accumulation in the lesion was shown to result from photosynthetic activity of cells within the lesion and not from the import of photosynthate from surrounding uninfected areas of the cotyledon.     

Metabolic regulation in diseased leaves. I. The respiratory rise in barley leaves infected with powdery mildew

Photosynthetic and respiratory activities have been measured in leaves of Hordeum vulgare L. var. Manchuria (barley) after infection with Erysiphe graminis var. hordei (powdery mildew). Two isogenic lines, one resistant to infection and the other highly susceptible, were examined.

These isogenic lines showed very different physiological responses following infection. Photosynthesis and the chlorophyll content of resistant leaves was unaffected by infection. Respiration increased slightly and this was accompanied by small increases in activities of enzymes of glycolysis, the pentose-P pathway and the tricarboxylic acid cycle.

The infection of susceptible leaves resulted in a slight increase in photosynthesis 48 hours after inoculation, but subsequently there was a progressive decrease in the photosynthesis of these leaves compared with that of noninfected leaves. The capacity of infected leaves for partial reactions of photosynthesis such as the Hill reaction and the photoreduction of nicotinamide adenine dinucleotide phosphate (NADP¹) decreased during the later stages of infection. The levels of chlorophyll, NADPH-diaphorase and aldolase also declined. There was no detectable difference in the respiration of infected and noninfected leaves until 48 hours after inoculation. After this time, the infected leaves showed a higher respiration, the maximum difference occurring about 144 hours after inoculation. The respiratory increase was not accompanied by significant changes in the levels of enzymes of glycolysis and the tricarboxylic acid cycle with the exception of malate dehydrogenase which was lower in infected leaves. In contrast, the activities of glucose-6-P dehydrogenase and 6-P-gluconate dehydrogenase showed changes similar to that observed for respiration.

The respiration and the activities of glucose-6-P dehydrogenase and 6-P-gluconate dehydrogenase did not increase in infected leaves of etiolated plants, even when excellent growth of the fungus was established by growing the plants in White's basal medium supplemented with sucrose. The respiration of a susceptible mutant barley (the yellow-green virescent mutant of the variety Himalaya) when grown in the light at 11° was not changed by infection although the characteristic respiratory rise occurred in plants grown at 15°. At the lower temperature chloroplasts fail to develop in this mutant, although development is normal at 15°.

It is suggested that the pathogen is not directly responsible for the increase in respiration in green leaves, rather that this is a response in the host cells to a loss of photosynthetic capacity.

     

Iron nutrition-mediated chloroplast development

Membrane development in chloroplasts was explored by resupplying iron to iron-deficient sugar beet (Beta vulgaris L. cv F58-554H1) and monitoring changes in lamellar components during regreening. The synthesis of chlorophyll a, chlorophyll b, and Q, the first stable electron acceptor of photosystem II, exhibited a lag phase during the first 24 to 48 hours of resupply. In contrast, the per area amounts of P₇₀₀ and cytochrome f increased linearly over the first 48 hours. During the early regreening period, the Q to P₇₀₀ ratio was 2.6 and decreased to 0.7 after 96 hours of regreening. The rate of photosynthesis (net CO₂ uptake) per chlorophyll increased during the first 48 hours of resupply, then by 96 hours decreased to values typical of control plants. The results suggest that there was preferential synthesis of the measured photosystem I components during the first 24 to 48 hours, while from 48 to 96 hours there was rapid synthesis of all components. The iron nutrition-mediated chloroplast development system provides a useful experimental approach for studying biomembrane synthesis and structural-functional relations of the photosynthetic apparatus.     

Carbon Dioxide and Ethylene Control of Spore Germination in Onoclea sensibilis L

Regulation of spore germination in the fern Onoclea sensibilis L. was investigated by applying CO₂ alone and in combination with ethylene. Sterile spores were sown aseptically on Knops solution in loosely capped culture tubes, enclosed individually in 2-liter chambers, and grown under continuous white light. When maintained in enclosed containers with the ethylene-absorbent mercuric perchlorate and with atmospheres enriched up to 2% CO₂ (v/v), spores germinated without any inhibition. Higher levels of applied CO₂ were progressively inhibitory. Inhibition by CO₂ was reversible. When CO₂ was permitted to escape and spores were exposed subsequently to ambient laboratory air, recovery from inhibition occurred within 48 hours. Also, inhibition by CO₂ was specific, since the same degree of inhibition resulted regardless of whether spores were treated with exogenous CO₂ for 48, 72, or 96 hours. The effect on germination of 1 μl/l added ethylene depended upon the amount of applied CO₂. When containers of KOH were enclosed and ambient CO₂ was absorbed, inhibition of germination by 1 μl/l exogenous ethylene was 90%. When CO₂ was applied in concentrations from 0.25 to 1.0% (v/v), CO₂ increasingly antagonized the inhibitory action of 1 μl/l added ethylene. Thus, photoinduced germination of spores was regulated by competitively interacting levels of CO₂ and ethylene.     

Ultrastructural Changes in Brassica Leaves Caused by Alternaria brassicae and Destruxin B

Alternaria blight in Brassica spp (caused by Alternaria brassicae) is characterized by dark brown to blackish necrotic lesions surrounded by chlorotic areas on the leaves. Similar chlorotic lesions were mimicked by a chlorotic toxin (destruxin B) purified from the culture filtrate. Ultrastructural studies were performed to study and compare the changes caused by A. brassicae inoculation in vivo and its host selective toxin under in vitro conditions. Electron microscopy of healthy, chlorotic and necrotic portions of B. campestris leaves naturally infected with A. brassicae revealed considerable differences at ultrastructural level. The necrotic lesions showed plasmolysis with total disruption of cell organelles. The chlorotic lesions had normal plasma membrane but swollen mitochondria with reduced number of cristae and vesiculation of the envelope. Chloroplasts showed degeneration of granal fretwork with an increase in the number of plastoglobuli. Chlorotic lesions due to foliar application of destruxin B induced indentical canges in leaves.     

Involvement of ethylene in the action of the cotton defoliant thidiazuron

The effect of the defoliant thidiazuron (N-phenyl-N′-1,2,3-thiadiazol-5-ylurea) on endogenous ethylene evolution and the role of endogenous ethylene in thidiazuron-mediated leaf abscission were examined in cotton (Gossypium hirsutum L. cv Stoneville 519) seedlings. Treatment of 20- to 30-day-old seedlings with thidiazuron at concentrations equal to or greater than 10 micromolar resulted in leaf abscission. At a treatment concentration of 100 micromolar, nearly total abscission of the youngest leaves was observed. Following treatment, abscission of the younger leaves commenced within 48 hours and was complete by 120 hours. A large increase in ethylene evolution from leaf blades and abscission zone explants was readily detectable within 24 hours of treatment and persisted until leaf fall. Ethylene evolution from treated leaf blades was greatest 1 day posttreatment and reached levels in excess of 600 nanoliters per gram fresh weight per hour (26.7 nanomoles per gram fresh weight per hour). The increase in ethylene evolution occurred in the absence of increased ethane evolution, altered leaf water potential, or decreased chlorophyll levels. Treatment of seedlings with inhibitors of ethylene action (silver thiosulfate, hypobaric pressure) or ethylene synthesis (aminoethoxyvinylglycine) resulted in an inhibition of thidiazuron-induced defoliation. Application of exogenous ethylene or 1-aminocyclopropane-1-carboxylic acid largely restored the thidiazuron response. The results indicate that thidiazuron-induced leaf abscission is mediated, at least in part, by an increase in endogenous ethylene evolution. However, alterations of other phytohormone systems thought to be involved in regulating leaf abscission are not excluded by these studies.     

The Stimulation of Ethylene Synthesis in Nicotiana tabacum Leaves by the Phytotoxin Coronatine

Coronatine is a chlorosis-inducing toxin produced by the plant pathogen Pseudomonas syringae pv atropurpurea. This bacterium is the causal agent of chocolate spot disease, in which brown lesions with chlorotic margins develop on the leaves of Lolium multiflorum Lam. Among the many physiological changes to plants caused by coronatine is the stimulation of ethylene production from bean leaves. The ethyl-substituted side chain of coronatine is an analog of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC). We have examined the question of whether part or all of the released ethylene comes from the breakdown of coronatine itself. The rate of ethylene release from leaves of Nicotiana tabacum was proportional to the concentration of coronatine applied to the leaf surface. The lowest effective concentration of coronatine, applied to leaves at 15 pmol cm⁻² of leaf area, resulted in the production of 44 pmol of ethylene cm⁻² over a period of 4 h. The maximum rate of ethylene production occurred 28 to 32 h after application of coronatine. The specific activity of ethylene produced by discs cut from coronatine-treated Nicotiana tabacum leaves floating on a solution containing 10 mm [U-¹⁴C]methionine was consistent with its exclusive origin from methionine. ACC accumulated in the coronatine-treated tissue. ACC synthase activity increased in Phaseolus aureus hypocotyls during a 6-h treatment with coronatine. Thus, coronatine induces the synthesis of ethylene from methionine.     

Therapeutic Effect of Kinetin on Tobacco Alternariosis

LESIONS produced by Alternaris tenuis on tobacco leaves consist of fungal invaded, necrotic centres surrounded by well demarcated, non-invaded, chlorotic haloes^1,2, though necrosis does occur without chlorosis following infection of leaves in advanced senescence³. Alternariol monomethyl ether (AME)⁴ and tenuazonic acid⁵ have been implicated as chlorosis inducing metabolites (CIM) but other metabolites^6,7 may be involved. CIM are readily detoxified in young, living tissue⁶, though tenuazonic acid seems to be fairly stable⁵. Under uniform conditions, infection of progressively older leaf tissues results in lesions with chlorotic halos of increasing width^1,3 which suggests that the CIM are detoxified less readily or are synthesized more abundantly as the host tissue ages.     

Influence of Iron Chlorosis on Pigment and Protein Metabolism in Leaves of Nicotiana tabacum L

Experiments were conducted on Nicotiana tabacum, L. to study the relation in the grana among chlorophylls, carotenoids, and proteins. The effect of iron chlorosis on protein and pigment synthesis was studied at different stages of chlorosis using glycine-U-C¹⁴. Pigments were separated by thin layer chromatography.

Chlorophyll a, chlorophyll b, carotenoid, and protein contents of chloroplasts from chlorotic tissue were less than those of normal tissues. A 25% decrease in protein labeling and a 45% decrease in chlorophyll labeling was noted in deficient tissue compared to normal tissue even before chlorosis was perceptible. Both normal and iron deficient leaf discs which received iron in the incubation medium incorporated higher amounts of radioactive glycine into chlorophyll a and chlorophyll b at all stages of development than their respective counterparts not supplied with iron in the incubation medium. The presence of iron in the incubation medium reduced the amount of glycine incorporated into carotenes and xanthophylls, except where the tissue was severely chlorotic. This may be attributed to active competition for glycine between the iron-dependent- (chlorophyll) and iron-independent-(carotenoid) biosynthetic pathways. Incorporation of glycine into chloroplast pigments was lowest at severe chlorosis, probably due to a reduction in the overall enzyme activity.

     

Intact Leaves Exhibit a Climacteric-Like Rise in Ethylene Production before Abscission

The rate of ethylene production by intact, attached leaves of cotton plants (Gossypium hirsutum L.) during aging and senescence was studied using a continuous flow system that allowed air around enclosed leaves to be scrubbed to collect and assay ethylene. Senescence of lower leaves began around 150 d after planting in a controlled environment room. A progressive decline in the ethylene production rate was observed when comparing the 3rd, 6th, and 10th leaves from the base with each other. Ethylene production rates of individual leaves also declined over a 50-d period. However, as leaves began to appear chlorotic, a peak of ethylene production occurred that lasted for about 4 d followed by abscission. This peak involved a 3-fold or greater increase in the rate of ethylene production. The data indicate that intact leaves experience a climacteric-like surge in ethylene production after visible symptoms of senescence appear. This “ethylene climacteric” is apparently the signal that initiates hydrolysis of cell walls in the abscission zone.     

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.     

Leaf position-dependent effect of Alternaria brassicicola development on host cell death,photosynthesis and secondary metabolites in Brassica juncea

During the first 24 hours of infection, Alternaria brassicicola developmental parameters such as conidial germination, germ tubes and appressoria formation on each of the five mature Brassica juncea leaves, correlated with a leaf position showing stronger development of the pathogen on older leaves than on young ones. As a consequence of fungal development, the black spot disease was observed during 96 hours of infection on a macroscopic scale, as well as via confocal microscopy. Degradation of the chloroplast thylakoids and plastoglobule appearance during infection, followed by the decrease in chlorophyll a fluorescence parameters i.e. maximum quantum yield of PSII (F_v/F_m), non-photochemical quenching (NPQ) and chlorophyll a:b ratio, have been observed. Also, after an initial increase of carbohydrates (glucose, fructose and sucrose), content far below the respective control values was found. The content of secondary metabolites such as flavonoids and glucosinolates increased in a leaf position-dependent manner in infected leaves, with a lower level in older leaves than in younger ones. Although, the total phenolic compounds (TPCs) content did not differ significantly in infected leaves compared to control leaves, TPCs level in both control and infected leaves was leaf position-dependent. To the best of our knowledge, this is the first report on leaf position-dependent effect on the B. juncea biochemical response to A. brassicicola infection.     

Iron availability in plant tissues-iron chlorosis on calcareous soils

The article describes factors and processes which lead to Fe chlorosis (lime chlorosis) in plants grown on calcareous soils. Such soils may contain high HCO₃ ^- concentrations in their soil solution, they are characterized by a high pH, and they rather tend to accumulate nitrate than ammonium because due to the high pH level ammonium nitrogen is rapidly nitrified and/or even may escape in form of volatile NH₃. Hence in these soils plant roots may be exposed to high nitrate and high bicarbonate concentrations. Both anion species are involved in the induction of Fe chlorosis.Physiological processes involved in Fe chlorosis occur in the roots and in the leaves. Even on calcareous soils and even in plants with chlorosis the Fe concentration in the roots is several times higher than the Fe concentration in the leaves. This shows that the Fe availability in the soil is not the critical process leading to chlorosis but rather the Fe uptake from the root apoplast into the cytosol of root cells. This situation applies to dicots as well as to monocots. Iron transport across the plasmamembrane is initiated by Fe^III reduction brought about by a plasmalemma located Fe^III reductase. Its activity is pH dependent and at alkaline pH supposed to be much depressed. Bicarbonate present in the root apoplast will neutralize the protons pumped out of the cytosol and together with nitrate which is taken up by a H⁺/nitrate cotransport high pH levels are provided which hamper or even block the Fe^III reduction.Frequently chlorotic leaves have higher Fe concentrations than green ones which phenomenon shows that chlorosis on calcareous soils is not only related to Fe uptake by roots and Fe translocation from the roots to the upper plant parts but also dependent on the efficiency of Fe in the leaves. It is hypothesized that also in the leaves Fe^III reduction and Fe uptake from the apoplast into the cytosol is affected by nitrate and bicarbonate in an analogous way as this is the case in the roots. This assumption was confirmed by the highly significant negative correlation between the leaf apoplast pH and the degree of iron chlorosis measured as leaf chlorophyll concentration. Depressing leaf apoplast pH by simply spraying chlorotic leaves with an acid led to a regreening of the leaves.     

The role of ethylene in the senescence of oat leaves

The evolution of ethylene, both from the endogenous source and from added 1-aminocyclopropane-1-carboxylic acid (ACC), has been followed in close relationship with the senescent loss of chlorophyll from seedling oat leaves. In white light, where chlorophyll loss is slow, the ethylene evolution increases slowly at first, but when the loss of chlorophyll becomes more rapid, ethylene evolution accelerates. CoCl₂ inhibits this increase and correspondingly maintains the chlorophyll content, with an optimum concentration of 10 micromolar. The rapid rate of chlorophyll loss in the dark is slightly decreased by 3-aminoethoxyvinyl glycine (AVG), by cobalt, and slightly stimulated by ACC. The slower chlorophyll loss in white light, however, is almost completely inhibited by silver ions, greatly decreased by cobalt and by AVG, and strongly increased by ACC. Since the chlorophyll loss is accompanied by proteolysis, it represents true senescence. Chlorophyll loss in light is also strongly antagonized by CO₂, 1% CO₂ giving almost 50% chlorophyll maintenance in controls, while in the presence of added ACC or ethylene gas, the chlorophyll loss is 50% reversed by about 3% CO₂. The ethylene system in leaves is thus more sensitive to CO₂ than that in fruits. Indoleacetic acid also clearly decreases the effect of ACC. It is shown that kinetin, CO₂, Ag⁺, and indoleacetic acid, all of which oppose the effect of ethylene, nevertheless increase the evolution of ethylene by the leaves, and it is suggested that ethylene evolution may, in many instances, mean that its hormonal metabolism is being prevented.     

Elemental 2-D mapping and changes in leaf iron and chlorophyll in response to iron re-supply in iron-deficient GF 677 peach-almond hybrid

Iron is an essential micronutrient for plant growth and development, involved in key cellular processes. However, the distribution of Fe in plant tissues is still not well known. In the so-called Fe chlorosis paradox, leaves of fruit trees grown in the field usually have high concentrations of Fe but still are Fe-deficient. Leaves of the Prunus rootstock GF 677 (P. dulcis?×?P. persica) grown in hydroponics have been used to carry out two-dimensional (2-D) nutrient mapping by synchrotron radiation-induced X-ray fluorescence. Iron-deficient leaves accumulated more Fe in the midrib and veins, with Fe concentration being markedly lower in mesophyll leaf areas. The effects of Fe deficiency and Fe re-supply on leaf chlorophyll concentration and on the distribution of Fe and other nutrients within different plant tissues have been investigated in the same plants. After Fe re-supply, leaf Fe concentrations increased largely in all leaf types. However, whereas re-greening was almost completely achieved in apical leaves, in some expanded leaves the increase in chlorophyll concentration was only moderate. Therefore, after Fe re-supply Fe-deficient expanded leaves of the Prunus rootstock GF 677 had significant increases in Fe concentration but were still chlorotic. This is similar to what occurs in leaves of peach trees in field conditions, opening the possibility that this system could be used as a model to study the Fe chlorosis paradox.