Showing papers in "Physiologia Plantarum in 1983"

Implications of water stress‐induced changes in the levels of endogenous ascorbic acid and hydrogen peroxide in Vigna seedlings

Abstract: Vigna cutjang Endl. cv. Pusa Barsati seedlings, subjected to increasing degrees of water stress (−0.5, −1.0, −1,5 MPa), produced an approximately proportional increase in glycolate oxidase activity, hydrogen peroxide (H2O2) and proline content but a decrease in catalase activity, ascorbic acid and protein content. Leaf water potential (leaf ψ) and relative water content (RWC) were also lowered with increasing stress. Pretreatment with l-cysteine and reduced glutathione (10-3 M) decreased glycolate oxidase activity, H2O2 content, ascorbic acid oxidase activity, proline content and also slightly improved the water status of leaves stressed (−1.0 MPa) for 2 days. Pretreatment of non-stressed seedlings with these antioxidants had little or no effect. These studies indicate that treatment with antioxidants makes the plant tolerant against water stress by modulating the endogenous levels of H2O2 and ascorbic acid in stressed tissue.

Internal filters: Prospects for UV‐acclimation in higher plants

Abstract: Wavelength-selective absorption of solar radiation within plant leaves allows penetration of visible radiation (400–700 nm) to the chloroplasts, while removing much of the damaging ultraviolet-B (UV-B, 280–320 nm) radiation. Flavonoids are important in this wavelength-selective absorption. Induction of flavonoid synthesis by solar radiation, and specifically by UV-B radiation, is discussed as this relates to the potential acclimation of plants to enhanced solar UV-B radiation that would result from stratospheric ozone reduction.

Effects of ultraviolet‐B radiation on the growth and yield of crop plants

Abstract: This paper reviews growth chamber, greenhouse, and field studies on the effects of ultraviolet-B (UV-B. between 280 and 320 nm) radiation on agricultural crop plants. Our understanding of the physiological effects of UV-B radiation comes primarily from growth chamber studies, where UV-B is artificially supplied via filtered lamps. Both photosystems I and II, as well as carboxylating enzymes, are sensitive to UV-B radiation. Ultraviolet-B radiation also affects stomatal resistance, chlorophyll concentration, soluble leaf proteins, lipids, and carbohydrate pools. In general, the effects of UV-B radiation are accentuated by the low levels of visible radiation typically found inside growth chambers. Ultraviolet-B radiation has also been shown to affect anatomical and morphological plant characteristics. Commonly observed UV-B induced changes include plant stunting, reductions in leaf area and total biomass, and alterations in the pattern of biomass partitioning into various plant organs. In sensitive plants, evidence of cell and tissue damage often appears on the upper leaf epidermis as bronzing, glazing, and chlorosis. Epidermal transmission in the UV region decreases in irradiated leaves. This decrease is primarily associated with a stimulation in flavonoid biosynthesis and is thought to be a protective, screening response to the deleterious effects of UV-B. A considerable degree of variability exists in sensitivity to UV-B radiation between different species. Approximately 30% of the species tested were resistant, another 20% were extremely sensitive, and the remainder were of intermediate sensitivity, in terms of reductions in total dry weight. In addition to this sizable interspecific variability, there appears to be a similarly wide intraspecific variability in UV-B response. The effects of UV-B radiation on crop yield have only been examined in a limited number of field studies, with ambient levels of UV-B radiation being supplemented with fluorescent sun lamps. Due to various deficiencies, all these field experiments to date have only limited utility for assessing the potential impact of enhanced levels of UV-B on crop productivity.

Effects of agar brand and concentration on the tissue culture medium

Abstract: In tissue-cultured Cynara scolymus L. (globe artichoke) vitrification (hyperhydric transformation) can only be overcome by increasing the concentration of agar. However, with increasing concentrations of Difco Bacto agar (8–15 g/I) the availability of labelled kinetin is decreased. There is some evidence for postulating that cytokinins under inductive conditions of low agar concentration or high matrix potential are evocators of vitrification. Both the brand and concentration of agar also affect the chemical and physical characteristics of a culture medium. Impurities introduced with the agar are responsible for significant differences in the concentration of an element in comparable media with different levels of agar, and are easily detected by conductance. Penetrometer measurements also show large ranges in solidity of media with increasing concentrations, not only within the type of agar, but also for the same concentration within different brands.

Varietal differences in the toxicity of sodium ions in rice leaves

Abstract: Visual damage is commonly used as the criterion for assessment of salinity resistance in rice breeding programmes. The use of other indicators, such as sodium uptake, is being evaluated: a correlation between initial sodium uptake to the third leaf and varietal survival was established and the physiological basis of this examined. Chlorophyll was used as an indicator of metabolic status and the relationship between leaf sodium and chlorophyll concentrations investigated for nine varieties differing in their resistance to salinity. By sampling a population of leaves having a wide range of salt concentrations, inverse relationships between chlorophyll and sodium concentrations were established. The salt concentration in the leaf tissue which causes equivalent toxicity (50% loss of chlorophyll) differed 3-fold amongst these varieties. Varieties showing the greatest tolerance to salt within the leaves were not necessarily those showing the greatest overall phenotypic resistance to salinity. The enzymes malate dehydrogenase and nitrate reductase were activated equally by sodium and potassium ions at 60–80 mol m−3 in both control and saline grown plants and severely inhibited at higher concentrations. If all the salt in the tissue of leaves with 50% chlorophyll remaining was in solution and uniformly distributed the concentration would be 135–500 mol m−3. This is improbable and some level of compartmentation is likely. It is concluded that while initial sodium uptake has predictive value for salinity resistance of varieties, there are other characteristics which are masked by excess salt entry and require independent assessment; no single factor confers resistance.

Characterization of ferric reducing activity in roots of Fe-deficient Phaseolus vulgaris

Abstract: Iron-deficient bean plants (Phaseolus vulgaris L. cv. Prelude) exhibited a ferric reducing activity in the roots, with kinetics characteristic for matrix-bound enzymes: the reaction rate was proportional to substrate (Fe-EDTA) concentration until 100 μM, and at higher concentrations it leveled off to a maximum; the Lineweaver-Burk plot yielded a non-linear relationship between rate −1 and substrate −1. The Arrhenius plot yielded apparent activation energies which were dependent on substrate concentration. No evidence was obtained for the secretion by roots of a low molecular weight metabolite involved in the reduction of iron prior to its uptake. The results are interpreted to indicate that the ferric reducing activity in the roots of iron-deficient bean plants is located in an enzyme in the plasmalemma of the cortex or epidermis cells.

Action of UV‐B radiation on photosynthetic primary reactions in spinach chloroplasts

Abstract: Spinach (Spinacia oleracea L. cv. Matador) chloroplasts were irradiated with several levels of UV-B radiation. Measurements which reflect characteristic steps of photosynthetic electron transport were made to localize the site of impairment of photosynthesis by UV-B radiation. Variable fluorescence, the μs-kinetics of the 320 nm absorption changes and also oxygen evolution were substantially reduced in chloroplasts irradiated with UV-B. It was not possible to restore the amplitude of the 320 nm absorption changes nor the signal of the transmembrane electric field measured at 520 nm by adding the photo-system II donor couple hydroquinone/ascorbate to UV-B treated chloroplast samples. This indicates that impairment of photosystem II activity is not caused by selective inhibition of the water-splitting enzyme system Y, but rather is due to blockage of photosystem II reaction centers. Photosystem 1 is inferred to be highly resistant to UV-B radiation. These results suggest that the reaction centers of photosystem II are transformed into dissipative sinks for excitation energy by action of UV-B radiation.

Breakdown of photosynthetic pigments and lipids in spinach leaves with ozone fumigation: role of active oxygens

Abstract: In spinach (Spinacia oleracea L. cv. New Asia) plants fumigated with ozone in light, destruction of chlorophylls and carotenoids and formation of malondialdehyde (MDA), an indicator of lipid peroxidation, were observed. Chlorophyll a and carotenoids in leaves started to be broken down 6–8 h after the commencement of 0.5 ppm ozone fumigation, whereas MDA formation in leaves increased linearly for the initial 8 h of fumigation followed by a more rapid increase. In leaf discs excised from 6-h fumigated plants, destruction of chlorophyll a and carotenoids and MDA formation proceeded in the light but were almost completely suppressed under an anaerobic condition. Effects of exogenously applied scavengers of active oxygen species suggest that active oxygens, especially superoxide radical (O2-), participated in both the destruction of chlorophyll a and carotenoids and the formation of MDA. Ozone fumigation reduced the levels of endogenous scavengers of O2-, superoxide dismutase (SOD) and L-ascorbate, in leaves to one-half the initial levels each by 3.5 and 8 h fumigation, respectively. The results indicate that the photosynthetic pigments and lipids were broken down by active oxygens accumulated in leaves as a result of the ozone-induced destruction of physiological defense against oxygen toxicity. Activity of polyphenol oxidase in chloroplast membranes of 4-h fumigated leaves increased to 240% of the initial level, suggesting that the thylakoid membranes had been affected severely before the pigment destruction. The relations between the pigment destruction and the disintegration of thylakoids are discussed.

Action spectra and estimation of biologically effective UV radiation

Abstract: The use of action spectra in calculating biologically effective ultraviolet radiation fluence is reviewed. In predicting biological damage due to ozone depletion, the most reliable action spectra are those derived from experiments utilizing polychromatic irradiation filtered by sharp cut-off UV filters. A new method is presented for optimum estimation of an action spectrum from data obtained in such experiments. Details of the mathematical procedures are discussed, and examples of their use on data from both plant and animal objects are given.

Current concepts on the role of potassium in stomatal movements

Abstract: A stoma opens when the surrounding guard cell pair increases in turgidity. The increase results from active accumulation of potassium in the guard cell vacuole. The intracellular compartmentation of potassium evokes compensatory accumulation of a yet-unidentified solute in the guard cell cytoplasm. The source of potassium is other epidermal cells; this indicates that stomatal movements in situ are under control of these cells also. Presumably, guard cell potassium uptake, which is from the apoplast, is mediated by a proton -extruding ATPase on the guard cell plasmalemma. The energy source is. oxidative phosphorylation and, to a lesser extent, photosynthetic electron transport. Except for high flux capacity and different responses to applied chemicals, potassium uptake by guard cells is similar to potassium uptake by other plant cells.

Inhibition by mevinolin of plant growth, sterol formation and pigment accumulation

Abstract: To obtain information on the importance of a functional mevalonate synthesis for plant growth and development, we investigated the effect of mevinolin, a highly specific inhibitor of 3-hydroxy-3-methylglutaryl (HMG) coenzyme A reductase (the mevalonate-producing enzyme) on growth, sterol accumulation and pigment formation of radish seedlings (Raphanus sativus L. cv. Saxa Treib) and in part also wheat seedlings (Triticum aestivum L. cv. Kolibri). Mevinolin applied during germination inhibits root elongation and development of lateral roots in etiolated and light-grown radish seedlings. This effect cannot be overcome by exogenous GA3, but by addition of mevalonic acid, the product of the internally inhibited reaction. This emphazises the specifity of the mevinolin effect and indicates that the biosynthesis of mevalonic acid is a mandatory requirement for root growth. In light-grown radish seedlings mevinolin also affects hypocotyl length-growth and inhibits sterol accumulation, but has little effect on the chlorophyll and carotenoid accumulation in the chloroplasts of the cotyledons. This indicates the possible presence of an independent mevalonate synthesizing pathway within the plastids and suggests a low transport rate of mevinolin from the radish roots to the cotyledons. When mevinolin is directly applied to the leaves at higher concentrations, it also reduces the light-induced chlorophyll and carotenoid accumulation as has been shown with etiolated primary leaves of wheat. This inhibition is age-dependent and proceeds to a higher extent in older than in younger etiolated leaf tissue. From our results we conclude that plastids possess an independent HMG-CoA reductase. In the cotyledons of radish, mevinolin seems to induce a senescence retardation and sun-type growth response, as has been evaluated by measuring the fast and slow chlorophyll fluorescence induction kinetics (Kautsky effect). These responses may be due to inhibitor-induced changes in the intracellular phytohormone balance.

Possibilities and constraints in the regeneration of trees from cotyledonary needles of Picea abies in vitro

Abstract: Although use of embryonic or seedling tissues for mass clonal micropropagation in vitro in conifer reforestation programmes is questionable, there is a potential application in the regeneration of plants from scarce and costly seed derived from controlled pollination. In addition, in vitro culture shortens considerably the lag phase in numbers during the initial stages of vegetative propagation via rooted cuttings. Successive steps of the present technique are described whereby cotyledonary needles (secondary explants) were subcultured on a hormone-free medium after administration of cytokinin or auxin to 14-day-old seedling (primary) explants of Picea abies. For bud induction, N6-benzyladenine (BA) was applied either as a short-duration (3 h), high-concentration (125 μM) pulse or by vacuum infiltration and incubation in a BA-containmg (5 μM) infusion medium. Induced adventitious shoots were elongated with the aid of far-red light and rooted in vivo after a long-duration (12 h), high-concentration (625 μM) application of indolebutyric acid. Pulse and infusion treatments resulted in the induction of greater numbers of adventitious buds (average of 12 per needle) over a three to four week shorter culture period than was the case with the conventional inclusion of growth regulators in the agarified medium. No exogenous auxin was required in the bud-induction programme; its inclusion even at nanomolar levels promoted histo- rather than morphogenesis. In cotyledonary needles, to the primary explants of which BA was applied as a pulse or by infusion, the cell divisions which gave rise to the meristemoids from which adventitious buds were produced, appeared to commence mainly in undifferentiated hypodermal layer cells but also in the mesophyll immediately below. By contrast, where BA was incorporated in the agarified medium the first divisions occurred mainly in cells of the epidermal layer. A number of factors affected plantlet regeneration, for instance seed variability, age of seedlings, and mode of application of growth substances. It should also be accepted that the xeromorphic nature of the conifer leaf might impose physiological and morphological constraints on its culture in vitro that could militate against easy morphogenic manipulation. It is deemed essential that the current mean ratio of regenerated plants to cotyledonary needles of 1:1 be increased 10 to 20 fold in order to approach commercial feasibility.

Membranes as targets of ultraviolet radiation

Abstract: In non-photosynthetic cells, evidence for UV (ultraviolet radiation) damage to membranes comes from electron microscopy, chemical analysis and observations of transport processes. Specific perturbations in transport across membranes occur quickly after a relatively low fluence of UV. As an example, irradiation of suspension-cultured rose cells with 500-2000 J m−2 (at 254 nm) causes an appearance of K+ in the extracellular medium at the rate of 5 × 10−10μmol cell−1 min−1 for 30 to 60 min and more slowly thereafter. The early, rapid phase of appearance of K+ reflects both an increase in efflux and a decrease in influx. The appearance of K+ is matched by an appearance of HCO−3 in the medium. The HCO−3 comes from respiratory CO2, which hydrates and dissociates in the cytoplasm, leading to a decrease in cytoplasmic pH. Overall, these results not only demonstrate UV damage to membrane function, but also suggest several ways by which UV may alter the general metabolic state of the cell. A demonstration of direct effects of UV on membrane components requires a purified system. At lower fluence, < 1800 J m−2 (254 nm), the ATPase of membrane vesicle preparations is inactivated in a two-phase process that suggests the presence of enzymes with different UV sensitivities. The existence of two non-mitochondrial enzymes in rose cell vesicles has been confirmed by solubilizing the vesicle proteins with 1% cholic acid and separating the components on G-150 Sephadex. One component of relatively high molecular weight is especially sensitive. The fact that it is still sensitive when it is dissolved in cholic acid strongly suggests that its sensitivity is intrinsic and does not depend on sensitization by other membrane components. The action spectrum for the inactivation of the ATPase has a major peak at 290 nm and extends into the UV-C and UV-A regions. The physiological effects of UV-stimulated membrane changes are uncertain. There is little evidence that the UV damage to membranes is responsible for cell death. A UV-induced loss of K+ from guard cells may result in lower stomatal conductance. UV-stimulated membrane changes may play a role in the UV-induced synthesis of anthocyanins.

Physiological changes in, and phosphate uptake by potato plants during development of, and recovery from phosphate deficiency

Abstract: The development of phosphate deficiency (P-stress) was observed in rooted sprouts of Solanum tuberosum L. cv. Desiree growing in solutions without phosphate. Shoot growth was inhibited by P-stress within 3 to 5 days of terminating the phosphate supply, while significant effects on root growth were not recorded until 7 to 9 days. Thus, the shoot:root dry weight ratio decreased from 4.3 to 2.6 over a 10-day period. Growth in the absence of an exogenous phosphate supply progressively diluted the phosphorus in the plant. The proportional decrease in concentration was similar in roots and shoots over a 7-day period, even though the former were growing more quickly. The potential for phosphate uptake per unit weight of root increased rapidly during the first 3 days of P-stress. When the plants were provided subsequently with a labelled, 1 mol m−3 phosphate solution, the absorption rate was 3 to 4-fold greater than that of control plants which had received a continuous phosphate supply. The increased rate of uptake by P-stressed plants was accounted for by an increase (3-fold) in the Vmax of system 1 for phosphate transport and by a marked increase in the affinity of the system for phosphate (decrease in Km). In the early stages of P-stress, before marked changes in growth were measured, the proportion of labelled phosphate translocated to the shoots increased slightly relative to the controls when a phosphate supply was restored. In the later stages of stress a greater proportion was retained in the root system of P-stressed plants than in that of controls. In plants with roots divided between solutions containing or lacking a phosphate supply, the increased absorption rate was determined by the general demand for phosphate in the plant and not by the P-status of the particular root where uptake was measured. By contrast, the poportion translocated was strongly dependent on the P-status of the root. The restoration of a phosphate supply to P-stressed plants was marked by a rapid increase in the P concentration in snoots and roots which returned to levels similar to unstressed controls within 24 h. The enhanced uptake rate persisted for at least 5 days, resulting in supra-normal concentrations of P in both shoots and roots, and in the formation of extensive necrotic areas between the veins of mature leaves. Autoradiographs showed accumulations of 32P in these lesions and at the points where guttation droplets formed on leaves.

Synchrony induced by double phosphate starvation in a suspension culture of Catharanthus roseus

Abstract: A synchronous cell division system was established using the double phosphate starvation method, based on the observation that one of the limiting factors in the growth of a suspension culture of Catharanthus roseus (L.) G. Don cells in the medium of Murashige and Skoog was phosphate. In the system, an increase in cell number took place in a short period of only 4 h, while the cell number remained almost constant during other periods of the cell cycle. The synchrony of the culture was confirmed by changes in mitotic index, which increased sharply prior to the increase in cell number. The S phase was determined by measuring incorporation of [3H]-thymidine into the DNA fraction during the cell cycle and synchrony of DNA synthesis was verified likewise. Synchronization by phosphate starvation is discussed in relation to the function of phosphate as a nutrient. The synchronous system thus established will be useful in biochemical studies of the cell cycle in higher plants.

Iron phosphate precipitation in Murashige and Skoog media

Abstract: Murashige and Skoog revised medium, a standard plant tissue culture medium, precipitated on keeping because iron was ineffectively chelated. Most of the precipitate formed after two days and analysis indicated that it was predominantly iron phosphate. It was found that, after precipitation, the supernatant medium had lost c. 45% of its original iron, 20% zinc and 13% phosphate. The following modifications prevented precipitation: lowering the pH to below 3.2, increasing to 3 the molar ratio of EDTA:Fe or preparing the stock solution of FeEDTA with inadequate heating. Most of these modifications had a deleterious effect on the growth and greening of Ocimum basilicum suspension cultures. However, when an increase in the molar ratio of EDTA:Fe was achieved by reducing the iron concentration to one-third, there was no inhibitory effect on growth or greening of these suspension cultures.

Cyanide-resistant respiration in roots and leaves - measurements with intact tissues and isolated-mitochondria

Abstract: A comparison was made between the oxygen uptake of roots and leaves and of mitochondria isolated from the same tissues. Ten species were included in this study: three legumes, one C3-monocotyledon, one C4-monocotyledon, the rest non-leguminous C3-dicotyledons. Root and leaf respiration in all species examined displayed substantial resistance to KCN (0.1–1.0 mM) and the cyanide-resistant respiration was completely inhibited by salicylhydroxamic acid (SHAM; 10–20 mM). SHAM alone inhibited oxygen uptake to varying degrees, depending on the species. Mitochondria were isolated from roots and leaves of many of the species examined and also displayed cyanide-resistant oxygen uptake, which was sensitive to both SHAM and tetraethylthiuram disulfide (disulfiram). Concentrations of SHAM greater than 2 mM caused inhibition of the cytochrome path as well as of the alternative path in isolated mitochondria. Respiration rates of intact roots and leaves in the presence of varying concentrations of SHAM alone were plotted against those obtained in the presence of both SHAM and KCN. This plot showed that in vivo the cytochrome pathway was not affected by 10 or 20 mM SHAM in the external solution. We conclude that the activity of the alternative pathway in intact roots and leaves can be reliably estimated by comparing SHAM-sensitivity and cyanide-resistance of respiration.

Parasitic and mutualistic associations between a mycorrhizal fungus and soybean: The effect of phosphorus on host plant‐endophyte interactions

Abstract: Soybean [Glycine max (L.) Merr. cv. Kent] plants were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe in pot cultures using an inert medium and a nutrient solution. Phosphorus was provided initially as 0, 25,50, 100 or 200 mg hydroxyapatite [HAP, Ca10(PO4)6(OH)2] per pot. Under the low (0 mg HAP) and high (100 and 200 mg HAP) P regimes, VAM plants showed 20, 25 and 38% growth retardation, respectively, relative to non-colonized controls. At 50 mg HAP, VAM plant growth was significantly enhanced (14%). Dry weight and P content of both VAM and control plants increased with increased P availability throughout the HAP gradient. Intraradical VAM fungal biomass increased linearly with increasing P availability. Extraradical VAM fungal biomass was smaller than the intraradical component of the fungus at the lowest and highest levels of P addition in the growth medium. The ratio of extra- to intraradical mycelium, a suggested index of VAM fungal effectiveness, was greatest for the 50 mg HAP treatment, coinciding with growth enhancement of the host plant. This enhanced growth of the host at an intermediate P level was apparently a result of increased P uptake by the endophyte.

Lipid changes during natural aging of soybean seeds

Abstract: When soybean seeds [Glycine max (L.) Merr. cv. Wayne] were stored at approximately 4°C and low humidity for 44 months (“natural aging”) there was a marked decline in vigor and viability which was associated with a decrease in the proportion of the polyunsaturated fatty acids. Other lipid parameters showed little change. Seeds subjected to high humidity at 40°C for several days (“accelerated aging”) experience a comparable decline in vigor and viability, but without the change in fatty acid polyunsaturation. These results indicate that accelerated aging may cause loss of vigor in a manner quite different from natural aging. The accelerated aging treatment is therefore of limited usefulness in investigations of the mechanism of natural seed aging.

Calmodulin dependent formation of membrane potential in barley root plasma membrane vesicles: A biochemical model of aluminum toxicity in plants

Abstract: Micromolar concentrations of aluminum ions interfere with calmodulin-stimulated, membrane bound ATPase activity which plays a role in the maintenance of the transmembrane potential of plasma membrane enriched vesicles isolated from barley roots. Calmodulin appears to be the major target for aluminum interaction resulting in pronounced changes in the exposure of a large, hydrophobic surface on this protein as determined with a fluorescent, hydrophobic surface probe. At a molar ratio of 3:1 [aluminum]/[calmodulin], the calmodulin stimulated enzymatic activity, probably associated with a Ca2++ Mg2+ATPase, is about 95% inhibited. Aluminum induced changes in calmodulin structure are reflected in reduced formation of the membrane potential when assayed with a fluorescent potential probe, oxonol VI. We hypothesize that the aluminum caimodulin complex represents a primary lesion in toxic responses of plants to this metal.

Comparative study of the action of ultraviolet‐C and ultraviolet‐ B radiation on photosynthetic electron transport

Abstract: The effect of ultraviolet-C (UV-C, mainly 254 nm radiation) and ultraviolet-B (UV-B, 290-320 nm) radiation on the photosynthetic electron transport reactions has been investigated. The rates of Hill activity mediated by ferricyanide and dichlorodimethoxy-p-benzoquinone (DCDMQ) were differently sensitive to UV-C but equally inhibited by UV-B. Replacement of water with diphenylcarbazide was ineffective in restoring the activity of dichlorophenol indophenol (DCPIP) Hill reaction in UV-B treated chloroplasts, but had significant effect in UV-C treated chloroplasts. Photobleaching of carotenoids in the presence of carbonyl cyanide-m-chlorophenyl-hydrazone, an indicator of the photochemical reaction associated with the reaction centre of photosystem II, was suppressed and is paralleled by the changes in Hill activity only in UV-B-treated chloroplasts. Carotenoid photobleaching occurred even in UV-C treated chloroplasts showing no measurable Hill activity. UV-C and UV-B irradiation diminished variable fluorescence. With UV-B treated, but not with UV-C treated chloroplasts, an increase in the fluorescence yield was observed upon the addition of 3-(3,4-dichIorophenyl)-l,l-dimethylurea (DCMU) and/or Na dithionite. Photosystem I activity was found to be unaffected by both UV-C and UV-B radiation at the fluences tested. Kinetics of P700 photooxidation and dark reversal in UV treated chloroplasts indicate that only the electron flow from photosystem II to photosystem I is impaired. It is concluded that while UV-B radiation inactivates specifically the photosystem II reaction centre, UV-C radiation acts at plastoquinone, the quencher Q, and the water oxidizing enzyme system.

Coaction between phytochrome and blue/UV light in anthocyanin synthesis in seedlings

Abstract: Light-mediated mass production of blue/UV absorbing pigments, anthocyanin and/or other flavonoid compounds, can be considered an adaptive mechanism to protect a plant against high levels of short wavelength sunlight. Comparative studies of light-mediated formation of anthocyanin in seedlings of higher plants have been performed. As a result of Darwinian evolution, a seedling may be expected to form considerable amounts of pigment only when necessary and only to the extent required for protection ('economy principle'). The four species investigated with regard to light-mediated synthesis of anthocyanin in seedlings (mustard, milo, tomato, wheat), differ greatly with regard to their photoperception. Phytochrome is involved in the photoresponse in all cases. We conclude that the Pfr-mediated differential gene activation leading to anthocyanin synthesis is the core of the response. However, the different species differ greatly with regard to the red, blue and UV light dependent processes they perform in order to establish sensitivity towards phytochrome (Pfr), or to amplify sensitivity towards Pfr.

Pathogen‐induced vascular gels: Ethylene as a host intermediate

Abstract: A cell free culture filtrate from 6-day cultures of Fusarium oxysporum f. sp. cubense was processed to give: (1) a heterogeneous enzyme mixture, (2) purified polygalacturonase (PG), (3) partially-purified polygalacturonate lyase and (4) β-1,4-xylanase. When introduced into explanted castor bean leaves each of these preparations was able to promote the formation of vascular system-obstructing gels. Exposure of castor bean leaves to ethylene (3 ppm) also triggered gel formation. Explanted leaves produced ethylene in response to the enzyme mixture and PG. Vascular gel formation did not occur when ethylene production in response to enzymes was prevented.

The effects of ultraviolet‐B radiation on plant competition in terrestrial ecosystems

Abstract: Evidence regarding the interaction of ultraviolet-B (UV-B, 280-320 nm) radiation and plant competition in terrestrial ecosystems is examined. The competitive interactions of some species pairs were affected even by ambient solar UV-B radiation (as exists without ozone depletion), when compared to control pairs grown without UV-B. Also, the total shoot biomass of these species pairs was depressed under ambient UV-B. Relatively large increases in UV-B radiation (approximating a 40% ozone layer reduction when weighted with the generalized plant action spectrum) altered the competitive interactions of some species pairs grown in pots under field conditions, but did not affect the total shoot biomass production of those pairs. Recent field experiments have examined the competitive interactions of wheat (Triticum aestivum L. cv. Bannock) and wild oat (Avena fatua L.) under a simulated increased UV-B regime resulting from a 16% ozone layer reduction when weighted with the generalized plant action spectrum. This increase in UV-B altered the competitive interactions of these two species without affecting the total shoot biomass production of the species pair. The manner in which increased UV-B affected the relative competitive abilities of the two species was highly dependent upon the environmental conditions during the early life stages of the plants. The implications of these results for both agricultural and natural plant communities are discussed.

The penetration of ultraviolet radiation to the ground

Abstract: The evolution of analytic formulas for characterizing the ultraviolet spectral ir-radiance penetrating to the ground is briefly described Analytic spectral functions for the extraterrestrial solar spectral irradiance, the ozone absorption coefficient, Rayleigh scattering coefficients and aerosol scattering and absorption coefficients, which are used as basic inputs, are given With Beer's law, these give immediately the direct solar spectral irradiance A ratio technique described in quantitative detail gives a procedure for calculating the skylight component of the UV radiation reaching the ground The influence of ground reflectivity, clouds and a possible connection between photobiology and radiological physics are discussed Finally the advantages of multiwavelength monitoring are described, using monochromators similar to those used in satellite ozone sounding to serve the needs of the photobiology and the atmospheric science communities

Effects of activated charcoal in anther cultures

Abstract: Embryogenesis in anther cultures of Anemone Canadensis L., Anemone hupehensis Lemoine and Nicotiana tabacum L. was shown to be inhibited by abscisic acid added to the medium. However, this inhibition was reduced in the presence of activated charcoal (AC). The presence of AC in the culture medium strongly promoted embryogenesis in anther cultures of Anemone Canadensis compared with other media combinations. Treatment of the agar medium with AC, which was removed before inoculation of the anthers, also stimulated embryogenesis, but treatment of the water constituent did not. The number of embryos produced in anther cultures of Anemone Canadensis and Nicotiana tabacum was shown to be positively related to the length of lime of incubation on medium supplemented with AC. In the case of Anemone Canadensis the stimulating effect of AC was most pronounced when the first visible embryos had emerged. The presence of anther-derived embryos from Anemone Canadensis in anther cultures of Anemone Canadensis and Nicotiana tabacum was shown to inhibit embryogenesis. It was also demonstrated that embryos from anther cultures of Anemone canadensis, Papaver setigerum DC and Clematis viticella L. produced phenolic substances, and that the concentration of these substances was higher in culture medium lacking AC. Treatment of such medium with AC could reduce the concentration of phenolic substances by more than 80%.

Effects of ultraviolet-B irradiation on plants during mild water stress.

Abstract: Cucumber (Cucumis sativus L. cv. Delikatess) and radish (Raphanus sativus L. cv. Saxa Treib) were grown in a factorial design under two ultraviolet-B (UV–B) irradiances and three levels of water stress. On a weighted, daily dose basis the UV–B radiation treatments were equivalent to ambient levels during the beginning of the growing season (controls) and those predicted for an 11.6% ozone depletion during the summer solstice at 49°N latitude. Water stress was achieved by varying the frequency of watering. The combination of UV–B radiation and water stress resulted in large species differences in the pattern of stomatal resistances. This study indicated that Cucumis is one of the most sensitive crop species to UV–B radiation yet identified and that the primary effect of UV–B radiation in this species is a decrease in the leaf diffusive resistance to water vapor. This, therefore, may result in reductions in growth via increased water stress.

The effect of brassinosteroid on auxin‐induced ethylene production by etiolated mung bean segments

Abstract: Brassinosteroid, an analogue of brassinolide, (BR) (2α, 3α, 22β, 23β-tetrahydroxy-24β-methyl-B-homo-7-oxa-5α-cholestan-6-one), was tested in conjunction with indole-3-acetic acid (IAA), naphthaleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), indole-3-butyric acid (IBA), indole-3-propionic acid (IPA), indole-3-pyruvic acid (IPyA), indole-3-aldehyde (IAld), indole-3-carbinol (ICB) or tryptophan (TRP) for its effects on ethylene production by etiolated mung bean (Vigna radiata (L.) Rwilcz cv. Berken) hypocotyl segements. The enhancement of ethylene production due to BR was greatest in conjunction with 1 μM IBA, 2,4-D, IAA, or NAA (these increases were 2580, 2070, 890, and 300%, respectively). When increasing concentrations of IBA, 2,4-D, IAA, or NAA were used, there was a decrease in the percentage stimulation by BR. Both IPyA and IPA had different optimal concentrations than the other auxins tested. Their BR-enhanced maximum percentage stimulations (1430 and 1580%) were greatest with 5 μM IPya and 10 μM IPA, respectively. There was a marked reduction in the percentage stimulation by BR with either 100 μM IPyA or IPA. The inactive indoles (IAld, ICB, or TRP) did not synergize with BR at any of the concentrations tested. Four hours following treatment those segments in contact with 1 μM BR with or without the addition of 10 μM IAA began to show a stimulation in ethylene production above the control and this stimulation became greater over the following 20 h. It was necessary for BR to be in continual contact with the tissue to have a stimulatory effect on auxin-induced ethylene production. When segments excised from greater distances below the hypocotyl hook, were treated with either IAA alone or in combination with BR, there was a decrease in ethylene production with increasing distance. There was no effect of hypocotyl length on BR stimulation of auxin-induced ethylene production; however, there was a definite decrease in ethylene production when IAA was applied alone.