Membrane Permeability--Regulation by Exogenous Sugars during Senescence of Oat Leaf in Light and Darkness

The effect of sugars (sucrose, glucose and fructose) on normalphysiological changes during senescence of foliar segments ofAvena sativa cv. Suregrain was studied. In general applicationof sugars raised tissue permeability both in the light and indarkness. This change was associated with increases in endogenoussugars, hydroperoxide content and lipoxygenase activity. Inthe light it was also associated with low catalase activity.Sugars did not influence superoxide dismutase activity. In thelight, sugars accelerated senescence, measured as decreasesin chlorophyll and increases in soluble amino acids. In darknesssugars delayed senescence. The effect of sugars in the lightseemed to result from an increase in photo-oxidations associatedwith the increase in permeability. The delaying effect on senescence,found in darkness, seemed to result from an increase in respiratoryactivity plus the lack of (or combined with the lack of) photo-oxidations. (Received March 18, 1985; Accepted June 3, 1986)     

The interaction of hormonal and environmental factors in leaf senescence

The work concerns the senescence of isolated young leaves of oats (Avena sativa) floated on water or solutions. Senescence is rapid in darkness but slow in white light; the effect of light is not due to photosynthesis, but is paralleled by stomatal opening. Closure of the stomata by osmotic or chemical means makes senescence in light proceed as fast as in darkness, while opening the stomata in darkness by cytokinins, fusicoccin,etc., delays senescence to rates typical of light. The osmotic closure in light is mediated by abscisic acid, and since this also accumulates in darkness it appears as a major factor controlling senescence. Efflux of ions into the solution; indicating increased permeability, occurs almost in parallel with senescence. Senescence in light is accelerated by 1-aminocyclopropane-l-carboxylic acid (ACC) and inhibited by cobalt, silver or aminoethoxyvinyl glycine (AVG) which interfere with ethylene production or action; however, ethylene’s role is unclear because some reagents, including kinetin, that delay senescence, actually increase ethylene production. At the endogenous level, therefore, ethylene may not be a limiting factor. Finally, a new ethylene-generating system is described in which the dehydrogenation of linoleic acid is coupled through manganese to the oxidation of ACC; it is probably activein vivo.     

The Senescence Process in Oat Leaves and its Regulation by Oxygen Concentration and Light Irradiance

The regulation of senescence by oxygen-concentration, lightirradiance and H₂O₂ has been studied in leaf segments of Avenasativa L. cv. Suregrain. The development of the components of the senescence process,for example chlorophyll breakdown, proteolysis (as soluble aminoacids), hydroperoxides (as malondi-aldehyde) and permeability(as conductivity) is accelerated in light as the O₂-tensionincreases. In darkness, 0.3% O₂ accelerates increases in hydroperoxides,permeability and proteolysis and delays the chlorophyll break-down,but 0.0005% O₂ delays all the components studied. In every casethe hydroperoxide content, permeability and proteolysis areclosely related. Any treatment inducing an increase in membranepermeability causes chlorophyll bleaching (photo-oxidation)if leaf segments are then treated with light in an atmospherecontaining oxygen. Light has a modulating effect on the senescenceprocess. An irradiance lower or higher than 40 W m^–2 hasan accelerating effect on the senescence process. (Received September 7, 1985; Accepted July 30, 1985)     

Effects of light and regulators on senescence-related changes in soluble proteins in detached oat (Avena sativa L.) leaves

The rate of senescence and the two-dimensional pattern of soluble proteins from detached oat leaves senescing in either darkness or light were analyzed, and compared to those of leaves in which senescence was delayed by application of the cytokinin benzyladenine or enhanced through the action of abscisic acid.Senescence of detached leaves in light did not differ significantly from senescence in attached leaves on intact plants. In darkness, protein was lost at a higher rate than in light, but several individual proteins showed relative increases. Notably, proteins previously characterized as high-molecular-weight proteins and senescence-associated proteins (Klerk et al., 1992) increased. Changes observed during incubation in light or darkness appeared to be related to this condition rather than the rate or progress of senescence. Cytokinins delayed and abscisic acid accelerated the changes in protein pattern compared to water. Beside changes previously identified in leaves senescing on the plant, detached leaves show alterations that reflect their condition of incubation rather than their developmental progress.Abbreviations 2D-PAG two-dimensional polyacrylamide gel electrophoresis - ABA abscisic acid - BA N⁶-benzyladenine - BSA bovine serum albumin - EDTA ethylenediamine tetraacetic acid - IEF isoelectric focusing - Rubisco ribulosebisphosphate carboxylase/oxygenase - SDS sodium dodecyl sulfate - Tris tris (hydroxymethyl) aminomethane     

Metabolism of Oat Leaves during Senescence: V. Senescence in Light

A comparison has been made of the progress of senescence in the first leaf of 7-day-old oat plants (Avena sativa cv. Victory) in darkness and in white light. Light delays the senescence, and intensities not over 100 to 200 ft-c (1000-2000 lux) suffice for the maximum effect. In such intensities, chlorophyll loss and amino acid liberation still go on in detached leaves at one-third to one-half the rate observed in darkness; however, when the leaves are attached to the plant, the loss of chlorophyll in 5 days is barely detectable. Transfer of the leaves from 1 or 2 days in the low intensity light to darkness, or vice versa, shows no carryover of the effects of the preceding exposure, so that such treatment affords no evidence for the photoproduction of a stable substance, such as cytokinin, inhibiting senescence. Light causes a large increase in invertaselabile sugar and a smaller increase in glucose, and application of 100 to 300 mm glucose or sucrose in the dark maintains the chlorophyll, at least partially. Correspondingly, short exposure to high light intensity, which increased the sugar content, had a moderate effect in maintaining the chlorophyll. However, 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) completely prevents the increases in sugars and yet does not prevent the effect of light on senescence, whether determined by chlorophyll loss or by protein hydrolysis. Light causes a 300% increase in the respiration of detached oat leaves, and kinetin lowers that only partly, but unlike the increased respiration associated with senescence in the dark, the increase in the light is fully sensitive to dinitrophenol, and therefore cannot be ascribed to respiratory uncoupling. The increased respiration in light is prevented by DCMU, parallel with the prevention of sugar formation. It is therefore ascribed to the accumulation of soluble sugars, acting as respirable substrate. Also, l-serine does not antagonize the light effect. For all of these reasons, it is concluded that the action of light is not mediated by photosynthetic sugar formation, nor by photoproduction of a cytokinin. Instead, we propose that light exerts its effect by photoproduction of ATP. The action of sugars is ascribed to the same mechanism but by way of respiratory ATP. This hypothesis unifies most of the observed phenomena of the senescence process in oat leaves, and helps to explain some of the divergent findings of earlier workers.     

Effect of Potassium Supply on the Acropetal Transport of Water, Inorganic Ions and Amino Acids in Young Decapitated Sunflower Plants (Helianthus annuus)

The effect of potassium supply on the exudation rate and the content of ions and amino acids in exudation sap was studied using young decapitated sunflower plants (Helianthus annuus L.). Plants were grown originally in a complete nutrient solution. After decapitation one set of plants was transferred to a nutrient solution without potassium and another set to a solution with potassium, all other nutrients remaining the same for both treatments. During an experimental period of 3 days the plants supplied with potassium showed exudation rates which were more than twice as high as plants in the nutrient medium without potassium. The potassium supply affected the potassium and nitrate contents of the exudation sap. Both were lower in the treatment without potassium. The calcium and magnesium contents of the exudation sap were not significantly influenced by the potassium treatment, whereas the phosphate content was higher in the treatment without potassium. At the beginning of the exudation period the potassium supply did not affect the content of amino acids in the exudate, but later the plants supplied with potassium showed higher contents of amino acids in the exudation sap. As the absence of potassium reduced the exudation rate considerably the acropetal transport of all exudation sap constituents analysed in this experiment was markedly reduced.     

The impact of light intensity on shade-induced leaf senescence

Plants often have to cope with altered light conditions, which in leaves induce various physiological responses ranging from photosynthetic acclimation to leaf senescence. However, our knowledge of the regulatory pathways by which shade and darkness induce leaf senescence remains incomplete. To determine to what extent reduced light intensities regulate the induction of leaf senescence, we performed a functional comparison between Arabidopsis leaves subjected to a range of shading treatments. Individually covered leaves, which remained attached to the plant, were compared with respect to chlorophyll, protein, histology, expression of senescence-associated genes, capacity for photosynthesis and respiration, and light compensation point (LCP). Mild shading induced photosynthetic acclimation and resource partitioning, which, together with a decreased respiration, lowered the LCP. Leaf senescence was induced only under strong shade, coinciding with a negative carbon balance and independent of the red/far-red ratio. Interestingly, while senescence was significantly delayed at very low light compared with darkness, phytochrome A mutant plants showed enhanced chlorophyll degradation under all shading treatments except complete darkness. Taken together, our results suggest that the induction of leaf senescence during shading depends on the efficiency of carbon fixation, which in turn appears to be modulated via light receptors such as phytochrome A.     

Relation between Respiration and Senescence in Oat Leaves

The respiration of excised oat (Avena sativa cv Victory) leaves and their sensitivity to inhibitors was followed during senescence under varied conditions. The respiration rate, which in controls reaches its peak on the third day in darkness, is lowered at the time of fastest loss of chlorophyll (as reported earlier) by seven unrelated reagents that all delay dark senescence. When senescence is delayed by white light or by cytokinins, the respiratory rise is correspondingly delayed. Kinetin and l-serine, which act as antagonists on senescence, also act as antagonists on the respiratory rate. However, an exception to this close correspondence between senescence and the respiratory rise is offered by the lower aliphatic alcohols, which delay dark senescence and yet accelerate the onset of the respiratory rise.     

The senescence of detached leaves of tropaeolum

The senescence of detached Tropaeolum majus leaves was compared with that described earlier for Avena. Tropaeolum was chosen as being not only a dicot but also as having a nearly circular leaf, thus needing only the smallest minimum of wounding, since wounding delays the loss of chlorophyll and protein in darkness. Tropaeolum resembles Avena in that closing the stomata osmotically or with ABA causes rapid senescence in light. As in Avena also, n-hexanol and α,α′-dipyridyl delay senescence in darkness but cause `bleaching' of chlorophyll in light. Unlike Avena, however, kinetin and gibberellic acid, which delay senescence in the dark in both species, do so in Tropaeolum without causing any significant stomatal opening. The senescence of Tropaeolum leaves is actually promoted by fusicoccin, which powerfully delays senescence in Avena, although fusicoccin does cause stomatal opening in darkness in both species. Thus, many of the phenomena of senescence are alike in the monocot and dicot, but there are several significantly different responses to the senescence-modifying reagents. It is concluded that while stomatal closure accelerates senescence in both species, stomatal opening is not directly linked to the prevention of leaf senescence.     

Effect of Na(3)VO(4) on the P State of Nitella translucens

The capacity of sodium orthovanadate to inhibit the plasmalemma H⁺ ATPase of Nitella translucens internodal cells in vivo was tested. Here we show that 1 millimolar vanadate added externally depolarizes strongly and permanently the membrane potential, both in dark and light, to the Nernst potential for potassium consistent with pump inhibition by vanadate. From the results it is clear that the H⁺ ATPase is always active, under light or dark conditions, in contradiction with the widespread idea of pump inactivation by darkness. The changes in conductance for light, dark, and vanadate-induced conditions are analyzed. The effect of dark on membrane passive permeabilities and on the possibility that some plasmalemma channels could be regulated by a phosphorylation-dephosphorylation process is discussed.     

锌营养状况对小麦根细胞膜透性的影响

小麦缺锌不仅导致根系K~ 和NO_3~-泌出量增加,而且低分子量有机化合物如氨基酸、糖类化合物和酚类化合物的泌出量也明显提高。重新供锌(ZnSO_4)12h后,根系K~ 、NO_3~-、氨基酸和碳水化合物的泌出量迅速减少,随着时间的延长,泌出量接近对照水平。结果说明锌对根细胞膜结构的稳定性及膜功能的完整性是必不可少的。     

Microbial products trigger amino acid exudation from plant roots

Plants naturally cycle amino acids across root cell plasma membranes, and any net efflux is termed exudation. The dominant ecological view is that microorganisms and roots passively compete for amino acids in the soil solution, yet the innate capacity of roots to recover amino acids present in ecologically relevant concentrations is unknown. We find that, in the absence of culturable microorganisms, the influx rates of 16 amino acids (each supplied at 2.5 microm) exceed efflux rates by 5% to 545% in roots of alfalfa (Medicago sativa), Medicago truncatula, maize (Zea mays), and wheat (Triticum aestivum). Several microbial products, which are produced by common soil microorganisms such as Pseudomonas bacteria and Fusarium fungi, significantly enhanced the net efflux (i.e. exudation) of amino acids from roots of these four plant species. In alfalfa, treating roots with 200 microm phenazine, 2,4-diacetylphloroglucinol, or zearalenone increased total net efflux of 16 amino acids 200% to 2,600% in 3 h. Data from (15)N tests suggest that 2,4-diacetylphloroglucinol blocks amino acid uptake, whereas zearalenone enhances efflux. Thus, amino acid exudation under normal conditions is a phenomenon that probably reflects both active manipulation and passive uptake by microorganisms, as well as diffusion and adsorption to soil, all of which help overcome the innate capacity of plant roots to reabsorb amino acids. The importance of identifying potential enhancers of root exudation lies in understanding that such compounds may represent regulatory linkages between the larger soil food web and the internal carbon metabolism of the plant.     

The effect of the phenolic compounds exuded by pea roots in darkness on the reproduction of Rhizobium

The exudation, composition, and biological activity of the phenolic compounds (PC) of pea (Pisum sativum L.) roots in the light and darkness were studied. The roots of leguminous plants grown for 5 days in darkness exuded a smaller amount of PC that displayed a weaker stimulation of Rhizobium reproduction. Moreover, the root exudates contained antimicrobial compounds, stilbenes. It is assumed that a lower PC exudation by roots and the specific features of PC composition influencing the biological activity are among the reasons causing a delayed nodulation of legumes grown in darkness.     

Metabolism of Oat Leaves during Senescence : VII. The Interaction of Carbon Dioxide and Other Atmospheric Gases with Light in Controlling Chlorophyll Loss and Senescence

In air largely freed from CO₂, senescence of isolated oat (Avena sativa cv Victory) seedling leaves is no longer prevented by white light; instead, the leaves lose both chlorophyll and protein as rapidly as in the dark. Senescence in light is also accelerated in pure O₂, but it is greatly delayed in N₂; 100% N₂ preserves both protein and chlorophyll in light and in darkness. In light in air, most of the compounds tested that had previously been found to delay or inhibit senescence in darkness actually promote the loss of chlorophyll, but they do not promote proteolysis. Under these conditions, proteolysis can therefore be separated from chlorophyll loss. But in light minus CO₂, where chlorophyll loss is rapid in controls, two of these same reagents prevent the chlorophyll loss. Unlike the many reagents whose action in light is thus the opposite of that in darkness, abscisic acid, which promotes chlorophyll loss in the dark, also promotes it in light with or without CO₂. Kinetin, which prevents chlorophyll loss in the dark, also prevents it in light minus CO₂. In general, therefore, the responses to light minus CO₂ are similar to the responses to darkness, and (with the exception of abscisic acid and kinetin) opposite to the response to light in air.     

Changes in ribulose-1,5-bisphosphate carboxylase and its translatable small subunit mRNA levels during senescence of mustard (Sinapis alba) cotyledons

During the senescence of detached first leaves of oat ( Avena sativa L. cv. Victory) seedlings (grown in continuous light) the protein is hydrolyzed and the proteases increase, but the expected simple relation between these two factors is not always realized. The present experiments examine the timing, the influence of light and darkness and the action of the protein synthesis inhibitors cycloheximide (CHI) and cordycepin. Transfer from dark to light delays the breakdown of both chlorophyll (Chl) and protein, but some residual proteolysis is ascribed to the enzyme initially present. Transfer to CHI resembles transfer to light, while the action of cordyceptin is similar but much weaker. Repeated determinations of the acid protease, which is the most active one and the first to appear, show that this enzyme is formed in the light about as rapidly as in the dark, though with different kinetics. In spite of this there is little proteolysis in light in the first 5 days. One possible explanation of that could be that protein is rapidly resynthesized in light, but treatment with [¹⁴C]-leucine shows that such resynthesis is no faster in light than in darkness. It is therefore concluded that the protease initially does not have access to its substrates and, as a corollary, that the senescence process must be controlled by the gradual impairment of the vacuolar membrane, allowing protease to enter the cytosol and attack the proteins there and in the organelles. This concept is supported by many observations on the timing and on the known changes in membrane permeability during senescence.     

Intracellular Distribution of Free Amino Acids between the Vacuolar and Extravacuolar Compartments in Internodal Cells of Chara australis

Distribution of free amino acids between the vacuolar and extra-vacuolar(cytoplasmic) compartments in internodal cells of Chara australiswas studied. Under the control conditions (14-h light : 10-hdark), most (90%) of the cellular free amino acids were foundin the extra-vacuolar compartment. The reverse was true forammonia. The major amino acids were isoasparagine, alanine,glutamic acid, aspartic acid, serine and glycine. The contentsof hydrophobic and basic amino acids were minor and relativelygreater proportions were found in the vacuole except when theircontents were extremely low. When cells were kept for 3 days under continuous light or incontinuous darkness, the total free amino acid content increasedto about 120% (light) and about 150% (dark) that of the control.These increases mainly took place in the vacuole, but the aminoacid species responsible for the increments differed with thelight conditions. In contrast, the cytoplasmic content was relativelyconstant (50–60 mM) even under continuous light or darkness.The results suggest that the vacuole acts in the homeostasisof the cytoplasmic amino acid content. As anion, amino acidsin the cytoplasm compensated for about 10–20% of the reported"anion deficiency" in the cytoplasm. (Received June 7, 1984; Accepted September 11, 1984)     

富钾基因型籽粒苋主要根系分泌物及其对土壤矿物态钾的活化作用

通过试验,研究了2种供K水平对籽粒苋(Amaranthus spp.)富K基因型和一般基因型根系分泌物含量变化的影响,以及在低K胁迫时3个生长期两类基因型主要根系分泌物含量的变化特点,模拟了籽粒苋根系分泌物对土壤矿物态钾的活化作用.结果表明,籽粒苋根系分泌物中可溶性糖、氨基酸和有机酸含量随供K水平的升高而降低,且富K基因型根系分泌物中3种物质的分泌量始终大于一般基因型;在正常供K条件下,两基因型根系分泌能力相近,但在低K处理时,前者显著高于后者,差异显著;在2种供K水平下,根系有机酸分泌量在3种分泌物中占绝对优势,分别是可溶性糖和氨基酸分泌量的几十倍和几百倍.籽粒苋生长到50 d时,一般基因型根系可溶性糖、氨基酸和有机酸的分泌量较40 d时迅速降低.富K基因型根系分泌物中可溶性糖、氨基酸和有机酸含量在3个生长时期均大于一般基因型,且随着生长时间的延长,两基因型间可溶性糖、氨基酸和有机酸含量的差异明显增大.两类基因型在3个生长时期均以分泌有机酸为主,其占总分泌量的93%以上.籽粒苋根系分泌物处理后的土壤速效钾含量均高于清水对照处理,富K基因型在低K胁迫时的根系分泌物对土壤K的活化作用明显大于一般基因型.     

The relation between nitrogen deficiency and leaf senescence

Because the "mobilization" of nitrogen resulting from nutritional nitrogen deficiency is also prominent during leaf senescence, the characteristics of these two syndromes were compared. Oat plants ( Avena sativa L. cv. Victory) were raised on a nutrient solution, complete except for nitrogen supply (i.e., with only the seed protein as nitrogen source), and the senescence of their leaves was compared with that of controls grown on a full nutrient solution. The N-deficient plants flowered after forming only 4 leaves and each set a single seed. The nitrogen lack affected the content of chlorophyll somewhat more than the content of the amino acids or protein nitrogen. However, spraying the plants with kinetin solution was able to retain 20–30% of the chlorophyll and protein. During senescence, the chlorophyll appears to be less stable in the N-deficient leaves than in the controls, while the protein is somewhat more stable than in the controls. Also, when the detached leaves from N-deficient plants senesced in white light or in darkness, kinetin delayed their senescence almost as effectively as that of control leaves. Most strikingly, the stomata of N-deficient leaves after detachment and floating on water were largely closed in light, just as in senescence, but could be partially induced to open by kinetin treatment. Since stomatal closure has earlier been shown to cause senescence, the characteristic syndrome of foliar nitrogen deficiency is concluded to be partly that of senescence.     

Light and dark assimilation of nitrate in plants

Abstract. Heterotrophic assimilation of nitrate in roots and leaves in darkness is closely linked with the oxidative pentose phosphate pathway. The supply of glucose-6-phosphate to roots and chloroplasts in leaves in darkness is essential for assimilation of nitrite into amino acids. When green leaves are exposed to light, the key enzyme, glucoses-phosphate dehydrogenase, is inhibited by reduction with thioredoxin. Hence the dark nitrate assimilatory pathway is inhibited under photoautotrophic conditions and replaced by regulatory reactions functioning in light. On account of direct photo-synthetic reduction of nitrite in chloroplasts and availability of excess NADH for nitrate reduclase, the rate of nitrate assimilation is extremely rapid in light. Under dark anaerobic conditions also nitrate is equally rapidly reduced to nitrite on account of abolition of competition for NADH between nitrate reductase and mitochondrial oxidation.     

The effect of the phenolic compounds exuded by pea roots in darkness on the reproduction of <Emphasis Type="Italic">Rhizobium</Emphasis>

The exudation, composition, and biological activity of the phenolic compounds (PC) of pea (Pisum sativum L.) roots in the light and darkness were studied. The roots of leguminous plants grown for 5 days in darkness exuded a smaller amount of PC that displayed a weaker stimulation of Rhizobium reproduction. Moreover, the root exudates contained antimicrobial compounds, stilbenes. It is assumed that a lower PC exudation by roots and the specific features of PC composition influencing the biological activity are among the reasons causing a delayed nodulation of legumes grown in darkness.