Probing Monocarpic Senescence and Pod Development Through Manipulation of Cytokinin and Mineral Supplies in Soybean Explants

Defined solutions containing cytokinin and/or mineral nutrientswere supplied in lieu of the roots through the cut stem baseof soybean explants (a leaf with associated pod and subtendingstem segment) in order to analyze the roles of cytokinin andmineral nutrients from the roots in pod development and foliarmaintenance. In explants cut at early-mid podfill, supplyingonly H₂O accelerated leaf senescence and pod maturation anddecreased seed d. wt relative to comparable parts of intactplants. Zeatin (Z) and/or minerals not only delayed leaf yellowingand the decline in foliar chlorophyll levels and photosyntheticrates but also inhibited leaflet and petiole abscission relativeto H₂O controls. Even large declines in foliar assimilatoryprocesses did not necessarily lead to abscission. Z and/or mineralsalso increased stomatal conductivity throughout podfill. Z showedsome positive synergistic effects with minerals on leaf maintenance.Pod wall, cotyledon and radicle yellowing were delayed by Zand/or minerals but not as much as leaf senescence. Mineralsonly or Z +minerals prolonged seed d. wt accumulation and increasedfinal dry seed wt to a level similar to that for intact plants.Seed growth showed a complex interrelation with pod wall andleaf f. wt and d. wt changes. A decline in cytokinin and mineralflux from the roots appears to be important for pod-inducedleaf senescence; however, pod development, foliar senescenceand their component processes may be affected differently. Thus,even though the explant is a physiological/nutritional moduleof the whole plant, it is influenced by cytokinin and mineralsfrom the roots and therefore only semiautonomous. Glycine max L. Merr. cv. Anoka, soybean, abscission, cytokinin, chlorophyll, mineral nutrients, seed development, semiautonomous physiological modules, senescence, stomatal resistance     

Cytokinin Biochemistry in Relation to Leaf Senescence: IV. Cytokinin Metabolism in Soybean Explants

When [³H]dihydrozeatin riboside and [³H]zeatin riboside were supplied to soybean (Glycine max L.) explants (comprising one leaf, associated pods, and subtending stem) via the xylem at mid to late podfill, 0.1% of the supplied ³H was extracted from the seeds. The distribution of ³H in the explants was similar to that bound previously following uptake of [³H]zeatin riboside at earlier stages of pod development. Metabolites formed in the explants from ³H-labeled zeatin, zeatin riboside, and dihydrozeatin riboside were identified and related to the endogenous cytokinins shown to be present. When zeatin riboside and zeatin were supplied for 1 hour, zeatin nucleotide was the principal metabolite formed and this appeared to be the precursor of the other metabolites detected subsequently. Explants supplied with zeatin riboside or dihydrozeatin riboside for 1 hour, and then transferred to water for 20 to 24 hours, yielded leaf blades in which the main metabolites were O-glucosyldihydrozeatin, adenosine, and adenine. The metabolism of zeatin riboside in blades of explants at pre-podfill, early podfill, and mid to late podfill did not differ appreciably. The results are discussed in relation to leaf senescence and seed development.     

Relative Activities of Xylem-Supplied Cytokinins in Retarding Soybean Leaf Senescence and Sustaining Pod Development

Soybean explants (leaf, pod and subtending stem segment) excisedat early-mid podfill and cultured on a mineral nutrient solutionsenesce much sooner than comparable intact structures. Pod developmentis also advanced, but seed yield is reduced. Cytokinin addedto the mineral nutrient medium retards leaf yellowing, bladeabscission, petiole abscission and to a lesser extent pod development.At 3?10^–7 M, dihydrozeatin riboside (diZR), benzyladenine,dihydrozeatin (diZ), zeatin riboside (ZR), zeatin (Z), isopentenyladenineor isopentenyladenine riboside retard leaf yellowing by 18,15, 12, 4–6, 2–5, 2 and 1 days, respectively. Thesecytokinins show a similar hierarchy of activity on abscission.Cytokinin also retards changes in stomatal resistance and transpirationrate. The activity hierarchy for the different cytokinins onstomatal resistance and pod development differs from that onleaf yellowing. When cytokinins are given as a long pulse (about3 days), they show a somewhat different activity pattern comparedwith a continuous supply of cytokinin. Clearly, the cytokininscoming up through the xylem from the roots play an importantrole in maintaining the foliage, and a decline in the supplyof cytokinins from the roots could be a major factor in monocarpicsenescence of soybeans. If Z. ZR, diZ and diZR are normallysupplied by the roots via the xylem, they must be importantfactors in sustaining leaf function and pod development. (Received June 10, 1983; Accepted November 22, 1983)     

Regulation of monocarpic senescence of Brassica campestris by the developing pods

Senescence of Brassica campestris L. cv. B-9 was studied with regard to seed maturation and source-sink relationships. In normal control plants leaf senescence (as determined by the change in chlorophyll level) started and proceeded in a progressive manner from base to apex during the period of early pod setting. Complete yellowing of the leaves occurred well before the seed maturation and pod wall senescence. The pod wall always senesced before the attainment of final seed weight. In two different sets of acrocarpous plants containing 65 pods and 10 pods, respectively, leaf senescence was delayed during the pod filling period. It started non-sequentially after complete yellowing and senescence of the pod wall. The degree of leaf senescence at the post-pod filling stage was almost proportional to the number of pods present. When peduncles of the acrocarpous 10-podded plants were removed after the pod filling stage of the plant, leaf senescence was delayed compared to plants whose pedicels were removed, although the senescence pattern of the upper three leaves was nonsequential in both cases. Defruiting at an early stage of development delayed leaf senescence, although the pattern of such senescence remained unaltered (i.e. nonsequential). Defoliation hastened the seed-filling process and pod wall senescence. Plants containing fewer pods had higher average seed weight, although yield per plant was reduced.
These results suggest that the pod wall serves as a temporary as well as intermediary storage organ and that foliar senescence is not directly related to seed maturation. The possible cause of uncoupling between foliar senescence and seed development is discussed.     

Transmission of the Monocarpic Senescence Signal via the Xylem in Soybean

During monocarpic senescence in soybean (Glycine max [L.] Merrill cv. Anoka) there is a remobilization of nitrogen from the leaves to the seeds, and it has been hypothesized that this loss of nitrogen from the leaves induces foliar yellowing. The phloem in a small segment of the petiole between the pods and the target leaf can be inactivated with a jet of steam. When a plant is depodded except for a single pod cluster in the center of the plant, the pod cluster induces yellowing of the nearest leaf even if the petiole contains a zone of dead phloem, whereas most of the rest of the plant remains green. The nitrogen content of these leaves with a dead phloem zone in their petioles does not decrease greatly, even though the leaves turn yellow. A similar treatment of a single leaf on a fully depodded plant (leaves stay green) does not cause that leaf to turn yellow. Since nutrients would have to be withdrawn from the leaves via the phloem, the pods do not induce yellowing by pulling nutrients out of the leaf and must be able to exert their influence via the xylem.     

Retardation of soybean leaf senescence and associated effects on seed composition

Soybean leaf senescence, leaf abscission, and pod yellowing were markedly delayed by sprays of 10^–4 M 6-benzylamino-9-(tetrahydropyran-2-yl)purine plus 5×10^–5 M -naphthalene acetic acid. The pods on the sprayed plants turned yellow 5–7 days later than those on the control plants, and the treated leaves remained dark green even when the pods had already desiccated. The antisenescence spray did not change pod numbers, seed numbers, seed size, or the yield. By retarding senescence, seed nitrogen content was increased in both TCA-soluble and TCA-insoluble fractions. Seed total protein, buffer-extractable total protein, and globulin were increased by 26, 28, and 33 mg/g of seed flour, respectively, and albumin was decreased by 6 mg/g. The overall increase in seed protein caused by spray treatment is confined to the globulin portion.     

EFFECT OF PETIOLE PHLOEM DISRUPTION ON STARCH AND MINERAL DISTRIBUTION IN SENESCING SOYBEAN LEAVES

Normally, starch (sugars) and minerals are redistributed from the leaves to the pods during monocarpic senescence in maturing soybean plants. Petiole phloem destruction (steam girdling), which blocked this redistribution by interrupting export through the petiole, altered the foliar senescence pattern producing a distinctive interveinal yellowing with green areas along the veins on pod-bearing plants. This suggests that blockage of the petiole phloem may cause nutrients to accumulate in the green zones along the leaf veins instead of being redistributed to the pods. In the leaves of untreated plants, starch showed the same distribution pattern as chlorophyll; however, starch was preserved in yellow areas as well as the green zones of the steam-girdled leaves. Mineral analyses of the veinal and interveinal zones of treated leaves and controls showed that the veinal green zones and interveinal yellowing in treated plants were not respectively enriched and depleted in minerals corresponding to a redistribution of minerals within the leaves. Depodding also blocked leaf yellowing, net mineral redistribution and starch breakdown. Thus, the pods are able to induce chlorophyll breakdown without net mineral redistribution or starch loss in leaves with petiole phloem destruction. This shows that chlorophyll breakdown is not obligatorily coupled with mineral redistribution or starch breakdown.     

Correlation of xylem sap cytokinin levels with monocarpic senescence in soybean

Cytokinins (CKs) coming from the roots via the xylem are known to delay leaf senescence, and their decline may be important in the senescence of soybean (Glycine max) plants during pod development (monocarpic senescence). Therefore, using radioimmunoassay of highly purified CKs, we quantified the zeatin (Z), zeatin riboside (ZR), the dihydro derivatives (DZ, DZR), the O-glucosides, and DZ nucleotide in xylem sap collected from root stocks under pressure at various stages of pod development. Z, ZR, DZ, and DZR dropped sharply during early pod development to levels below those expected to retard senescence. Pod removal at full extension, which delayed leaf senescence, caused an increase in xylem sap CKs (particularly ZR and DZR), while depodding at late podfill, which did not delay senescence, likewise did not increase the CK levels greatly. The levels of the O-glucosides and the DZ nucleotide were relatively low, and they showed less change with senescence or depodding. The differences in the responses of individual CKs to senescence and depodding suggest differences in their metabolism. Judging from their activity, concentrations and response to depodding, DZR and ZR may be the most important senescence retardants in soybean xylem sap. These data also suggest that the pods can depress CK production by the roots at an early stage and this decrease in CK production is required for monocarpic senescence in soybean.     

Pathway and regulation of phosphate translocation to the pods of soybean explants

We investigated the degree to which developing fruit compete directly with leaves for mineral nutrients, e.g. phosphate coming up from the roots. When soybean ( Glycine max (L.) Merrill cv. Anoka) explants cut at mid-late podfill were given a 15-min pulse of ³²Pi via the cut stem and then transferred to distilled water, 75% of the ³²P accumulated in the leaves and 21% in stem and petiole during the first hour. The amount of ³²P entering the seeds was low (1%) initially, but thereafter increased to 30% in 48 h. An accumulation of ³²P in the seed coats preceded its entry into the embryos. Disruption (with hot steam) of the phloem between the leaf and the pods after pulse labelling indicated that more than 80% of the ³²Pi pulse moved to the leaf before redistribution to the pods. Increasing "sink" size by adjusting the pod load from 1 to 2–3 did not increase the ³²P accumulated by the pods proportionally. Conversely, excision of the seeds after pulse labelling did not prevent translocation of ³²P out of the leaves. These results suggest that the rate of transport of phosphate to the pods at mid-late podfill is controlled primarily by factors in the leaves. The results are consistent with the observation that the relative size of the sink (pod load) does not regulate leaf senescence.     

Characterization and kinetics of soybean maturation and monocarpic senescence

Daring monocarpic senescence in potted soybeans ( Glycine maxi (L.) Merrill cv. Anoka) grown in controlled-environment chambers, foliar chlorophyll, soluble protein nitrogen, total nitrogen, and starch decline (roughly in that order). All of these precede visible yellowing and, of course, abscission. The pattern of yellowing within a leaf is not uniform and is closely paralleled by starch loss. Unexpectedly, acid-soluble nitrogen rises slightly before the total foliar nitrogen declines. Foliar fresh weight and total dry matter/cm2 of leaf surface decline little if at all before shedding. Preceding and even during the foliar yellowing, the seeds rapidly accumulate dry matter and nitrogen. Yellowing appears first in the radicle tip, then in the rest of the axis and the leaves and finally in the carpels. Ability to germinate is acquired at about the time the radicle + hypocotyl turns yellow. The relationship between these changes and their role in senescence is discussed.     

Cytokinin biochemistry in relation to leaf senescence. VII. Endogenous cytokinin levels and exogenous applications of cytokinins in relation to sequential leaf senescence of tobacco

The cytokinin complex in tobacco leaves of various maturities was characterized by radioimmunoassay and mass spectrometry. Zeatin was the major base, whereas zeatin riboside was identified as the main riboside. in leaves of all maturities studied. Relative to upper younger leaves, the basal yellow leaves had reduced levels of both cytokinin bases and ribosides. Exogenous applications of dihydrozeatin and zeatin to detached tobacco leaves in amounts sufficient to delay senescence, elevated cytokinin base and riboside levels 2–5 fold. Presenescent and senescent leaves of intact plants showed quantitatively similar changes in cytokinin content. which therefore appear to be of significance in control of senescence. When supplied exogenously, the principal cytokinin bases found to occur in tobacco leaves (zeatin and dihydrozeatin) were markedly more effective than auxins and gibberellic acid in retarding senescence. Localised application of cytokinins to leaf blades of detopped plants was much less effective than application to intact plants. The cytokinin induced senescence retardation in tobacco leaves was independent of effects on directed metabolite transport. Evidence that endogenous levels of active cytokinins in intact tobacco leaves are involved in control of sequential leaf senescence is discussed.     

Regulation of Mineral Redistribution in Pod-Bearing Soybean Explants

On the way from the roots to the seeds during reproductive developmentin soybean (Glycine max), a large proportion of the mineralspass through the leaves rather than travelling directly viathe xylem. This direct and indirect movement of mineral nutrientshas important implications for mineral redistribution, seeddevelopment and leaf senescence. Therefore, we have studiedthe role of cytokinin and mineral flux from the roots in regulatingmineral redistribution from the leaves to the seeds using explants,i.e. a leaf, a pod and a subtending stem segment, with theirbases immersed in treatment solutions. Thus, defined solutionscontaining cytokinin and/or minerals can be substituted forthe roots. When explants (excised at early-mid podfill) aresupplied H₂O only, leaf N, P, K, Mo, Mg, Zn, Fe, B, Cu, Ca,and Mn decline, ranging from 93% for Mo to 38% for Fe. In explantson H₂O, N, P, K, Mo, Mg, Zn, and Fe appear to be redistributedfrom the leaves to the seeds, while the B, Cu, Ca, and Mn lostfrom the leaves do not seem to move to the seeds. Although amixture of minerals resembling xylem sap can delay net lossof these elements from the leaves, it does not prevent the decreases.The cytokinin zeatin (4.6 µM) inhibits the loss of N,IC, Mo, Mg, Zn, Fe, B, Cu, Ca, and Mn from the leaves, but notthat of P. When combined with minerals, zeatin not only preventsthe loss of the minerals from the leaves but may even greatlyincrease them with the possible exception of Zn, Fe, and Cu.Supplying the mineral nutrient mixture increases the quantitiesof N, P, K, Mg, Cu, and B in the seeds but not Zn, Fe, Mn, Ca,and Mo. For those minerals, especially N, where zeatin inhibitsefflux from the leaves, it may reduce the amounts in the seeds,but it does not change P, K, Mg, and Ca. The accumulation andredistribution patterns of the different mineral nutrients showmany dissimilarities thereby suggesting differences in the controlof their distribution. Key words: Cytokinin, mineral transport, seed development, senescence     

EFFECTS OF SINGLE AND MULTIPLE EXPOSURES OF FERULIC ACID ON THE VEGETATIVE AND REPRODUCTIVE GROWTH OF PHASEOLUS VULGARIS BBL-290

Phaseolus vulgaris BBL-290 plants were grown in growth chambers in the Southeastern Plant Environment Laboratory and exposed to either single (at seedling, flower, or podfill) or multiple (biweekly or weekly) treatments of ferulic acid (FA). In the first experiment, plants were harvested one week after FA treatment (0, 1.0, 2.0 mM) and at final harvest (56 days old). FA delayed leaf expansion during the seedling and flowering stages. The total plant leaf area and the plant dry weight of plants treated with 1.0 and 2.0 mM FA as seedlings were reduced one week after treatment by 38–48%. The total plant leaf area and the plant dry weight of plants treated at flowering with 2.0 mM FA were reduced by 25% one week after treatment. Treatment with 2.0 mM FA at podfill caused the senescence and abscission of older leaves and reduced total plant leaf area, plant dry weight and mean pod dry weight by 54, 40, and 48%, respectively, one week after treatment. The plants treated at the seedling and flowering stages recovered by final harvest. In a subsequent experiment, FA (0, 0.50, 1.0, 1.5 mM) reduced total plant leaf area at the seedling and flowering stages but not at podfill. The youngest expanding leaves were most sensitive to FA at flowering. The leaf area of these leaves was reduced by 35 and 25%, one and two weeks after treatment, respectively. Their absolute growth rates were reduced from 31 to 56% one week after treatment at flowering. Their relative growth rates were reduced by 50% one week after treatment. Growth rates then recovered within two weeks after treatment. In the final experiment, biweekly exposures of FA (0.25, 0.50, 0.75, 1.0) reduced total plant leaf area but did not affect any other growth parameters. Weekly exposures of FA (0.25, 0.50, 0.75, 1.0) reduced total plant leaf area up to 34%, absolute growth rate up to 58%, leaf number up to 31% and pod number up to 58%. As the frequency of exposure to FA increased, the concentration necessary to affect bean plant growth and development decreased.     

Ethylene perception inhibitor 1-MCP decreases oxidative damage of leaves through enhanced antioxidant defense mechanisms in soybean plants grown under high temperature stress

Ethylene is a stress hormone involved in early senescence and abscission of vegetative and reproductive organs under stress conditions. Ethylene perception inhibitors can minimize the impact of ethylene-mediated stress. The effects of high temperature (HT) stress during flowering on ethylene production rate in leaf, flower and pod and the effects of ethylene inhibitor on ethylene production rate, oxidative damage and physiology of soybean are not understood. We hypothesize that HT stress induces ethylene production, which causes premature leaf senescence and flower and pod abscission, and that application of the ethylene perception inhibitor 1-Methyl cyclopropene (1-MCP) can minimize HT stress induced ethylene response in soybean. The objectives of this study were to (1) determine whether ethylene is produced in HT stress; (2) quantify the effects of HT stress and 1-MCP application on oxidative injury; and (3) evaluate the efficacy of 1-MCP at minimizing HT-stress-induced leaf senescence and flower abscission. Soybean plants were exposed to HT (38/28 °C) or optimum temperature (OT; 28/18 °C) for 14 d at flowering stage (R2). Plants at each temperature were treated with 1-MCP (1 μg L⁻¹) gas for 5 h or left untreated (control). High temperature stress increased rate of ethylene production in leaves, flowers and pods, production of reactive oxygen species (ROS), membrane damage, and total soluble carbohydrate content in leaves and decreased photosynthetic rate, sucrose content, F_v/F_m ratio and antioxidant enzyme activities compared with OT. Foliar spray of 1-MCP decreased rate of ethylene production and ROS and leaf senescence traits but enhanced antioxidant enzyme activities (e.g. superoxide dismutase and catalase). In conclusion, HT stress increased ethylene production rates, caused oxidative damage, decreased antioxidant enzyme activity, caused premature leaf senescence, increased flower abscission and decreased pod set percentage. Application of 1-MCP lowered ethylene and ROS production, enhanced antioxidant enzyme activity, increased membrane stability, delayed leaf senescence, decreased flower abscission and increased pod set percentage. The beneficial effects of 1-MCP were greater under HT stress compared to OT in terms of decreased ethylene production, decreased ROS production, increased antioxidant protection, decreased flower abscission and increased pod set percentage.     

Association of red coloration with senescence of sugar maple leaves in autumn

We evaluated the association of red coloration with senescence in sugar maple (Acer saccharum Marsh.) leaves by assessing differences in leaf retention strength and the progression of the abscission layer through the vascular bundle of green, yellow, and red leaves of 14 mature open-grown trees in October 2002. Computer image analysis confirmed visual categorization of leaves as predominantly green, yellow or red, and chemical quantification of leaf pigment concentrations verified that leaf color reflected underlying differences in leaf biochemistry. Significantly lower chlorophyll concentrations within red and yellow leaves indicated that senescence was more advanced in leaves from these color categories relative to green leaves. Among leaf types, only red leaves contained high concentrations of anthocyanins. There were significant differences in leaf retention capacity among color categories, with the petioles of green leaves being the most firmly attached to twigs, followed by red and then yellow leaves. Microscopic analysis indicated that yellow leaves had the most advanced extension of the abscission layer through the vasculature, with green and red leaves having significantly less abscission layer progression than yellow. A more limited progression of the abscission layer through vascular bundles may be evidence of delayed leaf senescence that could extend resorption of mobile leaf constituents. Together, results from this study suggest an association between leaf anthocyanin content and functional delays in senescence.     

Stomatal behaviour and leaf water potential of chilled and water-stressed Solanum melongena, as influenced by growth history

Abstract Leaf diffusion resistance and leaf water potential of intact Solanum melongena plants were measured during a period of chilling at 6 °C. Two pretreatments, consisting of a period of water stress or a foliar spraying of abscisic acid (ABA), were imposed upon the plants prior to chilling. The control plants did not receive a pretreatment. In addition to intact plant studies, stomatal responses to water loss and exogenous abscisic acid were investigated using excised leaves, and the influence of the pretreatment observed. Chilled, control plants wilted slowly and maintained open stomata despite a decline in leaf water potential to –2.2 MPa after 2 d of chilling. In contrast plants that had been water stressed or had been sprayed with abscisic acid, prior to chilling, did not wilt and maintained a higher leaf water potential and a greater leaf diffusion resistance. In plants that had not received a pretreatment, abscisic acid caused stomatal closure at 35 °C, but at 6°C it did not influence stomatal aperture. The two pretreatments greatly increased stomatal sensitivity to both exogenous ABA and water stress, at both temperatures. Stomatal response to water loss from excised leaves was greatly reduced at 6°C. These results are discussed in relation to low temperature effects on stomata and the influence of preconditioning upon plant water relations.     

Effects of morphactin and other auxin transport inhibitors on soybean senescence and pod development

Because triiodobenzoic acid increases pod number, albeit variably, in soybean (Glycine max), we tested other auxin-transport inhibitors. Morphactins, especially methylchlorflurenol (MCF), were found to be very active (optimal concentration 10 micromolar) when sprayed onto the foliage. Applications at 1 week after the start of flowering were most effective, producing a 40% increase in pod number with little inhibition (12%) of stem elongation. MCF increased the number of pods initiated (reaching 1 cm length) at least partially by prolonging the initiation period, while pod abortion (failure of pods > 1 cm long) remained low. Generally, MCF did not increase seed yield (dry weight/plant); more, but smaller seeds, were formed by the treated plants. The promotive effect of MCF on pod initiation seems to be independent of its inhibition of stem elongation, which is insignificant at 10 micromolar. MCF delayed pod maturation by 3 to 4 days, while foliar yellowing, blade abscission, and petiole abscission were retarded by 2, 4, and 2 days, respectively. MCF has only a small effect on senescence and that could be indirect, due to a delay in pod development. Other auxin-transport inhibitors tested, including N-1-naphthylphthalamic acid, produced little or no increase in pod number; however, 0.1 millimolar 5-[2′-carboxyphenyl]-3-phenylpyrazole caused a 27% increase. These results implicate auxin as a potential regulator of pod development, and they show that soybean seed yield is not simply sink limited.     

Effects of pod removal on metabolism and senescence of nodulating and nonnodulating soybean isolines: I. Metabolic constituents

Field studies were conducted in 1981 and 1982 to ascertain the effects of pod removal on senescence of nodulating and nonnodulating isolines of soybean (Glycine max [L.] Merr. cv Harosoy) plants. Specifically, the test hypothesis was that nodules act as a nitrogen source and a carbohydrate sink which would in turn prevent or delay senescence in the absence of pods. Senescence was judged by changes in metabolite levels, in dry matter accumulation, and by visual observation.

For both nodulated and nonnodulated plants, pod removal had no effect on the magnitude or rate of dry matter and reduced-N accumulation by whole plants. Phosphorus accumulation was significantly less in both nodulated- and nonnodulated-depodded plants, compared with respective control plants with pods. These data suggested a role for pods in phosphorus uptake. Accumulation of dry matter, reduced N, and phosphorus ceased at approximately the same time for all treatments.

Pod removal did affect partitioning of plant constitments, with leaves and stems of depodded plants serving as a major alternate sink for accumulation of dry matter, reduced N, phosphorus, and nonstructural carbohydrates (primarily starch). While depodded plants eventually lost a significant amount of leaves, leaf drop was delayed relative to plants with pods; and depodded plants still retained some green leaves at 2 weeks past grain maturity of control (podded) plants.

The results indicated that senescence patterns of soybean plants were the same for nodulated and nonnodulated plants, and that pods did not control the initiation of senescence, but rather altered the partitioning of plant constituents and the visual manifestations of senescence.

     

Cytokinin biochemistry in relation to leaf senescence. VI. Effect of nitrogenous nutrients on cytokinin levels and senescence of tobacco leaves

The effect of nitrogenous nutrients on endogeneous cytokinins and senescence of tobacco leaves was investigated. Ammonium nitrate was the most effective in retarding senescence and its activity was attributed principally to NH₄⁺ ions. Repeated applications or a continuous supply of ammonium nitrate was required for maximal retardation of tobacco leaf senescence. Ammonium nitrate solution supplied via the petioles reduced the senescence retarding effect of dihydrozeatin applied directly to the laminae of detached tobacco leaves. Ammonium nitrate also elevated the endogenous levels of cytokinins (especially zeatin and dihydrozeatin) particularly in growing tobacco leaves excised from near the apex of the plant. Ammonium nitrate induced retardation of leaf senescence may be mediated at least partly by its effect on foliar cytokinin content.     

花生衰老进程的研究

通过对鲁花11号和辐8707 2个高产花生品种的衰老进程研究表明：花生衰老具有地上部（叶片）渐进衰老和整株衰老的双重特点。花生从始花至花后60d左右为地上部（叶片）渐进衰老期：此期主茎高、侧枝长、分枝数、主茎、侧枝绿叶数、叶面积、茎、叶干重迅速增加,并接近或达到最大值,主茎及侧枝基部叶片逐渐由下向上开始衰老死亡,饱果开始出现,根系活力、固氮酶活性逐渐升高至接近最大值,始花后60－90d为整株缓衰期,此期地上部茎叶生长基因停止,逐渐开始衰老死亡,主茎、侧枝绿叶数开始减少,生殖体（荚果）干重迅速增长,根系活力、固氮酶活性缓慢降低;始花后90d以后称为整株速衰期,此期主茎、侧枝绿叶迅速减少,地上部迅速衰老死亡,生殖体（荚果）干重缓慢增长,根系活力、固氮酶活性迅速降低。地上部（叶片）渐进衰老期与开花及大量荚果形成相对应,整株缓衰期伴随着荚果迅速增重,整株速衰期与荚果缓慢增重一致。