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.

     

Effect of nodulation on assimilate remobilization in soybean

The objectives of this work were to determine the effect of nodulation on dry matter, reduced-N, and phosphorus accumulation and partitioning in above-ground vegetative parts and pods of field-grown soybean (Glycine max [L.] Merr. cv Harosoy).

From comparison of nodulated and nonnodulated isolines, it was estimated that nodulated plants attained 81 and 71% of total-plant (above ground) N from uptake of soil N in 1981 and 1982, respectively. These data, along with visibly greener leaves of nodulated plants, led us to assume that nonnodulated plants were under a moderate N stress relative to nodulated plants. Nonnodulated plants accumulated less total-plant N and partitioned less dry matter and N to the pods, compared with nodulated plants. This occurred even though net photosynthesis, as estimated by rate and amount of dry matter accumulation, was the same for both nonnodulated and nodulated plants. Rate of dry matter and reduced-N accumulation in pods was less for nonnodulated than for nodulated plants while duration of podfill was similar for both isolines. From these data we concluded that moderate N stress affected partitioning of photosynthate rather than net photosynthesis, and that N played a role in translocation of photosynthate to the pods. Total plants (above-ground portion) and pods of both nodulated and nonnodulated plants accumulated similar amounts of phosphorus, which indicated that phosphorus and N accumulation were independent.

Remobilization of nitrogen and phosphorus from vegetation to pods preceded dry matter remobilization. It appeared that either more nitrogen accumulation prior to podfill, or continued nitrogen assimilation during podfill would increase nitrogen and dry matter partitioning to pods, but that increasing photosynthesis without concomitantly increasing nitrogen input may not necessarily result in enhanced seed production.

     

Effect of pod removal on leaf photosynthesis and soluble protein composition of field-grown soybeans

Well nodulated, field-grown soybeans (Glycine max [L.] Merr. var Williams) were depodded just prior to seed development and near mid pod-fill. Both treatments caused a considerable increase in leaf dry weight, suggesting continued photosynthate production following pod removal. Moreover, depodding had a marked effect on leaf soluble protein without affecting total proteolytic activity. Early depodding caused a 50% increase in leaf protein, and both early and late depodding caused the retention of protein for several weeks following the decline in control leaves. But despite this retention of protein, leaves of depodded plants showed no difference in the onset of the irreversible decline in photosynthesis. Therefore, although depodding delayed the loss of leaf chlorophyll and protein, it did not delay the onset of functional leaf senescence and in fact, actually appeared to enhance the rate of decline in photosynthesis. There was a good correlation between the irreversible decline in ribulose bisphosphate carboxylase (activity and amount) and that of photosynthesis. In contrast, the correlation did not seem as good between stomatal closure and the onset of the irreversible decline in photosynthesis. The reason total soluble protein remained high following depodding while carboxylase, which normally comprised 40% of the soluble protein, declined was because several polypeptides increased in amounts sufficient to offset the loss of carboxylase. This change in leaf protein composition indicates a change in leaf function; this is discussed in terms of other recent findings.     

DEVELOPMENTAL CHANGES IN ENERGY (ATP AND ADP) AND NUTRIENT LEVELS PRIOR TO SENESCENCE IN PODDED AND DEPODDED PEA PLANTS

Energy (ATP and ADP) levels in stem apices of depodded pea plants (Pisum sativum L. cv. Little Marvel) were significantly higher than those of podded plants during the pod-filling stage before whole plant senescence. This difference in energy content appeared before decreases in leaf chlorophyll and soluble proteins occurred in plants of both treatments. In contrast, the mineral nutrient levels (N, P, K, Ca, Mg, Fe, and Mn) in stem apices of plants from both treatments were similar. Energy levels in reproductive leaves from podded and depodded plants were similar. The mineral nutrients in leaves with fruits in their axils and similar leaves of depodded plants were comparable except for nitrogen. Plant growth measurements—dry weight, leaf area, leaf dry weight, root/shoot ratio—were significantly higher in depodded than podded plants. Whole plant senescence occurred significantly earlier in podded than in depodded pea plants.     

Leaf Nitrate Reductase, d-Ribulose-1,5-bisphosphate Carboxylase, and Root Nodule Development of Genetic Male-Sterile and Fertile Soybean Isolines

The objectives of this study were to determine the effect of pod and seed development on leaf chlorophyll concentration, and on activities of leaf ribulose bisphosphate carboxylase, leaf nitrate reductase, and root nodule acetylene reduction in field-grown soybean (Glycine max [L.] Merr.). Two genetic male-sterile lines and their fertile counterparts (Williams and Clark 63) were compared in both 1978 and 1979. Two additional lines (Wells × Beeson and Wells × Corsoy) were compared in 1979.

The expression of male-sterile character was nearly complete as very little outcrossing due to insect pollinators was observed. Male-sterile plants showed a delayed late season decline in leaf chlorophyll content and ribulose bisphosphate carboxylase activity when compared with fertile plants. A slight delay in the loss of in vivo leaf nitrate reductase activity was also observed for male-sterile plants. Root nodule fresh weight and acetylene reduction activity declined slightly more rapidly for fertile lines than for male-sterile lines in both years with differences significant on the last two to three sampling dates as leaf loss occurred in the control plants.

Seed development was found to increase slightly, the rate of decline of metabolic activity in fertile lines compared with that of male-sterile lines. However, pod development was not an a priori requirement for leaf and root nodule senescence. Male-sterile plants also lost photosynthetic and nitrogen metabolic competence, but at a slower rate. These results support the concept that pod and seed development does not signal monocarpic senescence per se but rather affects the rate at which senescence occurs after flowering.

     

Photosynthesis, leaf resistances, and ribulose-1,5-bisphosphate carboxylase degradation in senescing barley leaves

The relationship between loss of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and the decline in photosynthesis during the senescence of barley primary leaves was assessed. Loss of RuBPCase accounted for about 85% of the decrease in soluble protein. RuBPCase was highly correlated with in vitro RuBPCase activity (r = 0.95) and gross photosynthesis (r = 0.96). However, the rate of photosynthesis per milligram RuBPCase increased during the early stages of leaf senescence. The concentration of nonreducing sugars was negatively correlated (1% level) with photosynthesis. Free α-amino N, in contrast to nonreducing sugars, declined markedly during senescence. A decrease in chlorophyll and an increase in in vitro protease activity was observed, but these changes did not appear to be closely related to the decline in photosynthesis and RuBPCase. Mesophyll resistance increased at the same rate that photosynthesis and RuBPCase declined. Stomatal resistance increased more rapidly than mesophyll resistance and accounted for about 24% of the total increase in resistance to CO₂ diffusion. The concentration of CO₂ in the intercellular air spaces decreased during the last stage of senescence. Although loss of RuBPCase probably is the primary event responsible for the decline in photosynthesis during leaf senescence, other factors such as in vivo regulation and stomatal aperture must also be considered.     

Effect of pod removal on leaf senescence in soybeans

Depodding soybean (Glycine max [L] Merr. cv Wye) plants results in an apparent inhibition of senescence as indicated by leaf chlorophyll and soluble protein retention. However, leaf photosynthesis and ribulose bisphosphate carboxylase (Rubisco) levels begin to decline earlier in depodded than in control, podded plants. The initial decline in photosynthesis is correlated with a decrease in leaf transpiration, while the latter decline is associated with the loss of Rubisco. Total soluble protein remains high in depodded plants because several polypeptides, three in particular, increase in amounts sufficient to offset the loss of Rubisco. Thus, depodding appears to change the function of the leaf rather than simply delaying or preventing the decline in leaf function. Changes in specific leaf weight and starch content following depodding suggest that the leaf may be changing to a storage organ.     

Ribulose Bisphosphate Carboxylase and Proteolytic Activity in Wheat Leaves from Anthesis through Senescence

Changes in ribulose bisphosphate carboxylase (RuBPCase) and proteolytic activity were followed in the flag leaf and second leaf of field-grown winter wheat (cv. Arthur). These changes were followed in relation to changes in leaf chlorophyll, protein, and photosynthesis, and seed development. Levels of RuBPCase were determined by rocket immunoelectrophoresis as described previously (Wittenbach 1978 Plant Physiol 62: 604-608). RuBPCase constituted 40 to 45% of the total soluble protein in the flag leaf and an even higher percentage of the soluble protein in the second leaf. This ratio remained unchanged until senescence when RuBPCase protein was apparently lost at a faster rate than total soluble protein. No change in the specific activity of RuBPCase on either a milligram protein or RuBPCase basis was observed until senescence. A close correlation existed among the various indices of senescence in the field, namely, the decline in chlorophyll, protein, photosynthesis, and RuBPCase activity. In addition, proteinase activity increased with the onset of senescence. These enzymes readily degraded RuBPCase, exhibiting a pH optimum of 4.8 to 5.0 and a temperature optimum of 50 C. Proteinase activity was modified by sulfydryl reagents suggesting the presence of sulfydryl groups at or near the active sites.     

Breakdown of Ribulose Bisphosphate Carboxylase and Change in Proteolytic Activity during Dark-induced Senescence of Wheat Seedlings

When 8-day-old wheat seedlings (Triticum aestivum L. var. Chris) are placed in the dark the fully expanded primary leaves undergo the normal changes associated with senescence, for example, loss of chlorophyll, soluble protein, and photosynthetic capacity (Wittenbach 1977 Plant Physiol. 59: 1039-1042). Senescence in this leaf is completely reversible when plants are transferred to the light during the first 2 days, but thereafter it becomes an irreversible process. During the reversible stage of senescence the loss of ribulose bisphosphate carboxylase (RuBPCase) quantitated immunochemically, accounted for 80% of the total loss of soluble protein. There was no significant change in RuBPCase activity per milligram of antibody-recognized carboxylase during this stage despite an apparent decline in specific activity on a milligram of soluble protein basis. With the onset of the irreversible stage of senescence there was a rapid decline in activity per milligram of carboxylase, suggesting a loss of active sites. There was no increase in total proteolytic activity during the reversible stage of senescence despite the loss of carboxylase, indicating that this initial loss was not due to an increase in total activity. An 80% increase in proteolytic activity was correlated with the onset of the irreversible stage and the rapid decline in RuBPCase activity per milligram of carboxylase. Delaying senescence with zeatin reduced the rate of loss of carboxylase and delayed both the onset of the irreversible stage and the increase in proteolytic activity to the same degree, suggesting that these events are closely related. The main proteinases present in wheat and responsible for the increase in activity are the thiol proteinases. These proteinases have a high affinity for RuBPCase, exhibiting an apparent K_m at 38 C of 1.8 × 10⁻⁷ m. The K_m for casein was 1.1 × 10⁻⁶ m. If casein is representative of noncarboxylase protein, then the higher affinity for carboxylase may provide an explanation for its apparent preferential loss during the reversible stage of senescence.     

Drought Stress and Elevated CO(2) Effects on Soybean Ribulose Bisphosphate Carboxylase Activity and Canopy Photosynthetic Rates

Soybean (Glycine max [L.] cv Bragg) was grown at 330 or 660 microliters CO₂ per liter in outdoor, controlled-environment chambers. When the plants were 50 days old, drought stress was imposed by gradually reducing irrigation each evening so that plants wilted earlier each succeeding day. On the ninth day, as the pots ran out of water CO₂ exchange rate (CER) decreased rapidly to near zero for the remainder of the day. Both CO₂-enrichment and drought stress reduced the total (HCO₃⁻/Mg²⁺-activated) extractable ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity, as expressed on a chlorophyll basis. In addition, drought stress when canopy CER values and leaf water potentials were lowest, reduced the initial (nonactivated) RuBPCase activity by 50% compared to the corresponding unstressed treatments. This suggests that moderate to severe drought stress reduces the in vivo activation state of RuBPCase, as well as lowers the total activity. It is hypothesized that stromal acidification under drought stress causes the lowered initial RuBPCase activities. The K_m(CO₂) values of activated RuBPCase from stressed and unstressed plants were similar; 15.0 and 12.6 micromolar, respectively. RuBP levels were 10 to 30% lower in drought stressed as compared to unstressed treatments. However, RuBP levels increased from near zero at night to around 150 to 200 nanomoles per milligram chlorophyll during the day, even as water potentials and canopy CERs decreased. This suggests that the rapid decline in canopy CER cannot be attributed to drought stress induced limitations in the RuBP regeneration capability. Thus, in soybean leaves, a nonstomatal limitation of leaf photosynthesis under drought stress conditions appears due, in part, to a reduction of the in vivo activity of RuBPCase. Because initial RuBPCase activities were not reduced as much as canopy CER values, this enzymic effect does not explain entirely the response of soybean photosynthesis to drought stress.     

Effect of multiple factor source-sink manipulation on nitrogen and carbon assimilation by soybean

The objectives of this study were to determine the effect of light enhancement and hastened reproductive development on nitrogen and dry matter accumulation by field-grown soybean (Glycine max [L.] Merr.). The impacts of photosynthate supply and reproductive development on change in the season-long profiles of in vivo leaf nitrate reductase (NR) activity and root nodule acetylene reduction (AR) activity were evaluated.

Light enhancement resulted in significant increases in dry matter accumulation, root nodule fresh weight and AR activity. Seed yield was increased in both light enhanced treatments in 1978 and in one in 1979.

Hastened flowering and seed development was accomplished through photoperiod manipulation within a single genotype. Seasonal decline in leaf NR activity was most rapid in plants entering reproductive development early. An early increase in root nodule fresh weight and AR activity was also observed in response to this treatment and was followed similarly by early decline.

The addition of high levels of soil-applied nitrogen increased leaf NR activity and delayed late season decline in NR activity for both control and early reproductive plants. Nitrate supply was therefore implicated as limiting to leaf NR activity during the decline associated with flowering and early seed development. A limited additional increase in leaf NR activity was observed in response to light enhancement plus soil-applied nitrogen. As no significant increase in leaf NR activity was observed in response to light enhancement alone, leaf nitrate supply was further implicated as more limiting to leaf NR activity than was photosynthate supply during flowering and early seed development.

     

Changes in the Number and Composition of Chloroplasts during Senescence of Mesophyll Cells of Attached and Detached Primary Leaves of Wheat (Triticum aestivum L.)

Changes in the number and composition of chloroplasts of mesophyll cells were followed during senescence of the primary leaf of wheat (Triticum aestivum L.). Senescence was due to the natural pattern of leaf ontogeny or was either induced by leaf detachment and incubation in darkness, or incubation of attached leaves in the dark. In each case discrete sections (1 centimeter) of the leaf, representing mesophyll cells of the basal, middle, and tip regions, were examined. For all treatments, senescence was characterized by a loss of chlorophyll and the protein ribulose 1,5-bisphosphate carboxylase (RuBPCase). Chloroplast number per mesophyll cell remained essentially constant during senescence. It was not until more than 80% of the plastid chlorophyll and RuBPCase was degraded that some reduction (22%) in chloroplast number per mesophyll cell was recorded and this was invariably in the mesophyll cells of the leaf tip. We conclude that these data are consistent with the idea that degradation occurs within the chloroplast and that all chloroplasts in a mesophyll cell senesce with a high degree of synchrony rather than each chloroplast senescing sequentially.     

Nitrogen metabolism of soybeans: I. Effect of tungstate on nitrate utilization, nodulation, and growth

The effects of N source (6 mm nitrogen as NO₃⁻ or urea) and tungstate (0, 100, 200, 300, and 400 μm Na₂ WO₄) on nitrate metabolism, nodulation, and growth of soybean (Glycine max [L.] Merr.) plants were evaluated. Nitrate reductase activity and, to a lesser extent, NO₃⁻ content of leaf tissue decreased with the addition of tungstate to the nutrient growth medium. Concomitantly, nodule mass and acetylene reduction activity of NO₃⁻-grown plants increased with addition of tungstate to the nutrient solution. In contrast, nodule mass and acetylene reduction activity of urea-grown plants decreased with increased nutrient tungstate levels. The acetylene reduction activity of nodulated roots of NO₃⁻-grown plants was less than 10% of the activity of nodulated roots of urea-grown plants when no tungstate was added. At 300 and 400 μm tungstate levels, acetylene reduction activity of nodulated roots of NO₃⁻-grown plants exceeded the activity of comparable urea-grown plants.     

Role of nitrogen assimilation in seed development of soybean

A nondestructive acetylene reduction assay for nitrogenase activity of soybean (Glycine max L. Merr) field plots is presented. Plots consisted of 120 × 150 × 30 centimeter boxes containing 65 plants. The plants were grown in a medium grade sand under controlled nutrient, moisture, and root temperature conditions. Acetylene at a concentration of 10 milliliters per liter was circulated through manifolds in the chambers; equilibration required 5 minutes, and activity was linear with time. Optimum growth and assay environments resulted in activity of 70 micromoles ethylene per plant per hour. Plant development and yield were comparable to soil-grown companion plots.

The well accepted hypothesis that developing seeds deprive the nodules of carbohydrate was not substantiated. The nondestructive acetylene reduction profile did not decline until 30 days after the onset of seed development (R-5). This result was consistent with reports from the literature which indicated that 60% of seasonal nitrogen was fixed after R-5. Further, a high correlation shown between integrated seasonal acetylene reduction and yield (r = 0.999) suggested a cooperative relationship between the roots and shoot. A reduction in source:sink ratio (60% defoliation) after R-5 had no effect on acetylene reduction. This showed that neither an increase in sink demand by the pods nor a carbon shortage during podfill decreased dinitrogen fixation. A conceptual model relating seed growth with carbon and nitrogen assimilation is proposed.

     

Light and Shade Effects on Abscission and C-Photoassimilate Partitioning among Reproductive Structures in Soybean

Field experiments were conducted in 1981 and 1982 to study the effects of low-irradiance supplemental light on soybean (Glycine max [L.] Merr. cv Evans) flower and pod abscission. Cool-white and red fluorescent lights illuminated the lower part of the soybean canopy during daylight hours for 3 weeks late in flowering. At the same time, flowers and young pods on half the plants were shaded with aluminum foil. Flowers were tagged at anthesis and monitored through abscission or pod maturity.

Responses to red and white lights were similar. Supplemental light tended to reduce abscission and increase seed weight per node compared to natural light. Shading flowers and pods increased abscission and reduced seed weight per node. Number of flowers produced per node, individual seed weight, and seeds per pod were not affected by light or shade treatments.

Further studies examined the effects of shading reproductive structures on their capacity to accumulate ¹⁴C-photoassimilates. Individual leaves were pulse labeled with ¹⁴CO₂ 1, 2, and 4 weeks post anthesis. Flowers and pods in the axil of the labeled leaf were covered with aluminum foil 0, 24, 72, and 120 hours before pulsing.

Shading flowers and pods resulted in a 30% reduction in the relative amount of radiolabel accumulated from the source leaf. The reduction in ¹⁴C accumulation due to shading was evident regardless of the length of the shading period and was most pronounced when the shades were applied early in reproductive development. We conclude that light perceived by soybean flowers and young pods has a role in regulating both their abscission and their capacity to accumulate photoassimilates.

     

Mobilization of Nitrogen in Fruiting Plants of a Cultivar of Cowpea

Patterns of flow of nitrogen were constructed for the post-anthesisdevelopment of symbiotically-dependent cowpea (Vigna unguiculataWalp. cv. Vita 3-Rhizobium CB756). Nitrogen fixed after floweringcontributed 40% of the fruits' total intake of N, mobilizationof N fixed before flowering the remaining 60%. Leaflets, nodulatedroot, stem plus petioles, and peduncles contributed mobilizedN in the approximate proportions 5: 2: 1: 1 respectively. Eachfruit drew on all available current sources of N, but N fromleaves was distributed preferentially to closest fruit(s), andlower fruits monopolized the N exported from nodulated rootsduring late fruiting. Rates of nitrogen fixation declined parallel with decreasingnet photosynthesis of shoots. Leaflets at upper reproductivenodes mobilized 70–77% of their N and declined steeplyin net photosynthesis rate per unit chlorophyll or per unitribulose-l, 5-bisphosphate carboxylase (RuBPCase)² before abscisingduring mid- to late fruiting, whereas leaflets at lower vegetativenodes (1–3) mostly failed to abscise, lost 44–57%of their N and maintained photosynthetic activity throughoutfruiting. Peptide hydrolase activity was examined in extracts of leaflets,roots and nodules, by autodigestion of extracts, or in assaysusing bovine haemoglobin and purified RuBPCase isolated fromcowpea as substrates. Hydrolase activities during fruiting werebroadly related to N loss from plant organs, but asynchronyin peaks of activity against different protein substrates indicateddistinct groups of hydrolases within single organs. Hydrolaseactivity of leaflet extracts against RuBPCase was highly andpositively correlated with in vivo rates of loss of RuBPCasefrom the same leaflets, and preferential degradation of thisprotein occurred during leaflet senescence. Key words: Nitrogen, Mobilization: Cowpea     

The Effect of Source—Sink Manipulations on Nitrogen Fixation in Peas

The effect on nitrogen fixation of excising leaves or pods in pea (Pisum sativum L. cv. Alaska) was determined over a 60-day period. Flower buds or their subtending leaves were removed, and C₂H₂ reduction, H₂ evolution and N accumulation were measured at weekly intervals. Highest percentage nitrogen content in all treatments coincided with time of maximal C₂H₂-reduction rates. Nitrogen fixation, calculated from C₂H₂ reduction and H₂- evolution data, was significantly lower in the partially defoliated and generally higher in the depodded plants than in the controls. Total N accumulation was greatest in the depodded plants and least in the defoliated ones. Percentage nitrogen content and N₂-fixation rates in the depodded plants were maximized approximately 10 days later than in the defoliated or control plants. The absolute rates of C₂H₂ reduction and H₂ evolution were significantly altered by plant organ removal, but the relative rates were proportional. As a result the ratios of H₂/C₂H₄ production and the related relative efficiency of N₂ fixation in the treatments were not significantly different from the controls.     

Natural and dark-induced nodule senescence in chickpea: nodule functioning and H2O2 scavenging enzymes

An investigation was carried out on chickpea (Cicer arietinum L.) cv. C-235 inoculated withRhizobium sp.Cicer strain cv 4 Az^r. Nodule functioning was monitored at 15 d intervals starting from 45 days after sowing (DAS) and inoculation in order to study nodule development and senescence under natural and stress conditions (dark treatments of 18 and 66 h). Maximum rate of N₂-fixation was observed between 50 - 60 DAS. After this acetylene reducing activity (ARA) fell and it was negligible 75 DAS. This decline in ARA with ageing of plants and nodules was accompanied by a decline in leghemoglobin content and greening of the nodules. When 60 % of the nodule tissue had turned green 75 DAS, a sharp increase in nodule peroxidase activity (3.7 fold) was observed whereas the catalase activity was reduced by 50 % in comparison with the control. The glutathione-reductase and ascorbate-peroxidase activity followed a trend parallel to that in N₂-fixation, but the variation was much smaller. The changes in the total soluble carbohydrates, cytosolic proteins and nitrogen content per se were not expressive. Dark treatments induced premature senescence of the nodules as was evident from the marked decrease in ARA. However, the decline in leghemoglobin content was relatively small as compared to ARA. The changes in cytosolic proteins, total soluble carbohydrates, peroxidase activity, catalase activity, glutathione reductase activity and ascorbate peroxidase activity of nodules under dark-induced nodule senescence were almost parallel to those observed under natural senescence.     

Roles for redox regulation in leaf senescence of pea plants grown on different sources of nitrogen nutrition

Leaf senescence and associated changes in redox components were monitored in commercial pea (Pisum sativum L. cv. Phoenix) plants grown under different nitrogen regimes for 12 weeks until both nodules and leaves had fully senesced. One group of plants was inoculated with Rhizobium leguminosarum and grown with nutrient solution without nitrogen. A second group was not inoculated and these were grown on complete nutrient solution containing nitrogen. Leaf senescence was evident at 11 weeks in both sets of plants as determined by decreases in leaf chlorophyll and protein. However, a marked decrease in photosynthesis was observed in nodulated plants at 9 weeks. Losses in the leaf ascorbate pool preceded leaf senescence, but leaf glutathione decreased only during the senescence phase. Large decreases in dehydroascorbate reductase and catalase activities were observed after 9 weeks, but the activities of other antioxidant enzymes remained high even at 11 weeks. The extent of lipid peroxidation, the number of protein carbonyl groups and the level of H(2)O(2) in the leaves of both nitrate-fed and nodulated plants were highest at the later stages of senescence. At 12 weeks, the leaves of nodulated plants had more protein carbonyl groups and greater lipid peroxidation than the nitrate-fed controls. These results demonstrate that the leaves of nodulated plants undergo an earlier inhibition of photosynthesis and suffer enhanced oxidation during the senescence phase than those from nitrate-fed plants.     

Chloroplast pigments,proteins, lipid peroxidation and activities of antioxidative enzymes during maturation and senescence of leaves and reproductive organs of Cajanus cajan L.

A comparative investigation was undertaken with pigeon pea leaves and attached flower buds/flowers/pods during their developmental stages including senescence in a natural system in experimental plots. Alterations in chloroplast pigments, total soluble proteins, lipid peroxidation, malondialdehyde (MDA) content and activities of guaiacol peroxidase (POD, EC 1.11.1.7) and superoxide dismutase (SOD, EC 1.15.1.1) were studied at 5-day interval from initial to 40-day stage. Chloroplast pigments and proteins of leaves increased upto 15 and 20-day stages respectively followed by a steady decline. Reproductive parts, however, exhibited rise in chloroplast pigments upto 25-day and protein till last stage as developing pods gain the amount from the senescing leaves which are nearest to them. Senescing leaves show very high POD activity than the developing and senescing pods and POD appears to be associated with chlorophyll degradation. Considerably higher activity and amount of LOX and MDA respectively have been noticed in senescing leaves than in flowers and pods. Increase in SOD activity during early stage of leaf growth and maturation indicates protective role that declined at senescent stages. Pods are unique in having very high SOD activity, only last stage of senescence does show a decline.