Effect of Pod Removal on Leaf Photosynthesis and Soluble Protein Composition of Field-Grown Soybeans

Abstract

Well nodulated, field-grown soybeans (Glycine max [L.] Meff. 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.Senescence of soybean leaves is normally characterized by a decline in photosynthesis and the loss of leaf protein and Chl (12), leading to death of the leaf. However, recently it was shown that following pod removal, the leaves lose the ability for photosynthesis but retain high levels of Chl and protein (7, 10), indicating a separation of functional senescence from death of the leaf. Therefore, removing the pods from soybeans apparently does not delay functional senescence of the leaves as has been claimed (5). Instead, depodding causes a change in the soluble protein pattern of the leaf suggesting a change in leaf function (10). The leaf appears to change from a photosynthesizing source organ to a sink organ.In the previous growth room study (10), Wye soybeans, a determinate variety, were grown with applied N which resulted in poor nodulation. Under these conditions, the plants had essentially only one sink, the pods, following flowering. Removing this sink caused a rapid decline of photosynthesis and a 'Contribution No. 3184

Regulation of Photosynthesis during Leaf Development in RbcS Antisense DNA Mutants of Tobacco

“…Consistent with this possibility is the well-established finding that source leaves adjust their photosynthetic rates in response to the demand for photoassimilates in the rest of the plant, inhibiting photosynthesis when the demand is low and increasing photosynthesis when the demand is high (sink regulation of photosynthesis) (reviewed by Turgeon, 1989;von Schaewen et al, 1990;Stitt et al, 1990;Stitt, 1991;Sonnewald and Willmitzer, 1992). It has also been observed that the rate of leaf senescence is affected by the sinksource status of the plant (Christensen et al, 1981;Bin Lazan et al, 1983;Wittenbach, 1983;Shibles et al, 1987;Guitman et al, 1991) and that photosynthetic gene expression can be modulated by carbohydrates at the transcriptional level (Sheen, 1990;Krapp et al, 1993). We are currently investigating the impact of carbohydrate levels on photosynthetic rates and Rubisco gene expression during leaf development in the antisense and WT plants.…”

Developmental and age-related processes that influence the longevity and senescence of photosynthetic tissues in arabidopsis.

“…This age-related, progressive loss of photosynthetic function from the time of leaf maturity may be a general feature of annual plants because this phenomenon has been observed in cucumber (Callow, 1974), Perilla (Batt and Woolhouse, 1975), barley (Friedrich and Huffaker, 1980), wheat (Peoples et al, 1980), maize (Crafts-Brandner et al, 1984a), and soybean (Wittenbach, 1982;Ford and Shibles, 1988). Wittenbach referred to the photosynthetic decline as "functional senescence" to distinguish this process from the subsequent rapid loss of chlorophyll and macromolecular turnover associated with the senescence syndrome (Wittenbach, 1983). The physiological basis for functional senescence is not known, although a number of explanations have been suggested (Callow, 1974;Woolhouse, 1982;Nooden, 1988b).…”

“…Interestingly, depodding of soybean plants will delay the final stages of senescence in the leaf but does not prevent photosynthetic decline (Wittenbach, 1982;CraftsBrandner et al, 1984b). In fact, the Rubisco enzyme may be degraded in leaves of depodded plants, but the mobilized N is apparently refixed in the paraveinal mesophyll as storage protein (Franceschi et al, 1983;Wittenbach, 1983). Thus, in soybean, depodding appears to result in uncoupling of photosynthetic decline from activation of the senescence syndrome.…”

Wheat Vegetative Nitrogen Compositional Changes in Response to Reduced Reproductive Sink Strength

“…The effects of an altered reproductive sink demand on accumulation of VSP have been limited to studies of soybean (5,18,19,25,26). Because depodding, JA, and MeJA elicit similar VSP accumulation responses in soybean, other monocarpic annuals that express MeJA-induced proteins also may accumulate proteins with VSP characteristics when reproductive sink demand is altered.…”

“…plants, abundant levels of the same specific vegetative proteins (molecular masses of 27, 29, and 94 kD) accumulate predominantly in the vacuoles of PVM cells of leaves (5,7,18,19,21). The 94-kD protein was recently identified as a member of the vegetative lipoxygenase family and proposed to be a bifunctional zymogen capable of catalyzing the hydroperoxidation of lipids and storing excess N temporarily (21).The effects of an altered reproductive sink demand on accumulation of VSP have been limited to studies of soybean (5,18,19,25,26). Because depodding, JA, and MeJA elicit similar VSP accumulation responses in soybean, other monocarpic annuals that express MeJA-induced proteins also may accumulate proteins with VSP characteristics when reproductive sink demand is altered.…”