Effect of Pod Removal on Leaf Senescence in Soybeans

Abstract

Senescence of soybean leaves is characterized by a decline in photosynthesis and the loss of leaf protein and Chl (11,22). Clearly, the most dramatic visual symptom is leaf yellowing, and because of this, it is widely used as an index of plant and leaf senescence. From our field studies with winter wheat (21) and soybeans (22), loss of Chl appears to be a good initial index of leaf senescence. Yet caution must be taken in using this as the only indicator of senescence since Chl loss is not always an inevitable event in senescence (18,19).Over 50 years ago, Molisch (10) recorded the observation that plants delayed in the reproductive stages showed delayed senescence. Leopold et al (7), following up on this report, were able to demonstrate a marked delay in soybean leaf and plant senescence following the continuous removal of either the flowers or pods. In recent years, this response in soybeans has been investigated further in an attempt to understand the 'senescence signal.' Lindoo and Nooden (9) were able to duplicate the pod removal effect by only removing the seeds from the pods, indicating that the senescence signal was associated with the developing seeds. The same laboratory (13) later provided evidence which separated seed dry matter accumulation from the senescence response, thereby contradicting the theory that seeds caused senescence by diverting or withdrawing needed nutrients from the leaves.Although these studies on pod and seed removal imply a maintenance of normal leaf function, this has never been conclusively demonstrated. In fact, Mondal et al. (1 1) found that depodding soybeans partially inhibited and caused photosynthesis to decline earlier than in control, podded plants.

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

“…Analysis of photosynthetic processes revealed that carbon fixation rates declined progressively from the time of full leaf expansion (Figure 4). 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).…”

“…This regulatory feature may be the critical one that distinguishes Arabidopsis from other monocarpic plants such as soybean, in which the senescence syndrome is correlatively controlled by reproductive development. 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).…”

“…The physiological basis for these processes has been extensively studied in a few annual crop species, such as soybean, cotton, and maize (Eaton, 1955;Wittenbach, 1982;CraftsBrandner et al, 1984a;Crafts-Brandner and Egli, 1987;Ford and Shibles, 1988). In general, monocarpic plant senescence involves a degeneration of existing vegetative tissues and a 1 To whom correspondence should be addressed.…”

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.…”