Leaf senescence

The productivity of the soybean canopy begins to decline shortly after the beginning of seed filling (Larson et al., 1981; Wells et al., 1982; Acock et al., 1985; Boerma and Ashley, 1988) as senescence destroys the photosynthetic machinery and nitrogen is exported from the leaf (Crafts-Brandner and Egli, 1987). The decline in photosynthesis represents an obvious enigma; the plant has progressed through the preliminary phases of the yield production process (phases I and II) and now, when the main event begins, the photosynthetic capacity of the plant is slowly destroyed. This does not seem to be, on the surface, a rational strategy for high yield, but it results in an efficient use of nitrogen and is a strategy that is followed by soybean and all grain crops.

The nitrogen exported from the senescing leaf is translocated to the developing seed (Morris and Weaver, 1983) where it can account for up to 100% of the seed nitrogen at maturity (Egli et al., 1978; McBlain and Hume, 1981; Zeiher et al., 1982; Egli et al., 1983). The remobilization of nitrogen is probably a result of senescence (Hayati et al., 1995, 1996) not a cause and, therefore, models based on a hypothesized seed nitrogen demand (Sinclair and de Wit, 1975, 1976; Frederick and Hesketh, 1994) probably do not provide an accurate depiction of the interaction of seed growth and leaf senescence in soybean.

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