Reproductive Yield

The question of how senescence affects reproductive yield (or even vegetative yield for certain crops) is an important underlying theme in much of whole plant physiology. Senescence is also an important issue in reproductive development. Here again, it is important to recognize physiological differences among monocarpic species but nonetheless to look for general principles.

In monocarpic species, there is a tradeoff or shift in allocation of resources from vegetative to reproductive growth, and this shift can be fairly abrupt. This allocation shift appears to be part of a strategy that optimizes the reproductive output (Begon et al., 1996; Barbour et al., 1999; Reekie, 1999). This shift in partitioning of resources is reflected in the cessation of shoot growth described above in Section IIIA, and the loss of assimilatory power (senescence) usually occurs later, apparently as a separate process. Field crops appear to have been selected not only to sharpen this allocation shift but also to synchronize seed maturation and facilitate harvest by dying (Hancock, 1992).

A key question is what limits seed yield, and there are two general perspectives on this; (1) sink limitation (e.g., seeds) and (2) source limitation (e.g., photosynthesis in leaves). Neither seed growth nor assimilation are simple one-component processes, and pinning down precisely which limits seed yields is difficult. There certainly is evidence to support both ideas. Indeed, some field crops, e.g., wheat (Kruk et al., 1997) may have selected for sink strength, so they are now source limited. Nonetheless, there has been a strong belief that sink limitation is the key, particularly in soybean. However, using morphactin (an auxin transport inhibitor) treatments, it is possible to increase the number of seeds set in soybean by about 40%, yet there is no increase in yield (Nooden and Nooden, 1985). The total dry weight of seeds produced is the same; there are more seeds, but they are smaller. Another viewpoint (Bloom et al, 1985) sees the multiple processes that go into reproductive yield as fairly well balanced, a sort of "just-in-time" system for a developing plant. This is also reflected in the complex, integrated correlative controls that regulate reproductive development in soybean (Nooden, 1984). From an economic or resource-use perspective, this balance is very efficient, but from a practical perspective, it means that increasing crop yields beyond exploitation of factors like disease resistance and lodging may be very challenging, requiring a better understanding of the controls at a whole plant level.

Nonetheless, delaying leaf senescence during monocarpic senescence does sometimes result in an increase in seed yield in rice (Dingkuhn and Kropff, 1996), corn (Prioul, 1996) and soybean (Nooden, 1985; Guiamet etal., 1990; Egli and Crafts-Brandner, 1996); however, these increases may not materialize under all circumstances, particularly in field conditions (Luquez and Guiamet, 2001).

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