Agriculture Reference
In-Depth Information
Senescence and Abscission
Whatever may be the rate of gradual decline in photosynthetic capacity, there is a
point in time for all leaves when much more rapid changes in both physiology and
appearance mark their impending death and abscission (Vincent 2006; Lim et al.
2007). Leaf senescence can be triggered by exogenous factors (seasonal changes in
climate, pathogen attack, herbivory) or by endogenous factors (self-shading, fruiting).
Whatever the trigger, senescence is intrinsically a process of genetically regulated
degradation (Nam 1997; Weaver and Amasino 2001; Nooden 2004; Vincent 2006)
involving upregulation of more than 800 genes (Lim et al. 2007). Senescence
allows orderly preparations for seasonal changes in environmental conditions,
including recovery of nutrients from senescing leaves and their recycling within the
plant. Many senescence-associated genes encode proteins that accomplish parts of
the recycling program such as proteases, nucleases, and proteins involved in metal
binding and transport (Guarente et al. 1998). Senescing foliage in broadleaf deciduous
forests often colors as chlorophyll degrades, no longer masking yellow and orange
secondary photosynthetic pigments, and as reddish anthocyanins are produced
de novo (Lee et al. 2003; Ougham et al. 2005). Coloring during senescence in species
with indeterminate shoot growth is weakly developed and usually initiated within
the tree crown or in the lower canopy, whereas in species with determinate shoot
growth coloring is strong and tends to occur first in the upper canopy (Koike 1990,
2004). The anthocyanins confer a degree of protection against photooxidation of
systems involved in the orderly breakdown and recycling of materials from the
senescing leaf (Pietrini et al. 2002). Leaf photosynthesis invariably declines
strongly with the onset of senescence (Makino et al. 1983; Hidema et al. 1991;
Hanba et al. 2004), and foliar nitrogen content decreases steadily as photosynthetic
systems shut down (Mae 2004).
