Leaves Aynchronous and asynchronous senescence; sequential or progressive senescence; deciduous senescence; top senescence; overall senescence. Patterns of leaf senescence relate directly to life form, annual/perennial habit and monocarpic (semelparous) versus polycarpic (iteroparous) reproduction. Foliar senescence within a canopy may be synchronous or, if asynchronous, may be random (often in response to stochastic biotic or abiotic stress) or progressive (Leopold 1961, Simon 1967). The leaves of many so-called evergreens have the capacity to undergo physiologically typical senescence (Nebel and Matile 1992, Pugnaire and Chapin 1993) but because it is not synchronised or seasonal, plants give a general impression of unvaryingly stable greenness. Patterns of leaf senescence A. Progressive or sequential senescence. Usually (but not always) seen in the vegetative phase of whole plant development. The gradient of leaf yellowing with age and position often reflects the distribution of nutrients such as N, indicating that this mode of senescence represents a kind of nitrogen-stat, maintaining optimal supply to young growing organs at the top of the plant. B. Synchronous or deciduous senescence, typical of temperate trees and shrubs. Nutrients are salvaged and stored in tissue of the shoot axis, which remains viable, though quiescent. Axillary and apical buds overwinter in a dormant condition. C. Top senescence. This is typical of biennials or perennials with underground resting organs such as rhizomes, tubers or corms. As well as leaves, buds and shoot axes die back. D. Overall or monocarpic senescence (sometimes called "big bang" senescence). This pattern is associated with reproduction and results in complete death of the parent plant leaving only the seeds to continue the lineage. The efficiency with which monocarpic senescence transfers nutrients from vegetative parts to seeds accounts for the predominance of this mode of senescence among the major crop species. As new young leaves develop in the growing vegetative plant, there is often a corresponding induction of senescence in the oldest foliage so that the shoot always bears the same number of green leaves. For example, tillers of temperate grasses commonly each carry about three mature leaves and as a new leaf emerges, so the green lamina at the lowest insertion point enters senescence (Johnson and Thornley 1983, Duru and Ducrocq 2000). The combination of leaf initiation and expansion at one level and progressive or sequential senescence in another, usually lower, layer results in zonation within the canopy (Thomas 1992). The generally low light fluxes deeper in a canopy do not seem to be particularly important for triggering senescence in lower foliage layers, and in some cases green leaves below the maximal penetration of illumination at the photosynthetic compensation point can survive for prolonged periods (Thomas and Sadras 2000). Light quality is known to influence the progress of sequential leaf senescence (Guiamét et al. 1989), as is the pattern of delivery of translocated materials in relation to leaf position and resistance to xylem flow (Neumann 1987). Canopy zonation during the plant life cycle is typically dynamic, reflecting changes in the rate of leaf turnover, which is related in turn to leaf lifespan. Extreme examples of the persistence of individual leaves are well documented. For example, Pinus longaeva leaves have been reported to survive for 45 years (Ewers and Schmid 1981) and the lifetime of the single large leaf of Welwitschia mirabilis is measured in decades (Molisch 1938). Amongst the distinctive features of long-lived leaves are a capacity to photosynthesise even when growth is limited, and relatively low rates of nutrient recycling (Monk 1966, Chabot and Hicks 1982, Jonasson 1989). This suggests that a defining characteristic of leaves with long lifespans is the relative insensitivity of their stress signalling mechanisms, including those regulating induction of senescence. Zonation of senescence is observed not only at the whole plant and canopy level but also within the tissues of the leaf, leading to quite complex interactions between waves of yellowing. Senescence in maize, for example, begins with colour change and loss of the lowest, oldest leaves and works its way up the plant. Then, as the rate of grain filling reaches its maximum, the uppermost leaves begin to senesce and there is simultaneous progression of senescence in both directions, converging on the leaves 2 or 3 nodes above the ear, which are the last to be lost. Superimposed on this is a gradation within each leaf, senescence beginning at the tip and moving basipetally as a V-shaped area of chlorosis (Feller et al. 1977, Wolfe et al. 1988). This higher-order control of pattern has a strong genetic element and can be altered in a heritable fashion (Gentinetta et al. 1986, Ceppi et al. 1987). Synchronous senescence is characteristic of temperate deciduous trees and shrubs. In this case terminal and axillary buds, the root system and cambial tissues in the shoot axis persist to the following season. Nutrients are resorbed from senescing foliage and stored in cambial tissues of dormant stems (Hagen-Thorn et al. 2006). Photoperiod or low temperature or both influence the onset and progress of synchronous senescence in these species (Olmstead 1951, Andersson et al. 2004, Heide and Prestrud 2005). It has been proposed that the bright autumnal colours of some temperate species have a protective function, allowing efficient nutrient recovery (Hoch et al. 2003). In species with underground perennating organs (geophytes in the Raunkiaer classification) there is often top senescence - complete senescence of the shoot (Leopold 1961). An example is potato, where the pattern of senescence is sequential until canopy closure, whereupon top senescence sets in and the entire shoot system dies back (Forbes and Watson 1992, p 155). The senescence pattern of monocarpic species is similar in principle to that of geophytes, but in this case germplasm survives in the form of seeds and the entire plant - leaves, buds, stems and roots - senesces and dies. Leopold (1961) calls this overall senescence. References
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