Introduction

Leaf senescence is a genetically coded process of the programmed death of a leaf (Chapter 1). Usually, it is measured as a decrease in photosynthetic rate or as other changes in chloroplasts (Thomas and Stoddart, 1980; Nooden, 1988). Concomitantly, senescence is influenced by micro-environmental conditions, particularly light availability, and these change as plants grow (Hikosaka et al., 1994). The available light often decreases due to shading by surrounding plants or self-shading. Limiting nutrients, especially nitrogen, are redistributed from old, shaded leaves at the bottom of a plant to new leaves at the top which can receive full light (Chapter 14). Here, leaf senescence and nutrient reallocation are considered to be an adaptive behavior in which a plant replaces leaves to adapt to environmental changes (Hikosaka, 1996).

The timing of leaf death differs among plant species. Some species exhibit short leaf longevities of less than 30 days, and others have leaf longevities greater than 10 years (Chabot and Hicks, 1982). Leaf longevity also differs among individual plants within a species depending on the environmental conditions. It also changes within an individual plant or with the development of a plant. The wide range of leaf life spans suggests that leaf longevity is an adaptive feature (Kikuzawa and Ackerly, 1999). In this chapter, I will review longevity of leaves of higher plants by considering the replacement of leaves to be an adaptive strategy of plants.

Since leaves are photosynthetic organs, leaf senescence and leaf replacement must be considered from the point of view of carbon economy of plants (Kikuzawa, 1991). Some plant species have leaves with a higher photosynthetic rate per unit leaf area, while other species have lower rates (Larcher, 1975; Ceulemans and Saugier, 1991). Since up to 75% of leaf organic nitrogen is present in the chloroplasts (Poorter and Evans, 1998), nitrogen contents are associated with rates of photosynthesis (Reich et al., 1995; Ninemets et al., 2002). The dependence of photosynthetic capacity on nitrogen content varies strikingly among species when both are expressed on a leaf area basis. In particular, leaves of evergreen trees and shrubs show very low photosynthetic capacities relative to those of crop and herbaceous plants (Hikosaka et al., 1998). Of the ten dicotyledonous species (four woody and six herbaceous species) compared by Poorter and Evans (1998), the herbaceous group had higher specific leaf areas (SLA: leaf area per unit leaf mass) and lower carbon concentration per unit leaf mass. This group allocated more nitrogen to thylakoids and to a key photosyn-thetic enzyme (Rubisco) while the woody group did the reverse (Poorter and Evans, 1998; Hikosaka et al., 1998; Evans and Poorter, 2001). In brief, the slow-growing species invest less in their photosynthetic apparatus and more in their non-photosynthetic functions such as defensive and supporting tissues (Chabot and Hicks, 1982). Such systematic differences in thickness of the photosynthetic apparatus (SLA), nitrogen investment patterns and pho-tosynthetic rates are observed between slow-growing and fast-growing species within herbs (Poorter and Evans, 1998; Rothstein and Zak, 2001), within trees (Poorter and Evans, 1998; Mooney et al., 1984; Atkin et al., 1998; Bauer et al., 2001) and even within a wider range of photosynthetic organisms including cyanobacteria. Photosynthesis rates (mg C g C-1 h-1) declined with increasing thickness of the photosynthetic tissue and were positively scaled to chlorophyll a (Enriquez et al., 1996).

The general course of the photosynthetic rate per unit leaf area during leaf ontogeny is a rather steep increase to a maximum value, often achieved before leaves complete their expansion and reach the greatest amount of chlorophyll. This peak usually is followed by a slower decline in photosynthetic rate (Sestak, 1981). Some tree species, in particular under-storey trees in tropical rain forests (Kursar and Coley, 1992a,b) and temperate evergreen broad-leaved trees (Miyazawa et al., 1998), showed delayed greening. Expanding leaves sometimes show a white or red color with no visible chlorophyll. The nitrogen contents and photosynthetic rates also remained low for 10-30 days after leaf expansion. In some evergreen coniferous trees, the time when the newly emerged leaves reach the maximum photosynthetic rate is also delayed (Kajimoto, 1990; Matsumoto, 1984). After reaching the maximum photosynthetic rate, photosynthesis usually declines with increasing leaf age (Matsumoto, 1984; Oren et al., 1986; Kajimoto, 1990). The decline in photosynthesis has been approximated by a straight line (Oren et al., 1986). Net CO2 uptake also decreased linearly with increasing leaf age in some tropical tree species (Zots and Winter, 1994; Kitajima et al., 1997) and temperate deciduous tree species (Koike, 1990). In some deciduous trees, however, the photosynthetic rate measured at light-saturation increased to a maximum near the completion of leaf expansion, was constant until autumn, and then rapidly declined until leaf death (Jurik, 1986).

Decline in photosynthetic rate with leaf age is associated with decrease in nitrogen content in the leaf (Field and Mooney, 1983). Some changes in chloroplast ultrastructure were observed such as swelling of the thylakoids (Nooden, 1988) and appearance of plastoglobuli (Nilsen et al., 1988; Nooden, 1988; Gepstein, 1988). In Rhododendron maximum, the shortest leaf longevity is associated with the most rapid reduction in photosynthetic capacity and the most rapid development of chloroplast plastoglobuli (Nilsen et al, 1988). The disappearance of chlorophyll is also a prominent feature of leaf senescence (Gepstein, 1988). In many cases, however, the decrease in chlorophyll is slower than that of some Calvin-cycle enzymes, soluble proteins, electron carriers and electron-transport capacity (Hikosaka, 1996). In erect herbaceous plants, the irradiance received by a leaf decreases as the plant grows because of shading by upper new leaves (Hikosaka, 1996). There is a gradient of photon flux density within a canopy of plant population. With the degradation of micro-environmental conditions, in particular light conditions, it is adaptive to remove nitrogen from older, shaded leaves and to reinvest it in newly emerged leaves (Mooney and Gulmon, 1982; Hirose and Werger, 1987). The whole canopy carbon gain will be maximized when nitrogen is distributed greatest to the leaves which receive the highest photon flux density (Field, 1983). In fact, photon flux density, leaf photosynthetic capacity and leaf nitrogen content all decrease with depth into a canopy (Kull and Kruijt, 1999). Actual leaf nitrogen distribution within a canopy of Solidago altissima was similar to that predicted from a model for optimal canopy-photosynthesis (Hirose and Werger, 1987).

Decline in photosynthetic rate with increasing leaf age is often attributed either to endoge-nously controlled senescence or to environmentally induced senescence, or both. As an illustration of these phenomena, the environmental effect can be distinguished from endoge-nously initiated senescence, by a unique experimental system in which a vine is grown horizontally to avoid mutual shading (Hikosaka et al., 1994; Hikosaka, 1996). When only a limited amount of nitrogen was supplied, the nitrogen content of leaves dropped drastically with decrease of photon flux density and also decreased with advance of leaf age and position along the vine even when the vine is horizontal, i.e., without shading. The result strongly suggests that this decline in photosynthetic rate can be attributed both to environmental factors and to endogenously controlled senescence.

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