The discussion in Section II shows that crops have a wide range of C:N ratios in the harvested portions. On both economic and on environmental grounds the N inputs to, and N losses from, an agroecosystem should be as small as is consistent with economically viable and environmentally sustainable crop production. The very low N content of wood only requires catalytic N in the photosynthetic and nutrient absorption apparatus, and in wood synthesis. Since the lifespan of leaves and fine roots is less than the time taken for a tree to produce useable wood, even in short-rotation coppice, minimizing N requirements and losses would best be achieved by maximizing internal recycling of N (and other nutrients). Moreover, minimization of the quantity of N in catalytic and structural components is consistent with delivering organic C to wood at an economic (to humans) rate.
For crops whose harvested portions are photo-synthetic, the requirement for minimal N in the photosynthetic apparatus is less stringent than in woody crops, especially if the consumer organisms obtain a significant fraction of their organic N from the crop. Any manipulations of N in leaf vegetables must be compatible with other nutritional requirements of the human (or other animal) consumer (Grusak and Dellapenna, 1999).
More complex in optimization terms is the allocation of N when the harvested product contains N as a desirable component but the harvested product does not perform much, or any, ofthe photosynthesis required in provisioning the harvested product with organic C. Here the sorts of models pioneered by Cohen (1966) are useful in indicating optimal N
allocation between the photosynthetic apparatus (and other essential components other than the harvested component) and the harvested product as a function of time.
These sorts of considerations, and especially those in which major temporal changes take place in the spatial disposition of N within the plant, are most readily modeled assuming a constant environment. Such assumptions are most reasonable for greenhouse crops, although even here the biotic environment (pests and pathogens) may be rather variable, and the light environment is not always controlled. In less managed crop environments the variability of the habitat is, of course, greater.
The optimization of C-N budgets in a variable environment requires that the organism not only deals with a temporarily restricted supply of a resource but also can deal with a temporary excess. Resource excess is perhaps most obvious with light in the form of photoinhibition (Long et al., 1994; Niyogi, 1999; Marshall et al., 2000). Accordingly, any optimization which focuses on maximizing C fixation per unit plant N in a given constant environment may not achieve the highest crop yields in a variable environment. This is exemplified by Mott and Woodrow (2000) for the large and frequent variations in photon flux density, and by Raven and Glidewell (1981), Cowan (1986), Majeau et al. (1994), Price et al. (1994), Evans and von Caemmerer (1996), Evans (1999) and Evans andLoreto (2000) for C02 transport in the liquid phase with varying intercellular space COj concentrations in C3 plants.
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