An inclusive definition of crops is that they are photosynthetic organisms cultivated, or otherwise deliberately encouraged to grow, by man. Most crops are thought of in terms ofa product, which is harvested by man directly or, indirectly, via domesticated grazing animals. This definition would include not only the 'obvious' crops such as cereals and 'root crops', but also managed pastures, trees used for wood or fuel, seaweeds cultivated for their wall polysaccharides, microalgae cultivated as food for maricultured invertebrates or as sources of human dietary supplements, and cut flowers. It may be stretching the definition of crops to include amenity plantings and sports turf (unless dislodging divots can be termed harvesting!), or any plantations for C sequestration.
The broad definition of crops includes plants in which the harvested material contains very little N (e.g. wood for construction or fuel) and where what N is present is of little or no significance for the uses to which man puts the material. The lack of N in wood is a result of effective N recycling and retranslocation, since, for example, every aromatic nucleus in lignin was produced from the action of phenylalanine ammonia-lyase on phenylalanine (Raven et al., 1992a). However, such crops are harvested by coppicing or, more usually by sacrifice of all of the above-ground parts of a tree. This means that significant N (in leaves and small stems) is discarded, although N has to a substantial extent been withdrawn from time-expired leaves before they are abscised, and retranslocated to new, growing leaves. At the other extreme are crops for which the harvested part is the main photosynthetic organs, e.g. Lactuca, Spinacia and pasture grasses. In an intermediate position are perennial crops in which the harvested structures are fruits, leaving the tree or shrub to produce a crop in a subsequent season. Most fruits of perennial plants fix by net photosynthesis much less than half of their C and rely on phloem (to a lesser extent xylem) for most of their N and the remainder of their organic C. Thus, the provisioning of the fruits is to some extent in competition for organic N and C with the assimilatory, and especially the photosynthetic, apparatus. Also in an intermediate position are the annual grain crops with fruits or seeds as the harvested structure. As with the fruits of perennial plants less than half of the organic C in the fruits or seeds of these annuals comes from in situ photosynthesis, so that much of the organic C and almost all the organic N comes from dedicated photosynthetic structures. This can, as for perennials, be regarded as competition between photosynthetic structures and grainfilling, although for the annuals no N (or C) in the vegetative plant is harvested, so that there can be a temporal distinction between vegetative growth and reproductive growth (Cohen, 1966). While Cohen (1966) dealt mainly with organic C, this model also applies to N. Thus, the optimal strategy for the annual as a wild plant in a variable habitat is to follow vegetative growth with reproduction. Such a use of N as a catalyst in photosynthesis followed by transfer to seeds and fruits in crops in producing seed and fruit protein may be the optimal strategy for the wild ancestors of annual grain crops (Cohen, 1966). Man has capitalized on these traits in the breeding of crops.
A good account of the regulation of fluxes between organs in a range of life-forms of higher plants can be found in Stitt and Schulze (1994). Extending the concept of crops to macroalgae and microalgae can use the same models of flux control as are used for higher plants (Stitt and Schulze, 1994), although the structures involved are different. Water flow over macroalgal thalli can influence allocation to wall polysaccharides (Kraemer and Chapman, 1991a,b), just as wind can influence allocation to wall materials in terrestrial vascular plants (Niklas, 1992).
A consideration of the nature of crops requires a consideration of the crop environment as well as of the organisms. This is especially the case where it is only the environment, which distinguishes crop plants from their wild relatives, e.g. in many micro- and macro-algal cultures. The crop environment frequently (in theory at least) involves monocultures, with high plant densities. Any selection, breeding or genetic modification programs related to manipulating C and N budgets must take into account the crop environment. An example is shading by the upper canopy in later growth stages of annual crops which impacts on C acquisition at the individual plant level, perhaps more than at the whole crop level. For N acquisition, the timing of nitrogenous fertilizer application in relation to crop growth can be very relevant to the effectiveness of use of the applied N, with benefits sometimes accruing from split applications and the use of slow-release fertilizers.
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