Adaptation of photosynthetic processes to stress.
I have focused on examples of plant adaptations to environmental conditions that range from adjustments in the allocation of metabolic resources and modification of structural components to entirely separate mechanisms. The result of these modifications is more efficient performance under the stresses typically encountered in the plants' native habitats. Such adaptations, for reasons which are not entirely clear, often lead to poorer performance in other environmental conditions. This situation may be a fundamental basis for the tendency toward specialization among plants native to specific niches or habitats. The evolutionary mechanisms that have resulted in these specializations are very large-scale processes. It seems reasonable to suppose that the plants native to particular habitats are relatively efficient in terms of the limitations imposed by those habitats, and that the adaptive mechanisms these plants possess are, compared to those which have evolved in competing organisms, the most succesful biological means of coping with the environmental stresses encountered. I believe that we can learn from nature and utilize the adaptive mechanisms of these plants in agriculture to replace in part our present reliance on resources and energy to modify the environment for plant growth. By analogy with natural systems, improved resource utilization will require specialization and greater knowledge of the limitations of a particular environment and plant genotype. For example, the cultural conditions, plant architecture, and physiological responses necessary to achieve high water use efficiency from our crop species with C(4) photosynthesis probably differ from those required to achieve maximum total growth. Also, efforts to control water application to eliminate waste carry with them the risk that the crop could be injured by inadequate water. Thus, greater demands would be placed on the crop physiologist, the plant breeder, and the farmer. Planting and appropriate management of adapted crop genotypes could enable cultivation of many areas presently considered unusable because of environmental extremes or shortage of resources, and may lead to more efficient resource utilization on land already under cultivation. The costs or benefits of this cannot yet be estimated. However, I suspect that the greatest potential for application of such techniques will be in the developing rather than the developed regions of the world. The genetic and functional diversity of plants is a tremendous biological resource. The capacity of plants to adjust in the future to changing environmental conditions depends on this diversity and on evolutionary processes of nature. Wild plants may provide a source of genetic material to improve crop plants. Also, as advocated by McKell (22), wild plants can be utilized to a greater extent directly by man. Long-term research efforts and commitment to preserve natural habitats and their populations of wild plants will be required to maintain and more effectively utilize this resource.
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