Man's dependence upon food resources from cultivated lands had its genesis some 10,000 years ago, and the resulting technology of crop hus bandry has been the principal factor permitting large increases in human population. The potential for food production in conventional agriculture and in alternative systems is undergoing intensive re-examination now be cause of current rapid population growth and attendant pressures for conver sion of highly productive, cultivated lands to other uses. The develop ment of plant physiology as a discipline has been closely linked to the de velopment of agriculture and agricultural research, so it is well that we attempt to take inventory in this review of how our research relates to cur rent and future problems in agricultural productivity.Let us focus our examination of agricultural ecosystems first upon several constraints which limit us in comparative analyses of diverse systems and which serve to shape research needs.First, agricultural systems are basically photosynthetic systems and hence must be assessed for their efficiency in conversion of solar irradiance in terms of both primary productivity and useful end products. This means focusing not only on basic aspects of photosynthesis at leaf and cellular levels but also at the community level. Responses to crowding, the dynamics of canopy development, and canopy persistence over the growing season become crucial matters. Time itself must be viewed as a resource.Second, the choice of crop species is heavily influenced by factors un related to primary production potential. Food, feed, and fiber preferences and secondary constraints such as pests, ease of culture, and the need for diversity in operations establish economic priorities which frequently over ride consideration of biological efficiency, Thus, in The Netherlands, grasses may accumulate 1000 g m-2 of dry matter before the time that maize can even be planted (210). However, significant amounts of maize are grown in The Netherlands because of its utility as a concentrated carbohydrate source. Also, it appears that genetic selection generally has favored quantity and quality of economic yield, variables which often correlate poorly with community productivity. These same factors encourage the culture of a desirable plant to be extended towards the limits of its climatic adaptation. Thus, economically attractive cropping systems may be transposed to en-431 7518 Annu. Rev. Plant. Physiol. 1971.22:431-468. Downloaded from www.annualreviews.org Access provided by North Dakota INBRE on 11/19/14. For personal use only. Quick links to online content Further ANNUAL REVIEWS