ABSTRACT
Several general principles are relevant to plant responses to environment. Crops should be studied at several levels of biological organization, for example, community, whole-plant, cellular, and molecular levels. Amplification of effects can occur with time at the whole-plant level but with less or no effect at the canopy level. Major advances in plant physiology have resulted from the recognition that some effects are in fact causes. Plant growth in the first part of the season was limited by low phosphorous, whereas growth in the last part of the season was limited by low nitrogen. The practical significance of the research in the Sahel was the demonstration that plant productivity is strongly limited by soil infertility, even in the presence of some drought. Slightly deficient plants have much less leaf area, but the supply of protein and enzymes per unit leaf area is regulated so that the plants maintain near-normal activities per unit leaf area.
