ABSTRACT
The necessity for plants to carry out photosynthesis under a wide range of environmental conditions has played a major role in the evolution of the diverse forms of life that exist. The rate at which a plant photosynthesises depends on its inherent capacity to utilise the available light energy; its ability to intercept light. Minimum and optimum light intensities for photosynthesis vary among species and even between leaves on the same plant. Plants or primary biological productivity is a function of the net photosynthetic production plus loss of parts of the plant biomass by death and by grazing. In 1964 the International Biological Programme was set up under the auspices of the International Council for Scientific Unions. Two other factors which can depress yield and which have a greater impact in agro-ecosystems and forest plantations than in unmanaged ecosystems are the so-called negative biological factors—the weeds, pests and pathogens—and the weather.
