The Effects of Artificial Shading on Some Growth Characteristics of Arabica and Robusta Coffee Seedlings. I. The Effects of Shading on Dry Weight, Leaf Area and Derived Growth Data

plants, can provide a more critical and more widely applicable analysis of some of the effects of solar radiation on the vegetative growth of crop plants than field experiments on crops with natural shade. The effects of natural shade on mature trees are immensely complicated, not only by self and mutual shading but by many factors other than shade per se. In such field experiments it is impossible to analyse the complex by testing the effects of one or a few factors at a time because so very many interactions are involved. Agriculturalists have often used shade trees in order to simulate 'llatural' conditions for coffee, as well as cacao and tea, because these species are all indigenous in forested regions. However, the spread of a species in space and time depends oIn the interplay of numerous environmental and plant factors. Any one ecotype exhibits certain degrees of genetic fitness and flexibility and the presence of a particular form in a community will be conditional on a number of limiting factors imposed from the time of distribution and establishment of the seed onwards. Any one of a multitude of such factors might account for the absence of a species, although no one factor alone would necessarily be responsible for its presence. Thus, the premise that because a crop species grows in shade in its natural association it must be shaded in an agricultural habitat is inadmissible. Despite the complexity of the shade problem as a whole, a knowledge of the effects of light intensity and total receipt on photosynthesis and growth has considerable value. This is so both in relation to management factors (such as pruning and spacing) and, additionally, in longer term breeding projects where the most efficient use of incoming radiation may be attempted by producing varieties with the appropriate habit and leaf attitude. Experimental evidence of the effects of artificial shade on photosynthesis and growth of coffee seedlings or of older plants is discussed in detail later, but the results of such experiments, which have been conducted in different parts of the world and under different nutritional conditions, have been conflicting. In order to investigate the problem more fully, the present studies were undertaken with seedlings of the two species Coffea arabica (Arabica coffee) and C. canephora (Robusta coffee), using growth analysis techniques, controlled nutrition and measured radiation income. The experiments were carried out at Makerere University College Farm, Kabanyolo, Uganda, during the period 1958-61.