DYNAMICS OF PLANT PERFORMANCE
During the early vegetative phase the dry matter mobilized from the seed and the dry matter produced in photosynthesis are distributed over leaves, stems, and roots. Rather soon the photosynthetic production is becoming the only source of available energy. Plants are the more successful in enlarging their absorbing area, the higher the leaf area production per gramme of produced dry matter is. More in particular the exposed area is the determining quantity, provided photosynthesis curves are similar (Brouwer et al., 1968). The orientation of the leaves, therefore, is important for the growth rate of individual plants. Mutual shading which increases with enlarging leaf area per plant causes a gradually decreasing relative growth rate and additionally in a crop shading by leaves of neighbouring plants leads to a further diminishing returns response per unit of leaf surface with increasing leaf area (Fig. 1). Crop growth rate expressed per unit of soil surface, increases up to a maximum with complete light interception (full cover). From thereon additional growth only increases biomass per m2 but does not contribute to more light absorption. Remarkably, growth rate shows a plateau response which indicates that respiration is not directly related to the amount of plant material present (De Wit et al., 1970) but more or less with photosynthesis.
The duration of the time interval between emergence and achievement of a closed crop surface depends on the growth rate during that phase and hence on the external conditions. After reaching full cover, production depends on the duration until ripening. Time of full cover coincides more or less with the transition to the generative stage and, depending on the type of plant (determined or not) and on external conditions (mineral supply, water supply, soil structure, temperature), leaves are ageing and will loose their photosynthetic capacity more or less readily. Crop growth rate during this stage depends largely on the percentage of soil cover with green surface.
The relative effect of unfavourable conditions during the vegetative phase depends largely on the time of harvest, whereas for seed or tuber-production the time of full cover determines yield (Fig. 2).
In this general pattern, environmental factors are interfering in all