Effects of partial shading of the potato plant on photosynthesis of treated leaves, leaf area expansion and allocation of nitrogen and dry matter in component plant parts

Literature shows that the distribution of nitrogen (N) over leaf layers tends to follow the distribution of light. Nitrogen is regarded as moving away from poorly illuminated leaves. If operative in plant canopies, such mechanisms affect leaf longevity and the allocation of N and dry matter to plant parts. To examine such mechanisms in potato (Solanum tuberosum L.) we conducted pot experiments with spaced plants in which the primary axis of the plant was subjected to shade treatments (50 or 90% shade), while the apical branches of the plant were illuminated as the control plants. N treatments were a limiting rate of N supply (N1) and a high rate of N supply (N2). Changes in leaf area, dry weight, N content (organic N and nitrate) and light saturated photosynthetic rate (Pmax) were recorded for particular leaf numbers. Leaf area, dry weights and total N content of all component plant parts were determined. Shaded leaves showed a lower specific leaf weight while leaf area was not affected. Fifty percent shade had little effect on age-related changes of leaf properties, but leaves senesced fast when subjected to 90% shade. Shading the primary axis enhanced apical branching, increased sizes of individual leaves and reduced stem:leaf weight ratio of non-shaded apical branches; partitioning of dry matter and nitrogen to tubers was less than in controls. It was concluded that these changes were not associated with enhanced remobilization of N from shaded plant parts; nor were they related to enhanced senescence of shaded leaves.