Optimality in the spatial leaf distribution of the weed Portulaca oleracea L.

In an open environment, a plant acquires a larger amount of sunlight if it spreads its leaves wide enough to avoid self-shading. Such an optimal form is not always attainable, however, because plant form is subjected to selection pressures such as structural stability; a plant must sometimes invest a higher proportion of its material in its stem tissue to maintain the structural stability needed to spread its leaves wide. In order to determine the existence of an optimal plant width that achieves maximum sunlight acquisition for a given plant biomass, we conducted a simple simulation based on the allometric growth of plant parts. The optimal value predicted by the simulation was somewhat higher than the observed value for the weed Portulaca oleracea L., but both predicted and observed patterns showed a gradual increase in leaf area index (total leaf area)/(ground area occupied by plant) with increasing plant width. This increase in leaf area index and self-shading occurs because stem weight increases faster than leaf weight making the maintenance of a wide leaf spacing structurally expensive in large plants.