A quantitative analysis of the three-dimensional spatial colonization by a plant as illustrated by white clover (Trifolium repens L.)

A new method is described for measuring spatial colonization by plants from three-dimensional (3-D) records of their aerial morphology. Nonparametric indices were devised to quantify the space colonized by the plant by measuring the spatial dispersion of sets of points representing relevant morphological features of its architecture, such as the nodes and petiole tips. These indices are defined as the square root of the second-order moments of these sets, i.e., as dispersion radii. They can be split into the components that characterize the major directions of colonization and those that analyze the contributions of the various morphological features. Horizontal dispersion can be visualized as an indicator ellipse ( to characterize the anisotropy), and vertical dispersion as equivalent heights. The kinetics of spatial colonization are then characterized as the changes in these dispersion radii during the development of the plant. This method is illustrated using isolated white clover plants. Two phases can be distinguished in the growth of this species: (i) a phase of rapid horizontal colonization involving the petioles and (ii) a phase in which the rate of horizontal colonization becomes slower and steady and involves both internodes and petioles. Petioles therefore make a major contribution to both vertical and horizontal colonization. We also show that these conclusions could not have been reached using only organ lengths and that the usual assumptions about the function of petioles in white clover should be reconsidered. We argue that this is a general method that can be applied to most species and growth forms of plants. It should be particularly useful for analyzing plastic morphogenetic responses to light conditions and their contribution to light foraging as a complement to estimating light interception.