A PHYSIOLOGICALLY EXPLICIT MORPHOSPACE FOR WATER TRANSPORT IN VASCULAR PLANTS

Morphometrics provides useful tools for quantifying differences among populations in space and/or time. Previous morphometric analyses of vascular plants have emphasized leaf form and venation. We focus here on water transport through conducting tissues in order to understand in a quantitative way how another key aspect of plant function has changed through time. We constructed a morphospace for xylem cells (tracheids and vessel elements) using a model of water transport based on three principal features: cell diameter, cell length and pit resistance. Physiological studies established that these three factors collectively determine overall conductivity through xylem elements. The model employed here yields information on conductivity that is independent of the environment. Moreover, every point in the resulting 3D morphospace can be explicitly interpreted in terms of a realizable plant cell conductance. Extant plants largely fall into two clusters: one characterized by wide, low-resistance conducting cells (mainly angiosperms) and a second group with narrow, relatively high-resistance xylem (especially conifers). This dichotomy reflects the physiological requirements of plants that use conducting cells primarily for high-volume water transport versus those that rely on xylem for structural support. In contrast, many fossil plants occupy portions of the morphospace that are empty today. At least in part, this reflects adaptation for high volume water transport independent of vessel evolution. There are many independent origins of tracheids that have transport rates comparable to vessel elements (e.g., the Carboniferous seed fern Medullosa); such adaptations may have been necessary to accommodate the evapotranspiration demands of large leaf areas.