Vascular Constraints And Long Distance Transport In Dicots

Plants are often assumed to be capable of freely translocating resources from one part of the plant to all other parts, a condition that we describe as integrated. In reality, many vascular plants are sectorial, meaning that the movement of nutrients, photosynthate, and other substances is restricted to specific vascular, xylem or phloem, pathways. Sectorial plants consist of numerous integrated physiological units (IPUs) whose pathways are relatively independent of each other. Integrated and sectorial define two ends of what in reality is a continuum among species. The patterns of sectoriality across plant species, the ecological consequences of sectoriality, or the evolutionary pressures selecting for greater sectoriality or integration have been examined. This chapter reviews why sectorial transport is common and briefly discusses how it interacts with environmental heterogeneity. It ends with a description of new techniques for quantifying sectoriality and a discussion of the ecological conditions that might favor evolution of independent sectors or increased integration. The chapter aims to stimulate research on (1) the ecological consequences of sectoriality, and (2) the evolutionary forces that may determine the extent of sectoriality. The discussion is restricted to dicots; monocots are generally integrated because of the large number of interconnected vascular traces and the numerous vascular bundles that come together in complex combinations at nodes.