Why Is Phloem Loading Passive In Most Tree Species?

Before sucrose can be exported from leaves, it is transferred into the phloem, a process known as phloem loading. Most herbaceous plants use one of the two active loading mechanisms, apoplastic loading or polymer trapping. Both loading mechanisms elevate the sugar concentration, and the hydrostatic pressure, of the phloem to very high levels. In contrast, phloem loading in most trees is passive: sucrose simply diffuses from mesophyll cells into the phloem. To date, there is no hypothesis that explains why most trees use passive phloem loading. Considering that phloem loading must operate within the constraint of water relations and trees generally have low whole-plant hydraulic conductance (K_p), we reasoned that, to maintain turgor, trees must accumulate high concentrations of sucrose (and sugar alcohol in some species) in leaf mesophyll cells, which provide the motivating force for the diffusion of sucrose (and sugar alcohol) into the phloem. We found, in a survey of 45 dicotyledonous species with different loading mechanisms that, herbs that load actively are characterized by high K_p, low leaf osmolality, and low foliar sugar concentration. In contrast, trees, and the few herbs that load passively, have low K_P, high osmolality, and high sugar concentration. These findings demonstrate a close link between phloem loading and plant water transport, and support the idea that accumulation of transport sugars in leaves of passive loading species serves two functions at the same time: to maintain leaf turgor and to drive the diffusion of transport sugars into the phloem for long-distance transport.