Variation Among Plant Species in Phloem Loading Strategies and Plant–Water Relations

Plants differ in terms of water relations and phloem loading strategies, and our recent survey of 45 dicotyledonous species demonstrates a close link between phloem loading strategy and plant water transport. Most herbaceous plants have a high whole-plant hydraulic conductance (K_p), which enables efficient transport of water from soil to leaves to replenish the water loss via transpiration. This allows for the operation of active phloem loading (apoplastic loading or polymer trapping) in these species to an extent that the concentrations of sugars are lowered to a level that is just enough for turgor maintenance. In contrast, most woody plants have a low K_p value due to a low root density and a long and convoluted flow path. This low water transport efficiency requires the leaves of these species to accumulate high concentrations of sugars, which are high enough to drive the diffusion of sucrose from mesophyll cells to sieve element-companion cell complexes for long-distance transport without the need to concentrate sucrose via active loading. Because sugar alcohols (sorbitol and mannitol) have twice the osmolality per unit carbon as sucrose, it renders a potential advantage in maintaining turgor in both mesophyll cells and phloem, which is important not only for water stress tolerance, but also for phloem loading and transport, particularly in passive loading species.