The role of phloem loading reconsidered.

It is generally assumed that the primary role of phloem loading is to drive long-distance transport by elevating hydrostatic pressure in sieve elements. This concept is consistent with the fact that, in many plants, energy is used to increase the concentrations of photoassimilates in the leaf phloem to levels well above those in mesophyll cells. However, on the basis of recent data, these fundamental assumptions need reevaluation. The data indicate that a large number of woody species—plants with the longest transport distances—load solute passively by maintaining high concentrations of Suc, and in some cases sugar alcohol, in mesophyll cells. Here I suggest that the adaptive advantage of active phloem loading, in the evolutionary sense, is to permit plants to maintain low foliar concentrations of nonstructural carbohydrates (NSCs), i.e. sugars, sugar alcohols, and starch. Economic considerations of inventory costs indicate that maintaining low NSC levels in leaves improves return on investment in carbohydrate synthesis, leading to a substantial increase in growth potential.