Carbohydrate Export from the Leaf: A Highly Regulated Process and Target to Enhance Photosynthesis and Productivity

The phloem is a central component of the plant9s complex vascular system that plays a vital role in moving photoassimilates from sites of primary acquisition to the heterotrophic tissues and organs of the plant. Indeed, as much as 50-80% of the CO2 photoassimilated in a mature leaf is transported out of the leaf in the phloem to satisfy the needs of the non-photosynthetic organs of the plant (Kalt-Torres et al., 1987). In recent years, new data has shown that the phloem also plays a key role in moving information molecules that coordinate many facets of plant growth and development (Turgeon and Wolf, 2009). This update will focus on phloem loading9s contribution to assimilate partitioning, and its role in balancing photosynthetic activity with sink utilization of photoassimilates.