-
Views
-
Cite
Cite
Elizabeth A. Ainsworth, Daniel R. Bush, Carbohydrate Export from the Leaf: A Highly Regulated Process and Target to Enhance Photosynthesis and Productivity, Plant Physiology, Volume 155, Issue 1, January 2011, Pages 64–69, https://doi.org/10.1104/pp.110.167684
Close - Share Icon Share
Extract
The phloem is a central component of the plant’s 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% to 80% of the CO2 photoassimilated in a mature leaf is transported out of the leaf in the phloem to satisfy the needs of the nonphotosynthetic organs of the plant (Kalt-Torres et al., 1987). In recent years, new data have 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 loading’s contribution to assimilate partitioning and its role in balancing photosynthetic activity with sink utilization of photoassimilates.
PHLOEM-LOADING MECHANISMS
The export of carbohydrate from photosynthesizing leaves (sources) provides the substrate for the growth and maintenance of nonphotosynthetic plant tissues (sinks), and the phloem is the delivery system for exported sugars. High concentrations of sugars, predominantly sucrose (Suc), in the sieve elements of source tissues raise turgor pressure, resulting in hydrostatic pressure-driven mass flow of sugars to the sieve elements of sink tissues, where sugars are unloaded and turgor pressure drops. Three different strategies for loading sugars into the phloem have been described (Rennie and Turgeon, 2009; Slewinski and Braun, 2010), which vary in the route that sugars take to enter the phloem and the energetics of accumulation. However, there is considerable flexibility in phloem loading across species (Rennie and Turgeon, 2009), and not all species strictly adhere to one strategy.
