9.2 - B - Translocation
Sucrose is the most abundant sugar found in phloem sap. The process of transporting sugars from source cells into the phloem is referred to as phloem loading.
In the loading process, active transport is used to form a concentration gradient of sucrose in an area. This involves two stages:
High concentrations of sugar in the phloem results in the movement of water from the xylem into the phloem through osmosis. Because of cell wall rigidity and the incompressibility of water, this results in an increase in the hydrostatic pressure within the phloem.
This pressure results in the movement of the sap throughout the plant.
Differences in hydrostatic pressure within the phloem results in the contents being transported. Sap moves from areas of high pressure near the sources to areas of low pressure near the sinks.
When phloem contents reach the sinks, the the sucrose is absorbed and use for metabolic processes or converted to starch for storage. The resulting decrease in solute concentration reduces osmotic pressure .
Water in the sap is then transferred back into the xylem where it becomes part of the transpiration stream again.
Follow the link below for an animated explanation of this entire process:
Compared with other plant products, phloem sap is very nutrient rich. Aphids are insects that are part of the group Hemiptera. They use mouth stylets to penetrate plant tissues and reach the phloem.
If an aphid is anesthetized and its stylet severed, the phloem will continue to flow out. In this case, the rate of flow and composition of the sap can be analyzed and quantified.