Summarizing Transport of Water and Solutes in Plants

Summary

Water potential (Ψ) is a measure of the difference in potential energy between a water sample and pure water. The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and matric potential. Water potential and transpiration influence how water is transported through the xylem in plants. These processes are regulated by stomatal opening and closing. Photosynthates (mainly sucrose) move from sources to sinks through the plant’s phloem. Sucrose is actively loaded into the sieve-tube elements of the phloem. The increased solute concentration causes water to move by osmosis from the xylem into the phloem. The positive pressure that is produced pushes water and solutes down the pressure gradient. The sucrose is unloaded into the sink, and the water returns to the xylem vessels.

Glossary

cuticle

waxy covering on the outside of the leaf and stem that prevents the loss of water

megapascal (MPa)

pressure units that measure water potential

sink

growing parts of a plant, such as roots and young leaves, which require photosynthate

source

organ that produces photosynthate for a plant

translocation

mass transport of photosynthates from source to sink in vascular plants

transpiration

loss of water vapor to the atmosphere through stomata

water potential (Ψ_w)

the potential energy of a water solution per unit volume in relation to pure water at atmospheric pressure and ambient temperature