Hydraulics of plant growth

Multicellular plants rely on growth in localised regions that contain small, undifferentiated cells and may be many millimetres from the nearest differentiated xylem and phloem. Water and solutes must move to these small cells for their growth. Increasing evidence indicates that after exiting the xylem and phloem, water and solutes are driven to the growing cells by gradients in water potential and solute potential or concentration. The gradients are much steeper than in the vascular transport system and can change in magnitude or suffer local disruption with immediate consequences for growth. Their dynamics often obscure how turgor drives wall extension for growth, and different flow paths for roots and shoots have different dynamics. In this review, the origins of the gradients, their mode of action and their consequences are discussed, with emphasis on how their dynamics affect growth processes.