Phloem and drought

Background/Question/Methods

Phloem transport failure may speed up plant mortality during drought by accelerating both hydraulic failure and carbon starvation. In all vascular plants, phloem and xylem tissues are located next to each other, and there is clear evidence of hydraulic connection between these tissues. Therefore, drought and high xylem water tension could lead to water shortage and impaired transport capacity also in the phloem. How this shortage might influence plant function and survival are rising as key questions for understanding drought responses of plants. Yet, there is little experimental information on phloem reactions to drought because measuring phloem transport is still a significant challenge to plant science. To shed light on these questions we reviewed existing data, and current theoretical models of phloem transport to discuss how phloem transport failure during drought might occur, how it could influence plant survival and what structural differences plants with different drought survival strategies might present.

Results/Conclusions

During drought phloem failure could occur either because of viscosity build-up at the source sites, which decreases flow rates and may block flow completely, or by a failure to maintain phloem water status and cell turgor. Which one of these mechanisms dominates depends on the hydraulic permeability of phloem conduit walls. Impermeable walls will lead to viscosity build-up, while permeable walls make the plant more susceptible to phloem turgor failure when xylem water tension increases. To prevent phloem turgor loss during drought, plants that allow large variation in xylem water tension (anisohydric plants) would benefit from phloem with impermeable conduit walls, but that would make them susceptible to viscosity build up. To reduce the effects of viscosity build-up on phloem transport, these plants should have high phloem conductive area. If phloem conduits have permeable walls, the capacity of a plant to osmoregulate phloem cells will determine when turgor loss occurs during drought.