The role of phloem failure in tree mortality during drought

Background/Question/Methods

Failure to maintain sugar transport in the phloem may promote tree mortality during drought. Impaired transport could leave sugar stores unutilized accelerating carbon starvation, or lead to hydraulic failure through diminished capacity to refill embolized conduits. Theoretically, drought hampers sugar transport in the phloem because increased solute concentrations are required for hydraulic function next to a xylem with decreasing water potential. High solute concentration could increase flow resistance to a point where transport is impossible because of the effect of carbohydrates on the viscosity of the solution. Currently, these hypothesized impacts of drought on phloem transport remain largely untested, and their role in determining tree mortality and survival during drought is unclear. To assess how drought affects phloem function we measured phloem turgor, leaf water potential and non-structural carbohydrate content of different tissues, and modeled phloem flow in pinon pine and one-seed juniper trees dying of drought.

 Results/Conclusions

Our results show that phloem turgor collapsed in trees during drought. The collapse occurred on average two weeks before permanent stomatal closure. The amount of stored sugars utilized depended on how long phloem turgor was maintained. Survival time also correlated with turgor maintenance, but drought limited respiration rate to different degrees, and the longest survivors were trees that utilized the available sugar reserves slowest. At very low xylem water potentials, solutes that do not affect fluid viscosity were needed to facilitate phloem transport. These solutes could also enhance recovery once precipitation occurs.  Based on these results we conclude that phloem failure is an important step in drought mortality of trees. Functionality of the phloem provides access to sugar reserves, and may prevent hydraulic isolation from the soil, but survival time during prolonged drought depends on how sugar consumption is controlled.