Water stress and the transport of carbohydrates: Phloem sap and sieve cell characteristics along a height and water stress gradient in a tall conifer

Background/Question/Methods

Drought has been implicated as the cause of recent large-scale decline in temperate forests, yet the physiological mechanisms involved in drought-related tree mortality and constraints on tree growth have not been resolved. Despite the critical role that phloem transport plays in plant productivity and survival, and the increasing focus on mechanisms involved in plant responses to drought, relatively few studies have examined how phloem transport is influenced by water stress. The research in the current study involves investigating trends in phloem sap properties and sieve cell anatomical characteristics in shoot tips along a height gradient in order to evaluate the influence of water stress on phloem transport capacity. The hydraulic status of phloem tissue is linked to that of the xylem because the two tissues are in water potential equilibrium.  Because xylem water tension increases with tree height due to both gravitational forces and frictional resistance to water transport, we analyzed phloem tissue from along a tree height gradient in order to evaluate the influence of water stress on key phloem characteristics that dictate the transport capacity of phloem tissue.

Results/Conclusions

Consistent with reductions in bulk shoot water potential with increasing tree height (p = 0.0003), phloem tissue moisture content decreased by 0.21% per meter height increase (p <0.001) and sieve cell lumen radius decreased by 0.03 mm m^-1 (p = 0.0085). Overall phloem sugar concentrations were not significantly correlated with tree height (p = 0.33), however sucrose content decreased (although not significantly) with increasing height, resulting in a significant height-related increase in the phloem concentrations of lower molecular weight sugars of 0.04% m^-1 (p < 0.01). These height-related trends in phloem water and solute content were used to obtain estimates of the vertical trend in phloem sap viscosity, which increased by 3x10^-4 MPa s m^-1 (p < 0.000001). Phloem sap viscosity and sieve cell anatomical characteristics were used to estimate vertical trends in phloem transport capacity. The observed patterns of sugar composition suggest that lower molecular weight sugars were enhanced at the expense of sucrose under conditions of increased water stress; which is consistent with height-related osmotic adjustment to maintain turgor and cell expansion. The observed trends in phloem sap properties and sieve cell anatomical characteristics indicate that water stress is associated with constraints on phloem transport capacity.