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Despite the critical role that phloem plays in a number of plant functional processes and the potential impact of water stress on phloem structural and phloem sap compositional characteristics, little research has been done to examine how water stress influences phloem transport. The objectives of this study were to develop a more accurate understanding of how water stress affects phloem transport in trees, both in terms of the short-term impacts of water stress on phloem sap composition and the longer-term impacts on sieve cell anatomical characteristics. Phloem sieve cell conductivity ( k  p ) was evaluated along a gradient of tree height and xylem water potential in Douglas-fir ( Pseudotsuga menziesii  (Mirb.) Franco) trees in order to evaluate the influence of water stress on phloem transport capacity. The Hagen–Poiseuille equation was used with measurements of sieve cell anatomical characteristics, water content of phloem sap, non-structural carbohydrate content of phloem sap and shoot water potential (Ψ l ) to evaluate impacts of water stress on  k  p . Based on regression analysis, for each 1 MPa decrease in mean midday Ψ l , sieve cell lumen radius decreased by 2.63 µm MPa −1 . Although there was no significant trend in sucrose content with decreasing Ψ l , glucose and fructose content increased significantly with water stress and sieve cell relative water content decreased by 13.5% MPa −1 , leading to a significant increase in sugar molar concentration of 0.46 mol l −1  MPa −1  and a significant increase in viscosity of 0.27 mPa s MPa −1 . Modeled  k  p  was significantly influenced both by trends in viscosity as well as by water stress-related trends in sieve cell anatomy.

The impacts of water stress on phloem transport in Douglas-fir trees