David R. Woodruff

Summary: Leaf hydraulics, gas exchange and carbon storage in Pinus edulis and Juniperus monosperma, two tree species on opposite ends of the isohydry–anisohydry spectrum, were analyzed to examine relationships between hydraulic function and carbohydrate dynamics. Leaf hydraulic vulnerability, leaf water potential (Ψ_l), leaf hydraulic conductance (K_leaf), photosynthesis (A), stomatal conductance (g_s) and nonstructural carbohydrate (NSC) content were analyzed throughout the growing season. Leaf hydraulic vulnerability was significantly lower in the relatively anisohydric J. monosperma than in the more isohydric P. edulis. In P. edulis, Ψ_l dropped and stayed below 50% loss of leaf hydraulic conductance (P₅₀) early in the day during May, August and around midday in September, leading to sustained reductions in K_leaf. In J. monosperma, Ψ_l dropped below P₅₀ only during August, resulting in the maintenance of K_leaf during much of the growing season. Mean A and g_s during September were significantly lower in P. edulis than in J. monosperma. Foliar total NSC was two to three times greater in J. monosperma than in P. edulis in June, August and September. Consistently lower levels of total NSC in P. edulis suggest that its isohydric strategy pushes it towards the exhaustion of carbon reserves during much of the growing season.

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.

We analysed concentrations of starch, sucrose, glucose and fructose in upper branch wood, foliage and trunk sapwood of Douglas‐fir trees in height classes ranging from ∼2 to ∼57 m. Mean concentrations of non‐structural carbohydrates (NSC) for all tissues were highest in the tallest height class and lowest in the lowest height class, and height‐related trends in NSC were most pronounced in branches. Throughout a 17‐month sampling period, mean values of branch NSC from the 57 m trees ranged between 30 and 377% greater than the 2 m trees. Branch NSC was inversely correlated with midday shoot water potential (Ψ_l), shoot osmotic potential at full turgor (Ψ) and shoot extension. Temporal fluctuation in branch NSC was inversely correlated with height, and positively correlated with middayΨ_l,Ψand shoot extension. The positive correlation between height and storage of NSC, and the negative correlation between NSC storage and shoot extension provide evidence that size‐related growth decline in trees is not strongly associated with constraints on photosynthesis. The negative correlation between height and fluctuation in NSC suggests that mobilization of photosynthate in taller trees is constrained by some factor such as reductions in turgor‐driven cell expansion or constraints on phloem transport.

Stomatal responses to leaf-to-air vapour pressure deficit (LVPD), leaf water potential components, and cuticular properties were characterized for Douglas-fir (Pseudotsuga menziesii) foliage collected from tree tops along a height gradient from 5 m to 58 m in order to explore height-related trends in stomatal sensitivity to LVPD and to investigate the role of bulk leaf turgor and leaf cuticle thickness in determining stomatal behaviour. There were three distinct phases in the response of stomatal conductance (g_s) to changes in LVPD. At low LVPD, g_s increased with increasing LVPD (phase one). During the second phase, g_s was maximal at low to intermediate LVPD and during the third phase g_s declined steadily as LVPD increased. The responsiveness of g_s to LVPD exhibited a height-related pattern such that maximum g_s (g_s-max) occurred at progressively greater LVPD with increasing height (r²=0.55, P=0.006). Bulk leaf osmotic potential at full turgor decreased with height (r²=0.77, P=0.00016), and LVPD at g_s-max and at maximum crown conductance (g_c-max) in the field were significantly correlated with leaf turgor (r²=0.92, P=0.0093). Cuticle thickness increased by 0.044 μm for every metre increase in height (r²=0.78, P=0.00015). The observed trends in the response of g_s to LVPD along a height gradient, and their consistency with height-related trends in foliar osmotic potential suggest that osmotic adjustment at the tops of tall trees influences the relationship between g_s and LVPD.