-Root hydraulic conductivity and xylem sap levels of zeatin riboside and abscisic acid in ectomycorrhizal Douglas fir seedlings

Mechanistic hypotheses to explain mycorrhizal enhancement of root hydraulic conductivity (L,) suggest that phosphorus (P) nutr i t ion, plant growth substances and/or al tered morphology may be responsible . Such ideas are based on work with VA (vesicular-arbuscular) mycorrhizas . S ince VA mycorrhizas and ectomycorrhizas di f fer in many respects , they may alter host plant water uptake via different mechanisms. This paper examines L, in various ectomycorrhizal associat ions whi le consider ing factors which are important to the VA mycorrhizal e f fect on L,. Douglas fir Pseudotsuga met&&i (Mirb.) France] seedlings inoculated with the ectomycorrhizal fungi Laccuriu bicolor (Maire) Orton and Hebelotna crustuliniforme (Bull. ex St. Amans) Quel. and non-inoculated seedlings infected naturally with Thelephora were grown under three low levels of P fertilization (1, 10 and IOO,uM P). Seedl ing morphology, t i ssue P levels , ,L, and plant growth substance levels in xylem sap were measured after nine months growth. Increased tissue P and decreased root/shoot ratio correlated with increased L, in each of the mycorrhizal treatments . When adjusted for the effect of these two factors , L, of Laccariu and Hebeloma seedl ings was st i l l lower than the Thelephora seedlings. In a subsequent experiment, the L, of seedl ings with Hebeloma and Rhizopogon vinicolor Smith mycorrhizas was compared to the L, of non-mycorrhizal seedlings.,(grown at 100 mM P).and no differences were found among treatments . The lack of an ectomycorrhizal effect on L, i s qui te different from the enhancement of host L, by VA mycorrhizas . Zeat in r iboside concentrat ions of Thelephoraand Hebeloma-infected seedl ings were s imilar , yet higher than with Laccuriu. There was no relationship between plant growth substances and L, in ectomycorrhizal Douglas fir, despite lower zeatin riboside concentrations for Laccariu-inoculated plants.