Anu S~ober, Julia Shilina

University of Tartu, Institute of Ecology and Earth Sciences, Tartu, Estonia


To create a driving force for water upward movement, the water potental of upper part of shoot must be more negative than water potential at the shoot base. Usually water potentioal of upper leaves is also more negative compared to that of lower leaves. However, our measurements showed reverse gradient of water potential between upper and lower leaves on terminal branches of 3 year old trembling aspen and hybrid aspen trees (in average -0,6 for upper and -2 MPa for lower leaves). To explain the reverse gradient in leaf water potential we proposed, that the difference in water potential between brshoot xylem and leaf must be much less in upper, developing leaves than in lower mature leaves. Also hydraulic resistance of leaves must be significant compared to hydraulic resistance in tree roots and trunk, The degree how low water potential drops in leaves depends on leaf hydraulic conductance and transpiration rate. Leaf hydraulic conductance and stomatal conductance were estimated and significant differences between developing and mature leaves were found. Hydraulic conductance was higher and stomatal conductance was lower in upper developing leaves compared to lower mature leaves. Differences in pathways of water movement in developing and mature leaves