Seasonal Osmotic Relations in Apple Leaves of Different Ages

Seasonal changes in the relationship between leaf conductance and total water potential in mature apple ( Malus domestica Borkh.) shoot leaves were found to be mediated by osmotic adjustment, not changes in the total water potential–relative water content relationship. Leaves at the growing shoot tip were found to have a much higher (1.0 to 2.0 MPa) osmotic potential than mature leaves and showed no seasonal osmotic adjustment, while mature, fully expanded leaves on the same shoots did adjust osmotically. Leaf osmotic potentials differed little among exposed leaves on a shoot once fully expanded.