Water Relations in Apple Seedlings: Changes in Water Potential Components, Abscisic Acid Levels and Stomatal Conductances under Irrigated and Non-irrigated Conditions1

Stomatal conductance, abscisic acid levels, and water potential components (water, osmotic, and rurgor potentials) were measured in irrigated and non-irrigated open pollinated seedlings of ‘Northern Spy’ apples ( Malus domestica Borkh.). Although non-irrigated seedlings typically displayed water potentials of 0.2 to 0.6 MPa lower than those of irrigated seedlings, turgor potentials remained comparable in both groups, Because diurnal osmotic adjustment was also greater for non-irrigated seedlings. Abscisic acid (ABA) levels in the leaf increased linearly in response to changes in leaf turgor, rather than water potential. Stomatal conductance was independent of bulk leaf ABA levels and was poorly correlated with leaf turgor potential above a critical value of 0.7 MPa.