Drought tolerance of clonal Malus determined from measurements of stomatal conductance and leaf water potential

We examined tolerance to soil drying in clonally propagated apple (Malus domestica Borkh.) rootstocks used to control shoot growth of grafted scions. We measured leaf conductance to water vapor (g(L)) and leaf water potential (Psi(L)) in a range of potted, greenhouse-grown rootstocks (M9, M26, M27, MM111, AR69-7, AR295-6, AR360-19, AR486-1 and AR628-2) as the water supply was gradually reduced. Irrespective of the amount of available water, rootstocks that promoted scion shoot growth (M26 and MM111) generally had higher g(L) and more negative Psi(L) than rootstocks that restricted scion shoot growth (M27 and M9). After about 37 days of reduced water supply, there were significant decreases in g(L) and Psi(L) in all rootstocks compared with well-watered controls. In all treatments, the slope of the relationship between log (g(L)) and Psi(L) was positive, except for rootstocks AR295-6, AR628-2 and AR486-1 in the severe-drought treatment, where the drought-induced change in the relationship suggests that rapid stomatal closure occurred when leaf water potentials fell below -2.0 MPa. This drought response was associated with increased root biomass production. Rootstock M26 showed little stomatal closure even when its water potential fell below -2.0 MPa, and there was no effect of drought on root biomass production. We conclude that differences among rootstocks in the way that g(L) and Psi(L) respond to drought reflect differences in the mechanisms whereby they tolerate soil drying. We suggest that these differences are related to differences among the rootstocks in their ability to control shoot growth.