Minimum Leaf Water Potential and Stomatal Closure in Citrus Leaves of Different Ages*

Minimum leaf water potentials at which stomata close were studied using citrus leaves in different stages of development throughout the year. Specific leaf weight and cuticle thickness were related to leaf age as well as to minimum leaf water potentials. Stomatal closure occurred consistently over a relatively narrow range of leaf water potential within each age class. Minimum leaf water potentials for stomatal closure of young leaves were as high as −1.6 MPa which coincides with previously measured leaf potentials where turgor reaches zero. Minimum leaf water potentials for mature leaves became as low as −3.5 MPa when leaves were 3–6 months old. This value is below that where leaf turgor is lost in mature leaves. It is possible that stomata of mature leaves remain open by maintaining higher guard cell turgor than mesophyll. The rate at which new leaves are produced and mature may be an important mechanism by which the tree can adjust to water stress.