Impact of drought stress induced by polyethylene glycol on growth, water relations and cell viability of Norway spruce seedlings.

We investigated physiological responses of 7-week-old Norway spruce seedlings to water deficits of different intensities. Hydroponically grown seedlings were subjected to mild (-0.15 MPa), strong (-0.5 and -1.0 MPa) and extreme (-1.5 MPa) water deficit induced by polyethylene glycol 6000, and their growth parameters, water status and physiological activity were analyzed. Seedlings effectively restricted water loss under drought, and even under extreme water deficit, shoot relative water content did not fall below 85%. Water stress induced substantial decreases in the osmotic potentials of root and needle cell sap, up to 0.3-0.4 MPa under extreme water deficit, though this did not result from water loss or accumulation of K and Na ions. Seedling growth was very susceptible to water stress because of poor capacity for cell wall adjustment. Water stress injured seedling roots, as evidenced by the loss of root cell physiological activity estimated by the ability to hydrolyse fluorescein diacetate and by increased root calcium content up to 8-10-fold under extreme water stress. At the same time, root hair growth was enhanced, especially under mild water deficit, which increased the root water-absorbing capacity. In summary, seedlings of Norway spruce were characterized by high susceptibility to water stress and concurrently by pronounced ability to maintain water status. These characteristics are fully consistent with spruce confinement to moist habitats.