Changes in morpho-physiological attributes of Eucalyptus globulus plants in response to different drought hardening treatments

Morpho-physiological attributes exhibited in response to drought hardening at the end of the growing season of Eucalyptus globulus Labill under nursery conditions were studied to evaluate the effect of three drought hardening treatments in morpho-physiological traits used as suitable indicators of drought hardiness, such as, plant growth, root growth potential, plant water relationships and survival. Freezing resistance of drought hardened plants was also studied in order to evaluate cross hardening effects in cuttings of Eucalyptus globulus Labill. Drought hardening consisted in induced water stress by watering restriction, until plant stem xylem water potentials (Psi(pd)) reached to-0.2, -1.3 and -2.4 MPa. Two water stress-rewatering cycles were applied during 54 days of treatment. The hardening treatments caused a significant reduction in plant height, leaf area, specific leaf area, plant, leaf, stem and root biomass. However, stem diameter was not affected. Root growth potential increased with the exposure to moderate water stress (-1.3 MPa). Drought hardening treatments have not effect on water relationship parameters such as saturation osmotic potential (Psi pi(sat)), volumetric module of elasticity (e), relative water content (RWCtlp) and osmotic potential (Psi pi(tlp)) at the turgor loss point. Only 1.7% and 6% of dehydrated dead plants were observed on treatments at -1.3 and -2.4 MPa respectively. Finally, the freezing damage index of leaves (LT50) was not significantly affected by drought hardening treatments. Furthermore, a reduction of 1.1 degrees C of supercooling capacity was observed at -2.4 MPa. As a conclusion, drought hardening is an important step of plants production programs during the final phase of nursery, because changes in morphological attributes caused by exposure to moderate drought, enable the plants to maintain the balance between transpiration and absorption areas and increase the capacity of plants to generate new roots.