Physiological plasticity in eucalyptus clones in the vegetative stage contributes to drought tolerance

With the expansion of eucalyptus crops to areas with severe water limitations, physiological studies involving eucalyptus clones to identify those that are tolerant to water stress become important. The objective of this study was to assess morphological and physiological responses by eucalyptus clones subjected to drought stress and rehydration. The experiment consisted of three eucalyptus clones: VC865, I224 and I144 and two water regimes: control and water stress followed by rehydration, with six replicates. Leaf water potential, gas exchange, maximum quantum efficiency of photosystem II and plant height and stem diameter were evaluated under drought stress and rehydration. After 6 d of rehydration, the number of leaves, leaf area and dry mass of root, leaf, stem and their total were evaluated. All clones showed intense reduction of gas exchange during the drought stress period, and only VC865 and I144 showed rapid recovery with 3 d of rehydration. Clone I224 showed greater reduction in height, stem diameter, number of leaves, water potential at midday (Ψw Midday), and maximum quantum efficiency of photosystem II (Fv/Fm). Clones VC865 and I144 showed lower reductions in Ψw Midday and Fv/Fm under stress. VC865 had lower reductions in leaf number, leaf area and higher leaf dry mass, while clone I144 had higher height and lower reduction in root dry mass under. Both these clones showed higher water use efficiency with 3 d of rehydration. These different phenotypic plasticities gave the clones VC865 and I144 efficient mechanisms of acclimatization to stress and more drought tolerance, enhancing their greater capacity for recovery after stress, which allowed lower dry mass reduction. Clone I224, however, was more susceptible to drought stress, undergoing greater physiological damage with only partial recovery during rehydration.