Drought effects on water use efficiency, freezing tolerance and survival of Eucalyptus globulus and Eucalyptus globulus x nitens cuttings

Genetic improvement of Eucalyptus genotypes for drought and frost resistance is essential for successful intensive management of commercial plantations. Understanding the physiological mechanisms that relate water use and frost resistance for highly deployed genotypes may allow for better prediction of their future performance, genetic selection and seedling management for site specific purposes. We studied whether instantaneous water use efficiency (WUE (i) ) may serve as drought, freezing and photoinhibition tolerance predictor by studying its response on six E. globulus clones (Eg1-Eg6) and four E. globulus x E. nitens hybrid seedlings (Egn1-Egn4) under drought and irrigated (control) treatments. Net photosynthesis (A) and transpiration (E) were studied using a gas exchange system in order to calculate WUE (i) (A/E). Simultaneous chlorophyll a fluorescence measurements were performed to assess the non photochemical quenching components. Frost tolerance of plants under control and drought treatments were evaluated by measuring temperatures that exert 50% photoinactivation of photosystem II. Finally, drought tolerance was evaluated by plant survival within each genotype after rehydration. Our results showed significant genotype variability in the rate of soil and xylem water potential decrease during drought. While most of the genotypes reached -4.0 MPa in about 35 days of drought, genotypes Eg6 and Egn4 required 56 days of drought to reach this xylem water potential. WUE (i) exhibited significant differences among genotypes and irrigation treatments. Genotypes Eg5 and Egn4 increased their WUE (i) between 70 and 80% after drought. This was associated with a more conservative control of water loss at the stomatal level combined with maintenance of relatively higher rates of net photosynthesis than the other genotypes under drought conditions. Plants exposed to drought were more freezing tolerant than control plants, having in average 3A degrees C lower LT50 than well irrigated ones. There was no a clear correlation between WUE (i) and drought tolerance or drought-induced photoinhibition, however WUE (i) was inversely correlated with LT50. Our results suggest that WUE (i) is not suitable by itself to select drought tolerant genotypes, but may provide evidence for discarding drought sensitive genotypes. In addition, it could provide valuable information to select for freezing tolerance.