Clonal differences in ecophysiological responses to imposed drought in selected Eucalyptus grandis×E. urophylla hybrids.

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Measuring ecophysiological responses of Eucalyptus clones grown under reduced water availability could assist in clonal selection for climate resilience. We hypothesized that clonal variation in chlorophyll a fluorescence was more readily detected than variations in leaf-level gas-exchanges when two-year-old E. grandis x E. urophylla hybrid clones (C1, C2, C3, and C4) were grown under rainfed (RF) and water-restricted (WR) conditions were evaluated during dry and rainy seasons, in the morning and midday diurnal periods. The C2 clone was the most drought tolerant as it had a similar net CO2 assimilation rate (A) considering the RF and WR conditions at midday during the dry season, while C1, C3, and C4 CO2 assimilation rates (A) decreased by 29.1 %, 28.3%, and 13%, respectively. This response was associated with a reduction to a lesser extent in leaf water potential, stomatal conductance (gs) and transpiration rates (E) (ca. 10%, 30% and 13% under WR, respectively), when compared to the other clones during the dry season at midday. The lower leaf to air vapor pressure deficit of C2 contributed to its greater water use efficiency (WUE), resulting in greater total dry mass gain. C1, C3 and C4 were less drought tolerant, decreasing gs, E and especially A under WR, resulting in lower WUE and total dry mass gain. Chlorophyll a fluorescence indexes were better indicators of drought tolerance compared to gas-exchange parameters in definition of drought tolerance of clonal Eucalyptus. Three drought sensitive clones showed low photochemical efficiency under WR, with the electron transport rate being impaired between photosystems II and I, indicated by the greater changes in photosynthetic performance index (PIabs). Under WR conditions, Fv/Fm, Ψ0, ΦE0, and PIabs decreased in all clones while ΦD0 and DI0/CS0 increased, with C2 showing the most stable responses suggesting that the photochemical apparatus was the less damaged by drought. Thus, C2 was the best clone for regions with water scarcity.