Responses to chilling of two Eucalyptus globulus clones with contrasting drought resistance.

The effect of chilling on growth and plant hydraulic properties in a drought-resistant clone (CN5) and a drought-sensitive clone (ST51) of Eucalyptus globulus Labill. was evaluated. Chilling (10/5°C, day/night) led to a general decrease in growth of both clones and significant reductions in root hydraulic conductivity, rate of photosynthesis and stomatal conductance in comparison to plants grown at control temperature (24/16°C). The drought-resistant CN5 clone maintained higher root growth and lower leaf-to-root-area ratio than the drought-sensitive ST51 clone, in both temperature treatments. Conversely, ST51 exhibited greater carbon allocation to the foliage and higher hydraulic conductance than clone CN5 at both temperatures. Plants of both clones, when acclimated to chilling, maintained a higher hydraulic conductivity than control plants exposed to chilling temperatures without acclimation. Under chilling, the main differences between clones were a higher water status and anthocyanin concentration in CN5 plants, and a stronger inhibition of root growth in ST51 plants. Except for roots, the hypothesis of a lower depression of growth rate in the drought-resistant clone under chilling was not verified. However, higher root growth under low temperatures, as observed in CN5, can be an advantageous trait in Mediterranean-type environments, protecting trees against summer water-stress.