Boron as mitigator of drought damage in Eucalyptus: a genotype-dependent mechanism?

The identification of morphological-physiological traits associated with drought resistance is extremely important for the selection of the most drought-tolerant Eucalyptus genotypes. Aside from the intrinsic characteristics of the species, external factors such as boron (B) supplementation can raise tolerance. This work aimed to evaluate the effects of water deficit on two Eucalyptus clones with differential drought tolerance, and the potential of B to mitigate the damage. Plants of clones i144 (tolerant) and gg100 (sensitive) were grown in soil, in a greenhouse, with and without water and B restriction. We evaluated the photosynthetic rates, the leaf water potential (psi w(leaf)), and the growth. In both clones, psi w(leaf) decreased drastically when soil moisture reached 65 % of the field capacity, regardless of B supply. The main mitigating effect of drought impacts of B on the tolerant clone i144 was to stimulate root and reduce leaf biomass production, optimizing water uptake and reducing transpiration losses. In both clones, gas exchange was affected by drought, but no differential responses were observed after B supplementation. In clone gg100, the CO2 assimilation rate and instantaneous water use efficiency were significantly reduced, whereas in clone i144 the intrinsic water use efficiency increased considerably. The transpiration rates dropped in both clones, which it was more evident in clone i144, as a result of the reduced stomata conductance. The electron transport rate was reduced and the effective quantum yield of photosystem II decreased in both clones when exposed to drought. In clone i144, boron attenuated the dissipation of thermal energy in the control and drought-stressed plants, which caused the values in the latter to become equal to those of the control plants. We concluded that the drought effects as well as the damage mitigation potential of B vary among genotypes, and that the tolerant clone i144 benefitted the most from B supplementation.