Hydraulic conductivity and contribution of aquaporins to water uptake in roots of four sunflower genotypes

Background: This article evaluates the potential of intraspecific variation for whole-root hydraulic properties in sunflower. We investigated genotypic differences related to root water transport in four genotypes selected because of their differing water use efficiency (JAC doi: 10.1111/jac.12079. 2014). We used a pressure-flux approach to characterize hydraulic conductance (L₀) which reflects the overall water uptake capacity of the roots and hydraulic conductivity (Lp_r) which represents the root intrinsic water permeability on an area basis. The contribution of aquaporins (AQPs) to water uptake was explored using mercuric chloride (HgCl₂), a general AQP blocker. Results: There were considerable variations in root morphology between genotypes. Mean values of L0 and Lpr showed significant variation (above 60% in both cases) between recombinant inbred lines in control plants. Pressure-induced sap flow was strongly inhibited by HgCl2 treatment in all genotypes (more than 50%) and contribution of AQPs to hydraulic conductivity varied between genotypes. Treated root systems displayed markedly different L₀ values between genotypes whereas Lp_r values were similar. Conclusions: Our analysis points to marked differences between genotypes in the intrinsic aquaporin-dependent path (Lp_r in control plants) but not in the intrinsic AQP-independent paths (Lp_r in HgCl₂ treated plants). Overall, root anatomy was a major determinant of water transport properties of the whole organ and can compensate for a low AQP contribution. Hydraulic properties of root tissues and organs might have to be taken into account for plant breeding since they appear to play a key role in sunflower water balance and water use efficiency.