Influence of saline drip-irrigation on fine root and sap-flow densities of two mature olive varieties

Salt stress is known to influence water use and carbon allocation in trees; however, information about the effects of salt exposure on water uptake and below-ground carbon investment is scant, especially for adult trees. Consequently, this study examined these variables in two mature olive varieties (Olea europaea L.) that differ in their NaCl tolerance: Barnea (tolerant) and Proline (sensitive). Trees were irrigated using water with electrical conductivities of 1.2, 4.2 (both varieties) and 7.5 dS m−1 (Barnea only) for 11 years. At each treatment level, we measured soil properties, root morphology, root biomass:necromass ratio, root xylem sap osmolality, and root sap-flow as well as leaf conductance and morphology. Both varieties exhibited reduced fine root biomass under salinity which was only partially compensated for by higher specific root areas under moderate salinity. Proline variety exhibited a smaller fine root system under moderate salinity than Barnea trees, likely causing the lower sap-flow density in coarse roots of Proline compared to Barnea. The higher biomass:necromass ratio of the Barnea root system under moderate salinity is indicative of lower root turnover rates and thus a more efficient carbon use than in Proline trees. Besides differences in ion exclusion capacities, the ability of the fine root system to resist the deleterious effects of salinity seems to affect the salt resistance of mature olive varieties by influencing water uptake and carbon allocation.

Research highlights
► Under salinity salt-tolerant olive trees have an higher fine root biomass than salt-sensitive trees.
► Under salinity salt-resistant olive trees sustain an higher root sap-flow density than salt-sensitive trees.
► Salt-sensitive olive trees possess a lower root biomass:necromass ratio under salinity than salt-tolerant ones.
► The ability of fine roots to resist the deleterious effects of NaCl affects the salt resistance of mature olives varieties.