Root structural plasticity enhances salt tolerance in mature olives

Roots are the first plant organ to encounter, sense, and respond to soil salinity. Like for many moderately salt tolerant species, roots of olive (Olea eurpaea) trees are the principal players in salt tolerance. We studied roots of mature olive trees in order to illuminate the yet vague mechanism(s) of root salt exclusion. Root structural traits were examined in olive trees grown in lysimeters and from a long-term salinity field trial. The distribution of salts was detected in root cross-sections using scanning electron microscopy combined with energy dispersive spectroscopy. Increased soil salinity caused a continuous decline in specific root length and an increase in root diameter, periderm thickness, and specific hydraulic conductivity. While periderm cell number was not modified, cell diameter increased linearly by a total of 50 % as root zone salinity increased nearly eight-fold. Strong compartmentation of Na within the periderm was found, with highest content in the outer periderm layer and lowest content in the stele. Increase in soil salinity was associated with decrease in stele area, yet, hydraulic conductivity was doubled due to higher fraction of large diameter xylem vessels. Our results indicate significant root structural plasticity, which underlies the salt resistance mechanisms. The dominant trait enabling salt tolerance of olive trees is the enlarged periderm serving as an apoplastic barrier that enhances salt compartmentation and exclusion from the vascular tissues.