Possible role of mannitol as an oxygen radical scavenger in olive

Along with mannose and sucrose, olive produces large amounts of mannitol in photosynthesizing leaves. Leaf mannitol shows large variation among olive genotypes and Sicilian cultivars with different leaf mannitol content were used in our trials to see whether mannitol may function as an additional non-enzymatic system to protect cells from oxygen radicals and photosystem damage. In one experiment, entire shoots of 'Passulunara', 'Castriciana', 'Nocellara del Belice', and 'Moresca' olive trees were taken to a greenhouse, and mature leaves were treated with paraquat solution, which generates oxygen radicals in presence of light. After 48 hours leaves were sampled, photographed and percent of necrosis was quantified by digital image analysis. Mannitol content was determined in control non-treated leaves positioned in the same node as the paraquat-treated ones. The same experiment was repeated using field-grown trees under full sun light. In both cases, a significant inverse relation was found between leaf necrosis and mannitol content suggesting that mannitol may provide some scavenging action on paraquat-generated oxygen radicals. In another experiment, leaf gas exchange and chlorophyll fluorescence were measured on field-grown 'Castriciana' (higher mannitol) and 'Nocellara del Belice' (lower mannitol) trees in the morning, at noon, and in the afternoon to quantify partitioning of absorbed energy among net photosynthesis (JCO2), photoprotection by heat dissipation (non-photochemical quenching, JNPQ) and by alternative electron transport and photorespiration (JNC), formation of reactive oxygen species (ROS) and heat re-emission inactive photosystem (PSII). 'Nocellara' reported higher JCO2 and JNC than 'Castriciana', whereas JNPQ was similar in the two cultivars. Despite a greater amount of energy for ROS formation, 'Castriciana' yielded a smaller percentage of inactive photosystems compared to 'Nocellara'. Also in this case mannitol may act as an additional oxygen radical scavenger and explain the differences in photosystem inactivation between the two olive cultivars.