Changes in non-structural carbohydrates in olive (Olea europaea) leaves during root zone salinity stress

Self-rooted olive ( Olea europaea L.) plants were grown in hydroponics at various NaCl concentrations (from 0 to 200m M ) for 28 to 32 days followed by 28 to 30 days of relief from salinity over two growing seasons. Olive leaves accumulated both glucose and mannitol during the period of salinity stress. The concentrations of fructose, myo -inositol, galactose, galactinol, sucrose, raffinose, and stachyose were not significantly affected by salinity. Starch content was decreased by salinity. The mannitol/glucose and mannitol/soluble carbohydrates ratios increased as the external NaCl concentration was increased, but returned to the control levels during the relief period. The increase in mannitol or glucose molar concentrations, expressed on a leaf tissue water basis, was partially due to a reduction in leaf tissue water content under salinity stress. However, an increase in mannitol concentration was also observed when expressed on a dry weight basis. The accumulation of mannitol in leaf tissue preceded any reduction in leaf area rate or net assimilation rate. The increase in leaf mannitol or glucose concentration was positively correlated with the increasing level of salinity at the root zone, but not with the accumulation of Na⁺ in the shoot. The role of mannitol. a potential osmoregulator in leaf mesophyll during salinity stress, is discussed in relation to the complex carbohydrate composition of olive leaves.