Effects of salinity and N on the growth, photosynthesis and N status of olive (Olea europaea L.) trees

The effects of NaCl salinity and N on the growth, ion concentrations and photosynthesis (Pn) in three olive (Olea europaea L.) cultivars (Mission, Manzanillo and Zard) were investigated. The experiment was conducted with four NaCl levels (0, 50, 100, and 150 mM) factorially combined with three N levels (100, 200, 300 mg l−1) as NH4NO3 by adding to the half strength of Hoagland solution. The plants were grown in 121 pots filled with sand and perlite mixture (1:1) for 12 months. Salinity had a statistically significant negative effect on all responses considered, although the severity of the effect varied among the cultivars. High concentration of salinity (150 mM) decreased the leaf area by 63%, 68% and 76% in cvs Mission, Manzanillo and Zard compare to that in control (Na0N100). The Plants growth were improved at 200 mg l−1 N concentration in cvs Manzanillo and Zard, but it was reduced when the N concentration increased up to 300 mg l−1. The growth of Mission progressively increased with the increasing both salinity and N levels. Both Pn and transpiration rate were significantly reduced by the increase of salinity in all three cultivars. Increasing of N levels in the solution had no effect on Pn in various salinity levels in cvs Mission and Manzanillo but in Zard cultivar Pn reduced as N level increased. Salinity in the root zone led to a significant decrease in both K concentration and K/Na ratio in the leaves in all olive cultivars regardless of the N levels. Within each N level, leaf Na concentration increased and K concentration decreased as salinity concentration in the root zone increased from 50 to 150 mM. Nitrate reductase (NR) activity in 150 mM treated plants decreased by 27%, 58% and 52% in cvs Mission, Manzanillo and Zard, respectively. The decreased activity of NR by the increased NaCl was accompanied by a decrease in total N and nitrate uptake. The deleterious effects of salinity on the plants growth appeared to be as the result of the reduction in Pn, K/Na ratio and NR activity in the salinity treated plants. It can be concluded that under salinity conditions increasing N concentration up to 200 mg l−1 in salt-sensitive cultivars to salinity is favorite in counteracting the adverse effects of salinity but the further increase of N concentration (300 mg l−1) may be ineffective or even harmful for the olive trees growth. In salt-tolerant cultivars increasing N fertilization can be an effective tool to restore the decreased growth caused by high salinity.