The Effect of Proline and Salt Stress on Growth Characteristics of Three Olive Cultivars at Three Different Stages of the Growing Season

Proline is a vital amino acid, commonly distributed in all plants. It is widely accumulated in plants under salt stress. It has been suggested that foliar application of proline has an important effect in reducing destructive effects of non-living stresses on plants. On the other hand, an excessive amount of free proline has negative effects on cell growth as well as protein functions. In this study, the six-months-old plantlets of three olive cultivars, including Arbequina, Arbosana and Koroneiki were sprayed with proline at 0, 100, and 200 mg/L for three times in intervals of 10 days. In addition, the samples were subjected to salinity at 0, 50, 100 and 200 mM sodium chloride for five months. Measurement of morphological characteristics of stems and leaves was conducted in three stages (4, 12 and 20 weeks after treatment). The results showed that stem length and number of nodes gradually increased over time at all concentrations of proline. Furthermore, at stage 3, stem diameter, number of leaves and branch number increased and leaf width decreased. The highest leaf thickness was observed at stage 1. However, no significant difference was found among the proline concentrations in the mentioned traits in any experimental stage. Plants sprayed with proline were later encountered the increased leaf necrosis. At stage 3, the control plants had a lower percentage of abscission than proline-treated plants. At stage 2, plants sprayed with proline had lower leaf thickness than control plants. Throughout the experiment, salinity, especially 200 mM, reduced cumulative stem length, number of shoots, internode length, number of nodes and number of leaves. The highest percentages of leaf abscission and necrosis, as well as the highest leaf thickness were observed at 200mM NaCl treatment. In general, despite the fact that proline increased in the plants under stress conditions, its external application was not significantly effective.