Effects of NaCl salinity on growth, ion content and CO₂ assimilation rate of six olive cultivars

Abstract

The effects of NaCl salinity on the growth, ion content and gas exchange of six olive (Olea europaea L.) cultivars (Koroneiki, Mastoidis, Kalamata, Amphissis, Kothreiki and Megaritiki) were studied. The plants were grown in 8.5 l pots containing sand–perlite mixture (1:3) for 5 months and were irrigated with half-strength Hoagland solution containing 0, 25, 50, 100 and 200 mM NaCl. Shoot length was reduced significantly above 25 mM for cvs Koroneiki, Kalamata and Megaritiki and above 50 mM NaCl for the others. Total plant leaf area was reduced significantly above 25 mM NaCl, reaching 85% at 200 mM NaCl for the cvs Mastoidis and Amphissis, due to defoliation. At high salinity the aerial part of the plants was more depressed than the root. The concentration of Na and Cl (% d.w.) was higher in roots than in other parts of the plant (shoots and leaves), and increased with the increase of salinity. In Kalamata, leaf Na and Cl concentration was very low for all salinity treatments. Salt injury symptoms (leaf tip burning) appeared in Koroneiki, Amphissis and Mastoidis at 100 mM NaCl, becoming more severe at 200 mM due to defoliation. The cultivars Kothreiki and Megaritiki at 200 mM NaCl exhibited only a few symptoms, while in Kalamata no any injury was observed. Assimilation rate of CO₂ and stomatal conductance were significantly reduced at high salinity levels in all cultivars. Gas exchange in the leaves was not highly correlated with chloride and sodium contents in all cultivars. Kalamata showed higher resistance to salinity, followed by Megaritiki and Kothreiki. Inferences on the mechanisms of action of salt on olive trees are discussed.

Introduction

Water scarcity in the Mediterranean basin appears as one of the main factors limiting agricultural development, particularly in the 2000–2025 period. During the next 25 years, although irrigated areas will increase, sustainable quantities of fresh water supplies will be diverted from agriculture to meet the growing water demand in the municipal and industrial sectors in the region (Hamdi et al., 1995, Correia, 1999). To overcome water shortages and to satisfy the increasing water demand for agricultural development, the use of marginal quality waters (brackish, reclaimed, drainage) will become necessary in many countries. However, the use of saline water for irrigation requires an adequate understanding of how salts affect soil characteristics and plant performance.

Olive tree is one of the major crops in Mediterranean region and its cultivation is continuously being extended to irrigated land. In most coastal areas, in which olive is cultivated, the increased need for good quality water for urban use, set limits or restricts the use of fresh water for irrigation. On the other hand, in those areas large quantities of low quality water—mostly saline—are available, which can and should be used for olive tree irrigation, since olive is considered as moderately tolerant to salinity (Mass and Hoffman, 1977, Rugini and Fedeli, 1990). Recent studies suggest that olives can be irrigated with water containing 3200 ppm of salt (ECw of 5 dS/m) with a SAR of 18, producing new growth at leaf Na levels of 0.4–0.5% d.w. (Al-Saket and Aesheh, 1987, Tattini et al., 1992). Therios and Misopolinos (1988) reported that 3-year-old olive plants did not suffer salt stress at NaCl concentrations lower than 80 mM during a 90-day culture period. Irrigation water with 8 g/l NaCl has been found to be the tolerance limit for olive trees (Rugini and Fedeli, 1990).

Tolerance to salt appears to be cultivar-dependent. Genotypic responses of olive to NaCl salinity has not been extensively investigated, and only recently some work have been published (Therios and Misopolinos, 1988, Benlloch et al., 1991, Tattini et al., 1992). Mechanisms of salt tolerance in olives are likely due to control of net salt import to the shoot. The mechanism is located within the roots and prevents salt translocation, rather than salt absorption (Benlloch et al., 1991, Tattini, 1994). It is probable that K–Na exchange at the plasmalemma is involved in the regulation of transport of Na to the shoot. Furthermore, the salt tolerance mechanism may be related to the capacity of olive to accumulate salt in the leaf vacuoles (Loreto and Bongi, 1987). The photosynthetic characteristics of salt-stressed olives did not change provided that Cl concentration was lower than 80 mM in total tissue water (Bongi and Loreto, 1989). Above this threshold, however, a reduction in photosynthesis and stomatal closure occurred and plant morphology was altered.

Although, there are interesting papers dealing with salt tolerance of olive cultivars (Tattini et al., 1992, Marin et al., 1995), only two refers to salt tolerance of Greek olive cultivars (Anagnostopoulos et al., 1955, Therios and Misopolinos, 1988). The objective of the present study was the comparative study on NaCl salinity tolerance and the photosynthetic performance in major Greek olive cultivars, since conditions such as those mentioned above (hot and dry climate) are likely to prevail in many semi-arid areas in Greece and especially in the island of Crete.