Physiological Responses of Two Olive Cultivars to Salt Stress

Abstract

The olive tree (Olea europaea L.) is the main fruit tree in most of the arid and semi-arid regions of Tunisia, which is where the problem of salinity is more pronounced. Salinity is one of the main factors that affects the productivity of olive trees, so the objective of this experiment was to study the effects of salinity on the photosynthesis, water relations, mineral status, and enzymatic activity of two cultivars of Olea europaea L., ‘Chemlali’ and ‘Koroneiki’. The trial was conducted under controlled conditions in a greenhouse for a period of 49 days and included two treatments: T0 control and T100 (irrigation with 100 mM of NaCl solution). Under salinity stress, the photosynthesis, stomatal conductance, and leaves of both cultivars were negatively affected. ‘Chemlali’ showed greater tolerance to NaCl salinity, based on a progressive decrease in osmotic potential (Ψπ) followed by a progressive and synchronous decrease in gs, without a comparable decrease in photosynthesis. The water use efficiency (WUE) improved as a result. In addition, the K+/Na+ ratio in ‘Chemlali’ rose. This appears to be crucial for managing stress. Conversely, enzymatic activity showed an accumulation of glutathione peroxidase (GPX) in stressed plants. The catalase (CAT) and ascorbate peroxidase (APX) content decreased in both stressed varieties. It can be concluded that the cultivar ‘Koroneiki’ is more susceptible to salt stress than the cultivar ‘Chemlali’, because the accumulation of GPX and the decreases in CAT and APX were more pronounced in this cultivar.

Tolerance Mechanisms of Olive Tree (<em>Olea europaea</em>) Under Saline Conditions

“…In most of the cultivars studied there is a decrease in CO2 assimilation with 100 mM NaCl [20,52,54,73,122,133,[137][138][139]. It has been reported that there is greater inhibition of CO2 assimilation rate in olive cultivars with high rates of photosynthesis and stomatal conductance [57,122,139,140]. In several studies it has been observed that photosynthesis inhibition is lower in the more tolerant cultivars compared to the more sensitive ones [57,122].…”

“…Salt stress tolerance is a multigenic trait in which different mechanisms are involved [48,49]. Olive is considered moderately tolerant to salinity [50], although differences in response to salt stress have been observed between cultivars [51][52][53][54][55][56][57].…”

“…The mechanisms of salt tolerance in olive are attributed to its ability to limit the flow of salt ions to sensitive shoot organs, which would accumulate in the root, as well as to its ability to maintain an adequate K + /Na + ratio in actively growing tissues [7,52,53,[58][59][60]. In addition, its tolerance to salt 6 stress can be improved through osmotic adjustment and reactive oxygen species (ROS) scavenging at the cellular level [21,48,56,57,[61][62][63][64].…”

“…Reduced growth under salinity stress is associated with reduced photosynthesis [52,54,105,132]. Studies with different cultivars have shown that photosynthetic rate decreased with increasing level of salt stress [7,20,[52][53][54]57,60,73,122,[134][135][136]. The effect of salinity on CO2 assimilation rate varies with the salt concentration, the cultivar, duration of the stress, and plant age [105].…”

“…It has been reported that there is greater inhibition of CO2 assimilation rate in olive cultivars with high rates of photosynthesis and stomatal conductance [57,122,139,140]. In several studies it has been observed that photosynthesis inhibition is lower in the more tolerant cultivars compared to the more sensitive ones [57,122]. Furthermore, in the salt tolerant cultivar 'Chemlali', a more rapid activation of the stress response was observed than in the moderately tolerant 'Koroneiki', which was manifested by a later inhibition of photosynthesis [57].…”