Influence of arbuscular mycorrhizal fungi inoculation on the control of stomata functioning by abscisic acid (ABA) in drought-stressed olive plants

Abscisic acid (ABA) is the key phytohormone modeling the stomata behavior under drought conditions. However, stomata closure is not correlated with leaf ABA content in mycorrhizal olive plant. The aim of the present study is to evaluate the impact of arbuscular mycorrhizal (AM) symbiosis on the control of stomata functioning by ABA in two olive tree (Olea europaea L.) cultivars ‘Zarrazi’ and ‘Meski’ subjected to dehydration–rehydration treatment. AM-inoculated (Myc +) and non-inoculated (Myc-) olive plants were subjected to water stress and then rewatered (recovery). Leaf ABA content, stomatal conductance and transpiration rate were measured in: (1) irrigated control, (2) moderately and severely stressed and (3) recovered plants. In both Zarrazi and Meski Myc– plants, ABA content increased in parallel with drought intensity and stomatal closure. However, an intra-specific variability appeared in inoculated plants; in Meski Myc + plants ABA content was not influenced by drought and recovery treatments, whereas in Zarrazi Myc + plants the ABA amount increased under moderate water stress and even further after water relief, but decreased under severe water stress. However, in Myc + plants of both cultivars, stomatal closure was not correlated with leaf ABA content. The results reveal that the ABA is not the key factor controlling the stomatal closure in AM-inoculated olive plants under drought conditions. In fact, other AM-related factors are involved in the control of stomata regulation in mycorrhizal olive plants exposed to severe drought. These factors act specifically in the drought-resistant cultivar ‘Zarrazi’ permitting a suitable stomata behavior.