Influence of arbuscular mycorrhizal fungi on the functional mechanisms associated with drought tolerance in carob (Ceratonia siliqua L.)

Arbuscular mycorrhizal symbiosis strengthened the mechanisms developed by carob to withstand drought stress, including improved water relations, increased cell wall rigidity and osmolytes accumulation, and enhanced oxidative stress alleviation. The present investigation was carried out to provide more insight into the influence of arbuscular mycorrhizal fungi (Funneliformis mosseae, Rhizophagus fasciculatus, and Rhizophagus intraradices) on drought tolerance of carob. Non-mycorrhizal (NM) and arbuscular mycorrhizal (AM) carob plants were subjected to two watering regimes, 75% of field capacity (well water) or 25% of field capacity (water stress). Obtained results showed that stressed AM plants exhibited increased performance in terms of growth and biomass production, water and nutrient acquisition, and oxidative stress alleviation compared to NM plants. In fact, under limited water regime, AM plants maintained high stomatal conductance and high relative water content (over 94%) due to their high water and nutrient uptake efficiency. Moreover, AM plants especially those associated with F. mosseae maintained high membrane integrity (over 80%), high cell wall rigidity, and high leaf water potential and osmotic potential at full turgor and at turgor loss, while these parameters steeply decreased in NM plants. Furthermore, drought-stressed AM plants showed decreased hydrogen peroxide and malondialdehyde contents associated with increased activities of superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, and catalase compared to their relative NM plants. Thus, AMF strengthened the mechanisms involved in drought tolerance of carob by improving water relations, increasing cell wall rigidity, and enhancing oxidative stress alleviation. Funneliformis mosseae was the most effective in improving carob tolerance to drought stress.