Olive is considered as a moderately salt tolerant plant, however, tolerance to salt appears to be cultivar-dependent and genotypic responses have not been extensively investigated. In this work, saline stress was induced in four olive cultivars: Arbequina, Koroneiki, Royal de Cazorla and Fadak 86. The plants were grown in 2.5 l pots containing 60% peat and 40% of pumice mixture for 240 days and were irrigated three times a week with half-strength Hoagland solution containing 0, 100 and 200 mM NaCl. The effects of salt stress on growth, physiological and biochemical parameters were determined after 180, 210, and 240 days of treatment. Saline stress response was evaluated in leaves by measuring the activity of GSH and CAT enzymatic activity, as well as proline levels, gas exchanges, leaves relative water content and chlorophyll content, and proline content. All the studied cultivars showed a decrease in Net Photosynthesis, leaves chlorophyll content and plant growth (mainly leaves dry weight) and an increase in the activity of GSH and CAT. In addition, the reduction of proline content in leaf tissues, induced an alteration of osmotic regulation. Among the studied cultivars Royal and Koroneiki better counteracting the effects of saline stress thanks to a higher activity of two antioxidant enzymes.

Salinity is one of the most impacting abiotic stresses regarding crop productivity and quality. Among the strategies that are attracting attention in the protection of crops from abiotic stresses, there is the use of plant biostimulants. In this study, Megafol (Meg), a commercial plant biostimulant, was tested on olive plants subjected to severe saline stress. Plants treated with salt alone showed substantial reductions in biomass production, leaf net photosynthesis (Pn), leaf transpiration rate (E), stomatal conductance (gs), and relative water content (RWC). In addition, samples stressed with NaCl showed a higher sodium (Na⁺) content in the leaves, while those stressed with NaCl and biostimulated with Meg increased the potassium (K⁺) content in the leaves, thus showing a higher K⁺/Na⁺ ratio. Salinity caused the accumulation of significant quantities of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) due to decreases in the activity of antioxidant enzymes, namely superoxide dismutase (SOD – EC 1.15.1.1), ascorbate peroxidase (APX – EC 1.11.1.11), guaiacol peroxidase (GPX – EC 1.11.1.9), and catalase (CAT – EC 1.11.1.6). When olive plants under saline stress were biostimulated with Meg, the plants recovered and showed physiological and biochemical traits much improved than salt stressed samples. Finally, Meg exhibited Ca²⁺-chelating activity in olive pollen grains, which allowed the biostimulant to exert this beneficial effect also by antagonizing the undesirable effects of hydrogen peroxide on Ca²⁺ metabolism.

Summary: In a number of compatible plant‐bacterium interactions, a rise in apoplastic Ca ²⁺ levels is observed, suggesting that Ca ²⁺ represents an important environmental clue, as reported for bacteria infecting mammalians. We demonstrate that Ca ²⁺ entry in Pseudomonas savastanoi pv. savastanoi ( Psav ) strain DAPP‐PG 722 is mediated by a Na ⁺ /Ca ²⁺ exchanger critical for virulence. Using the fluorescent Ca ²⁺ probe Fura 2‐AM, we demonstrate that Ca ²⁺ enters Psav cells foremost when they experience low levels of energy, a situation mimicking the apoplastic fluid. In fact, Ca ²⁺ entry was suppressed in the presence of high concentrations of glucose, fructose, sucrose or adenosine triphosphate (ATP). Since Ca ²⁺ entry was inhibited by nifedipine and LiCl, we conclude that the channel for Ca ²⁺ entry is a Na ⁺ /Ca ²⁺ exchanger. In silico analysis of the Psav DAPP‐PG 722 genome revealed the presence of a single gene coding for a Na ⁺ /Ca ²⁺ exchanger ( cneA ), which is a widely conserved and ancestral gene within the P. syringae complex based on gene phylogeny. Mutation of cneA compromised not only Ca ²⁺ entry, but also compromised the Hypersensitive response (HR) in tobacco leaves and blocked the ability to induce knots in olive stems. The expression of both pathogenicity ( hrpL , hrpA and iaaM ) and virulence ( ptz ) genes was reduced in this Psav ‐ cneA mutant. Complementation of the Psav ‐ cneA mutation restored both Ca ²⁺ entry and pathogenicity in olive plants, but failed to restore the HR in tobacco leaves. In conclusion, Ca ²⁺ entry acts as a ‘host signal’ that allows and promotes Psav pathogenicity on olive plants.

Selenium (Se) displays antioxidant properties that can be exploited, in plants, to counteract abiotic stresses caused by overly-produced reactive oxygen species (ROS). Here, we show that fertigation of maize crops with sodium selenate effectively protects pollen against oxidative stress. Pollen isolated from Se-treated plants (Se1) and untreated controls (Se0) was incubated in vitro with H₂O₂ to produce oxidative challenge. Given the impact of ROS on Ca²⁺ homeostasis and Ca²⁺-dependent signaling, cytosolic Ca²⁺ was measured to monitor cellular perturbations. We found that H₂O₂ disrupted Ca²⁺ homeostasis in Se0 pollen only, while Se1 samples were preserved. The same trend was observed when Se0 samples were treated with sodium selenate or Se-methionine, which recapitulated in vitro the protective capacity of Se-fertigation. Furthermore, we found that germination rates were much better retained in Se1 as compared to Se0 (46% vs 8%, respectively) after exposure to 20 mM H₂O₂. The same was observed with Se0 pollen treated with Se-methionine, which is the organic form of Se into which most fertigated sodium selenate converts in the plant. These results, together, show a close correlation between ROS, Ca²⁺ homeostasis and pollen fertility, and provide strong evidence that Se-fertigation is an excellent approach to preserve or enhance agricultural productivity.