Plant Growth Regulator Treatments Differentially Modulate the Responses of ‘Picual’ Olive Plants to Salinity

Salinity limits olive establishment and productivity, requiring strategies to improve acclimatisation. We evaluated treatments with five plant growth regulators (PGRs): gibberellic acid (GA3), indole-3-acetic acid (IAA), salicylic acid (SA), kinetin (KIN) and spermidine (SPD), in one-year-old ‘Picual’ plants exposed to 0, 100 and 200 mM NaCl. Salinity reduced above-ground growth, water status, pigment levels and photosynthesis, favouring biomass allocation to roots. At 100 mM NaCl, SA provided the greatest photosynthetic protection: it maintained net CO2 assimilation (A) and increased total plant dry weight (TPDW) and pigment content compared with plants without growth regulators at the same salinity. At 200 mM NaCl, IAA increased TPDW whilst maintaining leaf growth, A, and water- and nitrogen-use efficiencies. These benefits coincided with higher ascorbate peroxidase activity, higher proline concentrations and high phenolic contents, and lower ethylene production. GA3 preserved pigments and photosynthetic activity, KIN promoted leaf growth, and SPD increased root biomass. Descriptive estimates of the NaCl concentration corresponding to 50 % of reference A (C50) were higher with PGRs; neither reference threshold was reached with IAA or GA3 within 0–200 mM NaCl. Overall, the treatments promoted complementary responses, with SA standing out under moderate salinity and IAA under severe salinity.