A comparative study of the hormonal response to high temperatures and stress reiteration in three Labiatae species

Mediterranean plants are usually exposed to a combination of stresses, which may occur simultaneously or at different times throughout their life. Here, the hormonal response to high temperatures was compared in plants of three Labiatae species, including rosemary (Rosmarinus officinalis L.), sage (Salvia officinalis L.) and lemon balm (Melissa officinalis L.). Plants exposed to increasing temperatures for 5 days were subsequently exposed to heat stress and compared to plants experiencing heat stress for the first time (controls). Despite the three species showing a similar tolerance to a single heat stress event, stress recurrence had a different impact on each species. Lemon balm was the most sensitive species to stress reiteration, showing decreased relative water content upon heat stress repetition, together with enhanced levels of α-tocopherol and salicylic acid (SA). Some acclimation responses were observed in rosemary and sage, including improved water contents and reduced jasmonic acid levels in rosemary, and reduced abscisic acid (ABA) and malondialdehyde (MDA) levels in sage plants previously exposed to high temperatures. Furthermore, the response of plants to a combination of heat stress and water deficit was evaluated in plants previously exposed to heat stress and compared to controls. Rosemary and sage were much more resistant than lemon balm, which died when stresses were combined. Despite stress pre-exposure not having any effect on plant performance in terms of Fv/Fm, MDA and relative water content in rosemary and sage, it resulted in higher α-tocopherol levels in both species. The hormonal response differed between species: while the hormonal content did not change in sage, rosemary showed increased ABA and decreased SA levels as a result of repeated stress exposure. Overall, different stress imprints particular to each species and stress scenario were found in α-tocopherol and hormone levels, which led to similar protective effects in rosemary and sage.