Response to multiple stressors in Pinus pinaster Ait.: conflicts between drought and herbivory stress with Hylobius abietis

Because of their huge size and longevity, pines are exposed to numerous pests and pathogens, and have to cope with periods of severe abiotic stress such as water scarcity. Climate models predict an increase in mean annual temperatures and a decrease in precipitation, resulting in longer and more intense periods of drought. Associated to global change, an increase of the incidence of pests and pathogens has also been predicted. Understanding how pines are able to cope, at the same time, with both biotic and abiotic stress is, thus, necessary to know how they will finally respond to global change. Pines are isohydric species whose main mechanism to prevent water losses is the closure of stomata. Closing stomata under water deficit may imply a decrease in the carbon uptake needed for growth, reproduction and defenses. Hormonal cascading in response to both biotic and abiotic threats may also interact among each other altering the ability to tolerate each single stress. The aim of this study is to test how drought stress can constrain pine defensive responses to insect herbivory. For this purpose, 8 month-old seedlings of Maritime pine (Pinus pinaster Ait.) originating from three different populations known to differ in their tolerance to drought (San Ciprián, Coca and Oria) were submitted to three levels of water scarcity (control, moderate and severe drought stress) during four months and then subjected to herbivory (control and the pine weevil Hylobius abietis L.) for five days. Then, seedlings were harvested and biomass allocation, water potential, 13C concentration, non-structural carbohydrates and chemical defensive composition were studied. Water treatments promoted drought stress, measurable by a less discrimination to 13C, a decrease in water potential and reduced seedling dry weight in all populations. Moreover, the insect herbivore inflicted less damage on water-stressed than on well-watered plants. Pine responses to insect damage included alterations of the