The effect of simulated damage by weevils on Quercus ilex subsp. Ballota acorns germination, seedling growth and tolerance to experimentally induced drought

The main aim of this study is to assess the effect of moderate to low level of cotyledon damage (simulated weevils infection) on holm-oak seedling growth and physiological performance under conditions of soil water stress, a recurrent constraint in Mediterranean and other seasonally dry environment. Three levels of artificial damage were applied to the acorns (no damage NoD, low damage LD, and medium damage MD), and the germination, emergence and early seedling growth under controlled conditions were studied during ca. 1.5 months. On the other hand drought effect on seedling growth, leaf gas-exchange, PSII efficiency, photosynthetic pigments and electron transport energy fluxes was analyzed in a set of older seedlings (i.e., 6.5 months old) that also derived from treated acorns and were exposed to two irrigation treatments (well water WW and water stress WS) for 1.5 months. The results showed that LD and MD acorns germinated earlier than NoD ones but final seedling emergence was lowest in the MD treatment. Cotyledons exhibited increased level of necrotic tissue as physical damage (drilling) increased while seedling biomass and size-related traits tended to decrease. Under WS conditions seedlings derived from LD and MD exhibited higher decrease in above and belowground biomass, as well as on net photosynthetic rate (AN), stomatal conductance (gs), intercellular CO2 concentration (Ci) and PSII efficiency than those derived from NoD. These differences were much less conspicuous under WW treatment where all traits reached higher values. In view of the interactive effect of drought and acorn damage it is concluded that under natural conditions acorn infection by weevils and other insects may represent a highest limitation to holm-oak seedling recruitment than previously considered. In addition this limitation may become more important in the future whether warming and drought increase.