Genetic stability of physiological and plant secondary metabolite induced responses to defoliation in a eucalypt

Plant responses to defoliation may include recovery through increased growth rates, changes in resource allocation patterns and changes in photosynthetic rates. Induced defensive chemical responses are also common in many plant genera but, to date, there is little evidence of such induction in the ecologically dominant and commercially important genera, Eucalyptus. Further, there is little knowledge as to whether genetic variation within species affects the level of herbivory-induced plasticity in either physiological or chemical traits. In this study we investigated the genetic basis to induced variation in physiological and defensive chemical responses of E. globulus seedlings to artificial defoliation. Two treatments (control and 50% of leaves removed) were applied to seedlings sourced from eight families each from three populations (eight families per population) representing contrasting constitutive chemical resistance to mammalian herbivory. For old growth and regrowth foliage, net CO₂ uptake and chlorophyll content was assessed pre- and post-defoliation periodically up to 12 weeks and defensive chemistry was assessed pre-defoliation and at 12 weeks. Results revealed distinct physiological responses to defoliation, and although inherent genetic differences were evident, the three E. globulus populations did not vary significantly in their response to foliage loss. Defoliated plants exhibited significant up-regulation of photosynthetic rate and increased chlorophyll content of the old growth. An important finding was the significant increase in the content of a number of foliar chemical compounds in the regrowth foliage. This study shows multiple response strategies of E. globulus to foliage loss; rapid regrowth through up-regulated physiological responses matched with induced chemical defences.