Impact of leaf infestation by herbivorous insects on the elemental uptake of citrus trees

The elemental compositions of healthy and infested leaves of three species of lemon trees (Citrus aurantifolia, Citrus aurantium and Citrus medica) were compared to determine the impact of herbivorous insects on their uptake potentials. Systematically higher in the infested leaves than in the healthy, the total contents of major elements confirm a stress effect. The total amount of up-taken elements is increased by 8.2% in C. aurantium and by up to 30.9% in C. medica. Nutritive elements such as P and K decreased in C. aurantium, while they increased in the two other species by 5.3%, which is not representative analytically, and 61%. These notably different uptakes were obtained for two tree species infested by the same type of insect, demonstrating in turn that the type of insects does not determine the tree behavior. In turn, the elemental uptake by C. aurantium is almost systematically to the opposite of those of the two other species, which confirms that the defense process of the trees studied here is not insect specific but plant dependent. The different uptakes observed under defense stress do not suggest a standard behavior, reflecting a heterogeneous distribution of the major elements in the tree leaves. This interpretation is supported by the distribution of the rare earth elements of the infested leaves relative to that of the healthy equivalents. The varied patterns of the rare earth elements confirm that the trees do not develop a unique defense reaction. They display: (1) a flat distribution in C. medica without any fractionation induced by the insects, (2) a distribution characterized by an increased content in heavy rare earth elements in Citrus aurantifolia and (3) a pattern with a partially flat distribution but a significant decrease in the Ce content.