Efeitos da herbivoria e condições ambientais sobre a instabilidade no desenvolvimento e resposta vegetativa em Tithonia diversifolia (Asteraceae)

Abstract

Herbivory, edaphic competition and shading can induce phenotypic variation, decrease performance and increase levels of fluctuating asymmetry (FA, a stress measurer) in invasive plants. The association of these can further increase the FA, also harming severely or disrupting phenotypic variations of plants. On the other hand, fertilization can increase FA, performance and decrease the negative impacts of herbivory and edaphic competition on weeds. However, there are doubts about the relationship of FA with some of these factors. Thus, we used the invasive Tithonia diversifolia (Asteraceae) in greenhouse experiments to verify how herbivory, shading, edaphic competition and fertilization can influence fitness and phenotypic variation of this species. In the first chapter, we analyzed how herbivory can cause changes in foliar nitrogen (type of induced defense) and FA of injured leaves. For this, we used seedlings of this species and separated them in group treatment, where we caused simulated herbivory damage on leaves; and the control group, which did not suffer injuries. The FA of these leaves was followed from the first to the fourth week of study, when the nitrogen was also verified. Our results showed that plants of the treatment group had higher FA on the fourth week when compared to control group; but nitrogen did not differ between treatments. In this way, herbivory can increase FA. In the second chapter, we associate for four weeks, herbivory with shade condition, as well as for edaphic competition and fertilization. We believe that herbivory, shading and competition decrease performance and increase FA. We further hypothesized that fertilization increases fitness and levels of FA of T. diversifolia, and reduces negative impacts of herbivory and competition. For the herbivory-shade experiment, the treatment group suffered two simulated herbivory damage on leaves, with individuals distributed between sun and shade. The plants were followed for four weeks. We also checked the leaves that came up after treatments. For herbivory, edaphic competition and fertilization, we removed 50% area from the leaves of the plants in the first and third week of study, along with and without fertilization and competition. The results of these experiments revealed that shading causes negative impacts on T. diversifolia, and increases leaf FA. This condition also caused phenotypic variation in shoot structures. Herbivory and edaphic competition did not influence phenotypic variations and FA, but the two factors in the second experiment negatively impacted this species. Fertilization increased plant fitness and FA, and decreased the negative effects of herbivory and competition. Herbivory associated with shading and edaphic competition did not intensify negative fitness nor influenced phenotypic variation responses. Overall, this study revealed that herbivory, when caused directly on leaves, can increase FA, as well as fertilization and shading. However, we observed that shading, edaphic competition and herbivory may adversely affect seedlings of T. diversifolia, however, fertilization increases the performance of this species, as well as reduced the negative impacts generated by these factors.