Expansion Rates and Defenses in Young Leaves of Tropical Species

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Plants have evolved a battery of physical and chemical defenses which protect them against grazing damage (herbivory). Most of our understanding of herbivory and plant defenses derives from research on mature leaves. However, young expanding leaves are the most vulnerable stage during the life of a leaf. Eighty percent of the lifetime damage occurs during this short window, because expanding leaves suffer 10-100 times the rates of damage from fungi and herbivores as mature leaves. We are interested in understanding the effectiveness of young leaf defenses, and the interactions among defenses. In particular, we hypothesize that rapid leaf expansion, which reduces the vulnerable period, is incompatible with other defenses such as toughness, chemical defenses and greening. Although chemical defense and rapid expansion are the two most effective defenses for young leaves, synthesis of chemicals during leaf expansion may cause auto- toxicity as well as compete for resources used in rapid leaf construction. Species which have slowly expanding young leaves may therefore rely more extensively on effective chemical defense. We will characterize the chemical defense profiles of fast- and slow-expanding young leaves in rainforest plants, and quantify the effectiveness of these chemicals as defenses against insect herbivores and fungal pathogens. In addition to giving us insight into the ecology of this important interaction, the research will provide data on novel and effective chemical compounds. Out of convenience, most screening for biologically active compounds has used mature leaves, even though this age-class is primarily defended by toughness and tannins. Since young leaves are tender, they rely primarily on defensive chemicals to protect them from herbivores. We predict that the types of chemicals found in young leaves will be mobile, effective in small concentrations and of small molecular weight, all attributes that make them much more tractable for medicinal and agricultural development.