Expansion Rates and Defenses in Young Leaves of Tropical Species

植物已经进化出一系列物理和化学防御系统，以保护它们免受放牧破坏(食草动物)。我们对食草动物和植物防御的大部分了解来自于对成熟叶片的研究。然而，幼嫩的展开叶是叶片生命中最脆弱的阶段。80%的终生损害发生在这一短时间内，因为展开的叶子遭受真菌和食草动物的损害率是成熟叶片的10-100倍。我们感兴趣的是了解幼叶防御的有效性，以及防御之间的相互作用。特别是，我们假设，缩短脆弱时期的快速叶片展开与其他防御措施，如韧性、化学防御和绿化是不相容的。虽然化学防御和快速伸展是幼叶最有效的两种防御措施，但在伸展过程中合成的化学物质可能会导致自毒，并争夺用于快速构建叶片的资源。因此，幼叶缓慢展开的物种可能会更广泛地依赖有效的化学防御。我们将描述雨林植物中快速和缓慢扩展的幼叶的化学防御特征，并量化这些化学物质作为昆虫食草动物和真菌病原体防御的有效性。除了让我们深入了解这种重要相互作用的生态学，这项研究还将提供关于新型有效化合物的数据。为了方便起见，大多数生物活性化合物的筛选都使用了成熟的叶子，尽管这一年龄段的植物主要受到韧性和单宁的保护。由于幼叶很嫩，它们主要依靠防御性化学物质来保护它们免受食草动物的伤害。我们预测，在幼叶中发现的化学物质类型将是流动的，在小浓度和小分子量下有效，所有这些属性都使它们更容易用于医药和农业开发。

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.