Expansion Rates and Defenses in Young Leaves of Tropical Species

植物已经进化出一系列的物理和化学防御系统，以保护它们免受放牧（食草）的伤害。我们对草食和植物防御的了解大多来自于对成熟叶片的研究。然而，在叶子的一生中，初生的叶子是最脆弱的阶段。一生中80%的损害发生在这短暂的时间内，因为膨胀的叶子受到真菌和食草动物伤害的几率是成熟叶子的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，我们假设叶片的快速膨胀缩短了脆弱期，与韧性、化学防御和绿化等其他防御措施不相容。虽然化学防御和快速扩张是幼叶最有效的两种防御手段，但在叶片扩张过程中，化学物质的合成不仅会引起自毒性，还会争夺快速叶片构建所需的资源。因此，具有缓慢扩张的嫩叶的物种可能更广泛地依赖有效的化学防御。我们将描述雨林植物快速和缓慢扩张的幼叶的化学防御特征，并量化这些化学物质在防御昆虫食草动物和真菌病原体方面的有效性。除了让我们深入了解这种重要相互作用的生态学之外，这项研究还将提供有关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成熟的叶子，尽管这个年龄层主要是由韧性和单宁酸保护的。由于嫩叶很嫩，它们主要依靠防御性化学物质来保护自己免受食草动物的侵害。我们预测，在嫩叶中发现的化学物质类型将是可移动的，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.