Resumo Este estudo faz parte de uma longa investigação sobre a variação na composição da resina do gênero Hymenaea L. (Leguminosae — Caesalpinioideae) e os possíveis papéis ecológicos desta, nos diversos ecossistemas dos trópicos. O trabalho é o segundo de uma série em que será comparado o padrão de variação na composição entre a árvore-mãe e a progênie dela, tanto nas diferentes espécies de Hymenaea da Hiléia Amazônica, como em outros ecossistemas tropicais. Populações de H. intermedia Ducke e H. parvifolia Huber da Reserva Ducke, do Instituto Nacional de Pesquisas da Amazônia (INPA), próximas a Manaus, AM, foram estudadas no primeiro trabalho desta série, e são reexaminadas no trabalho atual dado o aparecimento de novos coortes de plântulas sob algumas destas árvores. Foram estudados também vários indivíduos de H. courbaril L. var. subsessilis Ducke situada no Km 7 da Rodovia Manaus-ltacoatiara, uma árvore de H. courbaril próxima a Aripuanã, MT, e uma população de H. courbaril var. stilbocarpa (Hayne) Lee & Langenheim em mata ciliar, na Estação Experimental do Instituto Florestal de São Paulo, no Município de Mogi-Guaçu. Este trabalho confirmou o padrão descrito anteriormente de que tipos químicos bem distintos ocorrem freqüentemente na floresta alta amazônica, mas não são comuns em outros habitats. Ausência de variação foi observada em adultos e plantas jovens de H. courbaril var. subsessilis e entre plântulas de H. courbaril var. stilbocarpa. Observações de atividade de insetos herbívoros na Reserva Ducke sugerem um modelo ecológico em que os padrões de variação das resinas foliares são mantidos pela interação coevolutiva com estes herbívoros.
Summary This paper is the second in a series of studies comparing leaf resin composition between parent trees and their progeny in the tropical leguminous genus Hymenaea. These studies form part of a long-range investigation of the variation in resin composition for this genus and the possible ecological roles of these resins within tropical ecosystems. Current work is emphasizing the leaf resin system rather than the fruit or trunk resin systems, because (1) the leaf resins of Hymenaea are relatively simple chemically, (2) it is possible to do experimental work with this system in seedlings, and (3) selective pressures are generally intense on seedlings and saplings in the field. The leaf resins of all species of Hymenaea contain the same 17 to 18 sesquiterpene hydrocarbons. Among individuals these sesquiterpenes vary quantitatively such that six discrete compositional types have been recognized. Other studies have shown that the phenotypic plasticity of these quantitative differences among individuals is very low, indicating a strong genetic control of quantitative composition. Previous work has shown centers of diversity in composition occur in Amazonia and southern Brazil in contrast to very little diversity in Central America and northern South America. The abundance and variety of secondary chemicals in tropical ecosystems, especially in tropical rainforests indicates that plants in these habitats allocate a greater proportion of their resources to chemical defense against predators and pathogens that plants of other habitat types. This is consistent with limited data indicating that herbivore and pathogen pressures are particularly great in humid tropical habitats. Another characteristic of herbivores and pathogens in humid tropical environments is their striking diversity. In such environments this diversity is matched by the highly diverse secondary plant chemicals. Several workers have hypothesized that the diversities of insects and pathogens and the variation in secondary plant chemicals are coevolutionarily related. One hipothesis, dealt with in this study, is that, for certain habitats, intrapopulational variation in leaf resin composition can constitute an important part of the defensive strategy of plants. Terpenoid resins have been shown to act as plant defenses against insects and fungi in temperate zone conifers. The leaf resins of Hymenaea courbaril are known to have toxic and deterrent effects on larvae of the generalist insect herbivore Spodoptera exigua (Hiibner), a noctuid lepidopteran. This supports the hypothesized ecological role of Hymenaea leaf resins in antiherbivore defense. In this study, leaf resin composition was compared between parent trees and their progeny in three species: H. intermedia and H. parvifolia from the rainforest ecosystem (high forest) at the Ducke Reserve near Manaus. Amazonas, Brazil and three population of H. courbaril also in Brazil, H. courbaril near Aripuanã, Mato Grosso do Norte, H. courbaril var. subsessilis near Manaus, Amazonas, and. H. courbaril var. stilbocarpa near Mogi-Guaçu, São Paulo. Gas-liquid chromatographic analyses were made of the leaf resins of the adult trees, seedlings collected under one of the trees (H. intermedia), and seedlings grown from seeds of the parent tree. All of the parent trees in the study were Type II (intermediate levels of caryophyllene and selinenes) with the probable exception of the H. courbaril var. stilbocarpa from the state of São Paulo which appears to be Type VI. Three different chemical types were represented among the seedlings. The seedlings collected under the H. intermedia had the some compositional type as that of the parent tree. This is in contrast to the pattern found in the first study of this series for trees in rainforest (high forest) where the parental type was not found in seedlings growing under the mature tree. However, seedlings of the present study were less than one year old and considerably younger than those of the first study. Thus, they had not been subject to selection for a long period of time. The H. parvifolia differed from its greenhouse-grown progeny in leaf resin composition as did the H. courbaril from Aripuanã to some extent. These two trees also occur in high rainforest. The tiees in other habitats, H. courbaril var. subsessilis in forest on sandy soils near Manaus and H. courbaril var. stilbocarpa in riparian forest near Mogi-Guaçu both showed great compositional stability. In our first paper it was hypothesized that the presence of leaf resin in Hymenaea acts as a defense against herbivores which would otherwise prevent seedling establishment under the parent tree. Is was futher suggested that the high degree of variation in leaf resin composition could be a significant part of this anti-herbivore defense. In our present work we refine a model in which discrete variation of quantitative leaf resin composition plays an important role in the defense against herbivores and thus an important role in seedlings establishment. In the relative dry season of the rainforest near Manaus, many trees lose their leaves and immediately produce a new set of leaves. For a period of about two weeks many trees have predominantly new, soft, and succulent leaves. During this period many trees are heavily attacked by lepidopteran larvae which completely defoliate the tree. After defoliation there is a mass migration of larvae to the forest floor where most of them pupate without further feeding. However, some larvae have not eaten sufficient amounts to pupate and actively search for food on the forest floor. These larvae are likely to be adapted to the compositional type of the parent and preferentially search for this type in the seedlings. This preferential herbivory increases the probability that seedlings which become established near the parent tree will be of a compositional type different from that of the parent. Thus, selection of this sort would favor the maintenance of several discrete compositional types in Hymenaea populations. At this time it has not been possible to explain the extreme constancy of the H. courbaril populations occurring outside the amazonian high rainforest. This and other questions raised by these first two studies are currently being investigated in studies which will more stringently assess the herbivore model presented here as well as examine the inclusion of pathogen pressures into the model.
