O vigor da planta altera a densidade de galhas em populações de Baccharis longiattenuata (Asteraceae) sob distintas condições de solo?

Abstract The plant vigor hypothesis suggests that more vigorous plants support higher density of galls. This study will evaluate such hypothesis using a mite-Baccharis longiattenuata system in two restinga populations under different environmental conditions. The measures of vigor are: height and stem diameter, volume and crown area, number of leaves per branch, nitrogen content and specific leaf area. The results evidenced the vigor effect on the density of galls. In addition to plant vigor, other factors may have indirectly influenced the observed pattern. In shrub-tree restinga, the highest water intake may have influenced the growth rates and formation of meristematic sites, which changes the gall-host dynamics. Different than expected in the literature, a population with less sclerophyllous leaves presented higher density of galls. On the other hand, in shrub restinga, the lowest SLA values, in response to high light incidence and lower soil water availability, may be influenced by higher density of mites per gall, since more sclerophyllous leaves have greater longevity, favoring in the time of permanence of the mature female inside the gall, increasing the density of mites per gall in this environment. A discussion is presented of environmental conditions that conform to the gall distribution pattern found.

摘要 植物活力假说（plant vigor hypothesis）指出，活力水平更高的植物可承载更高密度的虫瘿（gall）。本研究以两种处于不同环境条件下的海滨沙原（restinga）种群中的螨类-长细巴蒂斯塔（Baccharis longiattenuata）共生体系为研究对象，对该假说开展验证。本研究选取的植物活力指标包括：株高、茎粗、植株体积与冠幅面积、单枝叶片数、叶片氮含量以及比叶面积（specific leaf area, SLA）。
研究结果证实了植物活力对虫瘿密度的显著影响。除植物活力外，其他因素同样可能间接作用于本次观测到的分布格局。在灌乔混合型海滨沙原植被中，更高的水分摄入速率可能促进了植株生长与分生组织位点的形成，进而改变了虫瘿-寄主的互作动态。与已有文献的预期相悖的是，叶片硬叶性较弱的种群反而呈现出更高的虫瘿密度。
另一方面，在灌丛型海滨沙原植被中，植株因应对高光照射与土壤水分有效性降低的胁迫，其比叶面积（SLA）值更低，这一特征或与单瘿螨类密度更高相关：硬叶性更强的叶片寿命更长，可延长成熟雌螨在虫瘿内的驻留时长，最终提升了该生境下单瘿的螨类密度。本研究最后针对与本次观测到的虫瘿分布格局相符的环境条件展开了讨论。