Variation in root morphology of epiphytic orchids along small-scale and large-scale moisture gradients

ABSTRACT Root morphology is expected to respond to environmental conditions. Two earlier studies reported predictable changes in the structure of the velamen radicum (common in monocotyledons) along rainfall gradients, which was expected from its assumed role in plant-water-relations. The present study expanded on this idea by analysing nine root traits that can be linked to root function, including velamen structure, along two moisture gradients of different spatial scales: 1) along the vertical gradient of a lowland forest in central Panama and 2) along a strong regional rainfall gradient in western Panama. All studied traits (e.g. velamen width, number of xylem poles, lignification of the exodermis) of the 45 orchid species showed substantial intraspecific and interspecific variation, but none of this (within forest) or very little of this (regional gradient) was related to the gradients. Only the community weighted means of velamen width, stele width and xylem pole number varied significantly with rainfall, with substantial effect sizes only found in the latter two traits. Our results clearly disagree with those of earlier publications, which did not include species abundance and/or rigorous statistics in their analyses. This discrepancy highlights that our understanding of the link between velamen form and function is still rudimentary.

摘要：植物根系形态理应响应环境条件。此前两项研究报道了单子叶植物常见的根被皮(velamen radicum)结构沿降雨梯度发生可预测的变化，这一结果与其在植物水分关系中所承担的假定功能相符。本研究以此为基础，通过分析两种不同空间尺度的湿度梯度下与根系功能相关的9项根系性状（包括根被皮结构）展开研究：其一为巴拿马中部低地森林的垂直湿度梯度，其二为巴拿马西部显著的区域降雨梯度。所研究的45种兰科植物的全部性状（如根被皮宽度、木质部极(xylem poles)数量、外皮层(exodermis)木质化程度）均表现出显著的种内和种间变异，但在森林内部梯度上，这些变异均与梯度无关；而在区域梯度上，仅有极少部分变异与梯度相关。仅根被皮宽度、中柱(stele)宽度和木质部极数量的群落加权均值(community weighted means)随降雨变化呈现显著差异，且仅后两项性状的效应量(effect sizes)较为可观。本研究结果与此前发表的研究结论明显相悖，后者在分析中未纳入物种丰度数据，也未采用严谨的统计方法。这一差异表明，我们对根被皮形态与功能之间关联的认知仍处于初级阶段。