DISSERTATION RESEARCH: Evolution and Function of Farina in the Desiccation-Tolerant Notholaenid Ferns (Pteridaceae)

蕨类植物通常被认为局限于阴凉潮湿的栖息地，但notholaenid蕨类植物适应并在美国西南部和墨西哥北部的沙漠中多样化。Notholaenids蕨类植物在旱季处于休眠状态，但在水分恢复时迅速补水并恢复光合作用。据推测，这种干旱适应与一种叫做“farina”的蜡质物质有关，这种物质主要由植物色素组成，在叶子的下表面形成一层白色到黄色的层。当叶片因缺水而卷曲时，这个叶片表面直接暴露在阳光下。对于蜡质层，提出了两个主要的功能假设：1)它起到“屏障”的作用，帮助减少蒸发造成的水分损失；2)它起到“防晒霜”的作用，保护缺水的叶子免受过热和紫外线辐射造成的伤害。本研究旨在通过解剖学、生物化学、生物地理、生态生理学和分子系统发育等方法的结合，探讨notholaenid蕨类植物中淀粉的进化和功能，以解决植物如何应对干旱胁迫和高太阳辐照的关键问题。这些数据将为farina对蕨类植物叶片的功能作用和更普遍的抗旱性提供新的见解，这可能适用于其他植物群。一名研究生将接受多种方法的培训，并开发一个新的本科生培训模块，使用生态位模型来研究植物进化的模式。本研究有三个主要目标：第一是了解该群体中farina的进化，并利用来自核和质体标记的DNA序列重建notholaenid蕨类植物之间的种间系统发育关系。目的二将重点研究Notholaena standleyi，这是一种广泛分布的物种，具有三种公认的类黄酮化学型。将探讨这些化学型的系统发育关系、类黄酮成分、倍性水平和地理分布，以确定多倍体和非生物生态位差异是否有助于塑造它们当前的准种群分布。目的三将通过测量干旱处理前后、去除farina前后的气体交换率、水分利用效率和光合恢复速率，研究farina的生态生理功能。farina的详细解剖和光学特性也将被检查。

Ferns are commonly thought to be restricted to shady and moist habitats, but notholaenid ferns are adapted to, and have diversified within, the deserts of the southwestern United States and Northern Mexico. Notholaenids ferns become dormant during the dry season but quickly rehydrate and restore photosynthetic activity when moisture returns. This drought adaptation is hypothesized to be related to a waxy substance called 'farina', primarily composed of plant pigments, that form a whitish-to-yellowish layer on the leaf undersurface. When leaves curl up in response to water stress, this leaf surface is directly exposed to sunlight. Two primary functional hypotheses for the waxy layer have been proposed: 1) it acts as a 'barrier' to help reduce water loss from evaporation; 2) it acts as a 'sunscreen' to protect water-stressed leaves from damage caused by overheating and UV-radiation. This study aims to investigate the evolution and function of farina in notholaenid ferns and address a critical gap in understanding how plants cope with drought stress and high solar irradiance by using a combination of anatomical, biochemical, biogeographical, ecophysiological, and molecular phylogenetic approaches. These data will provide new insights into the functional role of farina on fern leaves and drought resistance more generally that may be applied to other plant groups. One graduate student will be trained in a diversity of approaches, and a new undergraduate training module will be developed using ecological niche modeling to investigate the patterns of plant evolution. This study has three primary objectives: The first is key to understanding the evolution of farina in this group, and involves reconstructing the interspecific phylogenetic relationships among notholaenid ferns using DNA sequences from both nuclear and plastid markers. Objective two will focus on Notholaena standleyi, a widespread species with three well-recognized flavonoid chemotypes. The phylogenetic relationships, flavonoid constituents, ploidy levels, and geographic distribution of these chemotypes will be explored to determine whether polyploidy and abiotic niche divergence have helped to shape their current parapatric distributions. Objective three will investigate the proposed ecophysiological functions of farina by measuring gas exchange rates, water-use efficiency, and photosynthetic-recovery rates after drought treatment, before and after removing the farina. Detailed anatomical and optical properties of farina will also be examined.