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

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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.