Sex-specific differences in functional traits and resource acquisition in five cycad species

• Published in: AoB plants Vol. 9; no. 2; p. 013
• Main Authors: Krieg, Christopher, Watkins, James E., Chambers, Sally, Husby, Chad E.
• Format: Journal Article
• Language: English
• Published: England Oxford University Press 01.03.2017
• Subjects: Bacteria; Carbon 13; Carbon content; Coevolution; Cycadophyta; Density; Ecology; Ecophysiology; Evolution; Females; Males; Nitrogen; Nitrogen fixation; Nitrogenation; Photosynthesis; Physiology; Resource allocation; Sex; Soil bacteria; Soil microorganisms; Stomata; Stomatal conductance; Water use; evolution; Dioecy; trade-offs; photosynthesis; root symbiosis; natural isotope abundance
• ISSN: 2041-2851; 2041-2851
• DOI: 10.1093/aobpla/plx013

The Mesozoic is often referred to as the “age of Cycads.” During this period, dinosaurs roamed vast Cycad forests, yet modern Cycads are a vestige of their Mesozoic glory. Extant species represent the oldest lineage of dioecious seed-bearing plants. This curious phylogenic position is matched by their unusual ecology: most species are extremely rare while others form dense colonies that may play important roles in forest function. In spite of this and quite remarkably, Cycads are the most poorly studied lineage of seed plants and almost nothing is known of their ecophysiology. The study by Krieg et al. is the first of its kind to examine sex-mediated ecophysiology in Cycads. Their results show unexpended differences in photosynthetic physiology and highlight the role that nitrogen fixing soil bacteria may play in Cycad reproduction and ecology. They found that species can vary markedly in nitrogen relations and that plant sex can drive unique leaf physiology. Their study is a cycadological call to arms for plant scientists to refocus efforts on this enigmatic group. Abstract Selective pressures acting on plant life histories can drive extreme specialization. One example of such specialization is the evolution of dioecious breeding systems. Evolutionary and ecological theory posits that dioecy may subject male and female individuals to different selective pressures and result in unique sex-mediated adaptive traits related to resource allocation and ecophysiology. Cycads are the earliest diverging lineage of seed plants with strict dioecy, yet we know almost nothing about the ecology and physiology of this group. Especially limited is our understanding of potential sex-specific differences and how such differences may influence species ecology. Here we examine the ecophysiology of male and female cycads to understand better, the role that dioecy plays in this group. We evaluated sex-specific differences in ecophysiological traits and resource acquisition in five species. Specifically, we compared photosynthetic physiology, nitrogen and carbon content, isotope discrimination (δ15N and δ13C), and stomatal density. In some cycads, (i) males and females have similar investments in leaf nitrogen but females exhibit greater incorporation of nitrogen from nitrogen-fixing soil bacteria, (ii) males display higher photosynthetic capacity but females show greater water-use efficiency, and (iii) males have higher stomatal conductance but similar stomatal density to females. This study is the first to examine the ecophysiological differences that have evolved in the oldest dioecious lineage of seed-bearing plants. Our results show unexpected differences in photosynthetic physiology and highlight the co-evolution with nitrogen fixing soil bacteria as a potential new key player in an old lineage.