Diversity in conduit and pit structure among extant gymnosperm taxa

Abstract

Gymnosperm taxa are incredibly diverse in many traits, including taxa with evergreen or deciduous leaves; broad, compound, needle‐like or scale‐like leaves; trees, shrubs, and lianas; and taxa with seed cones that range from fleshy to woody. Although less appreciated, xylem conduits are also diverse in structure among extant gymnosperm taxa. Within the xylem of gymnosperms, axial transport occurs predominantly via tracheids, although 10–40% of gymnosperm taxa, particularly within the Gnetophyta and Cycadophyta, also contain vessels. Gymnosperm taxa vary greatly in their inter‐conduit pit structure, with different types of pit membranes and pitting arrangements. While some taxa display torus‐margo bordered pits (60%), many others do not contain tori (40%), and at least some taxa without tori occur within each of the four extant phyla (Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta). Pit membrane types vary within families but appear relatively conserved within genera. Woody species with torus‐bearing pit membranes occur in colder environments (lower mean annual temperature) compared to those without tori; but occurrence does not differ with mean annual precipitation. Detailed descriptions of pit membrane types are lacking for many species and genera, indicating a need for increased anatomical study. Increased knowledge of these traits could provide a unique experimental context in which to study the evolution of conduit networks, the hydraulic implications of conduit and pit structure, and the diverse structural and functional strategies utilized by gymnosperms. There are myriad potential study questions and research opportunities within this unique and diverse group of plants.

Coordination of intertracheid pit traits and climate effects among cycads

“…Although pit aperture was not correlated with leaf anatomical traits (Table S4), significant negative correlations were found between F pa and A m (Figure 6D), D pal (Figure 6E) and stomatal conductance G s , for which species were reported with primitive vessels (Huang et al, 2010;Huang & Zhang, 1999;Jacobsen, 2021;Xu et al, 2005). No torusmargo structures were found in pits of the studied cycad species.…”

“…The breadth of climates occupied by each family and the differences in climate between families suggest that there may be differences in functional anatomy that influence cycad leaf water balance. Like other gymnosperms, cycads possess xylem tracheids for water transport, although vessels have also been reported in some species (Huang et al, 2010, Huang & Zhang, 1999, Jacobsen, 2021, Xu et al, 2005). The xylem organization in the rachis of cycads is typically shaped like an “omega,” resulting in the distal leaflets being irrigated by abaxial bundles, thus guaranteeing uniform water supply along the entire leaf length (Tomlinson et al, 2018).…”

“…Among plant lineages, pit membranes fall into one of two categories: the torus‐margo type found in most gymnosperms and the homogenous pit membranes characteristic of angiosperms (Choat et al, 2008, Pittermann et al, 2005), ferns (Carlquist & Schneider, 2007), and several cycad species (Jacobsen, 2021). More permeable pit membranes are found in basal tracheophytes, as well as in vesselless angiosperms and conifers, possibly to partially compensate for the higher hydraulic resistances imposed by a tracheid‐based vascular system (Brodersen et al, 2014, Hacke et al, 2007, Pittermann et al, 2005).…”

“…However, the connection between interconduit pit anatomy among cycads and their physiological and habitat affinities remains unclear. Although several cycad species have been shown to possess homogenous pit membranes similar to angiosperms (Jacobsen, 2021), these studies have been limited to just a few species without analyzing the possible connections between anatomical and physiological traits (Jacobsen, 2021). Comparative studies have revealed large intraspecific and interspecific variations in pit membrane and pit aperture size, shape, density, and the ratio of total pit aperture area to vessel wall area, likely reflecting differences in environmental adaptation and community filtering processes (Ellmore et al, 2006, Lens et al, 2011, Schmitz et al, 2007, Zhang et al, 2017).…”

Impact of freeze–thaw-induced pit aspiration on stem water transport in the subalpine conifer Abies veitchii

“…Conifers possess pit membranes consisting of a thick-walled "torus" in the central part and a fibrous "margo" in the periphery (Fig. 1B and C; Hacke et al, 2004;Jacobsen, 2021). When cavitation occurs in a tracheid, the large pressure difference forms between neighboring gas-and water-filled tracheids, which allows the torus to move towards the cell wall of the pit border due to the flexible structures of the margo and to close the pit aperture (i.e., pit aspiration; Fig.…”