Search for: All records

Abstract Interconduit pit membranes, which are permeable regions in the primary cell wall that connect to adjacent conduits, play a crucial role in water relations and the movement of nutrients between xylem conduits. However, how pit membrane characteristics might influence water‐carbon coupling remains poorly investigated in cycads. We examined pit characteristics, the anatomical and photosynthetic traits of 13 cycads from a common garden, to determine if pit traits and their coordination are related to water relations and carbon economy. We found that the pit traits of cycads were highly variable and that cycads exhibited a similar tradeoff between pit density and pit area as other plant lineages. Unlike other plant lineages (1) pit membranes, pit apertures, and pit shapes of cycads were not coordinated as in angiosperms; (2) cycads exhibited larger pit membrane areas but lower pit densities relative to ferns and angiosperms, but smaller and similar pit membrane densities to non‐cycad gymnosperms; (3) cycad pit membrane areas and densities were partially coordinated with anatomical traits, with hydraulic supply of the rachis positively coordinated with photosynthesis, whereas pit aperture areas and fractions were negatively coordinated with photosynthetic traits; (4) cycad pit traits reflected adaptation to wetter habitats for Cycadaceae and drier habitats for Zamiaceae. The large variation in pit traits, the unique pit membrane size and density, and the partial coordination of pit traits with anatomical and physiological traits of the rachis and pinna among cycads may have facilitated their dominance in a variety of ecosystems from the Mesozoic to modern times. 

Throughout leaf development, cell expansion is dynamic and driven by the balance between local cell wall mechanical properties and the intracellular turgor pressure that overcomes the stiffness of the cell wall leading to plastic deformation. The epidermal pavement cells in most leaves begin development as small, polygonally shaped cells, but in mature leaves epidermal pavement cells are often shaped as highly lobed puzzle pieces. However, the developmental and biomechanical trajectories between these two end points have not before been fully characterized. Here we characterized how epidermal pavement cell size and shape, cell wall thickness, and hydraulic traits change during leaf expansion in the tropical understory fern Microsorum grossum (Polypodiaceae). As fronds expanded by approximately two orders of magnitude in size, epidermal pavement cells became increasingly lobed as cell walls thickened. Furthermore, the timing of these developmental changes varied across the lamina, start first near the frond base and midrib, followed by more apical and lateral regions. During expansion, fronds also underwent substantial physiological changes: as cells expanded and cell walls thickened, intracellular turgor pressure and the bulk cell wall modulus of elasticity both increased while the water potential at turgor loss and the minimum epidermal conductance to water vapor both decreased. These results highlight the dynamic coordination between anatomical and physiological traits throughout leaf development, provide valuable data for biophysical modeling of leaf development, and highlight the vulnerability of developing leaves to drought conditions. 

• Drought-induced xylem embolism is a primary cause of plant mortality. Although ~70% of cycads are threatened by extinction and extant cycads diversified during a period of increasing aridification, the vulnerability of cycads to embolism spread has been overlooked. • We quantified the vulnerability to drought-induced embolism, pressure-volume curves, in situ water potentials, and a suite of xylem anatomical traits of leaf pinnae and rachises for 20 cycad species. We tested whether anatomical traits were linked to hydraulic safety in cycads. • Compared to other major vascular plant clades, cycads exhibited similar embolism resistance to angiosperms and pteridophytes but were more vulnerable to embolism than non-cycad gymnosperms. All 20 cycads had both tracheids and vessels, the proportions of which were unrelated to embolism resistance. Only vessel pit membrane fraction was positively correlated to embolism resistance, contrary to angiosperms. Water potential at turgor loss was significantly correlated to embolism resistance among cycads. • Our results show that cycads exhibit low resistance to xylem embolism and that xylem anatomical traits–particularly vessels–may influence embolism resistance together with tracheids. This study highlights the importance of understanding the mechanisms of drought resistance in evolutionarily unique and threatened lineages like the cycads.