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• 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.more » « lessFree, publicly-accessible full text available July 1, 2025
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Summary Drought‐induced xylem embolism is a primary cause of plant mortality. Although
c . 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 with other major vascular plant clades, cycads exhibited similar embolism resistance to angiosperms and pteridophytes but were more vulnerable to embolism than noncycad 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.
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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.