Stomatal protection against hydraulic failure: a comparison of coexisting ferns and angiosperms

• Hydraulic characteristics of pteridophyte (fern and Selaginella) foliage were investigated to determine whether the processes of water conduction and water loss are coordinated in these early vascular plants similarly to angiosperms. • Eight species of pteridophytes and associated woody angiosperms were examined from the sun and shade in a seasonally dry tropical forest. • Maximum leaf hydraulic conductivity (Kleaf) in the four pteridophytes was within the range of the sampled shade angiosperms but much lower than that of the sun-dwelling angiosperms. Hydraulic conductivity of both angiosperm and pteridophyte leaves showed a similar response to desiccation, with Kleaf becoming rapidly depressed once leaf water potential fell below a threshold. Stomatal closure in angiosperms corresponded closely with the water potential responsible for 50% loss of Kleaf while pteridophytes were found to close stomata before Kleaf depression. • The contrasting behaviour of stomata in this sample of pteridophytes suggest that this may be an intrinsic difference between pteridophytes and angiosperms, with lower safety margins in angiosperms possibly enhancing both optimization of gas exchange and xylem investment. © New Phytologist (2004).