Drought-induced lacuna formation in the stem causes hydraulic conductance to decline before xylem embolism inSelaginella

Lycophytes are the earliest diverging extant lineage of vascular plants, sister to all other vascular plants. Given that most species are adapted to ever-wet environments, it has been hypothesized that lycophytes, and by extension the common ancestor of all vascular plants, have few adaptations to drought. We investigated the responses to drought of key fitness-related traits such as stomatal regulation, shoot hydraulic conductance (K-shoot) and stem xylem embolism resistance inSelaginella haematodesandS. pulcherrima, both native to tropical understory. During drought stomata in both species were found to close before declines inK(shoot), with a 50% loss ofK(shoot)occurring at -1.7 and -2.5 MPa inS. haematodesandS. pulcherrima, respectively. Direct observational methods revealed that the xylem of both species was resistant to embolism formation, with 50% of embolized xylem area occurring at -3.0 and -4.6 MPa inS. haematodesandS. pulcherrima, respectively. X-ray microcomputed tomography images of stems revealed that the decline inK(shoot)occurred with the formation of an air-filled lacuna, disconnecting the central vascular cylinder from the cortex. We propose that embolism-resistant xylem and large capacitance, provided by collapsing inner cortical cells, is essential forSelaginellasurvival during water deficit.