Abstract Detail

Xylem apoplast-symplast interactions

Pittermann, Jarmila [1], Brodersen, Craig [2], Watkins, James [3].

Sporophytes, gametophytes and the symplast connection: evolutionary implications for water relations in ferns.

Seedless vascular plants (SVPs) such as ferns, lycopods and horsetails exhibit two lifestages, the vegetative sporophyte and the reproductive gametophyte. In contrast to their bryophyte ancestors, the evolution of vascular tissue allowed SVPs to support a diversity of morphologies and life history strategies. Indeed, vascular tissue in its symplastic and apoplastic forms was a key innovation that bridged the transition from simple non-vascular plants to derived woody vegetation. Here, we examine the role of the apoplast and symplast in the water relations of fern sporophytes and gametophytes in an effort to link these morphotypes in a physiological and evolutionary context. In angiosperms, symplastic tissues such as parenchyma and phloem are implicated in the repair of drought-induced xylem embolisms, that is air-filled dysfunctional conduits. Can we expect a similar pattern in ferns and lycophytes? Sporophytes are frequently desiccation tolerant, but patterns of embolism spread and the mechanisms supporting the hydraulic recovery of the xylem remain unclear. Using a combination of High Resolution Computed Tomography and image analysis, we peered into the xylem tissue of the redwood forest understory fern Polystichum munitum during an imposed drought stress and observed that within the large vascular bundles (VBs), the largest conduits are first to embolize, while smaller conduits in both large and small VBs are the last, possibly acting as a hydraulic reservoir. Half of the xylem hydraulic capacity was lost at -3 MPa, though water potentials in situ rarely exceeded -1.5 MPa. A rapid recovery from -3.1 MPa to -0.4 MPa was observed upon rewatering the greenhouse plants, suggesting that P. munitum may be able to refill embolized conduits. With phloem and parenchyma tissue surrounding the xylem in all SVPs, we suspect that hydraulic recovery was selected for in the earliest stages of the evolution of tracheophytes. Turning our attention to gametophytes, it is notable that gametophyte physiology is strictly symplastic. With little control over transpiration, thallus water relations are coupled to habitat water availability, relying on desiccation tolerance and variation in thallus morphology for survival during episodic water deficit. The functional coupling between the symplastic and apoplastic physiology of these two morphotypes has not been thoroughly explored but there is good evidence to suspect that drought tolerance in the fern gametophyte parallels that of the sporophyte in an evolutionarily meaningful way.

Broader Impacts:

1 - University of California, Integrative Biology, 1156 High Street, Santa Cruz, CA, 95064, USA
2 - University of California - Davis, Department of Botany, 2111 RMI North, One Shields Avenue, Davis, CA, 95616, USA
3 - Colgate University, Department of Botany, 129 Ho Science Center, 13 Oak Drive, Hamilton, NY, 13346-1338, USA, 315-228-7660

Keywords:
Desiccation Tolerance
Pteridophytes
xylem
Embolism
early land plants.

Presentation Type: Symposium or Colloquium Presentation
Session: C3
Location: Belle-Chasse/Riverside Hilton
Date: Monday, July 29th, 2013
Time: 9:15 AM
Number: C3006
Abstract ID:866
Candidate for Awards:None