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Tree Physiology Advance Access published online on March 25, 2009
Tree Physiology, doi:10.1093/treephys/tpp016
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Osmotic measurements in whole megagametophytes and embryos of loblolly pine (Pinus taeda) during seed development

Gerald S. Pullman1,2,3 and Shannon Johnson2

1 School of Biology, Georgia Institute of Technology, 500 10th Street N.W. Atlanta, GA 30332-0620, USA
2 Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th Street N.W. Atlanta, GA 30332-0620, USA
3 Corresponding author (Jerry.pullman{at}ipst.gatech.edu)


  Abstract

Water potential ({Psi}) and osmotic potential ({Psi}s) were measured weekly through the sequence of seed development in megagametophytes of loblolly pine (Pinus taeda L.). A Wescor 5500XRS vapor pressure osmometer, modified with a cycle hold switch, was used to measure {Psi} for whole megagametophytes containing embryos. The {Psi} measurements for megagametophytes with embryos removed were also attempted but readings were distorted due to cell lysates from the cut surfaces. Six seasonal sets of megagametophyte {Psi} profiles were generated. Megagametophytes from most of the trees examined showed a consistent {Psi} pattern: low measurements of –1.0 to –0.75 MPa during early embryo development in late June to early July when embryo Stages 1–2 occur; an increase for one to several weeks to levels of –0.5 to –0.75 MPa, beginning at Stages 3–5 when apical dome formation occurs; followed by a steady drop from –0.85 to –1.7 to –2.0 MPa from Stage 6 onward from late August until just before cone seed release. The {Psi}s was measured for supernatant from centrifuged frozen-thawed megagametophyte tissue (embryos removed). Megagametophyte {Psi}s profiles were similar for seeds analyzed from two trees and resembled {Psi} observations starting low, rising around Stages 4–7 and then undergoing a major reduction indicating a strong solute accumulation beginning at Stages 7–9.1. Somatic embryos stop growth prematurely in vitro at Stages 8–9.1. The major change in the accumulation of megagametophyte solutes at Stages 8–9.1 correlates with the halt in somatic embryo maturation and suggests that identifying, quantifying and using the major natural soluble compounds that accumulate during mid- to late-stage seed development may be important to improve conifer somatic embryo maturation.

Keywords: conifer, embryogenesis, gymnosperm, osmotic potential, somatic embryogenesis, water potential

Received January 26, 2009; Accepted February 27, 2009


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