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Linking Development and Determinacy with Organic Acid Efflux from
Proteoid Roots of White Lupin Grown with
Low Phosphorus and Ambient
or Elevated Atmospheric
CO2 Concentration1
Michelle Watt and
John R. Evans*
Environmental Biology Group, Research School of Biological
Sciences, Australian National University, P.O. Box 475, Canberra,
ACT, Australia 2601
White lupin (Lupinus
albus L.) was grown in hydroponic culture with 1 µM phosphorus to enable the development of proteoid roots
to be observed in conjunction with organic acid exudation. Discrete
regions of closely spaced, determinate secondary laterals (proteoid
rootlets) emerged in near synchrony on the same plant. One day after
reaching their final length (4 mm), citrate exudation occurred over a
3-d pulse. The rate of exudation varied diurnally, with maximal rates
during the photoperiod. At the onset of citrate efflux, rootlets had
exhausted their apical meristems and had differentiated root hairs and
vascular tissues along their lengths. Neither in vitro
phosphoenolpyruvate carboxylase nor citrate synthase activity was correlated with the rate of citrate exudation. We suggest
that an unidentified transport process, presumably at the plasma
membrane, regulates citrate efflux. Growth with elevated (700 µL
L 1) atmospheric [CO2] promoted earlier
onset of rootlet determinacy by 1 d, resulting in shorter rootlets
and citrate export beginning 1 d earlier as a 2-d diurnal pulse.
Citrate was the dominant organic acid exported, and neither the rate of
exudation per unit length of root nor the composition of exudate was
altered by atmospheric [CO2].
1
This study was funded in part by an Overseas
Postgraduate Award to M.W. from the Australian Government.
*
Corresponding author; e-mail evans{at}rsbs.anu.edu.au; fax
61-2-6249-4919.
Plant Physiol. (1999) 120: 705-716
Copyright Clearance Center: 0032-0889/99/120//12
© 1999 American Society of Plant Physiologists
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