PLANT PHYSIOLOGY , Vol 102, Issue 3 717-724, Copyright © 1993 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Magnitude and Kinetics of Stem Elongation Induced by Exogenous Indole-3-Acetic Acid in Intact Light-Grown Pea Seedlings
T. Yang, D. M. Law and P. J. Davies
Section of Plant Biology, Plant Science Building, Cornell University, Ithaca, New York 14853
Exogenously applied indole-3-acetic acid (IAA) strongly promoted stem
elongation over the long term in intact light-grown seedlings of both dwarf
(cv Progress No. 9) and tall (cv Alaska) peas (Pisum sativum L.), with the
relative promotion being far greater in dwarf plants. In dwarf seedlings,
solutions of IAA (between 10-4 and 10-3 M), when continuously applied to
the uppermost two internodes via a cotton wick, increased whole-stem growth
by at least 6-fold over the first 24 h. The magnitude of growth promotion
correlated with the applied IAA concentration from 10-6 to 10-3 M,
particularly over the first 6 h of application. IAA applied only to the
apical bud or the uppermost internode of the seedling stimulated a biphasic
growth response in the uppermost internode and the immediately lower
internode, with the response in the latter being greatly delayed. This
demonstrates that exogenous IAA effectively promotes growth as it is
transported through intact stems. IAA withdrawal and reapplication at
various times enabled the separation of the initial growth response (IGR)
and prolonged growth response (PGR) induced by auxin. The IGR was inducible
by at least 1 order of magnitude lower IAA concentrations than the PGR,
suggesting that the process underlying the IGR is more sensitive to auxin
induction. In contrast to the magnitude of the IAA effect in dwarf
seedlings, applied IAA only doubled the growth in tall seedlings. These
results suggest that endogenous IAA is more growth limiting in dwarf plants
than in tall plants, and that auxin promotes stem elongation in the intact
plant probably by the same mechanism of action as in isolated stem
segments. However, since dwarf plants to which IAA was applied failed to
reach the growth rate of tall plants, auxin cannot be the only limiting
factor for stem growth in peas.