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Plant and Cell Physiology, 1995, Vol. 36, No. 1 59-67
© 1995

Paclobutrazol- and Hardening-Induced Thermotolerance of Wheat: Are Heat Shock Proteins Involved?

Trevor E. Kraus1, K. Peter Pauls2 and R. Austin Fletcher1

1 Department of Environmental Biology, University of Guelph Guelph, Ontario, Canada N1G 2W1
2 Department of Crop Science, University of Guelph Guelph, Ontario, Canada N1G 2W1

The present report describes the effects of paclobutrazol and heat hardening treatments on the protein synthesis patterns in imbibing and germinating wheat seedlings (Triticum aestivum L. cv Frederick) during heat stress. A heat hardening treatment given during the imbibition period induced the transient expression of 118, 90, 70 and 18 kDa heat shock proteins (HSPs). However, the hardening and paclobutrazol treatments did not enhance the thermotolerance of imbibed seeds or etiolated seedlings. By contrast, the hardening and paclobutrazol treatments enhanced the thermotolerance of light-grown seedlings. While, both hardened and unhardened control seedlings synthesized several HSPs during a high temperature stress period, these proteins were not synthesized by the paclobutrazol-treated, light-grown seedlings. Thus, HSP synthesis during heat shock may have been a manifestation of stress perception by the seedlings and may not have mediated the thermotolerance induced by the triazole treatments. Since differential thermotolerance was only apparent in light-grown seedlings, it is suggested that chloroplasts may be required for the expression of paclobutrazol- and hardening-induced thermoprotection. Additional evidence indicating that chloroplasts are an important site of injury during high temperature stress was obtained from chlorophyll fluorescence measurements.

(Received July 11, 1994; Accepted October 26, 1994)
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