© 1998 Heron Publishing—Victoria, Canada
Paclobutrazol affects the resistance of black spruce to high light and thermal stress
Sean R. Mahoney (1), Sibdas Ghosh (2), David Peirson (3) and Erwin B. Dumbroff (1, 4)
1. Department of Biology, University of Waterloo, Waterloo, ON N2L 3G1, Canada / 2. Department of Biology, University of Wisconsin-Whitewater, Whitewater, WI 53190-1791, USA / 3. Department of Biology, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada / 4. Kennedy-Leigh Centre for Horticultural Research, Faculty of Agriculture, Hebrew University of Jerusalem, P.O. Box 12, Rehovot,
76100, Israel (dumbroff@agri.huji.ac.il) / Received February 4, 1997
Summary
Detached needles from 20-week-old black spruce (Picea mariana (Mill.) B.S.P.) seedlings root-drenched with 60 mg of paclobutrazol were exposed to two temperatures (22 and 50 °C) and two
light treatments (100 and 1900 μmol m–2 s–1 PAR) in a factorial combination for 4 h in vitro. Mean dry weights of individual needles from paclobutrazol-treated plants were approximately 1.9 times heavier than that
of needles from untreated controls at 22 °C, but no differences were observed following incubation at 50 °C. Numbers of cells
per needle remained constant in all treatments. Chlorophyll and carotenoid contents per needle were higher in seedlings treated
with paclobutrazol than in untreated control seedlings, and the differences were most pronounced in the high temperature plus
high light treatment. In low light at 50 °C, quantum efficiency of photosystem II was 45% higher in needles of paclobutrazol-treated
seedlings than in needles of untreated control seedlings, but quantum efficiency of needles from treated seedlings declined
when needles were exposed to high light at either temperature. Peroxidase and superoxide dismutase activities were up-regulated
by paclobutrazol, whereas catalase activities were depressed and no significant differences were observed between treated
and control needles at 50 °C in either light treatment. Paclobutrazol treatment did not moderate the depressive effects of
high temperature on total soluble protein or on the activity of ribulose-1,5-bisphosphate carboxylase. In contrast, high activities
of phosphoenolpyruvate carboxylase were maintained in paclobutrazol-treated needles under all stress conditions, whereas large
losses in activity were recorded in untreated needles at 50 °C. Collectively, these observations suggest that paclobutrazol
treatment may convey resistance to excessive light and high temperatures by increasing the potential of conifers to limit
damage caused by oxidative stress.
Keywords:
carboxylase enzymes, environmental stress, peroxidase, photosynthesis, Picea mariana, superoxide dismutase.
