© 1998 Heron Publishing—Victoria, Canada
Thigmomorphogenesis: a dose response to flexing in Ulmus americana seedlings
Frank W. Telewski (1) and Michele L. Pruyn (2)
1. W.J. Beal Botanical Garden, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824, USA / Received November 8, 1996
Summary
Six-week-old half-sib seedlings of Ulmus americana L. were subjected to different amounts of flexure daily for 3 weeks under controlled greenhouse conditions. The daily flexure
treatments were: no flexing in a staked stem, minimal flexing in a non-staked stem, and five, 10, 20, 40, and 80 flexures.
Seedling height and diameter growth and average leaf area were determined before and after the treatments. The ratio of the
change in height growth (ΔH) to the change in diameter growth (ΔD; (ΔH:ΔD)) before and after the 3-week treatments were calculated. At the end of the 3-week experiment, staked seedlings were significantly
taller and had smaller stem diameters than all of the flexed seedlings. Height growth tended to decrease exponentially with
increased flexure, with significant differences between the extremes of treatment. All of the flexure treatments significantly
increased stem diameter compared to staked seedlings. The ΔH:ΔD ratio exhibited an exponential function in response to increased flexure. Average leaf area decreased with increased flexure,
and seedlings in the 40× and 80× flexure treatments had significantly less leaf area than seedlings in all of the other treatments.
These data are similar to the dose responses previously observed in herbaceous species. The finding that trees exhibit greater
sensitivity to low doses of flexure than to high doses of flexure indicates that slight exposure to wind may result in a large
initial alteration in stem morphology, producing a thigmomorphogenetic effect. Trees will continue to respond to increasing
amounts of mechanical stress, but at an exponentially declining rate. Declining leaf areas in response to increasing amounts
of mechanical stress may result in a decrease in available photosynthate, resulting in a tree of smaller stature compared
to trees exposed to lower amounts of mechanical loading.
Keywords:
American elm, diameter, flexure, height, leaf area, mechanical loading, mechanical stress, wind.
