RESPONSES OF ASTER, DUSTY MILLER, AND PETUNIA SEEDLINGS TO DAILY EXPOSURE TO MECHANICAL-STRESS

Seedlings of aster [Callistephus chinensis (L.) Nees], dusty miller [Senecio bicolor (Willd.) Tod.], and petunia (Petunia Juss.) were subjected to brushing with burlap in two experiments. The spring experiment tested the daily duration of brushing (60, 120, or 180 minutes per day) and the autumn experiment tested the stimulus of brushing (brushing for 2 hours with single-layer, double-layer, or triple-layer burlap). Brushing was done with slow-moving apparatus. The increase in daily brushing duration led to a greater reduction of growth, although differences between the species were noted. Aster showed a growth reduction after 20 days of treatment, whereas dusty miller and petunia took longer to respond. Aster and petunia responded significantly even to the shortest daily brushing duration, whereas dusty miller needed 2 hours per day to achieve any significant growth reduction. Increasing the brushing stimulus resulted in injuries to aster leaves in atumn. Brushing with double-layer burlap caused maximum growth reductions in all the species. Brushing most clearly reduced the growth of dusty miller and petunia in autumn, but aster showed similar growth reductions in spring and autumn.