Tomatoes Remember being Brushed

Tomato seedlings grown in close proximity elongate rapidly in a shade-avoidance response. A daily touch stimulus can eliminate the extra growth associated with shade avoidance. Experiments to determine how the touch stimulus is integrated were performed on tomato seedlings grown in plug trays 22 mm apart, starting when two fully expanded leaves overlapped between plants to induce the shade-avoidance response. The standard touch stimulus was applied by brushing the surface of the canopy 10 times each morning with a piece of Styrofoam sheet. This treatment reduced the daily growth rate from 7.7 to 5.8 mm/day, but quadrupling the dose further reduced growth only slightly (J. Amer. Soc. Hort. Sci. 121:894). The ability of the plants to sum individual stimuli was tested by varying the interval between the individual strokes. Intervals of 0.01, 0.1, 1 and 10 min all produced the same growth response. Thus, the individual strokes were perceived as a single stimulus. Had they been perceived as separate stimuli, long intervals would have increased the response. There was no refractory period of insensitivity following the stimulus; that would have reduced the response at longer intervals. The height reduction was directly proportional to the number of days that the treatment was applied, indicating that each day of treatment reduced the growth rate for only 1 day (from 5.9 mm/day to 2.7 mm/day). Thus, brief stimuli are integrated during the day and expressed as a reduction of growth the next diurnal cycle.