Thigmomorphogenesis: Calibration of the parameters of the sensory function in beans

Abstract An instrument has been built which reproducibly exerts measured amounts of force in an up and down rubbing motion to bean plant stems (Phaseolus vulgaris L. cv. Cherokee Wax). The thigmostimulator has, for the first time, permitted an accurate determination of the relationship between the amount of mechanical stimulus or stress (MS) and the resulting elongation of the plant (E). Over the stress range used, the analyzed relationship is best expressed by the equation: E = m In MS + b, where m is the slope of the line and b is the E-intercept. The sensory function is saturated at very low forces (e.g. 2 rubs at 3.59 N). Reciprocity between force and the number of stimuli holds only at low forces and numbers of stimuli (e.g. 12 rubs at 0.97 N to four rubs at 2.92 N). Analysis using linear regression and differential calculus reveals that for five 1-cm rubs, the sensory system is saturated at 6 N and that there is a threshold of 0.3 N below which no response occurs. Using the thigmostimulator for calibration, two 1-cm rubs given by hand are equivalent to a force of 4.41 ± 0.60 N. The thigmostimulator also allows equivalent calibration of other types of mechanical perturbations.