Root elongation against a constant force: experiment with a computerized feedback-controlled device

Axial force was applied to the root tip of corn (Zea mays L. cv. Merit) see dlings using a computerized, feedback-controlled mechanical device. The sys tem's feedback capability allowed continuous control of a constant tip load , and the attached displacement transducer provided the time course of root elongation. Loads up to 7.5 g decreased the root elongation rate by 0.13 m m h(-1) g(-1), but loads 7.5 to 17.5 g decreased the growth rate by only 0. 04 mm h(-1) g(-1). Loads higher than 18 g stopped root elongation completel y. Measurement of the cross-sectional areas of the root tips indicated that the 18 g load had applied about 0.98 MPa of axial pressure to the root, th ereby exceeding the root's ability to respond with increased turgor pressur e. Recorded time-lapse images of loaded roots showed that radial thickening (swelling) occurred behind the root cap, whose cross-sectional area increa sed with tip load.