SIMULTANEOUS MEASUREMENT OF ROOT FORCE AND ELONGATION FOR SEEDLING PEA ROOTS

A new method was developed to measure simultaneously, continuously, and non-destructively the elongation rate and the force exerted by the roots of seedlings grown in moist air. A pea (Pisum sativum L. cv. Helka) seedling was suspended inside a modified sample tube on one side of a pulley, with the tip of the radicle pushing on to a force transducer through a hole in the tube. The force on the root tip was monitored by the force transducer and could be adjusted by adding or removing mass from the counterweight on the other side of the pulley. As the root grew, the sample tube was raised and the elongation of the root was monitored using a linear variable differential transformer (LVDT) attached to the thread connecting the sample tube and counterweight. The changes in elongation rate were recorded which occurred in response to increases and decreases in the applied force. Forces of up to 125 mN were exerted on the root, corresponding to forces per unit final cross-sectional area (i.e. root growth pressures) of up to 0.1 MPa. As soon as the force on the root was changed there was a rapid reversible compression or extension of the root. Superimposed on this elastic/viscoelastic deformation, the root elongation rate slowed by more than 50% within 30 min of increasing the force applied to the root by 100 mN. A similarly fast but smaller increase in growth rate occurred when the force was removed. Both of these 'fast' responses were followed by a longer period of more gradual change in the root elongation rate over a period of 20 h or longer. Both 'fast' and 'slow' responses may be explained in terms of a modified Lockhart model of growth. The initial 'fast' response of the root is probably due to the immediate change in the effective pressure (i.e. the turgor pressure minus the yield stress and external resisting pressure) available to drive cell elongation. The reason for the second slower adjustment of the elongation rate is not known, but is probably due to some combination of a decrease in the rate of cell production and/or a stiffening of the cell walls in the longitudinal direction with increasing mechanical resistance. The increase in root diameter in response to mechanical impedance decreased the root growth pressure that the root exerted, but was associated with a slower root elongation rate.