An insight into cell elasticity and load-bearing ability. Measurement and theory

We have studied the elasticity and load bearing ability of plant tissue at the cellular level, using onion (Allium cepa) epidermal cells. The Young's modulus and Poisson's ratio of the cells were obtained by loading a tensile force on onion epidermal peels of different turgor pressures, and measurin g the elongation and the lateral contraction of the peels. The Young's modu li and the Poisson's ratios ranged from 3.5 to 8.0 MPa and 0.18 to 0.30, re spectively. To determine the effects of cell elasticity and turgor pressure on the cell's ability to bear load, we loaded a small glass ball onto a ce ll and measured the projected contact area between the ball and the cell. U nlike previous studies, we considered the cell as a whole entity, and utili zed the Boussinesq's solution to derive the relevant equations that related the elastic parameters and cell deformation. For cells with a turgor press ure greater than or equal to 0.34 MPa, the predicted contact area agreed we ll with the measured area. The equations could also predict cell turgor pre ssure with a deviation from the measured value of 0.01 MPa. This study give s strong support to ball tonometry, a new method of measuring cell turgor p ressure.