Measurement of plant water status via static uniaxial compression of the leaf lamina

Turgor pressure is an essential, but difficult to measure indicator of plant water status. Turgor has been quantified by localised compression of cells or tissues, but a simple method to perform these measurements is lacking. We hypothesized that changes in leaf turgor pressure can be monitored by uniaxially compressing the leaf lamina and measuring the mechanical stress under a constrained thickness (stress relaxation); and that changes in leaf water content can be monitored by measuring the thickness of the leaf lamina compressed under a constant force (creep). Using a custom-built leaf squeeze-flow rheometer, we performed different compression tests on leaves from thirteen plant species. The equilibrium mechanical stress measured during stress relaxation was correlated with leaf turgor pressure (R2 > 0.95) and thus with leaf water potential (R2 > 0.94); the equilibrium leaf thickness measured during creep was correlated with relative water content (R2 > 0.74). The coefficients of these relationships were related to the leaf osmotic pressure at the turgor-loss point. An idealised average-cell model suggests that, under isothermal conditions, the bulk cell stiffness during compression is largely determined by the leaf osmotic pressure. Our study presents an inexpensive, accessible and automatable method to monitor plant water status non-invasively.