Running title: Leaf squeeze-flow rheometry 1 Measurement of plant water status via static uniaxial compression 2 of the leaf lamina 3

8 Turgor pressure is an essential, but difficult to measure indicator of plant water status. Turgor has been 9 quantified by localised compression of cells or tissues, but a simple method to perform these 10 measurements is lacking. We hypothesized that changes in leaf turgor pressure can be monitored by 11 uniaxially compressing the leaf lamina and measuring the mechanical stress under a constrained 12 thickness (stress relaxation); and that changes in leaf water content can be monitored by measuring the 13 thickness of the leaf lamina compressed under a constant force (creep). Using a custom-built leaf 14 squeeze-flow rheometer, we performed different compression tests on leaves from thirteen plant 15 species. The equilibrium mechanical stress measured during stress relaxation was correlated with leaf 16 turgor pressure (R 2 > 0.95) and thus with leaf water potential (R 2 > 0.94); the equilibrium leaf thickness 17 measured during creep was correlated with relative water content (R 2 > 0.74). The coefficients of these 18 relationships were related to the leaf osmotic pressure at the turgor-loss point. An idealised average- 19 cell model suggests that, under isothermal conditions, the bulk cell stiffness during compression is 20 largely determined by the leaf osmotic pressure. Our study presents an inexpensive, accessible and 21 automatable method to monitor plant water status non-invasively.