Trunk-diameter relative resistance to shrinking: an intrinsically derived plant water stress index, demonstrated in Ficus Carica cv. Black Mission

The accuracy of irrigation timing and the definition of the amount of water to apply depend on the precise assessment of plant water status within the soil–plant–atmosphere continuum. Among the widely reported indicators, trunk diameter–based metrics provide continuous and sensitive plant-based monitoring that can detect the onset of water stress at very early stages and thus improve precision irrigation and controlled deficit irrigation management. Nonetheless, these indicators still suffer from high variability and strong climatic dependence, as well as species- and growth stage–specific behavior. In the present study, intermittent water-deficit periods with varying intensities (Control, Mild, Moderate, and Severe) were applied to fig trees to generate a wide range of stem water potential (−0.5 to −2.5 MPa) and trunk diameter shrinkage values (100 to 900 μm), together with reference and crop evapotranspiration measurements. A third-degree polynomial trend was fitted to the relationship between trunk diameter shrinkage and accumulated reference evapotranspiration during the morning period to derive a new intrinsic and self-informative water stress indicator. The results confirmed the potential of trunk diameter variation to reflect internal hydraulic adjustments in the plant and supported the development of a new intrinsic water-stress indicator called diameter relative resistance to shrinking (DRS). This indicator is self-informative (no need for a well-watered reference plant), easy to interpret (ranges between 0 and 1), and better correlated with stem water potential (lower NRMSE) than the commonly used maximum daily shrinkage. The proposed indicator shows strong potential for supporting precise and adaptive irrigation scheduling under deficit irrigation conditions. However, further studies are required to evaluate the extent to which DRS depends on species- and cultivar-specific hydraulic traits.