Xylogenesis controlled by water potential gradients within the soil–plant–atmosphere continuum in one of the most widespread conifers

While numerous studies have explored the interplay between modelled temperatures and precipitation levels with xylogenesis, the synchronous tree–water balance (TWB), which is defined by the soil–plant–atmosphere continuum, remains poorly understood. The aim of this study is to delineate the hydro-limits in which the soil water potential (SWP), sap flow (SF), and air water potential (AWP) act as driving forces governing secondary xylem production, as evidenced by periodic cellular responses during the period from 2014–2016. We investigated the hydro-limits within which Scots pine regulates xylem cell differentiation and identified the key triggers influencing xylem morphological structures. Xylogenesis, which is monitored using microcores examination (cambial activity, cell enlargement, cell wall thickening, and maturation), identified SWP limit (< -0.5 MPa) as the primary regulator controlling the entire cell differentiation process, including the final dimensions of the tracheids. While the AWP showed minimal direct influence, the SF limit (< 20 kg day⁻¹) significantly reflected cell wall formation, highlighting its sensitivity to TWB levels, which are well detectable by the SF itself. Since seasonal water potential fluctuations regulate xylem cell wall thicknesses, we further demonstrated that thicker latewood tracheids result from accelerated wall thickening driven by improved TWBs. The SWP–SF–AWP interaction reveals the complex dynamics of the conduit system, as indicated by the morphological adaptations of developing tracheids under water depletion. Recognising the critical role of the TWB, this study underscores the need to reassess the synergy between conduit rehydration and functional‒morphological maintenance under water balance constraints.