This study considers citrus rootstocks as autonomous biological entities and examines whether, and to what extent, they differently regulate plant–water relations and biomass allocation as well as how such physiological variations translate into differences in vegetative vigor. To address these questions, four citrus genotypes—Sour Orange (SO), Volkamer Lemon (VL), Swingle Citrumelo (CTR), and Troyer Citrange (TC)—were compared with respect to their morphological traits, biomass distribution, and hydraulic properties. These four rootstocks were selected as they represent contrasting genetic backgrounds and well-documented differences in vigor, stress tolerance, and hydraulic behavior, providing an effective model for assessing intrinsic physiological variability. The findings reveal pronounced rootstock-specific differences in water acquisition, transport, and utilization, with direct implications for the hydraulic architecture, leaf water status, and partitioning of biomass between above- and belowground organs. CTR exhibited a highly integrated hydraulic strategy, characterized by elevated conductance across both aerial and root systems and accompanied by greater biomass allocation to the canopy and absorptive roots, resulting in an enhanced overall vigor. SO and VL displayed an intermediate physiological performance, whereas TC demonstrated a restricted hydraulic transport capacity, which is associated with lower biomass allocation, reduced leaf water potential, and diminished vigor. By assessing rootstocks independently of scion influences, this work demonstrates that variations between citrus rootstocks cannot be explained solely by morphological traits but instead reflect contrasting physiological strategies governing the coordinated management of water and carbon resources. These results highlight the rootstock as a central determinant of hydraulic functioning, biomass partitioning, and plant vigor and provide a conceptual basis for selecting rootstocks that are better suited to water-limited environments.
Gullo et al. (Wed,) studied this question.