Rootstocks and Root Systems in Citrus clementina (Hort ex Tan.) Plants: Ecophysiological, Morphological, and Histo-Anatomical Factors

Rootstock selection plays a pivotal role in determining the ecophysiological performance, growth dynamics, and hydraulic functioning of grafted citrus plants. This study evaluated three citrus rootstocks—Trifoliate Orange (TO), Swingle Citrumelo (SC), and Flying Dragon (FD)—grafted with Citrus clementina cv. SRA 63 (CLM), with the aim of elucidating how the rootstock genotype influences morphological traits, dry matter allocation, hydraulic conductance, and xylem anatomical features. Plants were monitored over two years under controlled agronomic conditions, and biometric, physiological, hydraulic, and anatomical traits were assessed. The results revealed distinct rootstock-dependent patterns. CLM/TO and CLM/SC exhibited greater vegetative vigor, higher total biomass, more extensive absorbing root systems, and larger conductive xylem areas, resulting in superior theoretical hydraulic flow. In contrast, CLM/FD demonstrated reduced growth, a smaller trunk diameter, lower biomass accumulation, and elevated hydraulic resistance in both root and graft union sectors, consistent with its known dwarfing behavior. Despite its lower hydraulic efficiency, FD promoted the highest stomatal conductance, suggesting a distinct water use strategy. Overall, the findings demonstrate that the rootstock genotype markedly influences the hydraulic architecture and growth partitioning of grafted Clementine plants. These insights contribute to our understanding of scion–rootstock interactions and support more informed selections of rootstocks in citrus orchards under diverse environmental and management scenarios.