Novel Citrus Rootstocks Mediate Distinct Physiological Strategies for Salinity Tolerance in Grafted Sweet Orange Trees

Salinity stress increasingly threatens world citrus production, particularly in coastal regions experiencing seawater intrusion. Because field testing is slow, initial greenhouse screening is critical to rapidly identify resilient rootstock candidates. We evaluated nine novel rootstock hybrids [US-1672, US-1673, US-1676, US-1680, US-1687, US-1688 (‘SuperSour 4’), US-2111, US-2132, and US-2137] grafted with ‘Valencia’ sweet orange against sour orange (tolerant) and Swingle citrumelo (sensitive) reference rootstocks under controlled greenhouse conditions. Trees were exposed to 0, 50, or 100 mM NaCl for 12 weeks. Both salinity treatments reduced scion growth, chlorophyll content, and fibrous-root biomass, while increasing leaf Na and Cl accumulation and fibrous-root respiration, indicating substantial metabolic costs. Rootstocks differed in their capacity to maintain grafted tree vigor, regulate ion balance, and adjust fibrous root morphology. US-1688 and US-1680 displayed a vigor-driven tolerance, allowing trees to retain high aboveground biomass. US-1672 exhibited a root-growth-driven response, characterized by a high proportion of fine roots. US-2132 demonstrated an ion-regulation strategy that restricted leaf Na accumulation. Conversely, Swingle and other rootstocks exhibited poor overall performance, characterized by ion accumulation and increased root respiration. Notably, rootstocks with identical parentage often exhibited completely different tolerance strategies, demonstrating that parentage alone does not consistently predict functional resilience. Stem dry weight, scion diameter, SPAD, leaf Na, leaf Cl, fibrous root biomass, fibrous root respiration, and fibrous root diameter distribution emerged as potential early screening indicators. These results support the screening framework and identify distinct tolerance strategies to accelerate rootstock selection for salt-affected areas.