Physiological responses of scion and rootstock in grafted grapefruit trees under salinity stress

: Soil salinity is a persistent global challenge in crop production, particularly in regions like coastal Florida (USA), where irrigation with high-salinity water adversely affects citrus production. This study aimed to evaluate the relative contributions of scion and rootstock to salinity tolerance in grafted grapefruit trees. Four scion/rootstock combinations were tested under controlled greenhouse conditions: ‘Star Ruby’ ( Citrus paradisi) grafted onto US-802 ( C . grandis × Poncirus trifoliata ), ‘Star Ruby’ grafted ontoto US-942 ( C . reticulata × P . trifoliata ), ‘Jackson’ ( C . paradisi ) grafted ontoto US-802, and ‘Jackson’ grafted onto US-942. The combinations were exposed to three salinity treatments: 4 mM NaCl (control), 50 mM NaCl, and 100 mM NaCl, over a six-week period. The experiment was arranged in a completely randomized design with seven replicates per treatment ( n = 7). Chlorophyll fluorescence (F v /F m ), gas exchange (E, A, gsw), and canopy volume were recorded weekly, while plant and soil Na and Cl contents, K/Na ratio, dry weights (leaf, stem, root), chlorophyll content (Chl a and Chl b ), and root apoplastic barrier formation were evaluated at the end of the experiment. Chlorophyll content and leaf dry weight were identified as primary indicators of salinity tolerance, with the rootstock showing a stronger influence than the scion. Among the rootstocks, US-942 exhibited greater tolerance than US-802. In contrast, the scion played a greater role in regulating Na accumulation and K/Na ratio, with ‘Jackson’ exhibiting greater tolerance than ‘Star Ruby’. Combinations showing an early decline in F v /F m and gas exchange also experienced significant reductions in dry weight and canopy volume. Root apoplastic barrier formation was found a potential marker for salinity tolerance. Salinity tolerance was expressed as a gradient among the graft combinations rather than as a simple rootstock effect, with ‘Jackson’/US-942 showing the greatest tolerance, followed by ‘Jackson’/US-802 and ‘Star Ruby’/US-942, whereas ‘Star Ruby’/US-802 was the most sensitive, emphasizing the importance of selecting optimal graft combinations for citrus production under saline conditions.