Interstock-induced mechanism of increased growth and salt resistance of orange (Citrus sinensis) trees.

Interstocks improve the growth and salt resistance of lemon (Citrus limon (L.) Burm. f.) trees, but their effects on orange (Citrus sinensis (L.) Osbeck) trees are unknown. We grew 'Cleopatra' mandarin (CM) seedlings, budded trees of 'Salustiano' orange (SAO) on CM, 'Valencia Late' orange (VLO) on CM (VLO/CM), and interstock trees VLO/SAO/CM in pots of sand watered with nutrient solution containing 5 (control) or 50 mM NaCl for 12 weeks. Plants were harvested on six successive occasions and the time trends in relative growth rate (RGR) and its components were estimated by fitting a Richards function regression to the harvest data. At low and high salinities, the VLO/SAO/CM combination had higher mean RGR than VLO/CM. Under control conditions, the increase in RGR caused by the interstock was the result of an increase in leaf mass fraction (LMF; leaf dry mass/plant dry mass ratio). Increases in net assimilation rate on a leaf mass basis (NARm) and LMF contributed equally to the increase in RGR in saline conditions, their growth response coefficients being 0.52 and 0.48, respectively. The structural modifications, specific leaf area (SLA) and leaf area ratio (LAR; leaf area:plant dry mass ratio), had a slight influence on the reduction in RGR by salinity. However, NARm had a large influence on RGR, except in CM. The interstock-induced mechanism increased biomass allocation to the assimilatory organs and, under saline conditions, increased Cl- and Na+ allocations to roots. Thus, the flux of ions to the leaves was either delayed or reduced or both. The dilution of imported ions by foliar growth reduced ion concentrations in leaves, resulting in higher NARm, which together with higher LMF, increased RGR.