Effects of Soil Salinity on Citrus Rootstock 'US-942' Physiology and Anatomy

The accumulation of salt deposits in the soil is becoming a major problem in agriculture. The problem can affect citrus production, which is already seriously hindered in Florida by the disease known as Huanglongbing (HLB, or citrus greening). With the citrus industry declining, it is imperative to study all potential environmental stresses affecting the citrus cultivation and to conduct screening tests to evaluate which rootstocks perform best under these challenging new conditions. This study aims to determine how the 'US-942' rootstock reacts to salinity. Thirty-six 3-month-old 'US-942' citrus rootstock seedlings were grown in a greenhouse. After 15 d of acclimation, plants were assigned to one of six groups and treated for 30 d with solutions of 25% Hoagland solution amended with 0 (control), 30, 60, 90, 120, or 150 mM NaCl. Higher NaCl concentrations significantly hindered plant growth and negatively affected some physiological processes [i.e., stomatal conductance (g(S))] and chlorophyll contents. Conversely, plants treated with mild concentrations (30 mM NaCl) had higher plant biomass and exhibited higher photosynthetic efficiency. Free hand sections of fresh roots were taken at the end of the experiment, and the suberin lamellae development was examined under a fluorescence microscope. In conclusion, results reported that 'US-942' rootstock is tolerant to mild salt stress and confirmed the hypothesis that the formation of root apoplastic barriers and the increase in the root biomass could be two possible mechanisms that give the 'US-942' a mild NaCl stress tolerance.