EFFECTS OF GIBBERELLIC ACID ON THE GROWTH TRAITS, PROLINE CONTENT, PHOTOSYNTHESIS INTENSITY, AND MIRCROMORPHOLOGY OF KING MANDARIN (<i>CITRUS NOBILIS</i> LOUR.) UNDER <i>IN VITRO</i> SALINITY STRESS CONDITIONS

King mandarin (Citrus nobilis Lour.) is one of the main fruit trees in agricultural production in Vietnam due to its high nutritional and economic value. Cultivation in the Mekong Delta, the largest citrus-producing region in Vietnam, is currently threatened by increasingly serious drought and salinity. Supplementing exogenous gibberellic acid helps stimulate the activity of antioxidant enzymes, improves stomatal conductance and photosynthesis intensity, and increases plant growth and salt stress tolerance. This study investigates the effects of gibberellic acid at concentrations of 0.0, 0.1, 0.3, and 0.5 mg.L-1 on the growth and the physiological and biochemical parameters of King mandarin in vitro under salinity stress conditions. The results indicate that the presence of gibberellic acid in the culture medium improved the germination and growth traits of King mandarin, particularly in the culture medium supplemented with 0.5 mg.L-1 gibberellic acid. Higher gibberellic acid concentration correlates with lower proline content. In addition, lignification and granular deposits in the root anatomical structure were lower with gibberellic acid treatment than with the control treatment. These results indicate that gibberellic acid treatment can be an effective tool for improving the growth and development and the physiological and biochemical parameters of King mandarin under salinity stress.