Pre-treatment of two contrasting water-stressed genotypes of cassava (Manihot esculenta Crantz) with ascorbic acid. I. Growth, physiological and antioxidant responses

Water deficit-stress at early growth stage is a major constraint of cassava production. Ascorbic acid is a non-enzymatic antioxidant that mitigates oxidative damage caused by water stress in plants. Growth, physiological and antioxidant defense system responses of two contrasting water-stressed cassava genotypes to pre-treatment with foliar application of ascorbic acid (AA) were investigated. The objectives of this study were to assess the growth, proline, photosynthesis pigments and antioxidant activities of young water-stressed cassava plants pre-treated with ascorbic acid. The study consisted of IITA-TMS-IBA980581 (drought tolerant) and IITA-TMS-IBA010040 (drought sensitive) cassava genotypes treated with six doses (0.00, 0.25, 0.50, 0.75 and 1.00 mM) of AA before being subjected to water deficit (45.0% field capacity) and a water sufficient AA-untreated control. In both genotypes, water stress reduced shoot height (40.3%), leaf area (42.5%), and number of root (54.5%), biomass (28.6%), relative water content (RWC, 3.2%) and photosynthetic pigments (300.0%). However, water stress increased proline (91.3%), endogenous AA (112.0%), catalase (CAT, 300.0%) and superoxide dismutase (SOD, 15.3%) in both genotypes. Compared with IITA-TMS-IBA010040, leaf area, biomass, number of root and shoot height of IITA-TMS-IBA980581 were higher by 7.3, 24.6, 25.9 and 13.1%, respectively. By less than a quarter, chlorophylls a and b, activity of superoxide dismutase and relative water content of IITA-TMS-IBA980581 were higher compared with IITA-TMS-IBA010040. However, proline content of IITA-TMS-IBA010040 was higher than IITA-TMS-IBA980581 by 14.3%. Pre-treatment with AA improved growth parameters, photosynthetic pigments, RWC, endogenous AA, activity of CAT and SOD, but decreased proline in both genotypes with an optimum concentration at 0.5 mM. Pre-treatment with 0.5 mM AA increased shoot height, area of leaves, leaf number, number of root and dry weight by 46.3, 44.7, 14.4, 88.2 and 37.5%, respectively. Pre-treatment with 0.5 mM AA doubled chlorophylls, tripled carotenoids content, doubled endogenous AA and slightly enhanced RWC (2.1%) and SOD (2.0%) when compared with AA-untreated water stressed plants. But pre-stress application of AA reduced proline content by one-fold, increased CAT activity by one-fold in IITA-TMS-IBA980581 and by one-third in IITA-TMS-IBA010040. The study concluded that pre-treatment of cassava young plants with AA before water deficit could alleviate oxidative stress.