PHYSIOLOGICAL MECHANISMS IN DROUGHT TOLERANT Digitaria exilis L.

Abstract

Drought-induced stress in plants is associated with excessive production of radicals resulting in oxidative damage. Digitaria exilis (DE) locally known as Acha is a drought tolerant food crop cultivated mainly in semiarid areas of West Africa. However, there is paucity of information on the radical scavenging mechanisms underlying its drought tolerance. Therefore, this study was aimed at investigating the physiological mechanisms that account for drought tolerance in various accessions of D. exilis.

Three DE accessions (Dinat Iburua-DI, Jakah Iburua-JI1 and Jiw Iburua-JI2) and two DE accessions (NG/11/JD/061 and NG/11/JD/062) were obtained from National Cereal Research Institute, Badeggi and National Centre for Genetic Resources and Biotechnology, Ibadan, respectively. Using in vitro techniques, Murashige and Skoog medium was supplemented with varied concentrations of poly- ethylene glycol 6000 to create osmotic conditions of -9.29, -13.93, -20.13, -26.32, -32.51 and 0MPa (control) in a completely randomised design. Seeds were inoculated and growth parameters (number of germinated seeds, leaf length, root and shoot weights) were measured after four weeks. In screenhouse experiments, plastic pots filled with sterilised top-soil (2kg each) were arranged factorially 5×3×4. The seeds were planted and subjected to 0, 4, 8 and 12-day water stress. Chlorophyll contents, free radicals (Hydroxyl (OH-) and 2, 2-Diphenyl 1-picryhydrazl hydrate (DPPH)), Catalase, Superoxide Dismutase (SOD), Anthocyanin and proline were determined following standard procedures. Nanodrop lite spectrophotometry was used to quantify the plant DNA. The drought tolerant index of 4.00-5.00 (drought tolerant), 2.00-3.99 (mild-drought tolerant) and below 2.00 (drought susceptible) were calculated using standard formula. Leaves were examined for anatomical features. Data were subjected to descriptive statistics and ANOVA at α0.05 was used to determine the best parameter.

The number of germinated seeds (5.00±0.61), leaf length (40.00±4.95cm) and fresh shoot weights (0.80±0.18g) of JI1 were significantly different from other accessions in the in vitro experiments. Accession JI1 had highest total chlorophyll (298.60μg/ml), Catalase (3.63units/mg protein), SOD (1.70units/mg protein) and proline (0.05mg/ml) compared with others. Proline concentration in JI1 was significantly higher (LSD 0.009) than other accessions; suggesting that JI1 exhibited high antioxidant enzymes which were osmoprotected by accumulation of proline. Scavenged radicals were high in JI1: 80% OH- and 78% DPPH compared to other accessions. In screenhouse experiment, DNA concentration (96mg/μl) and dry root weight (2.25g) were highest in DI which indicates well-developed deep and prolific root systems. The NG/11/JD/061 had the highest anthocyanin content (1114.65μg/ml) and low lipid peroxidation (0.0000069M), which lowered the osmotic potential of the leaves. The JI1 was drought tolerant (4.32); NG/11/JD/061 (3.93) and DI (2.12) were mildly drought tolerant, while JI2 (0.77) and NG/11/JD/062 (1.75) were susceptible to drought stress. Leaves of JI1 (113-175μm) and DI (132μm) had intercellular spaces which indicate rapid flow of gaseous exchange. All drought stressed accessions revealed turgid bulliform cells which indicate reduce transpirational loss.

Digitaria exilis accession Jakah Iburua had the most osmolyte which stabilised the activities of catalase and superoxide dismutase against hydroxyl radicals generated during oxidative stress.