EXOGENOUS USE OF SOME POTENTIAL ORGANIC OSMOLYTES IN ENHANCING DROUGHT TOLERANCE IN MAIZE (Zae mays L.)

Abstract

In the present study, a series of experiments were conducted in the Botanic Gardens of the Department of Botany, University of Agriculture, Faisalabad to test the relative effectivness of the three osmolytes in alleviating the adverse effects of drought stress on maize plants. The osmolytes were applied through different modes at different growth stages of maize plants under non-stress or water stress conditions. In the first experiment, varying levels (0, 30 and 60 mM) of each of the osmolytes were applied as foliar spray and presowing treatment of seeds of two cultivars (Agaiti-2002 and EV-1098). The plants raised from the treated seeds were grown in plastic pots filled with sandy loam soil under greenhouse conditions. After two weeks of the imposition of water stress, different levels (0, 30, and 60 mM) of each of the osmolytes were exogenously applied as a foliar spray to maize plants at the seedling, vegetative or both seedling and vegetative stages when grown under non-stress or water stress conditions (60% field capacity). Tween-20 (0.1%) was used as a surfactant for foliar spray. It was found that 30 mM of GB, or proline or trehalose was the most effective in promoting seedling growth under water stress conditions. However, GB and proline were more effective than trehalose in ameliorating the adverse effects of water stress on growth of both maize cultivars. It was found that effective concentration of exogenously applied osmolytes through seed soaking or through foliar application is 30 mM. During the 2 nd experiment the effective concentrations of these osmolytes (GB, proline and trehalose) as estimated from the 1 st experiment were applied as pre-sowing seed treatment and as foliar spray at different growth stages (seedling, vegetative and seedling+vegetative) as in the 1 st experiment to observe their effects in inducing drought tolerance maize plants in relation to various physiological and biochemical attributes. Exogenous application of osmolytes as presowing seed treatment or as foliar spray at different growth stages increased the photosynthetic rate of the plants of both cultivars that was associated with stomatal as well as non-stomatal factors. Plant water status as reflected by leaf water potential and leaf relative water content (RWC) was improved by the exogenously applied organic solutes and it was associated with leaf osmotic potential, endogenous GB and proline, which suggests the role of these osmolytes in osmotic adjustment. Exogenous application of these osmolytes increased the accumulation of nutrients particularly K, and shoot nutrient use efficiency in maize plants under water stress conditions. Exogenous application of compatible solutes enhanced the antioxidant capacity of drought stressed maize plants by increasing the activities of antioxidant enzymes (SOD, POD and CAT) and non-enzymatic compounds (phenolics, tocopherols and ascorbic acid) which is evident from the reduced MDA contents determined in the present study. Exogenously applied different osmolytes enhanced the seed and seed oil quality of both maize cultivars. Seed sugar, seed oil, seed protein, seed macro- and micronutriets increased due to exogenous application of these osmolytes. Moreover, improved seed oil quality was associated with enhanced oil unsaturation in terms of increased in oleic and linoleic acid contents as well as the oil lipophilic antioxidants (tocopherols, phenolics, flavonoids) which in-turn enhanced the oil antioxidant activity. Overall, exogenously applied different compatible solutes as a seed treatment or foliar application improved growth and yield of maize pants. Osmolyte-induced enhancement in growth and yield under water stress conditions was associated with increase in net CO 2 assimilation rate, plant water status, antioxidant capacity and nutrient accumulation, particularly of K. However, further studies are required to explicitly elucidate the mechanism of organic osmolytes influx through epidermis and target enzymes or metabolites to induce stress tolerance in plants. Moreover, further studies are important to work out cost-benefit ratios of the use of different organic osmolytes used in this study. This information will be undoubtedly beneficial for stakeholders, particularly the farmers.