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.