Exogenous Application Of Some Potential Growth Regulators For Improving Drought Tolerance In Maize And Soybean

World population is increasing at an alarming rate and is expected to reach about nine billion by the end of year 2050. On the other hand food productivity is decreasing due to the effect of various abiotic stresses; therefore minimizing these losses is a major area of concern for all nations to cope with the increasing food requirements. Environmental stresses trigger a wide variety of plant responses, ranging from altered gene expression and cellular metabolism to changes in growth rates and ultimately crop yields. Plants in nature are continuously exposed to several biotic and abiotic stresses, water deprivation being one of the commonest. Drought stress is perhaps the most severe threat for sustainable crop production in the conditions on changing climate. An efficient use of limited water resources and better growth under limited water supply are desirable traits for crops in drought environments. Crop production and sustainable development are severely constrained by water limitations during the growing season. Understanding the morphological, physiological, biochemical and molecular responses to drought is essential for a holistic perception of plant resistance mechanisms to water-limited conditions. The effect of drought at the whole plant and crop level is most complex because it reflects the integration of stress effects and responses at all underlying levels of organization over space and time. Plants have mechanisms that allow them to acclimate to drought stress. This is achieved by adjusting the photosynthetic apparatus, alteration in chlorophyll contents, changing the membrane lipid composition, accumulating proline, glycinebetaine and soluble sugars, and increasing levels of protective antioxidative system.Dry land crops such as maize and soybean play an important role in fulfilling the food and edible oil demand worldwide. Maize (Zea mays L.) is widely cultivated throughout the world and is the third most extensively planted field crop. It is regarded as an excellent model plant to examine the morphological, physiological and biochemical basis of water stress tolerance. Soybean is one of the crops where recent advances in crop physiology applications have been particularly important. It is one of nature's most versatile plants, and produces an abundant supply of protein and oil in both temperate and tropical environments. A cultivar is considered drought tolerant when the yield was significantly higher as compared to others in drought environment, but not in a non-drought environment. The gap between achievable and actual yields in farming systems is quite large because of various abiotic stresses, even when improved germplasm is available. Researchers are striving hard to develop resistance against water deficit in crop plants through application of potential growth regulators to ensure food security for the coming generations. Pot-culture experiments were carried out in rain-protected net-house of Southwest University, Chongqing, China, to investigate the possible role of exogenously applied brassinolide (BR), benzoic acid (BZA), fulvic acid (FA), Methyl jasmonate (MJ), and glycinebetaine (GB) in enhancement of drought resistance in maize and soybean.(1) Maize crop was subjected to moisture stress at tasseling followed by foliar spray of BR (0.1 mg L-1) to assess the changes in growth, yield, gas exchange, chlorophyll contents, protein, relative leaf water contents (RLWC), proline, malondialdehyde (MDA) and antioxidant enzymes activities. Drought stress severely hampered the maize growth, yield and yield components. It also exhibited a significant decline in gas exchange attributes and chlorophyll contents while increased the lipid peroxidation. Interestingly, exogenous application of BR considerably enhanced the gas exchange attributes and photosynthetic pigment contents, reduced due to water deficit conditions. These beneficial effects resulted in improved growth and yield under drought stressed as well as well-watered maize plants. It was further concluded that BR-induced drought resistance in terms of growth, yield, physiological and metabolic activities were mediated through enhanced activities of antioxidant enzymes and lowering the lipid peroxidation under moisture stress. Physio-biochemical damage caused by water deficiency in maize was ameliorated by exogenous application of BR.(2) Soybean plants were subjected to water stress at blooming and BZA was foliarly applied at the rate of 0.5 mM after one week of imposition of moisture treatments. Water stress exhibited a considerable decline in gas exchange and chlorophyll contents leading to reduction in plant growth and productivity. Nonetheless, BZA treatment considerably improved the gas exchange and chlorophyll contents possibly by enhanced levels of antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)] activities. Furthermore, BZA treatment remarkably lowered the lipid peroxidation by reduction in malondialdehyde (MDA) contents. These beneficial effects ultimately resulted in improved growth and yield in terms of plant height, stem diameter, biological yield per plant, grain yield per plant and harvest index in water-stressed soybean plants. It was worth noted that BZA treatment also improved the soybean performance under well-watered conditions. Hence BZA may be applied exogenously to induce drought acclimation in plants.(3) Maize plants were grown under normal conditions till tasseling and were then subjected to drought by cessation of water followed by foliar application of FA at the rate of 1.5 mg L-1. Drought stress disrupted of photosynthetic pigments and reduced the gas exchange leading to a reduction in plant growth and productivity. Nonetheless exogenous FA application substantially ameliorated the adversities of drought by sustaining the chlorophyll contents and gas exchange possibly by enhanced levels of antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)] activities and proline. These beneficial effects yielded in terms of plant growth and grain yield. It is interesting to note that FA application also improved the crop performance under well-watered conditions. Hence FA may be applied exogenously to develop resistance against drought in maize.(4) Soybean plants were grown under normal conditions till blooming and were then subjected to drought by withholding irrigation followed by foliar application of MJ at the rate of 50μM. Drought stress substantially suppressed the yield and yield related traits, while enhanced the membrane lipid peroxidation. MJ application triggered the drought induced suppression in yield and yield components. Nonetheless substantial increase in activities of enzymatic antioxidants [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)], proline, relative leaf water contents (RLWC) with simultaneous decrease in membrane lipid peroxidation were observed in MJ treated plants under drought. These beneficial effects led to improvement in biological and grain yield, and harvest index under drought. Interestingly MJ application also improved the performance of soybean crop under well-watered conditions. These results suggest the involvement of MJ in improving drought resistance in soybean by modulating the membrane lipid peroxidation and antioxidant activities.(5) The experiment was performed to explore the role of exogenously-applied glycinebetaine (GB) in improving drought tolerance in two contrasting maize cultivars viz:DD-60 and ND-95. The maize crop was grown with normal water supply till tasseling and then was exposed to two levels of soil moisture, well-watered control and drought-stressed, and then GB at the rate of 100 mM was foliar applied after five days of imposition of moisture treatments. The gas-exchange and chlorophyll contents were substantially declined in both maize cultivars subjected to water-stressed conditions in comparison to unstressed conditions. However, this reduction was comparatively less in DD-60 than ND-95. Nonetheless, GB-treated plants considerably maintained higher gas-exchange and pigment contents during drought stress than non-GB treated plants, thereby revealing in drought-stressed. The GB-induced improvement in gas-exchange and chlorophyll contents under water stress ultimately resulted in improved growth, yield and yield components in both maize cultivars. Furthermore, the positive responses of exogenous GB application were more pronounced in DD-60 as compared to ND-95 in all traits under water stress. Drought stress noticeably reduced the relative leaf water content (RLWC) but increased protein and proline concentrations in leaves of both cultivars. The reduction in RLWC in DD-60 was lower than ND-95, whereas, the accumulation of protein and proline was substantially higher in DD-60 over ND-95 during the course of experiment. Nonetheless, GB spray led to substantial enhancement in RLWC, protein and proline accumulation in DD-60 than ND-95. Prolonged water stress induced the membrane lipid peroxidation in maize leaves, which was more severe in drought sensitive variety (ND-95) than in tolerant one (DD-60). GB-treatments substantially ameliorated the lipid peroxidation and electrolyte leakage in DD-60 over ND-95 in water stress. The activity of superoxide dismutase (SOD), peroxide dismutase (POD) and catalase (CAT) was substantially enhanced in DD-60 than ND-95 with the progression of drought stress at first and then decreased afterwards. Nevertheless, GB-treated plants considerably maintained higher antioxidants activity during progressive drought stress than non-GB treated plants, thereby revealing in drought-stressed. These results possibly suggest that the drought-tolerant DD-60 showed a better protection mechanism against water-induced lipid peroxidation by maintaining higher constitutive activities of antioxidant enzymes, than the sensitive ND-95. Water stress tolerance can be induced in maize and soybean by the exogenous application of growth regulators by modulating morphological, physiological and biochemical attributes.