Submitted: 05 November 2019 Accepted: 20 March 2020 Published Online: 31 March 2020 40 Plants and Environment (2020) 2(1): 40-53 https://doi.org/10.22271/2582-3744.2020.mar.40 Research article Exogenous proline and hormone in combination with compost improves growth and tolerance of maize under heavy metal stress Sifau A. Adejumo*, Vitoria Awoyemi and Adeniyi O. Togun Environmental Biology Unit, Department of Crop Protection and Environmental Biology, Faculty of Agriculture, University of Ibadan, Ibadan, Nigeria *Corresponding Author: nikade_05@yahoo.com Abstract: Abiotic stress such as heavy metal contamination of agricultural soil limits crop production by disrupting plant’s physiological activities and seed germination. Exogenous proline, hormone and compost have been used to induce growth and stress tolerance in crop but little is known about their interactive and anti-oxidative roles in crop exposed to heavy metal stress. This study assessed the effects of pre-sowing seed soaking with exogenous proline (P) and hormones (Gibberellic acid; GA and Indole-acetic acid; IAA) on the Pb accumulation, osmolytes (P, phenolics; PH and Glycine betaine; GB), Photosynthetic pigments (chlorophyll and carotenoids) and stress indexes (H 2 O 2 and malonialdehyde; MDA) of maize grown on compost amended Pb (23,959 mg kg -1 Pb) contaminated soil. Compost was applied at 20 t ha -1 and 40 t ha -1 , P, GA and IAA were applied at 10 mM and 5 mM solely and in combinations to give a total of 34 treatments including control (Distilled water only) and replicated 3 times. Except in IAA treatment, seed pre-treatments with exogenous P and GA and soil amendment with compost generally enhanced chlorophyll production in maize and reduced GB, carotenoids, H 2 O 2 and MDA in treated seeds compared to control. With regards to stress indexes G2+C1 gave the lowest values. Sole application of P1, P2, C2, G1 and I2 enhanced the production of proline in treated plants compared to combined application. Combination with compost was better than hormonal combination. A combination of P2 with C (40 t ha -1 ) (P2C2) gave the maximum chlorophyll content and reduced the endogenous GB and P. Treatment with P2C2 also reduced Pb accumulation in plant. In conclusion, exogenous application of hormones and proline in combination with organic amendment induces heavy metal tolerance in maize and increased protection against oxidative stress compared to contaminated control. The technique that can enhance crop growth on contaminated soil, reduce metal uptake and provide osmo-protection could be a promising approach. Keywords: Contamination, Hormones, Osmolytes, Organic amendment, Seed soaking, Heavy metals. INTRODUCTION Soil contamination with heavy metals is one of the abiotic stresses that cause a reduction in crop production (Sandalio et al., 2001). They affect plant physiological processes and induce production of Reactive Oxygen Species (ROS) that cause oxidative damage and lipid peroxidation (Eun et al., 2000; Siedlecka & Krupa, 2002; Verma & Dubey, 2003; Souguir et al., 2011). The contamination of agricultural lands from different anthropogenic sources is most worrisome. Apart from reducing the amount of cultivable lands, it increases the risk of animal and human contamination with toxic heavy metals through food chain and poses a threat to food security. Heavy metal also causes reduction in photosynthesis and retard crop growth by competing with essential mineral nutrients and inducing phytotoxicity (Siedlecka, 1995; Sharma & Dubey, 2005, 2007). Different efforts have been made to remediate heavy metal contaminated soils (Adejumo et al., 2010) but none of the strategies proposed can give complete remediation. The development of metal-tolerant plants has also not been possible due to the complexity of the procedures involved. There is need therefore, for the development of simple and cost-effective methods for containing the contaminants, reduce/prevent plant uptake and enhance crop tolerance. Though the use of immobilizing agents to reduce metal solubility has been used effectively for reducing metal uptake by cultivated crops (Adejumo et al. 2010, Salati et al. 2010, Adejumo et al. 2018a) but, the first challenge in crop production on contaminated soil is seed germination and seedling development. Seed germination is the most crucial step in plant’s life cycle that ensures the survival of the next generation. If these hurdles can be overcome, plants have natural inbuilt ability to induce anti-oxidative mechanisms that can be used for survival in the face of different environmental challenges (Hossain et al., 2012). In this regard, the technique that can stimulate seed germination, induce seedling development, reduce metal uptake, ensure nutrient availability for crop development and provide osmo- protection could be an acceptable and comprehensive method for enhancing crop tolerance to heavy metal toxicity. Seed priming technique with osmo-protectants and growth elicitors like hormones have been implicated in the activation of some pre-germination metabolic processes and stress tolerance inducement (Tsegay & Andargie, 2018).