INTRODUCTION The climate changes would raise global air temperature by 1.1-6.4°C (Lobell and Field, 2007), and at the end of this century, it would rise up to 1.4 to 5.8°C (Houghton et al., 2001). Hot semi-arid conditions have a negative impact on the production of cotton crop. It is also grown successfully in arid-irrigated regions and semi-humid areas of the world (Hearn, 1994). Cotton crop likes hot climate but very high temperatures affect its growth and yield badly as there has been found a negative relationship between high temperature and the yield of cotton crop (Oosterhuis, 1999). The most favorable temperature for cotton growth and photosynthesis ranges from 20-30°C (Burke et al., 1988; Reddy et al., 1991) while above 35°C cotton growth starts affecting. Unpredictable year to year diversity in cotton yield is due to water and temperature stresses (Brown et al., 2003; Saleem et al., 2015). High day and night temperature is much responsible for year to year variation in cotton yield (Oosterhuis, 2002). When plant temperature exceeds the optimum, it causes reduction in vegetative and reproductive growth (Singh et al., 2007; Mahmood et al., 2014) leading to reduction in yield of cotton crop (Easterling et al., 2007; Ainsworth et al., 2008). Very high and low temperatures influence the efficacy of Bt cotton during different growth stages of crop (Mahon et al., 2002). Brown et al. (1995) reported that during the monsoon nights, cotton foliage is 4-5°C warmer than the foliage in summer nights. Under field conditions, sowing date is the most important factor for initiation of heat stress. Reproductive stages of early sown cotton come during the hottest months that can cause serious reduction in yield (Rahman et al., 2007; Khan et al., 2014). Growth regulators are organic or inorganic substances activating plant defensive system under abiotic stresses (Wahid and Shabbir, 2005). Hydrogen peroxide (H2O2) and salicylic acid (SA) are useful substances having role towards the release of stress (Gechev et al., 2006). The SA is a growth regulator within the plant body that regulates a number of phsyiological processes (Hayat et al., 2007) that palys an important role in stress tolerance in plants. Moringa leaf extract has great importance as its leaves contain Pak. J. Agri. Sci., Vol. 53(4), 767-776; 2016 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 DOI: 10.21162/PAKJAS/16.4974 http://www.pakjas.com.pk COMPARATIVE ABILITY OF SOME GROWTH REGULATORS FOR INDUCING THERMOTOLERANCE IN COTTON UNDER DIFFERENT THERMAL REGIMES M. Sarwar 1 , M. Farrukh Saleem 1,* , M. Ashfaq Wahid 1 , H. Naeem Asghar 2 and M. Shahid 3 1 Department of Agronomy, University of Agriculture Faisalabad, Pakistan; 2 Department of Soil and Environmental Sciences, University of Agriculture Faisalabad, Pakistan; 3 Department of Biochemistry, University of Agriculture Faisalabad, Pakistan. *Corresponding author’s e-mail: mfsuaf@yahoo.com Heat stress is the most important abiotic factor affecting cotton crop. Exposure of cotton crop to different thermal regimes at different growth stages influences growth and seed cotton yield. The foliar spray of growth regulators at these stages improves yield and growth of cotton crop by mitigating the adverse effects of heat stress. Therefore, a field study was conducted to evaluate the potential role of different growth regulators on thermo-tolerance in cotton during two consecutive years (2012 and 2013). Different thermal regimes were provided at three reproductive stages by overwhelming different temperatures under field conditions (April 2, May 3 and June 17 during 2012 and April 4, May 2 and June 19 during 2013). Foliar spray of water spray (control), hydrogen peroxide (30 ppm), salicylic acid (50 ppm), Moringa leaf extract (30 times diluted) and ascorbic acid (70 ppm) were studied at squaring, flowering and boll formation stages. April sown crop produced highest seed cotton yield, plant height, more number of nodes for the first fruiting branch and monopodial branches followed by May and June thermal regimes. June sown crop showed higher unopened bolls than others. Hydrogen peroxide (H2O2) produced higher seed cotton yield per plant and monopodial branches in high temperature sowing dates (April and May) than optimal thermal regime (June sown crop). Moringa leaf extract (MLE) and ascorbic acid (ASA) also produced similar results under high temperature sowing dates. The H2O2 and SA reduced unopened bolls in all thermal regimes over the control while MLE and ASA showed similar results like water spray. All the growth regulators showed non-significant effect on node number for first fruiting branch in all thermal regimes except in April sown crop where H2O2 and SA reduced node numbers over the water spray. Keywords: Growth substances, foliar spray, Gossypium hirsutum, high temperature stress, sowing dates, yield.