_____________________________________________________________________________________________________ *Corresponding author: E-mail: chandrumano11@gmail.com; International Journal of Environment and Climate Change 10(10): 77-84, 2020; Article no.IJECC.61239 ISSN: 2581-8627 (Past name: British Journal of Environment & Climate Change, Past ISSN: 2231–4784) Influence of Heat Units on Defoliation, Physiology and Yield of Cotton during Defoliants Application P. Chandrasekaran 1* , V. Ravichandran 1 , T. Sivakumar 1 , A. Senthil 1 , L. Mahalingam 2 and N. Sakthivel 3 1 Department of Crop Physiology, Tamil Nadu Agricultural University, Coimbatore - 641 003, Tamil Nadu, India. 2 Department of Cotton, Tamil Nadu Agricultural University, Coimbatore - 641 003, Tamil Nadu, India. 3 Department of Agronomy, Tamil Nadu Agricultural University, Coimbatore - 641 003, Tamil Nadu, India. Authors’ contributions This work was carried out in collaboration among all authors. All authors read and approved the final manuscript. Article Information DOI: 10.9734/IJECC/2020/v10i1030251 Editor(s): (1) Dr. Arjun B. Chhetri, Dalhousie University, Canada. Reviewers: (1) De Paiva Eliege Aparecida, State University of Londrina, Brazil. (2) Amey Kossi Bollanigni, Université de Lomé, Togo. Complete Peer review History: http://www.sdiarticle4.com/review-history/61239 Received 10 July 2020 Accepted 17 September 2020 Published 25 September 2020 ABSTRACT Aims: A study was conducted to determine the effect of temperature on defoliation process, physiology and yield of cotton during defoliants application. Study Design: The experiment was laid out in split-plot design with four replications. Place and Duration of Study: Field experiment was conducted at Department of Crop Physiology, Tamil Nadu Agricultural University, Coimbatore during 2018 to 2019. Methodology: The main plot comprised of three stage of defoliants spray (120 DAS, 127 DAS and 134 DAS) and the sub-plots were seven defoliant treatments (Control, 2, 4 D (0.5%), Ethephon (0.5%), Ethephon (0.5%) + Triiodbenzoic acid (TIBA) (450 ppm), Sodium chlorate (0.9%), 6 benzylaminopurine (BAP) (0.1%), Thidiazuron + Diuron (0.03%). Results: After 15 days of defoliants spray, Thidiazuron + Diuron (0.03 %) recorded highest defoliation percentage (99.3, 98.1 and 96.8%) followed by Sodium chlorate (0.9 %) (91.9, 94.6 and 93.6%) at three different stage of defoliants spray. Although a significant effect was observed on chlorophyll index value (SPAD) due to the favorable weather condition like minimum temperature Original Research Article