ORIGINAL RESEARCH published: 27 April 2022 doi: 10.3389/fpls.2022.855900 Edited by: Shabir Hussain Wani, Sher-e-Kashmir University of Agricultural Sciences and Technology, India Reviewed by: Noushina Iqbal, Jamia Hamdard University, India Muhammad Ali, Zhejiang University, China *Correspondence: Zhihui Cheng chengzh@nwafu.edu.cn Specialty section: This article was submitted to Plant Breeding, a section of the journal Frontiers in Plant Science Received: 16 January 2022 Accepted: 28 February 2022 Published: 27 April 2022 Citation: Amin B, Atif MJ, Meng H, Ali M, Li S, Alharby HF, Majrashi A, Hakeem KR and Cheng Z (2022) Melatonin Rescues Photosynthesis and Triggers Antioxidant Defense Response in Cucumis sativus Plants Challenged by Low Temperature and High Humidity. Front. Plant Sci. 13:855900. doi: 10.3389/fpls.2022.855900 Melatonin Rescues Photosynthesis and Triggers Antioxidant Defense Response in Cucumis sativus Plants Challenged by Low Temperature and High Humidity Bakht Amin 1 , Muhammad Jawaad Atif 1,2 , Huanwen Meng 1 , Muhammad Ali 1 , Shuju Li 3 , Hesham F. Alharby 4,5 , Ali Majrashi 6 , Khalid Rehman Hakeem 4,5,7 and Zhihui Cheng 1 * 1 College of Horticulture, Northwest A&F University, Yangling, China, 2 Horticultural Research Institute, National Agricultural Research Centre, Islamabad, Pakistan, 3 Tianjin Kerun Cucumber Research Institute, Tianjin, China, 4 Department of Biological Sciences, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia, 5 Princess Dr. Najla Bint Saud Al-Saud Center for Excellence Research in Biotechnology, King Abdulaziz University, Jeddah, Saudi Arabia, 6 Department of Biology, College of Science, Taif University, Taif, Saudi Arabia, 7 Department of Public Health, Daffodil International University, Dhaka, Bangladesh Environmental factors such as low temperature (LT) and high humidity (HH) hinder plant growth and development in plastic tunnels and solar greenhouses in the cold season. In this study, we examined the effect of melatonin (MT) on shoot-based tolerance to LT and HH conditions in cucumber (Cucumis sativus) seedlings and explored its underlying mechanism. LT and HH stress inhibited growth and biomass accumulation, produced leaf chlorosis, led to oxidative stress, lowered chlorophyll and carotenoid contents, reduced photosynthetic and photosystem II (PSII) activities, and increased the level of intercellular carbon dioxide and the non-photochemical quenching of photosystem I (PSI) and PSII. However, foliar application of MT significantly improved the morphological indices and photosynthetic efficiency of cucumber seedlings, which entailed the elevation of electrolyte leakage, lipid peroxidation, and reactive oxygen species accumulation by boosting the antioxidant enzyme defense system under LT and HH conditions. Additionally, the measurement of nitrogen (N), magnesium (Mg), and iron (Fe) contents in roots and leaves showed that MT significantly augmented the nutrient uptake of cucumber seedlings exposed to LT and HH stresses. Furthermore, MT application increased the transcripts levels of genes encoding antioxidant enzymes under LT and HH conditions, whereas treatment with LT and HH suppressed these genes, suggesting that MT application increases the LT and HH tolerance of cucumber seedlings. Overall, our results suggest that MT application increases the tolerance Frontiers in Plant Science | www.frontiersin.org 1 April 2022 | Volume 13 | Article 855900