*Article* **Hydrogen Sulfide, Ethylene, and Nitric Oxide Regulate Redox Homeostasis and Protect Photosynthetic Metabolism under High Temperature Stress in Rice Plants**

**Harsha Gautam, Mehar Fatma, Zebus Sehar, Iqbal R. Mir and Nafees A. Khan \***

Plant Physiology and Biochemistry Laboratory, Department of Botany, Aligarh Muslim University, Aligarh 202002, India; harshagautam99@gmail.com (H.G.); meharfatma30@gmail.com (M.F.); seharzebus5779@gmail.com (Z.S.); m3riqbal@gmail.com (I.R.M.)

**\*** Correspondence: naf9.amu@gmail.com

**Abstract:** Rising temperatures worldwide due to global climate change are a major scientific issue at present. The present study reports the effects of gaseous signaling molecules, ethylene (200 μL L<sup>−</sup>1; 2-chloroethylphosphonic acid; ethephon, Eth), nitric oxide (NO; 100 μM sodium nitroprusside; SNP), and hydrogen sulfide (H2S; 200 μM sodium hydrosulfide, NaHS) in high temperature stress (HS) tolerance, and whether or not H2S contributes to ethylene or NO-induced thermo-tolerance and photosynthetic protection in rice (*Oryza sativa* L.) cultivars, i.e., Taipei-309, and Rasi. Plants exposed to an HS of 40 ◦C for six h per day for 15 days caused a reduction in rice biomass, associated with decreased photosynthesis and leaf water status. High temperature stress increased oxidative stress by increasing the content of hydrogen peroxide (H2O2) and thiobarbituric acid reactive substance (TBARS) in rice leaves. These signaling molecules increased biomass, leaf water status, osmolytes, antioxidants, and photosynthesis of plants under non-stress and high temperature stress. However, the effect was more conspicuous with ethylene than NO and H2S. The application of H2S scavenger hypotaurine (HT) reversed the effect of ethylene or NO on photosynthesis under HS. This supports the findings that the ameliorating effects of Eth or SNP involved H2S. Thus, the presence of H2S with ethylene or NO can enhance thermo-tolerance while also protecting plant photosynthesis.

**Keywords:** ethylene; hydrogen sulfide; nitric oxide; photosynthesis; rice
