*3.2. Gas-Exchange Parameters and Chlorophyll Content*

Net photosynthesis rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and SPAD value decreased under HS by 35.0%, 26.9%, 23.2%, and 28.4% in Taipei-309 and 36.1%, 28.2%, 24.2%, and 29.2% in Rasi, respectively, compared to control plants (Table 1). In comparison to control and heat-stressed plants, the individual treatment of Eth, SNP, and NaHS increased these parameters significantly. The individual applications of Eth, SNP, or NaHS under HS showed a substantial increase in *Pn* (29.1% and 98.9%, 27.1% and 95.9%, 24.5% and 91.8%), *Gs* (25.6% and 71.9%, 25.0% and 71.8%, 24.4% and 70.3%), *Ci* (21.3% and 57.9%, 20.2% and 56.6%, 19.9% and 56.1%), and SPAD value (37% and 91.7% and 35.9% and 90%, 32% and 84.6%) in Taipei-309 and *Pn* (25.6% and 96.7%, 24.3% and 94.5%, 20.8% and 89%), *Gs* (23.1% and 71.6%, 21.9% and 70%, 21.4% and 69.3%), *Ci* (18.7% and 56.8%, 17.8% and 55.6%, 16.2% and 53.4%), and SPAD value (35.8% and 91.9%, 31.4% and 85.6%, 27.6% and 80.2%) in Rasi, respectively. Exogenously-applied Eth considerably alleviated more compared to SNP or NaHS, the decrement in the levels of *Pn*, *Gs*, *Ci*, and SPAD caused by HS. However, the potential effects of Eth or SNP on these parameters were significantly minimized by the H2S scavenger, HT. The treatment of NBD or cPTIO moderately reversed the mitigating effects of NaHS.

**Table 1.** Net photosynthetic rate (μmol CO2 m−<sup>2</sup> s<sup>−</sup>1), stomatal conductance (mmol m−<sup>2</sup> s<sup>−</sup>1), intercellular CO2 concentration (μmol mol<sup>−</sup>1), and chlorophyll content (SPAD value) of rice (*Oryza sativa* L.) cultivars Taipei-309 and Rasi after foliar treatment of plants with 200 μL L−<sup>1</sup> ethephon (Eth), 100 μM sodium nitroprusside (SNP) or 200 μM sodium hydrosulfide (NaHS) grown with or without high temperature stress (HS; 40 ◦C) or 100 μM hypotaurine (HT), 100 μM 2-4-carboxyphenyl-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) or 100 μM norbornadiene (NBD) scavengers of hydrogen sulfide (H2S), nitric oxide (NO), and ethylene action inhibitors, respectively, with HS at 15 days after sowing. Data are presented as treatments mean ± SE (*n* = 4). The values followed by the same letters did not differ significantly by LSD test at *p* < 0.05.


#### *3.3. Chlorophyll Fluorescence Parameters*

The measurement of chlorophyll fluorescence parameters was taken under both stress and without stress in leaves of rice cultivars (Tables 2 and 3). Heat stress exposure reduced the studied fluorescence parameters compared to control, but treatment with Eth, SNP, or NaHS increased these parameters under stress and no stress conditions. In contrast, NPQ increased under HS but decreased significantly with Eth, SNP, or NaHS application under no stress compared to control. Eth, SNP or NaHS treatments proved effective in improving (ΦPS II, Fv/Fm, Φesc, qP, and ETR) in heat-treated plants compared to controls. The data revealed that Eth, SNP, or NaHS treatments were essential to mitigate the negative effects of HS on the parameters mentioned above. Still, Eth was more effective than SNP or NaHS. The application of NBD or cPTIO reversed the positive effects of NaHS on the chlorophyll fluorescence parameters under HS; however, the addition of HT (100 μM) along with Eth or SNP entirely reversed the positive effects of Eth or SNP on these parameters in heat-stressed conditions.

**Table 2.** Actual efficiency of PSII, maximal efficiency of PSII, intrinsic efficiency of PSII, photochemical quenching, non-photochemical quenching, and electron transport rate of rice (*Oryza sativa* L.) cultivar Taipei-309 after foliar treatment of plants with 200 μL L−<sup>1</sup> ethephon (Eth), 100 μM sodium nitroprusside (SNP), or 200 μM sodium hydrosulfide (NaHS) grown with or without high temperature stress (HS; 40 ◦C) or 100 μM hypotaurine (HT), 100 μM 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) or 100 μM norbornadiene (NBD) scavengers of hydrogen sulfide (H2S), nitric oxide (NO), and ethylene action inhibitors, respectively, with HS at 15 days after sowing. Data are presented as treatments mean ± SE (*n* = 4). The values followed by the same letters did not differ significantly by LSD test at *p* < 0.05.


**Table 3.** Actual efficiency of PSII, maximal efficiency of PSII, intrinsic efficiency of PSII, photochemical quenching, non-photochemical quenching, and electron transport rate of rice (*Oryza sativa* L.) cultivar Rasi after foliar treatment of plants with 200 μL L−<sup>1</sup> ethephon (Eth), 100 μM sodium nitroprusside (SNP), or 200 μM sodium hydrosulfide (NaHS) grown with or without high temperature stress (HS; 40 ◦C) or 100 μM hypotaurine (HT), 100 μM 2-4-carboxyphenyl-4,4,5,5 -tetramethylimidazoline-1 oxyl-3-oxide (cPTIO) or 100 μM norbornadiene (NBD) scavengers of hydrogen sulfide (H2S), nitric oxide (NO), and ethylene action inhibitors, respectively, with HS at 15 days after sowing. Data are presented as treatments mean ± SE (*n* = 4). The values followed by the same letters did not differ significantly by LSD test at *p* < 0.05.

