**3. Results**

#### *3.1. Growth Parameters*

The rice plants' growth was analyzed in terms of shoot and root dry weight. The plants subjected to HS showed a reduction in shoot dry weight by (25.4%) in Taipei-309 and (28.0%) in Rasi, and root dry weight by (26.7%) in Taipei-309 and (30.3%) in Rasi, in comparison to control. However, Eth, SNP or NaHS application resulted in the reversal of the adverse effects of HS on rice growth; as the shoot dry weight recovered by Eth (29.0% and 28.0%), SNP (25.4% and 22.0%), and NaHS (21.8% and 20.0%) in Taipei-309 and Rasi, respectively, compared to control plants (Figure 1).

In Taipei-309 and Rasi, foliar applications of Eth, SNP, or NaHS lowered the effect of HS on root dry weight by (25.3% and 19.6%), (21.1% and 16.6%), or (18.3% and 13.6%), respectively, compared to controls.

This suggests that Eth, SNP, or NaHS can reduce the negative effects of HS on the biomass of both rice cultivars. Moreover, the application of HT exacerbated the deleterious effects of HS in Taipei and Rasi cultivars, resulting in reduced shoot dry weight by (12.7% and 14.0%) and by (16.3% and 18.0%) in the presence of Eth and SNP, respectively. Similarly, the root dry weight was reduced by (14.0% and 16.6%) and by (16.9% and 19.6%) in Eth and SNP-treated Taipei-309 and Rasi, respectively, relative to control plants. Therefore, combining HT with Eth or SNP completely reversed the mitigating effects of Eth or SNP. The application of NBD and cPTIO reduced shoot and root dry weights in the presence of NaHS in heat-treated plants in both the cultivars, reflecting that the application of NBD or cPTIO with NaHS did not entirely reverse the mitigating effects of NaHS on growth characteristics.

**Figure 1.** (**a**) Shoot dry weight and (**b**) root dry weight of rice (*Oryza sativa* L.) cultivars Taipei-309 and Rasi under control and high temperature stress (HS) supplied with 200 μL L−<sup>1</sup> ethephon (Eth), 100 μM sodium nitroprusside (SNP), 200 μM sodium hydrosulfide (NaHS) 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. 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.
