Sulfur deficiency reduces the thermotolerance of<i> Heliotropium thermophilum</i> to high temperatures

thermophilum, a thermotolerant plant, was subjected to low sulfate treatments to examine the effects of sulfur deficiency on the plant's thermotolerance. For this aim, two different concentrations (0.15 mM and 0.30 mM) of low-sulfate media as well as full nutrition (FN) medium were prepared and the plants were cultured at 25 degrees C for 60 days, then divided into 2 groups for temperature applications and kept at either 25 or 40 degrees C for 7 days. H. thermophilum could survive in low-sulfate media at 40 degrees C, but high temperature damages such as chlorosis, and green color retention were observed. In addition, heat treatment reduced plant fresh weight, relative water content, and total sugar contents in both low-sulfate media. Moreover, proline and hydrogen peroxide levels were the highest in plants grown in 0.15 mM sulfate at 40 degrees C. Peroxidase activities were increased in plants grown in low-sulfate media at high temperature, compared to values at 25 degrees C. In plants grown in 0.30 mM sulfate medium, catalase activity was elevated, whereas it was reduced in 0.15 mM sulfate-grown plants at 40 degrees C compared to the values at 25 degrees C. Glutathi-one reductase (GR) activity at 40 degrees C was downregulated in 0.30 mM sulfate-grown plants while in those grown in FN and 0.15 mM-sulfate media it did not change. Heat Shock Factor 4 (HSF4) and HSFA4A genes were also upregulated by low sulfate and high temperature although HSFA4A gene activation was lower under sulfur deficiency. Moreover, HSFA3 gene expression at 40 degrees C decreased upon the application of 0.30 mM sulfate. We hypothesize that sulfur defi-ciency makes H. thermophilum susceptible to high temperatures by decreasing chlorophyll and sugar contents, and reducing the activities of the antioxidant enzymes and thus plant growth. Moreover, the down-regulation of the HSFA3 and HSFA4A gene, caused by sulfur deficiency, confirms the negative effects of sulfur deficiency on plant response to high temperatures.