Figure 1.

Involvement of H₂O₂ in melatonin-induced C-REPEAT BINDING FACTOR (CBF) transcriptional regulatory cascade and chilling tolerance. (A) Dynamic changes in H₂O₂ accumulation and the expression of respiratory burst oxidase homolog D (ClRBOHD). Watermelon seedlings at three-leaf stage were sprayed with 150 µM melatonin (MT), and after 12 h, they were subjected to cold stress (CS) at 4 °C for 48 h. Seedlings sprayed with distilled water under temperatures of 28/18 °C (day/night) were set as control (CK). (B) The relative electrical conductivity (REC) and malondialdehyde (MDA) content. The seedlings were sprayed with 0.04, 0.2, 1, or 5 mM H₂O₂, and after 12 h, they were exposed to 4 °C for 48 h. (C) Chilling phenotypes. (D) Images showing the maximum PSII quantum yield (Fv/Fm). The false-color code depicted at the left of the image ranges from 0 (black) to 1 (purple). (E) The average values of Fv/Fm and MDA contents. (F) The expression of ClCBF1 and its regulons, including cold-responsive gene 47 (ClCOR47), early responsive to dehydration 10 (ClERD10), and cold induced gene 1 (ClKIN1). For (C–F), the seedlings were sprayed with 100 µM diphenyleneiodonium (DPI, an inhibitor of H₂O₂ production) 2 h before MT treatment. After 12 h, the seedlings were subjected to 4 °C. Relative expression of genes in CK plants was set as 1.0. Data show the means of three replicates ± standard deviation (SD). The different letters denote significant difference at p < 0.05 according to Turkey’s test.