Foliar application of biostimulants affects physiological responses and improves heat stress tolerance in Kimchi cabbage

This study investigated the effects of foliar applications, such as brassinosteroid (BR), methyl jasmonate (JA), salicylic acid (SA), and sodium nitroprusside (SNP), on the growth, yield, photosynthetic characteristics, and antioxidant enzyme activity of Kimchi cabbage (Brassica rapa L. ssp. pekinensis) exposed to heat stress. Kimchi cabbages were grown in extreme weather growth chambers under controlled conditions (air temperature, relative humidity, and solar radiation of the Daegwanryeong region, 37° 40′ N, 128° 32′ E). At 18 days after transplanting, Kimchi cabbages were treated with foliar application of biostimulants (vs. non-sprayed control) and exposed to heat stress (air temperature 30/25 °C) and mean daily air temperature under normal conditions (21.2 ± 1.6 °C day/night, respectively) for 5 days. The foliar application concentrations were 50, 100, and 200 nM BR; 50, 100, and 200 μM JA; 1, 2, and 4 mM SA; and 0.25, 0.50, and 1.00 mM SNP. At 38 days after foliar application, the shoot fresh weight of Kimchi cabbages treated with 200 nM BR was 2123 versus 1422 g plant⁻¹ (control). The head weight and yield were the greatest with 2 mM SA treatment, which produced 3806 versus 2184 kg ha⁻¹ (control). Heat stress induced a low photosynthetic rate in Kimchi cabbage, while foliar application of biostimulants enhanced photosynthesis recovery. During the 3 days after heat stress, catalase and peroxidase enzyme activities of Kimchi cabbage increased by 1.76- to 2.08-fold on average compared to the control. Results indicated that foliar application of biostimulants reduced physiological damage and enhanced the activity of the antioxidant enzymes, thereby improving heat stress tolerance in Kimchi cabbage.