Foliar application of zinc alleviates the heat stress of pakchoi (Brassica chinensis L.)

Abstract

Experiments were conducted to examine whether the foliar application of zinc (Zn) could mitigate the adverse effects of heat stress on pakchoi plants. Two varieties of pakchoi (Aikangqing and Wuyueman) were foliar applied with ZnSO4·7H2O (0%, 0.02%, 0.05%, 0.10%, 0.20%, 0.40%, 0.60%, and 0.80%), and then subjected to two temperature levels (22°C/16°C, day/night; 40°C/30°C, day/night). Heat stress decreased the net photosynthetic rate (Pn) (50.65% and 62.14% for Aikangqing and Wuyueman, respectively), chlorophyll content, chlorophyll fluorescence ratio (Fv/Fm), and effective quantum yield of PSII photochemistry (ΦPSII) of the leaves. Foliar application of ZnSO4·7H2O (0.02%–0.40%) effectively alleviated the heat stress in pakchoi by enhancing shoot Zn concentration, superoxide dismutase (SOD) activity, chlorophyll content, Fv/Fm, and ΦPSII. Pn increased by 12.61%–46.19% and 45.73%–119.01% in Aikangqing and Wuyueman compared with those without Zn treatments, respectively. Fuzzy comprehensive evaluation and the extreme model showed that Aikangqing and Wuyueman treated with 0.1218%–0.1220% ZnSO4·7H2O (approximately 0.004 M Zn2+) and 0.2178%–0.2744% ZnSO4·7H2O (approximately 0.008 M Zn2+) exhibited the most heat resistance, respectively. Furthermore, Zn (0.02%–0.80% ZnSO4·7H2O) application had no significant effect on most physicochemical parameters under normal temperature, which only increased shoot Zn and SOD. The results suggest that additional Zn would be required to fully protect plant growth from heat stress. Foliar application enhanced Zn concentration in leaves, thereby maintaining the SOD activity and membrane stability and protecting photosynthesis against heat damage.