Fulvic Acid Alleviates the Toxicity Induced by Calcium Nitrate Stress by Regulating Antioxidant and Photosynthetic Capacities and Nitrate Accumulation in Chinese Flowering Cabbage Seedlings

Abstract

Continuous cropping can lead to an excessive accumulation of nitrate in facility-cultured soil. Excessive accumulation of nitrate gradually becomes the main reason for crop failure in vegetables and endangers human health. Therefore, the exploration of effective measures to decrease abundant nitrate accumulation in Chinese flowering cabbage is indispensable. In this study, a kind of plant growth regulator, fulvic acid (FA), was used to study its positive effect on alleviating the growth inhibition induced by excessive Ca(NO3)2 in Chinese flowering cabbage. Meanwhile, we conducted hydroponic cultivation and measured the growth indices, photosynthetic and oxidation-reduction characteristics of Chinese flowering cabbage with different treatments. After determining the optimal treatment concentration, we mainly designed four treatment groups, including Con, FA, Ca(NO3)2 and FA + Ca(NO3)2 cotreatment, to explore the regulatory mechanism by which FA alleviates Ca(NO3)2 stress in Chinese flowering cabbage. The results showed that FA can effectively alleviate the inhibitory effect of excessive Ca(NO3)2 on the growth of Chinese flowering cabbage seedlings. FA recovered the photosynthetic capacity of seedlings under Ca(NO3)2 stress. In addition, FA depressed the accumulation of O2·−, H2O2, malondialdehyde (MDA) and relative electrical conductivity, but increased the activity of antioxidant enzymes, including SOD, POD, CAT and APX, which finally enhanced the stress resistance of Chinese flowering cabbage to Ca(NO3)2. The expression of nitrate-related transporters, BcNRT1.1 and BcNRT1.5, was depressed by FA, which inhibited redundant nitrate absorption and restricted more nitrate from being stored in the roots instead of being transferred to the shoot. Ultimately, nitrate accumulation in the edible part was reduced in Chinese flowering cabbage seedlings. In general, exogenous FA may alleviate nitrate stress by improving oxidation resistance, photosynthetic capacity and redundant Ca(NO3)2 accumulation in Chinese flowering cabbage.