Objectives This study explores the effect and mechanism of seaweed fertilizer on the yield and quality of flowering Chinese cabbage under drought stress condition using a combination of field and pot experiments. The aim was to provide theoretical support for applying seaweed fertilizer in leafy vegetables.
Methods Field experiments were conducted in Guangzhou and Ningxia in 2017, taking flowering Chinese cabbage as test materials. Under normal water and drought stress conditions (70%–75% and 50%–55% of the maximum field moisture capacity), the vegetable seedlings were subjected to treatments of seaweed extracts (SE), soluble chemical fertilizer (NPK), seaweed fertilizer (NPK+SE), and the control had no extract or fertilizer. The economic yield and quality of the vegetable were determined. Pot experiments were conducted in the greenhouse of South China Agricultural University in 2018. The fertilizer treatments were the same as those used for the field experiments under drought stress conditions and only a water control treatment under normal water condition (CK0). The leaf length, leaf width, and plant height of flowering Chinese cabbage were measured at 14 days, 21 days and 28 days after transplanting. The relative water content of leaf, the chlorophyll content, malonaldehyde (MDA), proline and free radical, antioxidant enzyme activities, and nutrient uptake were measured after harvest. The same pot experiment was also conducted using seaweed extracts divided into fractions of > 10 kDa, 5–10 kDa, 3–5 kDa and < 3 kDa molecular weight by ultrafiltration membrane.
Results In the two field experiments, NPK+SE treatment increased the yield and quality of flowering Chinese cabbage than NPK treatment, and the increment under drought stress was higher than under normal water condition. The results of pot experiments under drought stress condition showed that SE treatment did not exhibit growth and yield increase effect, compared with CK treatment. However, NPK+SE treatment significantly increased the plant height, leaf length and leaf width, compared with NPK treatment. Also, NPK+SE treatment increased chlorophyll contents in leaves by 10.24%, enhanced the antioxidant enzymes activities by 27.84%–43.40%, decreased free radical contents by 24.88%–41.56%, and improved the N, P and K uptake by 14.48%, 16.41%, and 35.37%, respectively. Among the four fractions of SE, the contents of alginate acid, polyphenols, and mannitols were higher in < 3 kDa fraction than those in the other fractions. The scavenging rate of superoxide anions and hydroxyl radicals of < 3 kDa fraction was significantly higher than those of other fractions, and the growth-promoting effect of < 3 kDa fraction was the strongest under drought stress condition.
Conclusions Seaweed fertilizer significantly increased the yield and quality of flowering Chinese cabbage under drought stress condition, in both field and pot experiments. SE could increase the chlorophyll content, reduce the accumulation of free radicals in leaves, increase the nutrient uptake of flowering Chinese cabbage, and thus improve the drought resistance of the crop. As the main active components existing in the SE fraction of molecular weight < 3 kDa, it has the most potent effect on improving the drought resistance of flowering Chinese cabbage.
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