Microalgae ( Chlorella vulgaris Beijerinck) alleviates drought stress of broccoli plants by improving nutrient uptake, secondary metabolites, and antioxidative defense system

Microalgae, such as Chlorella vulgaris Beijerinck (green algae), are beneficial microscopic organisms that may help plants to improve nutrient uptake, growth, and abiotic tolerance to stressors. The current study was performed to investigate the effectiveness of algae (Chlorella vulgaris Beijerinck) foliar applications [1%, 3%, or 5% (v/v)] on mitigation of drought stress in broccoli plants subjected to water deficit at 25% of field capacity. The results showed that the broccoli plants grown under drought stress alone exhibited severe disturbance in growth with considerable reductions in the shoot length, and fresh and dry weights, leaf area, relative water content, leaf water potential, and photosynthetic pigment contents and elevated levels of the lipid peroxidation product malondialdehyde in the leaves. Additionally, the foliar application of microalgae mitigated the detrimental effects of drought, leading to better growth performance (increase of 9%-132%) when compared with the drought-stressed plants that had not received an application of microalgae. Microalgae-mediated beneficial effects were particularly evident in the enhancement of the photosynthetic pigment contents, including chlorophyll-a (6%-60%), chlorophyll-b (19%-55%), and total carotenoids (26%-114%). Exogenous microalgae also contributed to the reduction of membrane damage, as proven by significantly decreased levels of malondialdehyde (10%-39%) in the leaves of the broccoli plants exposed to drought stress. The application of microalgae increased the total flavonoid and phenolic contents, and nutrition uptake. Furthermore, the activities of enzymatic antioxidants like ascorbate peroxidase, catalase, glutathione reductase, and superoxide dismutase increased in response to mediation, resulting in significant alleviation of drought-induced oxidative damage. The most effective application concentration of microalgae was determined as 5%. Overall investigations revealed that the foliar application of microalgae could be recommended as a sustainable strategy to improve the defense system of drought-stressed broccoli plants.