Foliar Spray or Soil Drench: Microalgae Application Impacts on Soil Microbiology, Morpho-Physiological and Biochemical Responses, Oil and Fatty Acid Profiles of Chia Plants under Alkaline Stress

SIMPLE SUMMARY: Chia is an important medicinal plant and is a rich source of omega-3 and omega-6 fatty acids. Alkaline soil inhibits the growth and productivity of all crops, including chia. Microalgae are a diverse group of photosynthetic microorganisms that can be used in modest doses to stimulate the growth and productivity of numerous crops in both normal and stressed conditions. Microalgae supplementation by two application methods (foliar spray and soil drench) resulted in an increase in the growth and productivity of chia plants cultivated under alkaline stress conditions, and caused an increase in the antioxidant levels in the chia seeds, although soil drenching gained the superiority in this respect. The oil content was increased following microalgae application with an increase in omega-3 proportion. Chia plants showed different responses to foliar and drenching applications. Microalgae would be a potential and eco-friendly approach for enhancing agricultural productivity in alkaline environments. Our findings also suggest that Arthrospira platensis supplementation via the soil drenching technique should be used in the future to enhance plant growth and productivity under alkaline soil conditions. ABSTRACT: Alkaline soil inhibits the growth and productivity of chia plants (Salvia hispanica L.). Microalgae as biofertilizers have been reported to induce alkalinity tolerance and enhance yield and quality. However, limited information is known concerning the influence of microalgae application on medical plants, including chia. Our experiments were performed to evaluate the effect of microalgae strains of Arthrospira platensis, Chlorella vulgaris, Nostoc muscorum, and Anabaena azollae with two application methods, foliar spray and soil drench, on morpho-physiological and biochemical parameters, yield, seed and oil quality, and fatty acid profiles of chia plants cultivated under alkaline soil conditions, as well as the on soil microbial activity. The results obtained reveal that both application methods positively influenced the growth and productivity of chia plants. However, the foliar application showed significant differences in the herb’s fresh and dry weights and leaf pigments, whereas the drenching application caused more effect than the foliar spray application at the reproductive stage. Untreated chia plants showed a slight decline in the growth, productivity, and antioxidant level with an increase in Na content. However, microalgae applications significantly ameliorated these impacts as they induced an enhancement in the growth, leaf pigments, total protein and carbohydrate contents, nutrient content, seed and oil yields, as well as an increase in linolenic and linoleic fatty acids, with a reduction in saturated fatty acids, namely, palmitic and lauric acid. Soil drenching generated an improvement in the soil microbial activity and caused a reduction in the pH. The treatment of A. platensis with drenching application resulted in higher seed and oil yield, with an increase of 124 and 263.3% in seed and oil yield, respectively.