Impact of green manure plants on the yield and bioactive compounds content of lettuce

Abstract

Background: Lettuce (Lactuca sativa L.) is a nutrient-rich vegetable that delivers a variety of essential vitamins, minerals, and bioactive compounds, including polyphenols, carotenoids, and chlorophyll. These compounds contribute to health benefits, particularly through their antioxidant properties, while keeping the vegetable low in calories. Organic farming practices, which exclude synthetic pesticides and fertilizers, have been demonstrated to enhance the levels of beneficial compounds like antioxidants and polyphenols in vegetables. Additionally, practices like green manuring improve soil health and fertility, contributing to the overall quality of the produce.

Objective: The primary objective of the research was to assess the impact of different green manure plants on the bioactive component content, including polyphenols, carotenoids, and ascorbic acid, of various lettuce varieties.   Additionally, the productivity of the various green manure plants was measured with the aim of enhancing food functionality.

Materials and Methods: Green manuring plants for fertilization in crop rotation with lettuce, bean (Phaseolus vulgaris L.), green pea (Pisum sativum L.), and soybean (Glycine max (L.) Merr.) were used. Soil analysis for nitrogen (N), phosphorus (P), and potassium (K), were performed using a Gallery Aqua Master Discrete Analyzer. Soil analyses were conducted both before the sowing of green manure plants and after the harvesting of lettuce. The content of ascorbic acid, polyphenols and carotenoids were identified with spectrophotometer.  

Results:  The agrochemical analysis showed that the use of green manure reduced the soil's bulk density by 9.2% to 14.4%. Green manuring plants demonstrated the most nitrogen accumulation, exceeding the control by 1.69-2.39 mg/100g, potassium by 1.2-3.1 mg and phosphorus by 0.1-0.05mg. The lettuce variety ‘Manushak’ from soybean-treated plots had the highest phenolic acid content, 1.68 times more than the control, and the greatest Vitamin C increase (13 mg/%).  In ‘Veradarz’ lettuce, leaves from plot manured with soybean Vitamin C increased by 11.5 mg/%, and β- carotinoids increased by 1.28 times. Green manuring contributed to increasing the yield of lettuce by 8.1%-30.0%. The highest lettuce yield (358.4 c/ha) was from the Veradarz variety obtained in the soybean-fertilized field.

Conclusion: Applying green manure plants, specifically soybean, enhanced the lettuce leaves’ functional properties by improving soil quality and increasing nutrients and bioactive compounds like phenols, carotenoids, and Vitamin C. This technology boosts lettuce quality, productivity, and functionality, making it a recommended method for producers.

Keywords: lettuce, green manuring, soil quality, carotenoids, phenolic acid, vitamin C, productivity