Effect of nitrogen and plant density on yield and post-harvest quality of basil and rocket

Abstract

Basil (Ocimum basilicum L.) and rocket (Diplotaxis tenuifolia (L.) DC.) are an important part of the human diet and consumed as herbs and vegetables. The cosmetic and pharmaceutical industries also use these herbs, and may help to prevent various forms of human diseases such as pancreas and colon cancer. In agricultural practice, the quality and yield of many herbs or vegetables depend largely on fertiliser application and plant density. Nitrogen (N) application is essential for growing healthy leafy vegetables, as it influences the green photosynthetic pigment, chlorophyll content, and marketable yield of herbs. Nutrient elements such as N, which is the basic component of chlorophyll, affect photosynthesis capacity. Poor management in plant spacing and N fertiliser application on high-value herbs, such as basil and rocket, can reduce the plant growth, yield, quality, and phytochemicals content. Basil is a popular culinary herbal crop grown for its fresh or dry leaf. It contains phytochemicals properties such as flavonoids and volatile terpenes, such as camphor and linalool. Known as a salad crop and for its healing properties, rocket contains phytochemicals properties, vitamin C and carotenoids. The N exerts its influence on vegetative aerial parts, and primary and secondary plant metabolites accumulation. There are few reports on the effect of N fertiliser application and plant density on yield, phytochemical content and antioxidant properties of basil and rocket during post-harvest storage, which are important quality aspects of herbs for human health. This study investigates the effects of N application (60, 90, 120, 150, and 180 kg.ha-1), plant density (40,000; 62,500 and 93,750 plants.ha-1 for basil; 40,000; 80,000 and 133,333 plants.ha-1 for rocket), and post-harvest storage duration (0, 5, 10, or 15 days) v on yield, phytochemical and antioxidant properties of basil and rocket. The arrangements of treatments were 5 × 3 × 4 factorial combinations in a randomised complete block design replicated three times. Parameters measured were fresh leaf mass, dry leaf mass, leaf area, fresh shoot mass, dry shoot mass and leaf chlorophyll content. The post-harvest quality, such as parameters, total phenolic content (TPC), flavonoids content (FC), free radical scavenging activity, and antioxidant activity were analysed. Leaf chlorophyll content of basil and rocket improved with the application of N at 120 kg.ha-1. Fresh leaf mass and area increased with increased plant density for both crops. Plant density at 40,000 plants.ha-1 for rocket, produced high K leaf content at 120 kg.ha-1 of N. Leaf N, calcium (Ca) and Magnesium (Mg) contents for basil improved at 62,500 or 93,750 plants.ha-1. N application at 60–180 kg.ha−1 did not affect leaf yield of basil and rocket. Application of 60 kg.ha-1 N at 93,750 plants.ha-1 for basil, and 60 kg∙ha-1 of N at 133,333 plants.ha-1 for rocket, improved leaf yield. Nitrogen application of 120 kg.ha-1 at day 0 of storage caused the highest accumulation of TPC, TFC, strong-free radical scavenging activity and antioxidant activity with limited effect by plant density on basil. In rocket, the application of 60 to 120 kg.ha-1 N at day 0 (freshly harvested) of storage had high TPC, while TFC was high at 90 to 180 kg.ha-1 N and 10 days of storage. Rocket had strong scavenging activity at 120 to 180 kg.ha-1 N at 15 days of storage, and for 60 and 90 kg.ha-1 N at 0 and 10 days of storage. Post-harvest quality of basil was sensitive to storage and high N application. TPC, TFC and free radical scavenging activity sagged continuously with the extension of storage days. The combination of 120 kg.ha-1 at day 0 of storage of basil, and for the rocket at 60 kg.ha-1 N and 10 days of storage were ideal. Spacing had limited response in all parameters recorded; there was no significant difference of phytochemicals on vi plant spacing. Increasing plant density and providing a reasonable amount of N could be useful to obtain high marketable leaf yield along with efficient N management. This study concluded and recommended that the most economic treatment for basil was 60 kg.ha−1 N for TPC, FC, FRS, and antioxidant activity, while for rocket, it was 120 kg.ha−1 with a longer shelf-life of 10 days of storage with regard to antioxidant activity. Optimising agronomic practices for improved production should consider phytochemical quality assurance to ensure there is no compromise in crop health benefits to consumers. The increased marketable yield, particularly at high plant density, may result in an increased profit margin and income generation for the producer.