Nitrogen levels, plant density and postharvest storage duration affect phytochemical and antioxidant properties of field-grown basil and rocket crops

ABSTRACT

Growers usually apply elevated nitrogen levels when plants are established at high densities to obtain improved yields. It is not known how this practice affects phytochemical and antioxidant properties of basil (Ocimum basilicum L.) and rocket [Diplotaxis tenuifolia (L.) DC]) during postharvest storage, which are important quality aspects of these crops for human health. This study investigated effects of N application (60, 90, 120, 150, or 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 postharvest storage duration (0, 5, 10 or 15 days) on phytochemical and antioxidant properties of basil and rocket. After harvest, leaves were packaged in biaxially oriented polypropylene (anti-mist) bags and kept at 12°C and 85% relative humidity in a cold temperature room. Nitrogen application of 120 kg^.ha⁻¹ at day 0 of storage, caused the highest accumulation of total phenolic content (TPC), flavonoid content (FC), strong free radical scavenging activity, and % antioxidant with limited effect due to plant density on basil. In rocket, application of 60 to 120 kg^.ha⁻¹ N at day 0 of storage had high TPC; FC was high at 90 to 180 kg^.ha⁻¹ N and 10 days of storage. Rocket had strong scavenging activity, as a result of 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. Postharvest quality of basil was affected by storage time and N application; TPC, FC, and free radical scavenging activity (FRS) were reduced as storage time lengthened. Rocket had improved postharvest quality at 10 days of storage. Spacing had limited effect on all parameters in basil and rocket. The most economic treatment in basil was 60 kg^.ha⁻¹ N for TPC, FC, FRS, and antioxidant activity; in rocket, it was 120 kg^.ha⁻¹ with longer shelf-life of 10 days of storage with regard to antioxidant activity. Optimization of agronomic practises for improved production should consider phytochemical quality assurance to ensure crop health benefits to consumers are not compromised.

KEYWORDS:

• Diplotaxis tenuifolia
• ocimum basilicum
• plant spacing
• total phenols
• total flavonoids
• antioxidant

Additional information

Funding

This work was supported by Agricultural Research Council, South Africa.