Seasonal variations of antioxidants and other agronomic features in soilless production of selected fresh aromatic herbs

The antioxidant activity, herbal fresh yield, water and nitrogen productivity were studied in selected aromatic herbs of the Lamiaceae family, namely basil (Ocimum basilicum), mint (Mentha viridis), balm (Melissa officinalis) and thyme (Thymus vulgaris), grown in soilless conditions in a typical Mediterranean climate. Seasonal variations of the bioactive substances of the produce were defined. In all crops, total antioxidant activity assessed by (i) ferric reducing antioxidant power/FRAP (μM ascorbic acid equiv/g FW) and (ii) 1,1-diphenyl-2-picrylhydrazyl/DPPH (mg ascorbic acid equiv/g FW), total phenolics (mg gallic acid equiv/g FW) and ascorbic acid content (mg/g FW) were greater in summer (avg. 82.3, 7.01, 8.59 and 0.75, respectively) and lower in winter (avg. 45.4, 2.93, 4.89 and 0.38, respectively), whereas values in spring and autumn were in between (avg. 62.9, 4.8, 7.3 and 0.53, respectively). Natural antioxidants were higher in thyme followed by balm and mint and lower in basil and total antioxidant activity was highly correlated in all cases with phenolics and ascorbic acid. The mean daily evapotranspiration of all crops ranged between 1.8-2.5 mm over the study period. The herbal fresh yield (kg m−2) was greatest in mint (avg. 22.8) followed by basil (avg. 19.3), balm (avg. 15.7) and lowest in thyme (avg. 10.2). Yield per unit of water consumed (kg tn−1) was greatest in mint and basil (avg. 25.5) followed by balm (avg. 21.3) and lowest in thyme (avg. 15.6). Accordingly, yield per unit of nitrogen uptake (kg kg−1 N) was highest in mint and basil (avg. 124.6), followed by balm (avg. 103.9) and lowest in thyme (avg. 76.3). These results may serve as a tool for the better management of greenhouse horticulture schemes under Mediterranean climatic conditions in terms of food-crops rich in antioxidants.