EFFECT OF DIFFERENT PERIODS OF IRRIGATION AND ASCORBIC ACID ON GROWTH, YIELD AND QUALITY OF ESSENTIAL OIL OF MENTHA PIPERITA L. PLANTS

The present study was carried out during two growing seasons (2019 and 2020) at the Nursery of Department of Floriculture, Ornamental Horticulture, Faculty of Agriculture, Alexandria University, Egypt to study the effect of irrigation intervals (4 days as control stress, 8 days as moderate stress and 12 days as severe stress) and ascorbic acid (AsA) at 0, 100, 150, 200, 250 mg l-1 on the growth, quantity and quality of the essential oil of Mentha piperita L. plants. The main results of this study indicated that most of the vegetative growth characteristics were increased when AsA was applied at 150 or 200 mg l-1 to the plants grown under control stress (irrigation interval at 4 days). AsA at 200 mg l-1 was more effective on leaves fresh weight than 150 mg l-1 which affects mostly the number of branches of the plants grown under moderate stress (irrigation interval at 8 days). Increasing irrigation interval to 12 days (severe stress) in addition to AsA at 200 mg l-1 decreased the harmful effects in most peppermint plants and leads to obtain high oil percentage and yield. Peppermint oil was analyzed according to GC analysis of fresh leaves grown under water stress and sprayed with ascorbic acid with different concentrations identified 9 components, the major component was menthol with an average of 34.55-52.67% during the three cuts. In general, there was a direct relationship between increasing irrigation intervals and oil components percentage (menthol, menthone and limonene). These results suggest that normal water deficit (4 days irrigation interval) in peppermint will be appropriate to enhance better growth and productivity. In addition, ascorbic acid treatment reduced the damaging action of water stress, especially at concentrations of 150 and 200 mg l-1, so we can conclude that these concentrations are sufficient enough to support peppermint growth, leaves fresh weight and consequently maximum oil yield.