Foliar application of plant-growth regulators: 4-PU-30, thidiazuron (DROPP) and 2,3,5-trijodbenzoic acid (TB), paclobutrazol (PBZ) and jasmonic acid (JA), was examined for their influence on the yield and composition of the peppermint essential oil. The following regulators have increased the total yield of fresh herbage: 4-PU-30 by 48.09%, DROPP by 40.49%, JA by 37.96% and TB by 34.42%. All investigated regulators improved the essential oil content: from 1.60 to 2.20%, as compared to the control 1.00%. The essential oil yield significantly increased in all variants by: 187.08% (4-PU-30), 186.30% (JA), 177.00% (DROPP), 126.49% (TB), and 35.53% (PBZ). GC analyses of oil composition demonstrated that qualitative oil alterations relate only to changes in the proportions of the main components: menthol, menthone, cineole, menthyl acetate, isomenthone and menthofuran.

In the 1970-1990 period different plant-growth regulators were examined for their influence on peppermint productivity. An increase of herbage, essential oil and rhizomes productivity of Mentha piperita L., without unfavorable changes in the essential oil composition after treatment with purine- and urea-type cytokinins, was achieved [1,2,3,4]. A stimulative effect of some phenylurea cytokinins (BAP, DPU, TDZ, and 4-PU30) was observed on the rooting process in peppermint in vitro propagation [5]. The insignificant influence of TDZ and 4-PU30 on the oil composition of in vitro produced peppermint plants was established [6].

El-Keltawi and Croteau reported the stimulate effects of foliar-apllied purine- and phenylurea-type cytokinins on plant growth and essential oil content, without alteration of the oil composition of some Lamiaceae species: Mentha piperita, M. spicata, M. suaveolens, Lavandula vera, Salvia officinalis [7,8]. It was found out that the primary influence of cytokinins was to stimulate the overall accumulation of monoterpenes typical of the respective essential oil. Phenylurea cytokinins were recommended for practical use because, as compared to the purine-type, they are more active, inexpensive and non-toxic.

The aim of the present study was to examine the influence of the foliar applied plant-growth regulators on the productivity and composition of peppermint essential oil under conditions of reduced irrigation. A comparative study was carried out using concentrations of the investigated plant-growth regulators with an already established positive effect [9,10].

The investigations were carried out in the Experimental Field of the Institute of Plant Physiology, Sofia, BAS during the 1996-1998 period using plant material from the Mentha piperita population of the Bulgaro-Mitcham-type which essential oil was composed preliminary of terpenic oxigen derivatives: menthon, menthol and menthyl acetate (over 70%). The experiments were conducted in 3 replications per parcel area of 3.20 m2, by the generally accepted agrotechnics, fertilizers in the amount of 300 kg/ha ammonium nitrate and 500 kg/ha superphosphate, no watering, and no herbicides. Treatments: var. 1. Control (water); var. 2. 4-PU30 [N-(2-chlor-4-pyridyl)-N'-phenylurea] (50 mg/l); var. 3. DROPP [50% thidiazuron - (N-phenyl-N'-(1,2,3-thidiazol-5-yl) urea] (50 mg/l); var. 4. 2,3,5-TB [2,3,5-trijodbenzoic acid] (25 mg/l); var. 5. PBZ [paclobutrazol] (500 mg/l); var. 6. JA [jasmonic acid] (25 mg/l).

Plants of 15 cm height were sprayed with water solutions of the tested plant-growth regulators in the spring (June 10th). Above-ground parts (herbage) were harvested twice in the vegetation period (July 22, and September 25).

The essential oil content was determined by one-hour hydrodistillation with a Clevenger-type apparatus. The qualitative analyses of essential oils were performed by PYE UNICAM gas chromatograph, 204 series, equipped with a flame-ionization detector, CARBOWAX capillary column, and hydrogen as the carrier gas at a flow rate of 1.3 ml/min. Temperature regime: of the detector 250°, of the injector 240°, maintaining the initial temperature of 50° for the column for 2 min, then increasing it by 16° per minute to 150° and maintaining it thus for 2 min. The volume of the injected sample was 0.05 ml.

The following regulators have increased the total yield of fresh above-ground mass: 4-PU30 by 48.09%, DROPP by 40.49%, jasmonic acid by 37.96% and 2,3,5-TB by 34.42% (Table 1). The influence of 4-PU30, DROPP and 2,3,5-TB was more significant at the 1st harvest, while JA demonstrated a reverse effect: the stimulate effect became more obvious at the second harvest. Despite the increasing second-harvest herbage yield by 16.05%, paclobutrazol caused a slight decrease in the total fresh-mass production and the effect was the same as in plant-height. Similar dependence of the stimulate effect of phytoregulators was observed in the dry-herbage yield.

All investigated regulators have increased the essential oil productivity (Table 2). The content of essential oil from air-dried plant material varied from 1.60 to 2.20%, as compared to the control 1,00%. As a result, the essential oil yield significantly increased in all variants by: 187.08% (4-PU-30), 186.30% (JA), 177.00% (DROPP), 126.49% (2,3,5-TB), and 35.53% (PBZ).

GC analyses of the essential oil composition demonstrated that qualitative oil alterations related only to insignificant changes in the proportions of the main components: menthol, menthone, menthyl acetate, cineole, isomenthone and menthofuran (Table 3). As compared to European Pharmacopoeia (1997), the content of each compound varied within the permissible limits: menthol 30.0 to 55.0%, menthone 14.0 to 32.0%, menthyl acetate 2.8 to 10.0%, cineole 3.5 to 14.0% and menthofuran + isomenthone 2.5 to 19.0%.

1. We consider foliar application of 4-PU 30 (50 mg/l), jasmonic acid (25 mg/l), DROPP (50 mg/l) and 2,3,5-trijodbenzoic acid (50 mg/l) as promising for agricultural practice; it will more than double the annual essential-oil yield;

2. The sum total of main essential-oil components varied insignificantly in all variants from 86.93% to 89.93%, as compared to the control 88.11% and thus the specific pharmacological activity of the peppermint oil should be guaranteed;

3. The obtained results should contribute to the improvement of peppermint cultivation technology, especially in cases of limited irrigation or drought.

This study was kindly supported by Grant No. CC-645/96 of the National Science Fund, Bulgaria.

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