EFFECT OF NUTRIENT SOLUTION\'S IRON CONCENTRATION ON GROWTH AND ESSENTIAL OIL CONTENT OF OREGANO PLANTS GROWN IN SOLUTION CULTURE

An experiment was done with oregano [Origanum vulgare spp. hirtum (Link) Ietswaart] grown in NFT under 3 different target levels (2.5, 5.0 and 11.0 mg/l) of Fe-EDTA in the nutrient solution to study the effect of iron on growth and essential oil content of the plants. Iron uptake by plants was associated with the Fe concentration in the nutrient solution and the uptake rate during the growing season was maximal in the seed formation stage. Iron content in leaves and roots increased with increasing Fe concentration in the nutrient solution. Top (younger) leaves had less iron content than the older ones. Roots presented much higher accumulation of iron than leaves. Increasing iron concentration to 11.0 mg/l resulted in decreased plant growth parameters (fresh and dry mass of shoots, dry mass of roots), but increased the ratio of dry leaves/stem. Essential oil content of the shoots decreased in the highest (11.0 mg/l) Fe nutrient solution concentration. It can be concluded that under the conditions of the experiment the high iron concentration applied reduced both biomass and essential oil yield. INTRODUCTION The growing market of medicinal, aromatic plants and their derivatives has stimulated studies aimed at elucidating the factors affecting plant productivity and quality. Among other exogenous factors, fertilizers, especially macronutrients, have long been known as powerful means of increasing medicinal and aromatic plant productivity and quality (Economakis, 1993, Ichimura et al., 1995, Fournaraki, 1998, Economakis et al., 1999, Mairapetyan et al., 1999). Investigations, dealing with effect of trace elements on the performance of these plants are scarce and have primarily appeared during the last 10-20 years, mainly. Micronutrients, applied both as complex and single fertilizers have been proven effective in affecting productivity, yield of secondary metabolites, and, in some cases, quality of those metabolites (Haykazyan et al., 1994; Misra and Srivastava, 1991). Iron is a trace element, required in higher amounts by plants than other micronutrients. The role of iron in biological redox systems (electron transfer chain in photosynthesis and respiration), enzyme activation, N2 fixation, chloroplast development, heme proteins (cytochromes, catalase, peroxidase), iron-sulphur proteins (ferredoxin, isoenzymes of superoxide dismutase, aconitase), is well known and documented (Welch, 1995). Less is known about the effect of iron on the secondary metabolism, either indirectly, by affecting availability of photosynthates provided by primary metabolism (Srivastava et al., 1993), or directly, through some factors responsible for efficient utilization of precursors coming from primary synthesis (Srivastava et al., 1998). The literature on the effect of iron on growth parameters and essential oil yield is contradictory. It was reported by Misra and Sharma (1991) that the optimal concentration of iron for Japanese mint (Mentha arvensis L.) was responsible for the fresh and dry matter yield, essential oil and menthol content. In another experiment, additional foliar application of iron gave the best results for Cymbopogon citratus Stapf. productivity and essential oil yield (Mairapetyan and Tadevosyan, 1999). On the contrary, in case of Proc. Int. Conf. on MAP Eds. J. Bernáth et al. Acta Hort. 576, ISHS 2002 278 Rosmarinus officinalis, the foliar application of Fe-DTPA did not show marketable increase in essential oil content (Moretti and Peana, 1998). Addition of Fe-EDTA to the nutrient solution of peppermint in hydroponic culture resulted in essential oil decrease (Mairapetyan Tadevosyan, 1999). The present study deals with iron nutrition of Greek oregano [Origanum vulgare spp. hirtum (Link) Ietswaart], a plant of high economic value, which already is being cultivated commercially in the Mediterranean area. The effect of iron on herbage yield and essential oil quantity of oregano plants grown in the solution culture is discussed. MATERIALS AND METHODS Experimental Lay-out The experiment was conducted in an unheated greenhouse at the Subtropical Plant and Olive Trees Institute, Chania, Greece. Economakis (1989) previously described the NFT system used in trials. The experiment occupied three PVC channels. The cultivation period in NFT lasted from 04.12. 2000 to 14.06. 2001. Photosynthetically active radiation (PAR), air temperature and relative humidity were recorded over the growing period (Table 1). A modified nutrient solution formulated for oregano in NFT culture (Fournaraki, 1998) was used. Three levels of iron, i.e. 2.5 mg/l (Fe1); 5.0 mg/l (Fe2) and 11.0 mg/l (Fe3), were the target values in the nutrient solution (Fig. 1). Approximately once per month the old nutrient solution was dumped and a fresh one was prepared. The target pH and EC values of all the three nutrient solutions were 6 and 2 mS/cm, respectively, measured by portable pH and EC meters. Target pH and EC were controlled by daily addition of 5% HNO3 and the required amount of corresponding stock solutions. The plant material used consisted of single plant cuttings of Origanum vulgare spp. hirtum rooted in a peat-based rooting media under mist propagation and transferred to the NFT channels 50 cm apart. Statistical analysis was done by SPSS. Nutrient Solution and Tissue Analysis Samples of nutrient solution were taken weekly and analyzed by an Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Leeman Labs Inc., Model PS 1000-AT. Analytical lines for each element described were chosen according to EPA 200.7 method. The nutrient uptake/plant curve for iron was plotted by using differences of concentration values for two subsequent measurements, summed up by the concentration of iron in the stock solution added for the referred week and dividing by the number of plants in the channel. Leaves collected for tissue analysis were gathered from all plants and mixed together. Collected leaves were sorted into “top”, “middle” and “bottom” depending on their position on the stem. Two chemical tissue analyses were performed for each treatment. Roots were thoroughly washed under tap water prior to analyzing. Samples were placed in an oven at 45 C for 1 week. After drying the tissue was homogenized and taken for ashing in a muffle furnace, which was slowly brought to 500 o C and kept there for 5 hours. The remaining ash was dissolved in 10 ml 2 NHCl. The volume was brought to 50 ml by addition of distilled water. The tissue elemental content was determined by ICP. Essential Oil Distillation Shoots subject to essential oil analysis were taken separately from each individual plant and dried on a lab bench at ambient temperature. Stems were removed from the dry shoots and 10 g of the remaining leafy fraction was taken for essential oil determination