Study of physiological characteristics of marjoram (Origanum majorana), as a medicinal plant in response to zinc levels under drought stress conditions

Introduction Marjoram is an annual medicinal plant of Labiatae family. This plant is native of south-east of Mediterranean regions and grown in different countries including Iran. Marjoram has great economical and industrial importance and since ancient times was known for its medicinal properties (Baatour et al., 2011). Water deficit is one of the most important factors limiting crop yield (Munns, 2002). Drought stress can cause changes in the morphological, physiological and biochemical characteristics of plants including medicinal plants (Hasani and Omidbaigi, 2002). In drought conditions, chemical compounds, including sugar, protein and chlorophyll content differ between plants and this could be involved plant resistance to drought. Plant nutrition with microelements such as Zn, can improve plant tolerance to drought stress (Alloway, 2004). Some researcher indicated that Zinc contributed in improvement of plant growing conditions especially in drought conditions (Misra, 1992). The present study was conducted in order to evaluate the physiological responses of marjoram to Zn application levels under drought stress conditions. Materials and methods In order to study the physiological responses of marjoram (Origanum Majorana) to use of zinc under drought stress, a factorial experiment was conducted as a completely randomized design with three replications during 2014-15 in the greenhouse of the University of Hormozgan. Experimental factors were included three drought stress levels (soil moisture equal to 50, 75 and 100% of field capacity) and three concentrations of zinc fertilizer (concentrations of 0, 1 and 3 per thousand as foliar spraying). Drought stress was applied in 8 to 10 leaves stages. Fertilizer solutions sprayed on the 20th day after onset of drought stress for four times (before flowering) at intervals of ten days. Ten days after the third application of zinc solutions, physiological traits such as chlorophyll, soluble sugars, anthocyanin contents and catalase activity were measured. In order to measure dry weight, essential oil content, the marjoram plants were harvested in complete flowering stage. Data analyses were carried out using SAS. Means were compared with LSD (Least significant difference, P=0.05) values. Results and discussion Drought stress had significant effect on all traits except the essential oil content. Spraying of Zn fertilizer had significant effects on all evaluated traits with the exception of chlorophyll a content and catalase activity. Interaction of Zn fertilizer with drought stress had significant effects on chlorophyll a, b and total and anthocyanin contents, and essential oil percent. Results of this study showed that when marjoram grown in soil with moisture equal to 75% of field capacity, chlorophyll a, b and total increased significantly, while in 50% of field capacity soil moisture, carotenoid, soluble sugars, anthocyanin content and catalase activity increased significantly. Some researchers indicated that in drought stress conditions synthesis and accumulation of plant pigments and soluble sugars can increase plant tolerance to stressful conditions through adjusting and maintaining osmotic pressure and maintain cell membrane stability (Kleff et al., 1994). Results of mean comparison indicated that Zn spraying at three per thousand concentration increased chlorophyll b and total significantly, while Zn spraying at one per thousand concentration increased soluble sugar and essential oil content significantly. Spraying of Zn solution, improved physiological characteristic of marjoram plants, so that at drought stress condition equal to 75 and 100% field capacity Zn spraying at one per thousand concentration, increased chlorophyll a, b and total and anthocyanin content and essential oil percentage, significantly. Spraying of marjoram plants with low concentration of Zn increased plant tolerance to drought stress and therefore increased plant growth characteristics via increasing in chlorophyll, carotenoid, anthocyanin and soluble sugar. Cakmak (2000) reported that Zn can improve physiological characteristics of plants via cell protection against reactive oxygen species. Conclusion Based on these results, the effect of drought stress on all traits except for essential oil content was significant. As well as the effect of different levels of zinc fertilizer on chlorophyll b, carotenoids, soluble sugars and anthocyanin were significant. These results indicated that Zn application at drought stress condition may be through promote plant pigment synthesis improved marjoram growth and therefore essential oil percentage.