Effects of different levels of NaCl and CaCl 2 on seed germination characteristics of Melissa officinalis L . and Ocimum basilicum

Recently, in Iran there is increasing interest in cultivation of medicinal and aromatic plants due to their economic advantages compared with the other horticultural crops (Ibrahimi, 2002). Within these medicinal species, lemon balm and sweet basil are two of the more common and easy to grow ones in Iran’s medicinal plants section. Melissa officinalis L. (lemon balm) from the family Lamiaceae is an aromatic perennial subshrub native to the eastern Mediterranean region and Western Asia. The leaves are used as a herbal tea for their aromatic, digestive, and antispasmodic properties in nervous disturbance of sleep and for gastrointestinal disorders (Mencherini et al., 2007). Ocimum basilicum L. (sweet basil) is an aromatic annual plant belonging also to the family Lamiaceae. It is native in tropical Asia, perhaps North-East Africa, and cultivated mainly in North-Africa, Europe, and South-West Asia. The species is an important source of essential oils used extensively in perfumery, food and cosmetic industries (Hiltunen & Holm, 1999). Low degree of viability and unsuccessful germination in different regions of Iran, especially in the irrigated areas, which are covered by saline and alkali soils have always been a problem for producers. In irrigated areas of arid and semiarid regions salinity causes osmotic stress and reduction in plant germination and subsequently in plant growth and yield productivity (Çiçek & Çakirlar, 2002). Water deficiency is occurring in arid and semiarid parts of the world with an estimate of 6150 million hectares (Koochaki, 2000). On the other side, most saline water and soil resources of the world are located in these areas. From the total areas of saline soils of the world in arid and semiarid regions, which is about 955 million hectares, approximately 18 million hectares belong to Iran (Koochaki & Mahallati, 1994). During plant life, seed germination is one of the most sensitive stages of plant growth which is controlled by environmental factors as well as by physiological processes. Germination includes those events that commence with imbibitions of water by the dormant, usually dry seed and terminate with the elongation of the embryonic axis (Lambers et al., 2008). Water availability is basically important for seed germination. Since salinity is one of the most important environmental factors influencing water availability via osmotic stress, testing its effect on germination is considered a crucial step in research works. In discussing the effects of salts in the soil, it is distinguished between high concentrations of Na+, referred to as sodicity and high concentration of total salts, referred to as salinity. The two factors are often related, but in some areas Ca2+, Mg2+ and So4 -, as well as NaCl can contribute substantially to salinity. The high Na+ concentration of a sodic soil can not only injure plants directly but also degrade the soil structure, decreasing porosity and water permeability (Taiz & Zeiger, 2002). Effects of different levels of NaCl and CaCl2 on seed germination characteristics ofMelissa officinalis L. and Ocimum basilicum L.