Marigold is a hardy, predominantly cross-pollinated, erect and branched flowering annual. Marigold is grown as an ornamental crop for loose flowers and as bedding plant in landscape. French marigold (Tagetes patula) is ideal for rockeries, edging, hanging baskets and in window boxes. Both leaves and flowers are equally important from medicinal point of view (Terschuk et al., 1997). African marigold petals are commercially valuable as a natural source of xanthophylls pigments used primarily in poultry industry to intensify yellow colour of egg yolk. The essential oil present in different species of Tagetes is largely used in compounding of high-grade perfumes (Afzal et al., 2009). Due to the short period needed for its cultivation it is conveniently grown as part of a multicrop system, rotated with other agricultural or horticultural crops. It is also grown as a mixed crop on the borders with other plants (e.g. tomatoes) with beneficial effects to the latter. The plant also has some resistance to saline and other adverse conditions (Girwani et al., 1990). Agricultural practices, especially chemical and biochemical treatments for improving the flower yield have been reported by Gowda and Jayanthi (1994). Bioactive extracts of different Tagetes parts exhibit nematocidal, fungicidal and insecticidal activity. Nematocidal activity of roots is attributed to thienyls while the biocidal components of the essential oil from flowers and leaves are terpenoids. Also carotenoid pigments from Tagetes are useful in food coloring (Vasudevan et al., 1997). There is strong evidence that a particular species of Tagetes exerts anti-inflammatory action towards acute and chronic conditions (Kasahara et al., 2002).
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Todayâ€™s bedding plant market place demands 100% filled plug cells in a tray. Bedding plant producers pay attention to seed germination, because what is harvested is what initially emerges. Unfortunately, all thoughts of a uniform crop stand have long ago vanished due to thermodormancy and variation in seed vigor of the same marigold seed lots, which make this goal difficult to achieve (McDonald, 1999).
One factor limiting field seedling emergence is cool soil temperatures in early spring. Suboptimal temperatures have been correlated with a reduction in germination rate, total germination percentage, and seedling emergence. While early sowing could result in poor stand establishment and plant death due to cold temperatures, it could also result in earlier harvests. In addition, direct sowing is more economical for the producer than using transplants. Labor costs are reduced, especially if a precision seeder is used, and there are no propagation and transplant production expenses. Thus, direct-seeding on the earliest optimum sowing date is beneficial for producers (Bosma et al., 2003a). Seed priming is being used successfully for broaden the temperature range for germination of various crops (Bray, 1995).
There is a lot of diversity in physiological activities in seed, depending on the moisture level provided, during the germination process (Leopold and Vertucci, 1989). Seed priming is a technique to control the hydration level within seeds so that the metabolic activity necessary for germination can occur but radicle emergence is prevented. Seed priming is known to improve final germination, reduce emergence time and better stand establishment in many horticultural and field crops (Bradford, 1990; Afzal et al., 2002). Primed seeds germinate in a wider range of temperature (Bray, 1995) and are less sensitive to oxygen deprivation (Corbineau et al., 1993) than unprimed ones. The beneficial effect of priming has been associated with various biochemical, cellular and molecular events including synthesis of DNA and proteins (Bray et al., 1989). Priming is also known to enhance respiratory activity of seeds (Halpin-Ingham and Sundstorm, 1992) and when applied to aged seeds, restores activities of enzymes involved in the cell detoxifying mechanisms such as superoxide dismutase, catalase and glutathione reductase (Bailly et al., 1997).
In hydropriming, seeds are soaked in aerated water for specific period of time followed by re-drying. It is a simple technique which does not require technical expertise (Basu, 1994). Osmopriming or halopriming is a special hydration technique in which seeds are soaked in aerated low water potential solution for a specific period of time; osmotica CaCl2, NaCl, KCl, and KNO3 etc. are added to control the hydration process (Warley, 2002). Physiological and biochemical basis of seed invigouration in primed maize seeds under low temperature conditions might be ascribed by lower T50, higher reducing and total sugars as well as higher Î±-amylase activity (Afzal et al., 2008). Marigold diploid and triploid cultivars and mimulus were morphologically more variable than the other bedding plant species because of variation in temperature under greenhouse conditions (Meritt and Ting, 1995).
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Pre-sowing magnetic field (MF) treatments enhanced the growth and yield of tomato crop (De Souza et al., 2006). MF-pretreatment increased seed germination rate, seedling growth and development, although also increased lipid oxidation and ascorbic acid contents (Yinan et al., 2005). This might trigger auxin action that could cause rapid increases in plant cell wall extensibility, alter ion flux at the plasma membrane, and regulate a diverse set of developmental processes via changes in cell division, cell elongation, cell differentiation, and cell and organ polarity (De Souza et al., 2008).
In view of previous work done on horticultural crops like asparagus (Evans and Pill, 1989), pepper (Smith and Cobb, 1991), cucumber (Yinan et al., 2005), tomato (Moon and Chung, 2000), pansy (Yoon et al., 1997) and impatiens (Frett and Pill, 1989), it can be assumed that seed priming and magnetic seed stimulation can provide early establishment of vigorous, healthy and uniform marigold seedlings. Limited work has been reported regarding biochemical and physiological changes in marigold induced by priming and magnetic seed stimulation. Therefore, this study was carried out to investigate the effect of different priming techniques and magnetic seed stimulation on enhancing germination and seedling vigor of marigold species and to explore the possible biochemical basis of this enhancement under normal and suboptimal temperature regimes.