Sea kale (Crambe maritima L.) is a perennial edible halophyte belonging to the Brassicaceae family with a natural growth habitat along gravel and shingle (beach gravel consisting of large, smooth pebbles unmixed with finer material) sandy beaches in Scandinavia, the United Kingdom, and along the English Channel and the northern part of the Black Sea. It is mainly distributed on the high tide line above the water splash zone on the beach. The hydrophobic surface of the thick leaves provides high salt tolerance and the leaf thickness has been shown to increase when exposed to salt spray (400 mm) imitating exposure to sea gust (de Vos et al., 2010). This edible plant has been grown in home gardens or collected along beaches around the Victorian era (1837–1901) in England and used as a vegetable; young shoots were used as asparagus, young inflorescences as broccoli, and fresh green leaves were blanched to reduce bitterness and used like spinach (Maher, 1812). The plant is almost forgotten as a crop today, but trends and studies on local and reinvented foods have increased in the last 20 years. Collectively, this can offer sea kale a renaissance as a vegetable.
Sea kale contains important secondary metabolites, especially glucosinolates, acting as defense mechanisms against insect predation, similar to other species of the Brassicaceae family. The glucosinolates are stored in vacuoles, and when leaves are disrupted, the vacuole and adjacent myrosinase-containing cells burst and the glucosinolates are released and hydrolyzed by myrosinases to isothiocyanates. This group of chemicals is associated with cancer-preventive activity and gives Brassicaceae species the characteristic mustard-like flavor (Falk et al., 2007; Lee et al., 2008; Song and Thornalley, 2007; Verkerk et al., 2009).
Sea kale is self- or cross-pollinated (Bond et al., 2005) and in fall, when the leaves wilt, the hard flower stalks with mature siliques and seeds remain over winter (Briard et al., 2002; de Vos et al., 2010). The wind disperses the hard cork-like fruits along the coasts or into the sea water. The seeds inside the fruits are able to survive in the sea water as a result of the hard pericarp (Bond et al., 2005), which also imposes dormancy and affects germination and aging (Fusheng et al., 1998; Walmsley and Davy, 1997b). Pericarp removal and soaking in gibberellic acid, water, or NaOCl solutions are some of the methods used to overcome dormancy (Fusheng et al., 1998). Although such treatments have shown to increase germination, the side effect, especially for gibberellic acid, has been high seedling mortality (22% to 77%) caused by the surface-borne fungus Phoma lingam. This fungus causes black leg disease in Brassicacae species that is transmitted by fruits and remains viable up to 14 months (Lloyd, 1959).
The intake of cabbage species per capita is relatively low in the Scandinavian population (Meyer and Astrup, 2011), and sea kale could be a dietary supplement to traditional cabbage species. Most research on sea kale has been ecological studies (Bond et al., 2005; Briard et al., 2002; Walmsley and Davy, 1997a, 1997b, 1997c). A few studies describe sea kale being grown as Belgian endive (Cichorium intybus L.) by forcing it in dark rooms to produce etiolated shoots (Fusheng et al., 1998; Fusheng and Péron, 1998; Péron, 1989, 1990). This method is said to be the most economic and profitable, but still no commercial production occurs (Briard et al., 2002). No other uses of sea kale as a vegetable have apparently been reported. Walmsley and Davy (1997c) published an ecological study on the plant in response to substrate composition around its known habitat and fertilizer. They found the highest dry mass at shingle-dominated soils and no response to fertilizer. The plant growth response has not been studied and the growth of sea kale in less sandy soil types is apparently unknown.
The objective of this study was to examine the potential to introduce the species as a crop in agriculture by an organic approach and on other soil types than it normally thrives. We focused on seed germination ability, growth rate, and biomass production at low fertilizer levels. Most vegetable crops respond to soil types and the amount of available nutrients, but no published studies on sea kale growth responses on agricultural land are apparently available. This study includes a germination experiment with seeds without pericarp at 10, 15, and 20 °C, an organic fertilizer experiment simulating the effect on PAN application of 15 and 30 kg·ha−1, and a soil type experiment based on four soil types (sandy loam, gravel, fine and loamy sand). All experiments were conducted to examine response of the plants to specific growth conditions.
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