Farmers throughout the world have traditionally used fire and smoke in grain drying practices. It is thought that these methods improve germination and seedling vigor (Paasonen et al., 2003). Recently, the germination response to smoke has been studied through the use of smoke-water, which is derived from burning plant material and bubbling the smoke through water. This has been shown to promote germination in wild plant species from many ecosystems (Light and Van Staden, 2004) and also to improve the growth and yield of agricultural and horticultural species (Brown et al., 2003), although it is necessary to dilute the concentration of the smoke-water before application to many species (Lloyd et al., 2000).
The main active germination compound of smoke-water derived from burned plant materials (Van Staden et al., 2004) and cellulose (Flematti et al., 2004) has been identified as butenolide [3-methyl-2H-furo(2,3-c)pyran-2-one], which is effective at very low concentrations (1 ppb). The compound has recently been referred to as "karrikinolide" (Commander et al., 2008). The action of smoke in promoting seed germination in many species is attributed to the presence of this compound (Soos et al., 2009). Dixon and Roche (1995) reported that the promotive effect of smoke is independent of seed size, seed shape, seed type, phylogenetic position, geography, and plant life form, i.e., whether annual, perennial, herbaceous, fire-sensitive seeder, or fire-tolerant resprouter. Smoke from a wide variety of biotic sources, including wood, straw, mixtures of dry and fresh plant material, and charred wood, can stimulate germination (Brown and Van Staden, 1997).
Papaya (Carica papaya) seed germination is affected by many environmental factors such as temperature, light, pH, oxygen, and soil moisture (Lange, 1961). Under normal conditions, germination can occur as soon as in 16 to 20 d. (Bhattacharya and Khuspe, 2001). Lange (1961) also found that the percent germination may vary from 3% to 71% depending on the cultivar and temperature. In unfavorable conditions, germination has been reported to be slow, erratic, and incomplete (Lange, 1961).
In Taiwan, the rice production area is more than 200,000 ha, creating a problematic volume of dry straw residue (Council of Agriculture Statistics Year Book, 2010) that would better serve agriculture if it could be repurposed. Papaya in Taiwan is grown on 3,287 ha and fruits yield 129,322 tons. The papaya industry is limited by seed germination rates. Therefore, the objective of this study was to evaluate the efficacy of smoke-water prepared from dry rice straw on papaya seed germination and the growth of the ‘Tainung No. 2’ papaya seedling.
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