Lagenaria siceraria (Mol.) Standl., a viny species of the Cucurbitaceae family, is thought to be among the first domesticated plant species (Cutler and Whitaker, 1967). Its dry, mature fruit is used by people throughout the world for making jars, utensils, tubes, and musical instruments, is used as a vegetable, and is commonly known as the bottle gourd (Decker-Walters et al., 2001, 2004; Erickson et al., 2005). It is also known as the white-flowered gourd or calabash (Jeffrey, 1967). Bottle gourd is indigenous to Africa (Morimoto et al., 2006; Richardson, 1972); however, remains found in archeological digs point to the possibility that it reached temperate and tropical areas in Asia and the Americas over 10,000 years ago. The increased geographical distribution of the bottle gourd plant is likely due to human migration (Erickson et al., 2005), though dispersion of bottle gourd could have occurred through movement across oceans. Bottle gourd fruits are known to have the capacity to float on seas for many months without losing seed viability (Decker-Walters et al., 2004). The crop thrives in a wide range of soil types and is more tolerant of a high water table (Yetisir et al., 2006) and to salt than watermelon Citrullus lanatus (Thunb) Matsum and Nakai (Colla et al., 2005). In addition to the aforementioned comparative advantages of bottle gourd, research should be conducted focusing on the response of bottle gourd to herbicides that are registered in watermelon. This research would provide knowledge for making selections of bottle gourd rootstocks for grafting with watermelon.
More than 105 million tons of watermelon are produced annually on a global scale (Food and Agricultural Organization; http://faostat.fao). The herbicide clomazone (Anonymous, 2005) is widely used in cucurbit crop production in the United States. Clomazone controls many important annual weeds such as barnyardgrass [Echinochloa crus-galli (L.) Beauv], velvetleaf (Abutilon theophrasti Medicus), and cocklebur (Xanthium strumarium L). The recommended use rates for watermelon are 0.17–0.28 kg·ha−1 depending on the soil type. Clomazone can cause moderate injury to young plants that is expressed as chlorosis of leaves and growing tips (Grey et al., 2000). A study performed at the University of Florida demonstrated that preemergence (PRE) applications of clomazone at 0.28 kg·ha−1 to transplanted watermelon grafted to bottle gourd resulted in ≈8% injury (Adkins, 2011).
Previously, differences were reported in clomazone tolerance among genotypes of watermelon and its relatives (Harrison et al., 2011). The most tolerant genotypes were accessions of the African watermelon Citrullus lanatus var. citroides obtained from the U.S. National Plant Germplasm collection; however, other accessions of C. lanatus var. citroides were highly susceptible. About 5-fold higher clomazone concentrations were required to cause equivalent injury and growth reduction in the most tolerant genotypes when compared with susceptible genotypes in a greenhouse concentration–response experiment. Hines and Wilson (1992) also reported that watermelon cultivars varied in clomazone injury in a greenhouse experiment. Staub et al. (1991) screened the U.S. National Plant Germplasm collection of cucumber for clomazone tolerance in the field and identified tolerant and susceptible accessions based on injury ratings. In a greenhouse evaluation of selected accessions, the relative differences in tolerance of the accessions did not range as much as those observed in the field. Al-Khatib et al. (1995) reported differences in clomazone tolerance among five cucumber cultivars in greenhouse and field experiments. Following the observation of differences in clomazone injury in a field trial, Harrison and Keinath (2003) evaluated 67 pumpkin (Cucurbita maxima Pang) cultivars for tolerance in the greenhouse. Differences in tolerance were greatest between the species marketed as pumpkins; however, differences in tolerance were also observed among cultivars within species. Thus, there is a precedent of clomazone tolerance being variable among varieties of cucurbit species. Watermelon was the most sensitive to clomazone when compared with honeydew (Cucumis melo L. var. inodorus Naud), zucchini (Cucurbita pepo L.), winter squash (C. maxima Duch, Ex. Poiret), cucumber (Cucumis sativus L.), and muskmelon (Cucumis melo L. var. reticulatus Naud) (Figueroa and Kogan, 2005) based on early season injury ratings and growth measurements. However, clomazone at up to 0.9 kg·ha−1 did not affect watermelon crop yields. Boyhan et al. (1995) reported that watermelon yields were reduced by clomazone applied PRE at 1.1 kg·ha−1 to direct seeded watermelon. Grey et al. (2000) reported that clomazone applied preplant incorporated at 0.84 kg·ha−1, preplant to the soil surface, and posttransplant to the soil surface caused minor foliar bleaching (14% injury or less), but did not reduce the yields of seeded or transplanted ‘Royal Sweet’ watermelon. Cohen et al. (2008) concluded from a greenhouse study that clomazone applied at 1.0 and 2.0 kg·ha−1 was too injurious to grafted and nongrafted watermelons. Variation among cultivars in clomazone tolerance has been reported in several non-cucurbit crops, including bean (Sikkema et al., 2006), cabbage (Hopen et al., 1992), corn (Keifer, 1989), rice (Mudge et al., 2005; Scherder et al., 2004; Zhang et al., 2004), and sweetpotato (Harrison and Jackson, 2011). The variable tolerance observed in other crops necessitates screening a large number of bottle gourd accessions for clomazone sensitivity to increase the probability of finding a line with high tolerance.
The objectives of these greenhouse experiments were to assess the variability of clomazone tolerance in an extensive bottle gourd germplasm collection and to identify lines with high levels of tolerance. The potential end application would be to select clomazone-tolerant bottle gourd lines that could be used as rootstock to graft to watermelon scions. In the past, bottle gourd rootstock had effectively improved salt tolerance to the watermelon scion (Colla et al., 2010). Certain bottle gourd lines are resistant to multiple viruses that would infect economically important watermelon lines (Ling et al., 2013). Crown rot (Phytophthora capsici Leonian) disease, root-knot nematodes, and whiteflies are reduced with certain bottle gourd varieties as well (Kousik et al., 2012; Levi et al., 2009). A dose–response experiment was conducted to quantify the difference in tolerance between the most and least tolerant accessions.
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