Sweetpotato [Ipomoea batatas (L.) Lam] is an economically important crop in the United States; in 2017, its worth was more than $733 million (U.S. Department of Agriculture, 2018), and a total area more than 60,000 ha was planted throughout the United States (U.S. Department of Agriculture, 2019). Sweetpotato is transplanted using vegetatively propagated stem tip cuttings (slips) and requires ≈2 to 6 weed-free weeks to maximize yield (Harrison and Jackson, 2011a; Smith et al., 2009). The impact of weed competition on yield has been well documented. For example, Cyperus esculentus reduces yield by up to 80% (Meyers and Shankle, 2017). However, Meyers et al. (2010) reported a 36% to 81% reduction in marketable yield of Beauregard and Covington sweetpotato cultivars with Palmer amaranth [Amaranthus palmeri (S.) Wats] interference.
Because relatively few herbicides are registered for sweetpotato, chemical weed management is challenging (Harrison and Jackson, 2011b). Flumioxazin, S-metolachlor, and clomazone are PRE herbicides registered for application on sweetpotato that control troublesome weeds, such as Amaranthus palmeri and annual grasses (Kemble, 2017; Meyers et al., 2010). There is a lack of POST herbicides to control Cyperus esculentus in sweetpotato (Webster, 2010). A potential herbicide that is used to evaluate sweetpotato and could control or suppress yellow nutsedge is bentazon. The range of tolerance to bentazon in multiple sweetpotato cultivars has been reported by Motsenbocker and Monaco (1991). However, the safety data from this study warrant further exploration of bentazon tolerance in more cultivars with alternative strategies to reduce bentazon injury in sweetpotato. One possible means to increase POST herbicide options for sweetpotato is the incorporation of herbicide safener concepts, which, ideally, would protect sweetpotato from POST herbicides while not antagonizing weed control (Parker, 1983). Numerous studies of monocot species have reported that safeners increase the activity of cytochrome P450s, resulting in increased tolerance to multiple herbicide modes of action through conjugation and metabolism of the herbicide molecule (Hatzios 1991). Furthermore, Dubleman et al. (1997) recorded the capacity of the safener furilazole to enhance P450 activation, resulting in the de-esterification of halosulfuron-methyl to halosulfuron acid in corn seedlings. A plant hormone that has been shown to increase cytochrome P450s and potentially sequester reactive oxygen species in broadleaf vegetable crops is melatonin.
Arnao (2014) highlighted the antioxidant capacity of melatonin (N-acetyl-5-methoxytryptamine), which is able to scavenge reactive oxygen species (ROS), reactive nitrogen species (RNS), and detoxify various chemical contaminants as a response to environmental stress. Abiotic or biotic stresses (e.g., low temperatures, plant competition, and chemical applications) can impact the photosynthetic rate and increase the production of ROS. Increasing ROS production can result in lipid peroxidation of membranes, DNA damage, and inactivation of various enzymes (Cheng and Song, 2006; Foyer and Noctor, 2003). Turk et al. (2014) suggested that melatonin can enhance plant resistance to cold stress in wheat (Triticum aestivum L.) seedlings by directly scavenging ROS and by modulating redox balance and other defense mechanisms. A tissue culture experiment conducted by Erland et al. (2019) reported that exogenous melatonin was taken up by a specific transport mechanism. That mechanism involved active internal transport, which dispersed melatonin as a response to environmental stresses, resulting in the accumulation of this antioxidant substance in endodermal cells. Mandal et al. (2018) noted that the external application of melatonin could directly impact the genes involved in biotic and abiotic stress response, and they demonstrated that melatonin could increase cytochrome P450 activity in watermelon (Citrullus lanatus L.). In that same study, powdery mildew (Podosphaera xanthii) disease severity significantly decreased when melatonin was applied exogenously. Transgenic rice (Oryza sativa L.) that overexpressed melatonin contained lower levels of H2O2 when treated with butafenacil, thus confirming that a cellular increase in the melatonin level in plants results in resistance to oxidative stress (Park et al., 2013).
The identification and selection of bentazon herbicide tolerant lines would be beneficial for managing weeds in sweetpotato. In vitro methods are efficient for screening stress tolerance in different plants because they require lower resources and materials than field trials (Cutulle et al., 2020; Sakhanokho and Kelley, 2009). Rajasekaran et al. (2005) reported the efficiency of tissue culture to analyze cotton (Gossypium arboretum L.) plant interactions with multiple antifungal compounds. Cutulle et al. (2009) described the in vitro technique as the most accurate assessment for evaluating resistance to mitotic-inhibiting herbicides on annual bluegrass (Poa annua L.). A significant limitation to herbicide programs in sweetpotato is the lack of registered POST herbicides that control Amaranthus spp. and yellow nutsedge. Therefore, expanding POST herbicide options would provide growers with more flexibility in their weed management program. Bentazon is a photosystem II inhibiting herbicide with activity on Cyperus spp. and would benefit growers if the label was expanded to include sweetpotato.
Understanding the interactions between melatonin and bentazon herbicide in sweetpotato may lead to improvements in weed management. Therefore, the objectives of this study were to: 1) determine the effects of the bentazon rate on sweetpotato clones and 2) characterize the response of ‘Beauregard’ to bentazon and exogenous applications of melatonin.
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