Ultraviolet radiation affects intumescence development in ornamental sweetpotato (Ipomoea batatas)

Intumescences are a physiological disorder characterized by hypertrophy and possibly hyperplasia of plant tissue cells. Ultimately, this disorder results in the death of the affected cells. Previous observations and research suggest that the quality and quantity of light to which plants are exposed may be a factor in development of the disorder. The purpose of this study was to assess the preventive effect of ultraviolet-B (UVB) radiation on intumescence development in ornamental sweetpotato (Ipomoea batatas). Two sweetpotato cultivars, Sidekick Black and Ace of Spades, were grown under light treatments consisting of 1) normal greenhouse production conditions; 2) supplemental UVB lighting; 3) supplemental UVB lighting with Mylar sleeves over the lamps to block UVB radiation; and 4) control lighting with full spectrum lamps. Treatments were administered for 2 weeks, and the experiment was repeated twice. ‘Ace of Spades’ was highly susceptible to intumescence development, whereas ‘Sidekick Black’ was much less susceptible to the disorder. For ‘Ace of Spades’, the addition of UVB radiation significantly reduced the number of leaves affected with intumescences when compared with plants grown under the other light treatments; this UVB effect was not apparent for ‘Sidekick Black’. Furthermore, there was no evidence for reduced plant growth under UVB light in either cultivar, but side effects from the radiation included leaf discoloration and deformities. This study indicates a cultivar-specific effect of UVB light in preventing intumescence development on ornamental sweetpotato, therefore suggesting a potential genetic component in intumescence susceptibility. These results provide further insight in better understanding intumescence development and how to prevent the disorder. Intumescences are a physiological disorder that develops sporadically on the leaf tissue of many plant species, including some varieties of tomato (Solanum lycopersicum ‘Maxifort’; Rud, 2009), sweetpotato (Wetzstein and Frett, 1984), and cuphea (Cuphea sp.; Jaworski et al., 1988). This disorder is often described as abnormal, translucent outgrowths on the leaf surface with a gall or wart-like appearance (J.K. Craver, C.T. Miller, K.A. Williams, and D.L. Boyle, in press; Morrow and Tibbitts, 1988; Wetzstein and Frett, 1984). This disorder was first observed and named as intumescence by Sorauer (1899) and was found at that time to develop on numerous plant species (La Rue, 1932). Although the term intumescence is commonly used to describe this disorder, other common and interchangeably used nomenclature in the published literature includes excrescences, neoplasms, galls, genetic tumors, leaf lesions, enations, and oedemata (Pinkard et al., 2006). Intumescences can have a substantial impact on both the economic and aesthetic value of affected plants (Balge et al., 1969; Rangarajan and Tibbitts, 1994). Many of the species most susceptible to this disorder are grown solely for ornamental purposes, and intumescences can greatly impair the overall aesthetics of the crop. Additionally, severe cases of intumescence development can result in impaired photosynthesis (Pinkard et al., 2006; Roloff and Scherm, 2004). Thus, this physiological disorder presents a substantial problem for growers attempting to produce crops that may be prone to its development. The causative factors related to intumescence development remain somewhat elusive. One key finding has been that this disorder predominantly occurs on plants being produced in controlled environments (Jaworski et al., 1988; Lang and Tibbitts, 1983; Petitte and Ormrod, 1986; Wetzstein and Frett, 1984). For the most part, no pathogen has been found to be involved in intumescence development, which leads most to agree that this is a physiological disorder (Rangarajan and Tibbitts, 1994). Some of the most commonly proposed causative factors include air contamination (Kirkham and Keeney, 1974; Lang and Tibbitts, 1983), hormones and hormone concentrations (Kirkham and Keeney, 1974; Lang and Tibbitts, 1983; Morrow and Tibbitts, 1988; Petitte and Ormrod, 1986; White, 1951), humidity (Douglas, 1907; Eisa and Dobrenz, 1971), temperature (Balge et al., 1969; Eisa and Dobrenz, 1971), and light (Lang and Tibbitts, 1983; Morrow and Tibbitts, 1987; Rud, 2009; Wheeler, 2010). Excess water has also been cited as a potential causative factor and is commonly found when referring to the development of oedema on geranium (Pelargonium sp.; Balge et al., 1969; Metwally et al., 1970, 1971). Light quality is one of the strongest candidates as a causative factor or, conversely, as a potential preventive measure for intumescence development. Ultraviolet (UV) light, in particular, is thought to be connected to the disorder because many greenhouse-glazing materials block ultraviolet light wavelengths (100 to 400 nm) and intumescences occur in protected culture. Lang and Tibbitts (1983) found that ultraviolet light, specifically UVB, effectively prevented intumescence development on tomato (var. hirsutum and var. esculentum ‘Oxheart’). Similar results have been observed by Rud (2009) on tomato ‘Maxifort’ and by Morrow and Tibbitts (1987) on tomato var. hirsutum. However, the exact mechanism by which ultraviolet light effectively inhibits intumescence development remains uncertain. Ornamental sweetpotato is an annual ornamental crop commonly produced in greenhouses during the spring season; it is a popular species because of its trailing habit and striking foliage colors with several new cultivar introductions in recent years. However, the species is prone to intumescence development when produced in greenhouses, as indicated in a recent patent application for a new variety (Yencho et al., 2012). Intumescences on sweetpotato have not been extensively studied, and very few published research articles discuss the development of this disorder on the crop. One paper, ‘‘Intumeszenze fogliari di ‘Ipomoea batatas,’’’ Received for publication 14 May 2014. Accepted for publication 17 July 2014. This research was funded by The Fred C. Gloeckner Foundation and K-State Research and Extension. This manuscript has been assigned Contribution no. 14-379-J from the Kansas Agricultural Experiment Station (KAES). We thank the following individuals, all from Kansas State University, for their assistance: Lea Westervelt and Olivier Francescangeli. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable. Graduate Research Assistant. Assistant Professor. Professor. To whom reprint requests should be addressed; e-mail ctmiller@ksu.edu. HORTSCIENCE VOL. 49(10) OCTOBER 2014 1277 CROP PRODUCTION