North Carolina ranks number one in sweetpotato production in the United States with 87,000 acres planted ($332 million in gross farm value) in 2015 and accounts for more than half of the total acreage planted nationwide (U.S. Department of Agriculture, 2016). ‘Covington’ sweetpotato is the most commonly grown cultivar in North Carolina (North Carolina Crop Improvement Association, 2014). The wide adoption of ‘Covington’ is because of its disease resistance and consistency to produce a high percentage of no. 1 grade sweetpotato roots that result in excellent economic return. For improved storage root yield and quality, growers must closely follow recommended growing practices in the production field (Kemble, 2013). However, production practices in the propagation bed and transplant (nonrooted stem cuttings, which are also referred to as slips) cutting practices are often overlooked both in research and in practice within production fields. The practice of holding transplants for a few DBP is thought by many North Carolina growers to allow for transplants to produce adventitious roots before planting in the production field (J. Jones, personal communication). These growers believe that transplants that have been held to initiate adventitious roots before planting in the field will establish more successfully than transplants cut from propagation beds and planted on the same day. The beneficial effects of holding transplants before planting were reported to increase storage root yields in sweetpotato (Beyene et al., 2015; Hammett, 1983; Nwinyi, 1991; Ravindran and Mohankumar, 1989). However, there is limited information with respect to the optimum HD of ‘Covington’ transplants before planting under North Carolina sweetpotato growing conditions.
Storage roots are the principal carbohydrate storage organ in sweetpotato, and each root is a length of adventitious root that forms a localized carbohydrate storage structure. They are defined by their distinctive lateral growth (Lewthwaite and Triggs, 2009). The role of adventitious root formation is critical to storage root formation. Villordon et al. (2009) reported that adventitious roots started within 7 d after planting (DAP) comprised 86% of final storage root yield of ‘Beauregard’ sweetpotato under certain conditions. The initial adventitious roots originate from preformed root primordia that are commonly visible on the aerial stem at the time of cutting (Belehu et al., 2004; Hahn and Hozyo, 1983). These root primordia typically form in pairs on either side of the stem just below the nodes or leaves (Togari, 1950). Adventitious rooting can also develop within the callus tissue that forms on the buried end of a transplanted sweetpotato stem cutting. However, early storage roots develop from the initial adventitious roots preformed at nodes before planting rather than from those initiated later in the stem end callus (Lewthwaite and Triggs, 2009). Any event that limits carbohydrate deposition, or the cambial activity associated with the development of a fully formed storage root will temporarily or permanently obstruct the progress of a storage root during the season (Kays, 1985; Meyers et al., 2017; Togari, 1950). Togari (1950) reported that management and environmental variables within the first 20 DAP influenced adventitious root cambium activity in sweetpotato and the rate of storage root initiation, which in turn determined yield. These conditions include water-logged soils that expose roots to prolonged periods of oxygen deficiency or long periods of drought that can lead to the formation of pencil roots. Pencil roots have limited carbohydrate storage function as the stele may be partially lignified, which allows for some lateral thickening to occur (Wilson, 1970).
Environmental conditions such as precipitation and initial soil moisture have been shown to be the key components for transplant survival and storage root set in sweetpotato (Gajanayake et al., 2013; Thompson et al., 2017). Soil moisture stress is detrimental during plant establishment and can limit the initial growth and development of sweetpotato roots that eventually affect storage root yield (Gajanayake et al., 2013; Pardales and Yamauchi, 2003; Pardales et al., 2000). Villordon et al. (2012) reported that available soil moisture at the time of planting through 20 DAP was critical in influencing whether adventitious roots became storage or pencil roots. Sweetpotato is considered a relatively drought-tolerant crop (Pardales et al., 2000) and average yields have been reported in production systems suffering drought stress (Bouwkamp, 1985). Previous studies have demonstrated average yields in drought conditions; however, yield benefits from supplemental irrigation have been demonstrated when rainfall is not adequate or where moisture distribution is erratic and unpredictable (Bouwkamp, 1985). Rainfall can be sporadic in North Carolina after sweetpotato planting or during the growing season. The use of additional irrigation after planting and through the growing season is not usually practiced by growers in North Carolina. However, in California, 100% of the sweetpotato crop is irrigated through the use of drip or furrow irrigation because of very little rainfall during the growing season (Stoddard et al., 2013). The use of irrigation would not only maximize plant stand and storage root yield, but also allow transplants that have been held for long periods of time to have the ability to survive and produce good quality sweetpotato storage roots.
A significant number of transplants are shipped out of state and comprise an important portion of the North Carolina sweetpotato transplant production business (J. Jones, G. Warren, and R. Glover, personal communication). The authors are unaware of any literature that addresses the effect of shipping duration, which increases transplant holding time, on sweetpotato transplant survival. In general, sellers suggest planting of transplants as soon as they arrive or storing transplants for not more than 10 d (if planting conditions are not suitable) to avoid any negative effect on sweetpotato root quality and yield (Sand Hill Preservation Center, 2017). Therefore, it is important to determine how longer durations of transplant holding affect plant stands and yields.
Thus, the objective of this research was to determine the effect of ‘Covington’ sweetpotato transplant HD before planting on plant survivability, and storage root yield and quality. The effect of transplant HD on plant survivability, storage root yield, and quality was also investigated with and without PI at the time of sweetpotato transplant planting.
Belehu, T., Hammes, P.S. & Robbertse, P.J. 2004 The origin and structure of adventitious roots in sweetpotato (Ipomoea batatas) Austral. J. Bot. 52 551 558
Beyene, K., Nebiyu, A. & Getachew, M. 2015 Effect of number of nodes and storage duration of vine cuttings on growth, yield and yield components of sweet potato (Ipomoea batatas L.) at Jimma, southwest Ethiopia J. Biol. Agr. Healthcare 22 51 64
Bouwkamp, J.C. 1985 Production requirements, p. 1–33. In: J.C. Bouwkamp (ed.). Sweet potato products: A natural resource for the tropics. CRC Press, Boca Raton, FL
Gajanayake, B., Reddy, K.R., Shankle, M.W. & Arancibia, R.A. 2013 Early-season soil moisture deficit reduces sweetpotato storage root initiation and development HortScience 48 1457 1462
Hahn, S.K. & Hozyo, Y. 1983 Sweet potato and yam, p. 319–341. Symp. on Potential Productivity of Field Crops under Different Environments. Intl. Rice Res. Inst., Los Banos Laguna, Philippines
Hall, M.R. 1985 Influence of storage conditions and duration on weight loss in storage, field survival and root yield of sweet potato transplants HortScience 20 200 203
Kays, S.J. 1985 The physiology of yield in the sweet potato, p. 79–132. In: J.C. Bouwkamp (ed.). Sweet potato products: A natural resource for the tropics. CRC Press, Boca Raton, FL
Kemble, J.M. 2013 2013 Southeastern U.S. vegetable crop handbook. Vance Publ., Lincolnshire, IL
Meyers, S., Shankle, M.W., Main, J., Gajanayake, B. & Reddy, K.R. 2017 Sweetpotato storage root initiation. Mississippi State Univ. Ext. Publ. 2809
Milholland, R.D. & Averre, C.W. 2008 Sweetpotato operations manual for the production of micropropagated-certified planting stock. 2nd ed. Dept. Plant Pathol., North Carolina State Univ., Raleigh, NC
Nakatani, M., Oyanagi, A. & Watanabe, Y. 1987 Holding of cut-sprouts in sweet potato (Ipomoea batatas Lam.) II. Physiological changes in cut-sprouts during the holding period Jpn. J. Crop. Sci. 56 244 251
North Carolina Crop Improvement Association. 2014. Seed list 2013–2014. 22 Apr. 2015. <http://www.nccrop.com/varieties.php/8/Sweetpotato>
Nwinyi, S.C.O. 1991 Pre-planting method and duration of storing sweet potato (Ipomoea batatas L. Lam) shoot cutting planting materials for increased tuber yield Trop. Sci. 31 1 7
Pardales, J.R., Banoc, D.M., Yamauchi, A., Iijima, M. & Esquibel, C.B. 2000 The effect of fluctuations of soil moisture on root development during the establishment phase of sweetpotato Plant Prod. Sci. 3 134 139
Pardales, J.R. & Yamauchi, A. 2003 Regulation of root development in sweetpotato and cassava by soil moisture during their establishment period Plant Soil 255 201 208
Ravindran, C.S. & Mohankumar, C.R. 1989 Effect of storage life of vines with and without leaves on the establishment and root yield of sweet potato J. Root Crops 15 145 146
Sand Hill Preservation Center. 2017. Sweet potato. 1 Nov. 2017. <http://www.sandhillpreservation.com/2017_pdf/Sweetpotato2017.pdf>
Stoddard, C.S., Davis, R.M. & Cantwell, M. 2013 Sweetpotato production in California. Univ. California, Agr. Natural Resources, Veg. Production Ser. Bul. 7237
Thompson, W.B., Schultheis, J.R., Chaudhari, S., Monks, D.W., Jennings, K.M. & Grabow, G.L. 2017 ‘Covington’ sweetpotato plant survival and yield response to preplant irrigation, planting depth, and transplant size HortTechnology 27 824 830
U.S. Department of Agriculture. 2005. United States standards for grades of sweetpotatoes. U.S. Dept. Agr., Washington, DC
U.S. Department of Agriculture. 2016. Crop production 2015 summary. U.S. Dept. Agr., Washington, DC
Villordon, A., LaBonte, D.R. & Firon, N. 2009 Development of a simple thermal time method for describing the onset of morpho-anatomical features related to sweetpotato storage root formation Scientia Hort. 121 374 377
Villordon, A., LaBonte, D.R., Solis, J. & Firon, N. 2012 Characterization of lateral root development at the onset of storage root initiation in Beauregard sweetpotato adventitious roots HortScience 47 961 968