Influence of Prohexadione Calcium on Sweetpotato Growth and Storage Root Yield

Excessive vine growth in sweetpotato has been associated with lower storage root yield. Prohexadione–calcium (Pro-Ca), a plant growth retardant, has been used to reduce vegetative growth and increase harvest efficiency and yield in many fruit and row crops. The influence of Pro-Ca on sweetpotato (Ipomoea batatas Lam.) vegetative growth and storage root yield was evaluated in this study. In 2010 and 2011, the sweetpotato cvs. Beauregard, Porto Rico, O’ Henry, and SC1149-19 were sprayed with 0 and 810 mg a.i./L Pro-Ca in a split plot randomized complete block. Each plot received two sprays, the first at 2 weeks after transplanting and the second at 6 weeks after transplanting. When averaged across cultivars, Pro-Ca significantly (P £ 0.05) reduced vine length and vine yield and increased total root yield compared with the control. There was no significant Pro-Ca 3 cultivar interaction on any trait. Total biomass was not significantly (P £ 0.05) different between Pro-Ca-treated and control plants. However, the treated plants had more total root yield, whereas the control plants had more vine yield, suggesting that ProCa treatment affected the distribution of assimilates. Data from this study suggested that it may be possible to use Pro-Ca to manipulate the source-sink relationship for higher storage root yield in sweetpotato. The sweetpotato is among the top seven main crops in the world with more than 130 million tons produced each year (All-aboutsweet-potatoes.com, 2011). It grows well in poor soils and harsh environments (CIP, 1999) and, depending on the cultivar and growth conditions such as soil nitrogen, soil moisture, and photoperiod, will produce a huge amount of interlocking vines (Somasundaram and Santhosh Mithra, 2008). Uncontrolled vegetative growth presents multiple problems of great economic importance, including encroachment that can increase harvest cost and reduce harvest efficiency (Smith and Wright, 1994) and increase photoassimilate sink capacity of non-economical (vegetative) plant parts (Renter and Stassen, 1998). Although several plant growth inhibitors have been found to be effective in controlling vegetative growth, there are few without health and environmental concerns (Miller, 2002). Prohexadione–calcium (calcium 3-oxido5-oxo-4-propionylcyclohex-3-enecarboxylate) belongs to a new class of plant growth retardants called acylcyclohexanetriones (Nakayama et al., 1992; Rademacher, 2000). Pro-Ca is believed to act by competing with the natural cosubstrate, 2-oxoglutarate, at the active site of dioxygenases, which catalyzes the later stages of the biosynthesis of gibberellins (GA) (Rademacher, 2000). Pro-Ca inhibits the conversion of biologically inactive GA20 into biologically active GA1, involved in shoot elongation in many plant species (Nakayama et al., 1991, 1992; Rademacher, 2000). Unlike other plant growth retardants, Pro-Ca is only taken up by the green tissues, breaks down shortly without leaving a residual effect, and is registered for use in horticulture crops (Rademacher, 2000). Pro-Ca has been used successfully to reduce labor cost in many fruit and row crops including apples (Evans et al., 1999; Miller, 2002), pears (Smith et al., 2005), and peanuts (Beam et al., 2002; Jordan et al., 2001). In fruit trees, the reduction in labor cost was achieved through reduction in pruning time. In peanuts, the reduced labor cost was attributed to increased harvesting efficiency (Beam et al., 2002). Vegetative growth reduction in fruit trees has been shown to reduce competition between vegetative and reproductive sinks (Hicklenton, 1990; Williams, 1988). Manipulation of sinks capacity may enhance sweetpotato yield through assimilates repartitioning. The sweetpotato is a storage root and a major carbohydrate sink. Suppressing the growth of vegetative material may have the potential to reduce the aboveground sink capacity thereby sending more assimilates to roots (Williams, 1988). Pro-Ca application was found to increase the chlorophyll contents of apple and pear leaves, resulting in a net increase in photosynthesis (Sabatini et al., 2003). If Pro-Ca has the same effect in sweetpotato, it will strengthen the source capacity and increase the amount of assimilate available to grow the storage root. Pro-Ca has been used to reduce pruning time in apple (Miller, 2002), increase fruit yield (Renter and Stassen, 1998), and increase harvesting efficiency (Beam et al., 2002). It is expected that using Pro-Ca to limit vine growth in sweetpotato may redirect more photoassimilates down to form and/or grow storage roots, resulting in increased yield. This study examined the influence of Pro-Ca on sweetpotato vegetative growth and storage root yield among four cultivars. Materials and Methods Two experiments were conducted in a field on the campus of Alcorn State University (lat. 31.8769 N, long. 91.1411 W) located in Lorman, southwest MS, in 2010 and 2011 to determine the effects of Pro-Ca (Apogee ; BASF Corp., Research Triangle Park, NC) on sweetpotato vegetative growth and storage root yield among four cultivars. The experiments were planted on Memphis silt loam soil (Fine-silty, mixed, active, thermic Typic Hapludalfs). The experiments were planted on 1 and 6 June for 2010 and 2011, respectively. The two experiments were conducted in different sections of the same field having the same soil type. Plant material. Four sweetpotato cultivars, three of which are commonly grown in Mississippi, were used in this study. ‘Beauregard’, a bush vine-type sweetpotato (Rolston et al., 1987), matures in 90 d and has excellent storage root yield; ‘Porto Rico’, a compact bush vine-type sweetpotato with small runners (Nielson and Pope, 1960), matures in 110 d and has good storage root yield; ‘O’Henry’, a white-flesh variant mutant of ‘Beauregard’ with the same growth characteristics as ‘Beauregard’ but matures in 100 d and has good storage root yield; and ‘SC1149-19’, a prolific vine producer with very low storage root yield was developed in South Carolina. Experimental design and plot management. The experiments were conducted in a split plot, randomized complete block design with two replications. Pro-Ca treatments (0 and 810 mg a.i./L) were the main plots and cultivars were the subplots. Pro-Ca-treated and control plots were separated by a 3-m buffer zone to prevent drift during spraying. Sweetpotato was transplanted to conventionally prepared raised beds at a rate of 10 plants per row with 0.45 m between plants within the row and 1.06 m between rows. Each plot consisted of two rows. Before plowing the field, glyphosate was applied to burn down all weeds. Post-transplant weed control was accomplished by hand-hoeing. Soil nutrient amendments were based on Mississippi Received for publication 29 Aug. 2012. Accepted for publication 15 Oct. 2012. Thanks to the USDA/NIFA for providing the funds (Grant Numbers 2010-38821-21478 and 2009-38863-20054) necessary to conduct this study. Thanks to all the students who participated in this work. To whom reprint requests should be addressed; e-mail vicnji@alcorn.edu. HORTSCIENCE VOL. 48(1) JANUARY 2013 73 GROWTH REGULATORS