The sweetpotato is among the top seven main crops in the world with more than 130 million tons produced each year (All-about-sweet-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-oxido-5-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.
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