There is increasing interest in off-season production of raspberry, necessitating the need for new cropping systems. In subtropical areas, an annual production system has been examined (Darnell et al., 2006; Knight et al., 1996). This system uses prechilled, dormant, long-cane raspberry plants obtained from northern nurseries, thus eliminating problems associated with insufficient chilling and dormancy release. Plants are field-planted in January and fruit harvest occurs as early as March (Darnell et al., 2006; Knight et al., 1996). In this annual system, raspberry plants are removed after harvest and are replaced with new prechilled long-canes for the next season. Previous work has shown that yields in this annual system are less than yields observed in perennial red raspberry production systems (Alvarado-Raya et al., 2007; Darnell et al., 2006). This may be from disturbance of the root system during digging and shipment from the nursery, which can lead to significant root loss. This, in turn, can result in decreased root carbohydrate reserves and decreased yield.
Many studies have shown that root pruning in temperate crops affects shoot growth and yield. Dormant root pruning, such as what occurs in the above described annual raspberry production system, reduces vegetative growth and fruit size in apple (Schupp and Ferree, 1987; 1989), grape (Ferree et al., 1999; Lee and Kang, 1997), and sweet cherry (Webster et al., 1997). This may be from removal of a large source of reserve carbohydrate in the roots that would normally be used to support vegetative or floral budbreak.
In raspberry, spring vegetative and reproductive growth are concomitant (Atkinson, 1973), and both need carbohydrate for production of new biomass. Primocanes also begin growth at this time and are an additional sink for root carbohydrates (Whitney, 1982). Root pruning could further exacerbate carbohydrate competition because root growth also increases after root pruning (Schupp et al., 1992), resulting in additional sink activity. Given this scenario and the low photosynthetic activity in floricane leaves before bloom (Fernandez and Pritts, 1994), there is a high probability that roots of raspberry plants, especially in an annual production system, may be carbohydrate depleted by the beginning of bloom.
Root carbohydrate depletion before bloom would potentially decrease flower bud number (and therefore fruit number) and fruit size in raspberry. Flower bud initiation/differentiation in the lower portion of raspberry floricanes continues during early bloom (Williams, 1959; Qingwen and Jinjun, 1998), and work with other fruit crops indicates that adequate carbohydrate supply is critical for this process (Darnell, 1991; Ooshiro and Anma, 1998). Therefore, the elimination of part of the root system during dormancy could decrease the carbohydrate reserves required for early floricane lateral growth, including flower bud initiation and consequently, fruit number and yield.
Additionally, root pruning may also affect cane yield by decreasing fruit size. Fernandez and Pritts (1996) found that the maximum demand for assimilates in red raspberry is at the onset of fruiting while primocanes, roots, and fruits are all growing. In an annual cropping system, the elimination of part of the root system and the consequent intensive root regeneration (Schupp et al., 1992) during the high sink activity of the raspberry plant could decrease carbohydrate availability for the developing fruit.
The hypothesis tested in the present experiment was that low yields observed in an annual red raspberry production system are because of dormant root pruning that occurs during removal from the nursery, and a resultant decrease in root carbohydrate availability for floricanes during budbreak and early bloom. Our objectives were to assess the effect of dormant root pruning on plant carbohydrate allocation, fruit number, fruit size, and overall yield.
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