Sweet cherry production relies on the availability of stored non-structural carbohydrates to support and maintain growth (Flore and Layne, 1999). Throughout the growing season, carbohydrate concentrations fluctuate, corresponding to seasonal phenology (Gent and Seginer, 2012). The efficiency in producing, transporting, using, and storing of carbohydrates significantly influences the growth rate and quality of both vegetative and reproductive organs (Clair-Maczulajtys et al., 1994). In the period leading up to dormancy, fruit trees will accumulate total NSCs, which are then translocated into storage areas including buds, trunk, and roots (Gaudillère et al., 1992; Loescher et al., 1990). NSCs play an essential role in initiating and maintaining growth (Flore and Layne, 1999; Keller and Loescher, 1989; Maust et al., 2000; Rady and Seif El-Yazal, 2013) during periods of stress (such as dormancy, the breaking of dormancy, and initial stages of bud burst). In many fruit trees, including sweet cherry, flower buds develop earlier than vegetative buds; therefore, bud NSC reserves are critically important in initiating bud burst and must be sufficient or remobilized from other storage organs (Ben Mohamed et al., 2012; Cerasoli et al., 2004; Marafon et al., 2011; Richardson et al., 2010; Vasudevan et al., 1998) until photosynthetic leaf area can provide adequate photosassimilate to developing sinks (Dejong and Grossman, 1995; Flore and Layne, 1999).
Carbohydrate dynamics have been widely investigated at the whole-tree scale (Kozlowski, 1992; Lacointe et al., 1993). In contrast, less is known about the fine-scale carbohydrate dynamics during winter dormancy in and around the meristematic zone within buds (Bonhomme et al., 2005). However, studies addressing this are becoming more frequent with recent work in grape (Vitis vinifera), pear (Pyrus pyrifolia), apple (Malus ×domestica), and kiwifruit (Actinidia deliciosa) (Ben Mohamed et al., 2010; Ito et al., 2012; Rady and Seif El-Yazal, 2013; Richardson et al., 2010). Carbohydrate availability in these structures is of major importance to the control of bud growth and development during dormancy and dormancy release (Ben Mohamed et al., 2012; Marquat et al., 1999). A pivotal paper (Marquat et al., 1999) showed that the sucrose stored in buds of peach (Prunus persica) was used during dormancy release to synthesize the sorbitol and raffinose that are required for bud burst.
Concentration and mobilization of carbohydrates such as sugars and starches within tissues are affected by biotic and abiotic factors (Daie, 1985). Strategic pruning is important for optimizing relationships among shoot growth, source leaf area, current photosynthesis, annual building of storage reserves, and ultimate realization of good yields with high-quality fruit (Costes et al., 2006; Lang, 2001). Pruning, like all stresses, induces hydrolysis of reserves on one hand and an accumulation of certain metabolites on the other (Clair-Maczulajtys et al., 1994; Daie, 1985). Timing and severity of pruning can play a crucial role in the partitioning of the carbohydrate reserves. Early summer pruning can stimulate the reallocation of reserves to renewing leaf growth, whereas late summer pruning may reduce leaf demand. Late pruning has been shown to increase assimilates to the current season’s sweet cherry fruit (Measham et al., 2013), which, at this time, is at maximum sink strength (Ayala and Lang, 2008; Flore and Layne, 1999).
Cropload (or simply the number of fruit per tree) can also influence fruit sugars in the current season (Bound et al., 2013; Measham et al., 2012; Roper et al., 1988). Klages et al. (2001) found a decrease in starch and an increase in glucose in apple fruit from trees with low croploads, whereas Roussos et al. (2011) found a decrease in glucose in apricot (Prunus armeniaca) fruit from low-cropping trees but no impact on total soluble solids. The impact of cropload on sweet cherry fruit quality appears to be a complex issue with inconsistent results, which may be explained partly by tree architecture (Bound et al., 2013). Manipulating cropload can additionally encourage bud development and flowering for the next season (Proebsting and Mills, 1981; Smith et al., 2007). Therefore, carbohydrates are posited as influencing flower bud differentiation, fruit set, and fruitlet abscission (Bustan et al., 2011) and fruit. However, a thorough investigation of varying carbohydrate concentration in developing buds and subsequent impact on the next phenological stages such as flowers, fruit set, and fruit has not occurred.
To better determine whether it is possible to improve bud burst through use of standard orchard practices to manipulate bud reserves of carbohydrate, and ultimately improve fruit yield and quality (Usenik et al., 2008; Vitasse et al., 2011), carbohydrate levels of buds were ascertained under different pruning regimes. The aim of this study was to determine the effect of pruning treatments (applied at different phenological stages) and cropload on the starch and soluble sugar contents of buds pre-dormancy (summer) and during dormancy (winter) and fruit quality at harvest (the next summer). In addition, the impact of any resultant changes in bud carbohydrates during dormancy on bud burst (spring) was investigated.
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