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Abstract
Bud break timing in peach [Prunus persica (L.) Batsch] is determined by the sequential fulfillment of a chilling requirement (CR) and a heat requirement (HR) for development. Genotypic variation in CR has been well characterized in peach. Adapting peaches to low chilling environment through reduced CR can make them susceptible to crop destroying spring frosts, if bloom occurs too early. Potential variation in HR between accessions has received less attention due to the methodological difficulty in assessing HR independently of CR. HR could vary in the magnitude of growing degree hours (GDHs) and/or the base temperature at which GDH accumulation begins. Characterizing HR traits in peach accessions could allow improved bloom time modeling and selection of phenotypes with improved spring frost avoidance through delayed bloom. We estimated GDH and apparent base temperature for floral bud break by observing time to floral bud break at several constant forcing temperatures. We evaluated 54 peach accessions (representing a range of CR) in which chilling had been saturated after >1,700 h at 3°C. Accessions differed widely in both the GDH requirement (2,015 to 11,191°C⋅h) and apparent base temperature (−1.85 to 8.69°C) for GDH accumulation. GDH and apparent base temperature were negatively correlated. A simulation exercise was performed to assess relative importance of varying base temperature vs. GDH for delaying bloom at different chilling accumulations at three locations in the southeastern United States using 30 years of historical weather data. The aim of this study was to determine whether there may be unrecognized diversity in peach germplasm for two HR traits (base temperature and thermal time) to enable breeding efforts to delay floral bud break and reduce the frost exposure risk of developing flowers and fruits. Our results suggest that selecting cultivars for increased GDH would be a safer, more reliable strategy for delaying bloom than increasing base temperature for GDH accumulation.
Highlights
Peach floral buds are cold hardy during endodormancy, but lose cold hardiness during bud burst and bloom after which open flowers and developing fruit are highly sensitive to freezing temperatures (Lang, 1987)
Diversity in heat requirement (HR) for floral bud break among peach cultivars has been assumed to primarily be based on different thermal time requirements calculated from a common base temperature (Okie and Blackburn, 2011a; Maulión et al, 2014)
While we have highlighted an important area of unrecognized diversity in heat requirement behavior in peach, we should note that the methods used in this study have two important caveats
Summary
Peach floral buds are cold hardy during endodormancy, but lose cold hardiness during bud burst and bloom after which open flowers and developing fruit are highly sensitive to freezing temperatures (Lang, 1987). The interval between floral bud break and the last frost of the spring is, a critical period of risk exposure to producers. Two temperature requirements act to determine floral bud break and avoid potentially lethal freezing temperatures in the spring. Peach floral buds must experience a quantitative exposure to chilling temperatures [chilling requirement (CR)] to allow bud development to proceed in response to quantitative accumulation of warm temperatures [heat requirement (HR)]. Variety CR has long been recognized as a major determinant of bloom timing, with breeding for later bloom dates resulting in selection for increased CR (Topp et al, 2008)
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