Some caneberry (Rubus L.) production regions experience dramatic seasonal variation in bloom date and fruit ripening time. Phenology and biology of flowering in both cultivated and wild raspberries (Rubus idaeus L.) and blackberries have been studied under diverse environmental conditions in Scotland (Robertson, 1957) and throughout the United States, including in Maryland (Waldo, 1933), Missouri (Warmund and George, 1990; Warmund and Krumme, 2005), Oregon (Takeda et al., 2002; Waldo, 1933), and West Virginia (Takeda et al., 2002; Takeda and Wisniewski, 1989). Hoover et al. (1989) reported that when primocane-fruiting raspberry cultivars were grown at a range of sites across North America, temperature emerged as the critical factor for cane growth because it was correlated with heat unit accumulation (base 5 °C) throughout the season. Under temperate-zone growing conditions, floricane-fruiting blackberries have a clearly defined seasonal pattern of dormancy, entering the endodormant phase as a result of a shortened photoperiod and low and moderate temperatures in fall, and exiting after sufficient winter chilling (Moore and Caldwell, 1985). In floricane-fruiting blackberries, budbreak occurs from late March to early April in temperate regions of North America. Fruiting lateral shoots emerge from the axillary buds along the length of floricanes and their lateral branches. Over the next 4 to 5 weeks, the inflorescence develops distally on fruiting lateral shoots, and flower buds on the inflorescence open from mid-May to mid-June. Jennings (1979) compared bloom dates and fruit ripening times in eight blackberry cultivars of diverse genetic origin over three seasons and concluded that accumulated heat units (the accumulated temperatures above 6 °C) successfully quantified fruit ripening time (time from flowering to ripe fruit) but were not useful in predicting bloom date. However, no details were provided as to how these accumulated heat units were calculated. In eastern West Virginia, flowering in semierect ‘Black Satin’, ‘Dirksen Thornless’, and ‘Hull Thornless’ blackberries occurred over a 5-week period and fruits with bloom dates of 7, 14, and 21 June needed 50, 55, and 61 d to reach maturity (Takeda, 1987). It appeared that bloom date affected the onset of fruit development as well as maturation rate, but that study did not use temperature recordings to account for seasonal variations in the onset of bloom in the spring and the fruit-ripening period in summer.
Models that reliably predict bloom and fruit ripening are useful in understanding the environmental constraints on crop development, which in turn is useful in matching genotypes to their environment for optimum cropping. An understanding of how environmental conditions affect plant development has also been used in an attempt to predict the potential impacts of global climate change (Hanninen, 1995). Furthermore, increased interest in producing blackberries in protected cultivation (Strik et al., 2007) requires a better understanding of the relationship between temperature and crop development. This is particularly important in managing greenhouses and high tunnels designed to advance fruiting (Heidenreich et al., 2008).
For processes that are primarily temperature-driven, a measure of heat unit accumulation accounts for seasonal differences in development time. The basic concept in heat unit accumulation is that growth occurs when temperatures are above some minimum (base temperature) and that growth rate increases with temperature. Further refinements provide for optimum and critical temperatures (Richardson et al., 1975). The optimum temperature is the point at which there is no longer a continued increase in growth rate (plateau), whereas the critical temperature is the maximum temperature at which growth will continue. Historically, temperature data were often limited to daily maximum and minimum, in which growing degree day accumulation was based on averaging the daily maximum and minimum subtracting the base temperature and summing this value for each day (Arnold, 1960). A refinement on this method is according to Baskerville and Emin (1969) in which a sine curve is fitted to daily maximum and minimum temperatures to approximate diurnal fluctuations and thereby estimates the amount of time each day when temperatures exceed the base temperature for growth. With improvements in automated weather monitoring, it is now practical to calculate heat unit accumulation on an hourly basis and sum growing degree hours (GDH), eliminating the need to mathematically approximate diurnal fluctuations.
A linear model for accumulated heat units (Anderson and Seeley, 1992) can be defined by a basal, optimum, and critical temperature (Fig. 1). A further refinement of this model is the use of an asymmetric curvilinear relationship (ASYMCUR) between temperature and growth (Anderson et al., 1986). The purpose of this work was to determine heat requirements for bloom in Rubus, primarily blackberry, under both controlled environment and field conditions.
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