291 J. AMER. SOC. HORT. SCI. 128(3):291–296. 2003. The Effect of Temperature, Photosynthetic Photon Flux Density, and Photoperiod on the Vegetative Growth and Flowering of ‘Autumn Blissʼ Raspberry J.G. Carew, 1 K. Mahmood, J. Darby, 2 P. Hadley, and N.H. Battey Soft Fruit Technology Group, Centre for Horticulture and Landscape, The University of Reading, Whiteknights, Reading, RG6 6AS, U.K. ADDITIONAL INDEX WORDS. Rubus idaeus, Richards function, growth modelling, crop prediction ABSTRACT. The effects of temperature, photosynthetic photon flux density (PPFD) and photoperiod on vegetative growth and flowering of the raspberry (Rubus idaeus L.) ‘Autumn Blissʼ were investigated. Increased temperature resulted in an increased rate of vegetative growth and a greater rate of progress to flowering. Optimum temperatures lay in the low to mid 20°C range. Above this the rate of plant development declined. Increased PPFD also advanced flowering. While photoperiod did not significantly affect the rate of vegetative growth, flowering occurred earliest at intermediate photoperiods and was delayed by extreme photoperiods. These responses suggest that there is potential for adjusting cropping times of raspberry grown under protection by manipulating the environment, especially temperature. The raspberry (Rubus idaeus L.) has a clearly defined annual cycle of growth. In primocane fruiting cultivars, canes are pro- duced in early spring and grow vegetatively until midsummer when floral primordia are initiated. These grow out to produce fruit in the autumn. In biennial fruiting cultivars flower initia- tion is delayed until autumn and, as a result, fruiting occurs in the canesʼ second year of development (Carew et al., 2000a). The timing of flower initiation depends on cultivar and location (Ourecky, 1976). This presumably reflects the fact that factors such as temperature, light and water availability, which obviously vary between sites, affect cane development (Hoover et al., 1989; Privé et al., 1993). Growth rate in both biennial and primocane fruiting cultivars increases with temperature (Carew et al., 2000b; Kershaw, 1991; Ourecky, 1976; Williams, 1959a). For example, Ourecky (1976) found that as temperature increased from 13 to 25 °C the plantsʼ growth rate increased in ‘Heritageʼ. Williams (1959b) observed even greater effects with the biennial fruiting raspberry ‘Malling Promiseʼ. A temperature of 24 °C encouraged vigorous vegetative growth. However, 11 °C caused canes to stop elongating beyond 1 to 2 cm in height, suggesting that dormancy had been induced. In previous work, Carew et al. (2000b) described the effect of temperature on node production in ‘Autumn Blissʼ. The rate of node production increased to an optimum of ≈22 °C. As tempera- ture continued to increase, however, it became supraoptimal and node production slowed. This effect of high temperatures has also been observed in commercial production systems with the cultivar Heritage (Nonnecke and Taber, 1989). Increasing temperatures also advance flowering (Kershaw, 1991; Lockshin and Elfving, 1981) and fruiting in primocane fruiting cultivars, but above an optimum temperature, fruiting is delayed (Carew et al., 2000b). In biennial fruiting cultivars, where flower initiation is more distinct from flowering and fruiting, the temperature at which flower initiation is delayed or prevented seems to be much lower than for primocane fruiting cultivars (Williams, 1959b). While previous work has described the effects of tempera- ture on fruiting (Carew et al., 2000b), the flowering response to temperature and the effects of light intensity and photoperiod have not been described. Williams (1959b) found that at 24 and 11 °C, photoperiod did not affect development of the biennial fruiting cultivar Malling Promise. However, when an intermediate temperature was used (17 °C), a 9-h photoperiod caused early cessation of cane growth when plants were 1 to 2 cm in height, but a photoperiod of 14 h supported vigorous growth. These data point to potentially important effects of photoperiod on raspberry. A quantitative analysis of the effects of temperature, photope- riod and light intensity, on raspberry growth and development is therefore required. Without this quantitative approach, optimization of fruit production is not possible. This is especially the case where the aim is for an extension in cropping season. To satisfy demand for a 12-month supply of fruit, growers are now using heated polytunnels or glasshouses to extend cropping into the colder months (Allen and Raffle, 2000). In this context, the ability both to predict when fruiting will occur and to schedule flowering and fruiting become critical. The utility of this approach is shown for the strawberry by the model developed by Le Mière et al. (1998), which predicts strawberry yield and time of cropping based on the temperature that plants receive. The two series of experiments described here were carried out to determine the effects of temperature, photosynthetic photon flux density (PPFD) and photoperiod on cane growth and flowering in raspberry ‘Autumn Blissʼ. This information was used to examine the potential for manipulating environmental variables to schedule cropping in primocane fruiting raspberry cultivars. Materials and Methods SERIES 1: PHOTOPERIOD. Two experiments were conducted to determine the effect of photoperiod on the growth and flowering of ‘Autumn Blissʼ. EXPERIMENTAL METHOD. One hundred and fifty plants of ‘Autumn Blissʼ were obtained from Darby Farms Brothers Ltd., Methwold, U.K. on 15 Oct. 1996 (Expt. 1) and on 28 Oct. Received for publication 10 June 2002. Accepted for publication 7 Jan. 2003. We acknowledge financial support from The University of Reading Research Endowment Trust Fund and Darby Brothers Farms Limited. We would like to thank Rex Brennan (SCRI, Dundee, Scotland) and Steve Adams (HRI, Welles- bourne) for their comments on the manuscript. 1 Corresponding author 2 Darby Brothers Farms Ltd., Bars Hall Farm, Basil Road, West Dereham, Kings Lynn, Norfolk, PE33 9RP, U.K. J. AMER. SOC. HORT. SCI. 128(3):291–296. 2003.