The requirement for exposure to a particular photoperiod in order to effect induction can be profoundly modified by temperature. Examples include the SDP strawberry, which is strictly photoperiodic only at temperatures above about 15°C (Guttridge, 1985) and Clarkia amoena, which is day-neutral at 20-24°C but is an LDP at lower temperatures (Halevy and Weiss, 1991). Many other similar interactions have been recorded (Vince-Prue, 1975 and see Appendix I). In a few cases, temperature has been shown to influence the induction process in the leaf. Perilla (green) is a SDP at 22°C but will flower in continuous light when maintained at 5°C; a single leaf from a plant induced to flower by exposure to low temperature acted as a donor of flowering when grafted to a receptor plant maintained in continuous light at 22°C (Deronne and Blondon, 1977). Similarly, in Pharbitis, there was little flowering response when cotyledons were removed after 10 days in continuous light at 13/14°C but the number of flower buds was the same as in intact plants when they remained for a further 2 days at 23°C before excision (Shinozaki and Takimoto, 1982). In the majority of cases, however, it is not known whether changes in temperature modify the induction process in the leaf or other components of the overall flowering response. Interactions between temperature and photoperiod are not always found, however, even when flowering is responsive to both environmental factors (Roberts et al., 1985).
Insensitivity to temperature is a characteristic of circadian rhythms and temperature usually has little effect on the time of maximum sensitivity to a night-break (Chapter 2). In contrast, the critical nightlength has been shown to be markedly influenced by temperature, although there is considerable variation between species in the extent to which this occurs (Fig. 6.5). Little effect was observed in the SDP Chenopodium, Lemna and Xanthium, but both the SDP Pharbitis and the LDP Hyoscyamus showed a strong dependence of CNL on temperature. The effect is presumed to occur in the leaf, although this has not been strictly demonstrated.
FIG. 6.5. Effects of temperature on the length of the critical dark period for flowering. Adapted from King, 1979.
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