A model of the effects of a wide range of constant and alternating temperatures on seed germination of four Orobanche species

Seeds of the obligate parasitic plants, Orobanche spp., were conditioned in water or GA(3) for 2 or 12 weeks and then stimulated to germinate by the synthetic stimulant GR24. Temperature treatments during the germination tests comprised 169 different constant and alternating temperature regimes on a two-dimensional gradient plate. Optimum temperatures for germination of seeds of O. aegyptiaca and O. crenata were 18-21 degrees C and 18 degrees C, respectively. However, longer conditioning periods slightly lowered the optima in both species, and the maximum germination percentage was also reduced due to an induction of secondary dormancy. At a given mean temperature, more seeds germinated at constant than at alternating temperatures. Results were analysed in terms of characteristics of alternating temperatures that appeared to control germination, i.e. mean temperature, maximum temperature, amplitude (difference between daily maximum and minimum temperatures) and thermoperiod (the time spent at the maximum temperature each day). Final germination was modelled on the basis of there bring two prerequisites for germination: a minimum mean temperature which must be exceeded and a maximum temperature above which the seed will not germinate. These two requirements were assumed to be independent and to be normally distributed in the seed population so that final germination could be described by a multiplicative probability model. Because of the response to maximum temperature, inhibitory effects were more evident at alternating temperatures. Amplitude and thermoperiod influenced this effect of maximum temperature. The implications of the detrimental effect of alternating temperatures for germination of Orobanche spp. in the field are discussed. (C) 1999 Annals of Botany Company.