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 s ynthetic stimulant GR24. Temperature treatments during the germination test s comprised 169 different constant and alternating temperature regimes on a two-dimensional gradient plate. Optimum temperatures for germination of se eds of O. aegyptiaca and O. crenata were 18-21 degrees C and 18 degrees C, respectively. However, longer conditioning periods slightly lowered the opt ima in both species, and the maximum germination percentage was also reduce d due to an induction of secondary dormancy. At a given mean temperature, m ore 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 g ermination was modelled on the basis of there bring two prerequisites for g ermination: a minimum mean temperature which must be exceeded and a maximum temperature above which the seed will not germinate. These two requirement s were assumed to be independent and to be normally distributed in the seed population so that final germination could be described by a multiplicativ e probability model. Because of the response to maximum temperature, inhibi tory effects were more evident at alternating temperatures. Amplitude and t hermoperiod influenced this effect of maximum temperature. The implications of the detrimental effect of alternating temperatures for germination of O robanche spp. in the field are discussed. (C) 1999 Annals of Botany Company .