A quantitative model for loss of primary dormancy and induction of secondary dormancy in imbibed seeds of Orobanche spp

Abstract

(Roberts, 1988; Murdoch and Ellis, 1992). In seeds of parasitic plants such as Striga and Orobanche, a moist Seeds of three species of Orobanche were conditioned pretreatment known as ‘conditioning’ is a prerequisite for (i.e. stored fully imbibed in darkness) for periods up to germination (Joel et al., 1991). The seeds become respons210 d in order to model relief of primary dormancy and ive to a germination stimulant which, in the natural induction of secondary dormancy. The data were con- environment, originates from the host plant. Extending sistent with the hypothesis that periods for loss and the pretreatment also results in reduced germination due induction of dormancy and loss of viability are normally to secondary dormancy which was called ‘wet dormancy’ distributed in populations of imbibed seeds and that by Vallance (1950). For example, Brown et al. (1951) these three processes are independent. There was a reported that O. minor Sm. seeds conditioned at 25°C positive, linear relationship between the rate of loss showed a maximum loss of dormancy after 14 d and that of primary dormancy and temperature from 10‐30°C the germination capacity of some seed lots decreased in O. aegyptiaca and O. cernua and 10‐25° Ci n rapidly thereafter. Van Hezewijk (1994) also reported O. crenata. In all three species, the rate of induction that prolonged conditioning resulted in a decrease in of secondary dormancy was highest at 10°C and germination of O. crenata Forsk. This decrease in gerdecreased with increase of temperature up to about mination was greater at 15°C and 10°C than at 20° Ci n 20°C, above which there was little further change in one of the two seed lots tested. Vallance (1950) observed the rate with temperature. The resulting model that germination of Striga hermonthica (Del.) Benth. explained over 90% of the variation in germination seeds was lower when the conditioning period was longer after conditioning in both O. aegyptiaca and O. cernua. than that necessary to obtain maximum germination, In O. crenata, however, this model was only satisfact- which at 22°C was 6 d. ory at 10 and 15°C. At higher temperatures, dormancy During conditioning the two processes of loss of primwas relatively stable for periods of conditioning from ary dormancy and induction of secondary dormancy are 70 to about 154 d. Possible explanations for this are occurring perhaps simultaneously, perhaps independently discussed. Applications of these models for estimating and certainly at diVerent rates. As such, there is a close the time required to reduce Orobanche infestations in analogy with the prechilling response of Rumex and Picea the field are also briefly discussed.