Annals of Botany

Seed strategy centred on cycling sensitivity

Plants whose seeds form long-lived seed banks exhibit a range of strategies to spread germination potential over a protracted period. One of these, seasonal cycling in germinability, has been studied by Jayasuriya et al., at Lexington, USA (pp. 341–352) in Ipomoea lacunosa, a member of the Convolvulaceae and a troublesome weed in the USA. It forms seed banks and the seeds may remain viable for 40 years. Dormancy is physical: the hard seed coat prevents water uptake. Dormancy may be broken by scarification of the seed coat or by opening of the water gap. It has been claimed that the seeds exhibit dormancy cycling but, as the authors point out, it is impossible to re-impose physical dormancy once the seed coat has been breached or the water gap opened. If the seeds do exhibit cycling of germinability, then there must also be either a reversible physiological aspect to dormancy or a reversible sensitivity to dormancy break. It is the latter that the authors’ research has demonstrated in I. lacunosa. Space does not permit extensive discussion of their work but the key features are as follows. Incubation of physically dormant seeds under appropriate conditions at 35/20 °C resulted in >90 % germination; this figure falling significantly as temperature decreased, with the exact percentages varying between seed batches. Physically dormant seeds exposed to dormancy-breaking incubation on wet sand at 35 °C for 2 h exhibited >95 % germination even at 25/15 °C. Seeds incubated in moist conditions at lower temperatures were sensitive to this dormancy-breaking treatment but seeds incubated dry were not. Further, seeds in which sensitivity had been induced could be made insensitive again by incubation under dry conditions. The data lead the authors to develop a model that shows that this strategy fits I. lacunosa very well for its habitat and lifestyle.