Annals of Botany

Once more unto the breach

The genus Cuscuta shows a strong evolutionary commitment to a particular lifestyle: all 145 species are holoparasitic vines and many of these are significant pests in agriculture. Further, along with other genera in the Convolvulaceae, at least some Cuscuta species exhibit physical dormancy of seeds, as discussed by Jayasuriya et al. (Lexington, Kentucky, USA and Taipei, Taiwan, pp. 39–48). A common feature of seeds with physical dormancy is the presence in the seed coat of a water gap, the initial point of entry of water into the seed, but it is not known whether this exists in Cuscuta. The authors have therefore investigated this and other features of dormancy in C. australis. The majority of seeds exhibited physical dormancy, which could be broken by scarifying the seed coat or by dipping the seeds into boiling water for 10 s or, more ‘naturally’, by incubating them wet in a temperature regime of 35/20 °C. Seeds were made sensitive to the latter dormancy-breaking treatment by dry storage at ambient laboratory temperature for 2 months. Breakage of dormancy was associated with opening of the hilar fissure; staining with an aqueous solution of aniline blue demonstrated that this was the route for water entry. No dye entered dormant seeds nor non-dormant seeds in which the hilar fissure had been sealed. The hilar fissure thus functions as a water gap, equivalent to the bulge adjacent to the micropyle in other members of the Convolvulaceae. Finally, there was evidence for sensitivity cycling, regarded as a strategy that enables dormant seeds to sense favourable conditions for germination and seedling growth. It is a relatively common phenomenon amongst physiologically dormant seeds but unusual in physically dormant seeds. Indeed, the authors state that this is the ‘second example of a species in Convolvulaceae that can undergo sensitivity cycling and the first demonstration in the only holoparasitic genus whose seeds have physical dormancy’.