The effects of external cues on moss stomatal apertures: a reappraisal of existing data and new experimental evidence

Stomatal function varies widely across land plants. Fixed apertures in hornworts suggest a primary role in spore discharge and dispersal whereas actively regulated pores control water loss in angiosperms. Many spore-bearing vascular plants exhibit passive aperture movements. Mosses, belonging to an earlier lineage, are anomalous in having supposedly actively controlled apertures. The notion of the acquisition of active regulation early in land plant evolution comes mainly from the presence of stomata-related genes common to all land plants. In stark contrast, only three papers present data on changes in aperture dimensions and these involve very limited sampling of only two taxa, Physcomitrium patens and Funaria hygrometrica, both of which have anomalous unicellular guard cells. To overcome this lack of data, we undertook a rigorous experimental study of stomatal aperture dimensions in mosses to answer the following questions: do apertures in mosses change with sporophyte age? Do apertures respond to external stimuli known to elicit stomatal closure in angiosperms? Are the responses the same in mosses across a broad taxonomic range? Because moss stomata vary greatly in size, to minimize possible biased sampling errors we measured every stoma on every capsule and used vertically split capsules to compare treated and untreated halves. Vascular plant leaves were used as controls. Overall, we measured over 25,000 apertures from 12 moss species. Our results demonstrate unequivocally that moss stomata have fixed apertures and do not respond to external cues. Aperture dimensions are unchanged by all the treatments applied, regardless of the age of the sporophytes and whatever the nature of the stomata in terms of numbers, position, size, guard cell morphology, sporophyte size and position of taxa in the moss tree of life and their ecology. Post mature moss apertures become occluded with additional wall materials and closed stomata are extremely rare. Our split capsule data indicate that previous reports of ABA-induced changes in aperture dimensions in Funaria and Physcomitrium are most probably attributable to insufficient sampling that inadvertently failed to take account of the considerable inter- and intra-sporophyte variations in stomatal dimensions in mosses. The absence of any functional evidence for regulation of gaseous exchange in mosses strongly favours the notion of gradual rather than early acquisition of stomatal function. The principal function of moss stomata seems to be the facilitation of sporophyte desiccation in contrast to the active control of water loss in angiosperms.