Pseudostomata in Sphagnum (Bryophyta)

 

Exploding a myth: the capsule dehiscence mechanism and the function of pseudostomata in Sphagnum

by Duckett J. G., Pressel S., P’ng K. M., Renzaglia K. S. (2009)

in New Phytol 2009, 183:1053-1063.

(PubMed Abstract | Publisher Full Text)

Figure 4. A series of scanning electron micrographs illustrating the changing appearance of the capsule walls and guard cells in Sphagnum subnitens as they dry out. (a) Fully hydrated capsule with swollen guard cells. (b,c) After 2–3 h drying out (20–30% water loss) the guard cells have central depressions. (d,e) Just before dehiscence (60–80% water loss) the wall has deep longitudinal grooves and the guard cells have collapsed completely. (f,g) At the point of dehiscence (80–90% water loss) the guard cells often have ruptured outer walls. (h,i) Post-dehiscence, slit-like depressions mark the position of the epidermal cell lumina. The outer walls of the guard cells are almost always ruptured but splitting of their inner periclinal walls to form open pores never occurs . Bars, (a,c,e,g,i) 20 µm, (b,d,f,h) 50 µm. – http://onlinelibrary.wiley.com/store/10.1111/j.1469-8137.2009.02905.x/asset/image_n/NPH_2905_f4.gif?v=1&t=ieytu6mj&s=b4aadb87879dd689c82abdf2e5363b87e9ab2d47

Summary

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  The nineteenth century air-gun explanation for explosive spore discharge in Sphagnum has never been tested experimentally. Similarly, the function of the numerous stomata ubiquitous in the capsule walls has never been investigated.
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  Both intact and pricked Sphagnum capsules, that were allowed to dry out, all dehisced over an 8–12 h period during which time the stomatal guard cells gradually collapsed and their potassium content, measured by X-ray microanalysis in a cryoscanning electron microscope, gradually increased. By contrast, guard cell potassium fell in water-stressed Arabidopsis.
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  The pricking experiments demonstrate that the air-gun notion for explosive spore discharge in Sphagnum is inaccurate; differential shrinkage of the capsule walls causes popping off the rigid operculum. The absence of evidence for a potassium-regulating mechanism in the stomatal guard cells and their gradual collapse before spore discharge indicates that their sole role is facilitation of sporophyte desiccation that ultimately leads to capsule dehiscence.
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  Our novel functional data on Sphagnum, when considered in relation to bryophyte phylogeny, suggest the possibility that stomata first appeared in land plants as structures that facilitated sporophyte drying out before spore discharge and only subsequently acquired their role in the regulation of gaseous exchange.

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