Ion-adsorbent substomatal structures in Tradescantia pallidus

Abstract

STOMATAL movements result from turgor imbalances occurring within the stomatal complex which are largely brought about by differential potassium distribution between the guard cells and adjacent epidermal cells^1–4. The guard cell vacuole has been established as the sink of the potassium flux during stomatal opening^5,6, but the source of the ions has not been identified. It is generally believed that the ion flux is apoplastic because plasmodesmata have only occasionally been observed between the guard and adjacent epidermal cells^7–9. We have described substomatal ion-adsorbent sites in some species of filicopsid ferns and Commelinaceae which we believe could act as the immediate sink/source of potassium ions involved in stomatal closing and opening¹¹. Discrete accumulations of Macallum's stain for potassium¹² were demonstrated at both poles of the guard cell complex on the face adjacent to the mesophyll. These sites corresponded to substomatal endocuticular swellings observed by scanning electron microscopy which, when collapsed, revealed the presence of a disk-shaped structure on the outer face of the guard cell walls at the site of ion adsorption. Direct observation of the ion-adsorbent sites was prevented by the overlying endocuticular substomatal sacs. We have now observed these sites directly in Tradescantia pallidus and obtained evidence which strongly implicates them in stomatal functioning.