Elemental analysis of freeze-dried thin sections of Samanea motor organs: barriers to ion diffusion through the apoplast.

Open Access
Publisher Website
Google Scholar
Abstract
Leaflet movements in the legume S. saman are dependent upon massive redistribution of K, Cl and other solutes between opposing (extensor and flexor) halves of the motor organ (pulvinus). Solutes diffuse through the apoplast during redistribution. To test the possibility that solute diffusion might be restricted by apoplastic barriers, elements were analyzed in the apoplast in freeze-dried cryosections of pulvini using scanning electron microscopy/X-ray microanalysis. Large discontinuities in apoplastic K and Cl at the extensor-flexor interface provide evidence for a barrier to solute diffusion. The barrier extends from the epidermis on upper and lower sides of the pulvinus to cambial cells in the central vascular core. It is completed by hydrophobic regions between phloem and cambium, and between xylem rays and surrounding vascular tissue, as deduced by discontinuities in apoplastic solutes and by staining of fresh sections with lipid-soluble Sudan dyes. Thus, symplastic pathways are necessary for ion redistribution in the Samanea pulvinus during leaflet movement. In pulvini from leaflets in the closed state, all cells on the flexor side of the barrier have high internal as well as external K and Cl, whereas cells on the extensor side have barely detectable internal or external K or Cl. Approximately 60% of these ions migrate to the extensor during opening; all return to the flexor during subsequent closure. Solutes lost from shrinking cells in the outer cortex may diffuse through the apoplast to plasmodesmata-rich cells of the inner cortex, collenchyma, and phloem; solutes cross the barrier by moving through plasmodesmata.