The effects of pressure release on the sieve plate pores of Nicotiana

Abstract

Segments of petioles of Nicotiana tabacum L. (var. Wisconsin 38) were fixed in formaldehyde-glutaraldehyde and postfixed in osmium tetroxide for electron miscroscopy. Sieve elements located close to cut surfaces show several effects of the strong hydrostatic flow which occurs in them when they are cut. The P-protein components are massed against the sieve plate and forced into the sieve plate pores forming “slime plugs”. Strands of P-protein may extend “downstream” from the sieve plate pores. Plastids generally rupture, and their starch grains are swept into the slime plugs. Small starch grains pass through the sieve plate pores, but larger starch grains block the pores. If pores are blocked by a starch grain they commonly contain little or not P-protein, indicating that they were unplugged in vivo. There is only slight callose deposition near the cut surface. The observations strongly support the view that phloem sieve plate pores are unplugged in vivo and thus provide anatomical evidence in favor of the mass flow concept.