Distribution of P-protein in Mature Sieve Elements of Cucurbita maxima Seedlings Subjected to Prolonged Darkness

Abstract

A study has been made of the structure of the sieve tubes in the phloem of seedlings of Cucurbita maxima kept in total darkness for 2 or 3 days. All cytoplasmic components were found to be parietal in their distribution. The parietal system was closely applied to the cell membrane and appeared to be supported by a continuous framework of endoplasmic reticulum (ER) with which P-protein was intimately associated. The ER-P-protein complex was highly compact in some sieve elements and loosened to various degrees in others. The pores in the sieve plates were either unobstructed or occluded by components of the parietal complex in various ways, occlusion not always being accompanied by noticeable disruption of the parietal system. In visibly undisturbed sieve tubes, in which the ER-P-protein complex was in a highly compact state, occlusion appeared accidental, arbitrary and without any alignment of the components present in the pores. It is suggested that the distribution of the cytoplasmic components in the parietal position represents a true-to-life condition of the sieve tube, preserved due to control of the ‘surge’ artefact to which transporting sieve tubes are susceptible. However, the organization of sieve tube probably changes with the state of transport and the highly compact condition of the ER-P-protein complex as well as unobstructed or arbitrarily obstructed sieve plate pores represent a state of ‘rest’ or low transport.