Phloem unloading in aerial roots of Monstera deliciosa

Plants of Monstera deliciosa Liebm. pruned to exemplars with one leaf and one aerial root were labeled with 7.4 MBq (14)CO2 over the leaf blade. Microautoradiographs of soluble and insoluble radioactivity were prepared from three different regions of the aerial root. In addition, histochemical localization of ATPase was carried out on similar aerial roots. Vigorously growing aerial roots grew as fast as 26 mm d(-1), and zones of differentiation extended more than 10 cm from the root tip. In the region 2-3 cm from the root tip, in which only protoelements of the vascular tissue were differentiated, (14)C-label was restricted to the protophloem. The activity of ATPase was recognized in many different cellular organelles of the meristematic phloem parenchyma. In the region 5-6 cm from the root tip, in which the first metaelements differentiated, all parenchyma cells of the central cylinder and many cortical cells showed (14)C-label, in addition to the densely labeled protophloem. Differentiating vessels were heavily labeled at sites where secondary walls were formed. In this region of the root, ATPase activity was concentrated on the plasmalemma and cortical cytoplasma of the sieve tubes, and on the tonoplast of the phloem parenchyma cells. In contrast, the strands of internal metaphloem with giant sieve tubes, which are scattered among the metaxylem, were neither labeled nor did they show ATPase activity. In the zone 19-20 cm from the root tip, regions of cell differentiation in the sclerenchymatic mantle of the inner cortex, the late-formed metaxylem vessels and some strands of the internal metaphloem could be identified by dense (14)C-label. Low ATPase activity was found in the plasmalemma of practically all living cells. In this nearly mature region, a strong peroxidase activity was observed in the radial walls of the endodermis. The results indicate that phloem unloading was strongest at sites of root differentiation, where ATPase activity was concentrated in the plasmalemma of sieve tubes and the tonoplast of phloem parenchyma.