Versuch zur Bestimmung der Enzymverteilung zwischen Geleitzellen und Phloemparenchymzellen bei Cucurbita pepo

Abstract

Summary If stems of Cucurbita pepo are cut exudate flows out from the phloem. Herein enzymes were found which in the course of exudate formation were washed out of the companion cells and phloem parenchyma cells. During the first two minutes of the exudation the activities of the triosephosphate isomerase, glucose-l-P-uridylyltransferase, phosphoglycerate kinase, and malate dehydrogenase differ only little. After the stop of exudation it began anew after cutting off a thin section (1-2 mm) from the stem (repeated 5 times). In these successively obtained exudates the activities of the four called enzymes decrease very strong. In the phloem tissue the loss of enzymes was studied histochemically. Closest to the incision the loss amounts almost up to 100 p.c. and decreases to 50 p.c. in the phloem parenchyma cells at a distance of about 10 cm and in the companion cells at 20 cm. Using cross sections of stems of 1.3–1.7 cm in diameter the volumes of the total phloem and its cell types were calculated for a length of 20 cm. For the whole phloem 860 mm3 were obtained, the share of the phloem parenchyma cells being 660 mml, that of the sieve tubes 177 mml, and of the companion cells 24 mm 3 . The quantity of the exudate originating from such a stem was ascertained as a mean volume of 0.414 ml. These values and the histochemical studies allowed the calculation of the enzyme activity occuring in the companion cells and in the phloem parenchyma cells. Adult sieve tubes do not contain enzymes of this nature as tested here. The volume of the companion cells of Cucurbita pepo stems amounts only to 2.8 p.c. of the whole phloem, nevertheless, about 87 p.c. of the total activity of some enzymes of the primary metabolism are localized in them. A connection between the high metabolic capacity in the companion cells and the A TP supply to the sieve tube elements is suggested.