THE NACREOUS SIEVE-ELEMENT WALL IN HEALTHY AND IN MLO-INFECTED APPLE-TREES

The development, ultrastructure, and histochemical composition of the nacreous sieve-element wall were studied in axial and petiolar phloem of young shoots of healthy and mycoplasma-like organism (MLO)-infected apple trees, Malus domestica Borkh. In the protophloem, the sieve-ele ment walls are single-layered and consist of transversely oriented fib rils. In the first-formed sieve elements of the metaphloem, a segregat ion of the fibrils that constitute the cell wall leads to the formatio n of a middle wall layer of uneven thickness. The sieve-element walls now appear three-layered, with a thin inner, a thick middle, and a thi n outer layer. The outer and the inner layers are rich in cellulose an d pectin and consist of densely packed fibrils arranged perpendicular to the cell axis. The middle layer is cellulose- and pectin-poor and c onsists of an electron-translucent matrix with few, loosely embedded f ibrils of variable orientation. The three-layered wall reaches its max imum thickness at sieve-element maturity but diminishes as the sieve e lement ages. The sieve elements of the axial phloem lose their wall th ickenings faster than the sieve elements of the petiolar phloem. In he althy trees, only the youngest sieve tubes of the axial phloem have wa ll thickenings in autumn, and collapsed sieve tubes never have wall th ickenings. In MLO-infected apple trees, however, the shoots that devel op during the second flush (witches' brooms) show wall thickenings in autumn in most living sieve tubes, and in some collapsed sieve element s, of the axial phloem. This is a result of the younger developmental stage of the second flush shoots and of disease-related premature siev e-tube collapse, respectively. No differences in the ultrastructure or in the staining properties of the sieve-element wall were found betwe en healthy and diseased apple trees. Therefore, we conclude that MLOs do not influence the sieve-element wall in M. domestica.