Light and scanning electron microscopy of the telial, pycnial, and aecial stages of Melampsora lini

Abstract

Urediospore infection by Melampsore lini on stems or leaves of Linum usitatissimum produced uredia which quickly converted from urediospore to teliospore production. The telium was composed of a palisade of laterally united, sessile, prismatic teliospores (ca. 50 × 6 μm) positioned between the epidermis and the stem cortex. Teliospores were induced to germinate by subjecting telia to a series of freeze–thaw and wet–dry cycles. Germination began after 6–10 such cycles and continued until 20–24 cycles had been performed. Teliospores germinated apically to form basidia and smooth, ovate to elliptical basidiospores (ca. 6 × 5 μm). Basidiospore germlings penetrated directly through the host epidermis and formed intercellular and intracellular hyphae. The haploid intracellular structures formed were irregular and hypha like, often septate, and occasionally grew into adjacent host cells. Several morphological variants of intracellular hyphae were observed; all such structures were distinct from the more specialized dikaryotic haustoria derived from either dikaryotic aeciospore or urediospore infections. Development of pycnia occurred primarily in the substomatal cavities of leaves. Flexuous hyphae extended through stomata; concomitantly, pycniospores produced in chains from a palisade of pycniosporophores exuded through the stomata onto the leaf surface in droplets of honeydew. Pycniospores were smooth, ellipsoidal, and ca. 3 × 1.5 μm. Caeomoid aecia developed on both sides of flax leaves and on stems. Short irregular chains of aeciospores alternating with smaller wedge-shaped intercalary cells were produced from sporogenous hyphae at the base of aecia. Mature aeciospores were globose to ovate (ca. 16 × 13 μm) and were densely ornamented with smooth, cog-like verrucae.