Effects ofFusarium solani naphthazarin toxins on the cytology and ultrastructure of rough lemon seedlings

Abstract

Two naphthazarin phytotoxins (dihydrofusarubin and isomarticin) produced byFusarium solani were used to determine their effects on the cytology of leaves of rough lemon (Citrus jambhiri Lush) seedlings maintained in a dilute solution of the toxins. Dihydrofusarubin alone or in combination with isomarticin (80:20, v/v) caused cell necrosis above the midveins and lateral veins, plasmolysis or collapse of spongy mesophyll cells, collapse of phloem, depletion of starch, swelling of chloroplasts and disruption of cellular organization. At the ultrastructural level, the toxins affected chloroplast membranes by causing swelling, breaks in outer membranes, granal stack disorganization and swelling of intergranal membranes. The interstromal lamellae appeared as vesicles and sometimes as peripheral reticulum, with an increase in plastoglobuli. The tonoplast was broken or vesiculated. The only membranes not affected by the toxins were those of the nucleus and the mitochondria. This study establishes that the initial toxin effects of these fungal phytotoxins are on organellar membranes, primarily those of the chloroplasts, plasmalemma and tonoplast.