CHANGES IN CELL ULTRASTRUCTURE AND ZEATIN RIBOSIDE CONCENTRATIONS IN HEDERA-HELIX, PELARGONIUM ZONALE, PRUNUS-AVIUM, AND RUBUS ULMIFOLIUS LEAVES INFECTED BY FUNGI

Ultrastructural alterations in epidermal and mesophyll cells and varia tions in endogenous zeatin riboside (ZR) concentrations were studied i n leaves of Hedera helix, Pelargonium zonale, Prunus avium, and Rubus ulmifolius infected by Colletotrichum trichellum, Puccinia pelargoniiz onalis, Cercospora circumscissa, and Phragmidium violaceum, respective ly. Infected tissues showed a marked increase in vesicles, myelin-like structures, and electron-dense bodies associated with plasma membrane s. The main changes to the chloroplast included thylakoid swelling and disruption of the chloroplast envelope. The ZR content of the green i slands was always higher than that of the yellow, senescent parts of t he same leaves; the highest levels of ZR were observed in the green ar eas of infected Prunus avium (462.2 pmol g(-1) fresh weight [FW]) and Rubus ulmifolius (441.6 pmol g(-1) FW), followed by Pelargonium zonale (263.8 pmol g(-1) FW) and Hedera helix (219.8 pmol g(-1) FW); the yel low zones of the same leaves had lower ZR contents (78.3, 73.9, 73.6, and 18.1 pmol g(-1) FW,respectively). The green islands had almost the same ZR content as the controls (green healthy leaves). These results suggest a relationship between ultrastructural alterations and ZR con tent of these plant species (blackberry, cherry, English ivy, geranium ) in reacting to this type of biotic stress and could confirm the role of cytokinins as senescence-delaying hormones.