CHANGES IN XANTHINE OXIDASE AND URICASE ACTIVITIES IN POTATO VIRUS Y INFECTED TOBACCO LEAVES

The activities ofxanthine oxidase and urate oxidase (uricase) were determined in tobacco leaves (cvs Havana 425 and Samsun NN) infected by a common (PVY°) or a veinal necrotic (PVY N) strain of potato virus Y. Neither enzymatic activity changed before expression of symptoms for up to 5 days after inoculation. Xanthine oxidase, with some differences between the cultivars considered, increased from 6 to 10 days after inoculation in PVY N - infected fully developed inoculated leaves at the onset of vein clearing (in the noninoculated leaves) and veinal necrotic symtom expression (in cv. Samsun NN the highest infected/healthy ratio was 3.1). During this period a less enhanced activity was also detected in PVY°-infected leaves. However, xanthine oxidase did not change in new-growing systemically-invaded leaves where necrotic symptoms in PVN N -infectedleaves were also expressed almost simultaneously with fully developed inoculated leaves. Uricase activity increased during symptom expression to the same extent in both cultivars, irrespective of whether common or veinal necrotic PVY strains were involved, reaching values 1.2-2.2 times higher than control. Moreover, the enhancement of uricase activity was detected both in inoculated and systemically-invaded leaves. Allopurinol, a specific inhibitor of xanthine oxidase, applied to plants by root absorption 10 days before inoculation, did not inhibit reinal necrotic symptom expression in PVYN-infected leaves. All the results obtained are consistent with the hypothesis that changes in neither enzymatic activities tested - nor in general oxidative purine catabolism - are involved in veinal necrogenesis associated to PVYN infection but represent a reflex of a particular metabolic status induced at late stages of PVY infection when high amounts of virus accumulate in the infected tissues. Sono state determinate le attività della xantina ossidasi e urato ossidasi (uricasi) nelle foglie di tabacco delle cultivar Havana 425 e Samsun NN infette o da un ceppo cumune (PVY°) o da un ceppo della necrosi nervale (PVYN) del virus Y della patata. Le attività dei due enzimi non subivano variazioni fino a 5 giorni dall'inoculazione, prima dell'espressione dei sintomi. La xantina ossidasi, con qualche differenza tra le due cultivar, subiva invece un incremento nelle foglie inoculate (plenamente sviluppate) con PVYN alla comparsa dello schiarimento (nelle foglie non inoculate) e della necrosi delle nervature (6-10 giorni dopo l'inocul