Regeneration of vascular tissues in relation to Fusarium wilt resistance of carnation

Fusarium wilt-resistant 'Novada' carnations responded both to stem inoculation with a conidial suspension of Fusarium oxysporum f. sp. dianthi or F. oxysporum f. sp. lycopersici and to root inoculation by planting in soil infected with F. oxysporum f.sp. dianthi by means of a localization mechanism comprising gel formation in the xylem vessels and hyperplasia of adjacent parenchyma cells. Dye translocation experiments showed that xylem transport was limited by the presence of vascular gels, although wilting did not occur. Overcapacity of the vascular system apparently allowed for sufficient water transport to compensate for local vascular dysfunction. Also, vascular regeneration in the hyperplastic tissue next to occluded xylem vessels created new pathways for water transport to compensate for those lost by occlusion. Regeneration of xylem vessels was eventually followed by regeneration of xylem fibers, xylem parenchyma, cambium, and phloem cells. 'Early Sam' carnations, susceptible to Fusarium wilt, responded to stem inoculation with F. oxysporum f. sp. lycopersici by similar localization of infection and vascular regeneration. Stem inoculation with F. oxysporum f. sp. dianthi, however, resulted in colonization of the xylem vessels followed by lysis of the vascular tissues. Vascular gelation, hyperplasia of parenchyma cells, and vascular regeneration did generally not occur. However, if some hyperplasia occurred in attempted defence, some differentiation of hyperplastic cells into single xylem vessel elements was observed which only rarely resulted in complete vascular regeneration next to colonized xylem. In the absence of hyperplasia, differentiation of medulla parenchyma cells bordering destroyed vascular tissue into xylem vessel elements was even more exceptional. Apparently, vascular regeneration in carnation is a normal defence reaction to fungal invasion.