HISTOCHEMICAL DEFENSE FACTORS ON TOLERANCE TO DAMPING-OFF IN WATER-RAISED PLUG SEEDLINGS OF BROCCOLI

Plug seedlings of broccoli (Brassica oleracea var. italica, cv. Kairyo-ryokuen) grown by only watering with limited fertilization period acquired characteristics of long-period preservation under room temperature and tolerance to damping-off caused by Rhizoctonia solani. In this study, histochemical defense factors on the damping-off tolerance were investigated. Plug seedlings were prepared by custom raising method with successive fertilization and by water-raised method, i.e., 30, 45, 60 day-raised seedlings with water only-raised period after fertilization. Incidence of damping-off became lower in water-raised plots than in custom one after Rhizoctonia inoculation. In water-raised plots, the secondary walls in xylem, pith and cortical cells were thickened compared with that in custom plot. As for histochemical aspects in hypocotyl, content of pectic substances became higher in water-raised plots; HCl-soluble pectin showed considerably higher values. Little difference occurred in cellulose and hemicellulose content among the plots, however, lignin content became significantly higher in 45 d and 60 d plots. These findings suggest that in water-raised plots, accumulation of lignin and pectic substances in cortical cells, xylem and pith resulted in the increase of the rigidity in tissue and cells, which leads to the damping-off tolerance caused by suppression of Rhizoctonia infection in hypocotyl.