MECHANISMS OF RESISTANCE TO PHYTOPHTHORA-CINNAMOMI IN CLONAL, MICROPROPAGATED EUCALYPTUS-MARGINATA

Plants of the eucalypt, Eucalyptus marginata, selected through a glass house screening procedure for resistance or susceptibility to Phytopht hora cinnamomi, were established in tissue culture and micropropagated . After inoculation with P. cinnamomi, root lesions in clonal lines se lected as resistant (RR) to P. cinnamomi were restricted and became co ntained within four days after inoculation while lesions in roots of t hose lines susceptible (SS) to P. cinnamomi continued to extend rapidl y. Activity of phenylalanine ammonia-lyase (PAL) was increased above c ontrols in root segments of the RR lines 48 h after inoculation with P . cinnamomi while activity in unselected seedlings and the SS lines wa s reduced or unchanged. After inoculation, lignin concentration was in creased and reached high levels compared with uninoculated control lev els in roots of the two RR lines tested. Constitutive levels of phenol ics in roots of the RR lines were up to 94% higher than in seedling ro ots and levels were further increased after inoculation. Levels of phe nolics in the other lines and seedlings were unaltered by inoculation. A line derived from resistant seedlings from a susceptible family (RS ) had the highest constitutive levels of lignin. which were further in creased after inoculation. Resistance to P. cinnamomi in clonally prop agated E. marginata seedlings is based on similar mechanisms to those of field resistant species.