ARACHIDONIC-ACID INDUCES LOCAL BUT NOT SYSTEMIC SYNTHESIS OF SALICYLIC-ACID AND CONFERS SYSTEMIC RESISTANCE IN POTATO PLANTS TO PHYTOPHTHORA-INFESTANS AND ALTERNARIA-SOLANI

When the lower leaves of young potato plants (cv. Bintje) were treated with an aqueous suspension of the polyunsaturated fatty acid arachido nic acid (AA), a threefold increase in the amount of free salicylic ac id (SA) in the treated leaves was observed 3 h after treatment. The hi ghest level of free SA was measured 6 h after treatment, reaching 3.21 gig fresh weight compared to 0.15 g/g fresh weight in controls. The l evels of conjugated SA in the treated leaves increased up to 51 g/g fr esh weight 3 days after treatment compared to 0.83 g/g fresh weight in controls. Four days after treatment with AA the upper leaves became r esistant to the potato late blight pathogen, Phytophthora infestans, b ut no accumulation of SA was observed. This systemic protection was ob served in plants treated with AA at concentrations as low as 10 ppm an d is unlikely to have arisen from AA translocated in the plant. In add ition, treatment with AA protected potato plants against the early bli ght pathogen, Alternaria solani, indicating a broader spectrum of prot ection. Local but not systemic accumulation of a pathogenesis-related (PR)1-like protein was observed 24 h after treatment with AA. Applicat ion of SA on potato leaves did not confer resistance against P. infest ans, but a local accumulation of a PR1-like protein was observed 24 h after treatment. Younger leaves of healthy susceptible and resistant p otato plants contained higher levels of SA than older leaves, and this gradient correlated with their natural resistance to P. infestans. In general, cultivars showing field resistance contained higher amounts of conjugated SA than did susceptible cultivars.