ABSTRACT
Inoculation of one true leaf of cucumber (Cucumis sativus L.) plants with Pseudomonas syringae pathovar syringae results in the systemic appearance of salicylic acid in the phloem exudates from petioles above, below, and at the site of inoculation. Analysis of phloem exudates from the petioles of leaves 1 and 2 demonstrated that the earliest increases in salicylic acid occurred 8 hours after inoculation of leaf 1 in leaf 1 and 12 hours after inoculation of leaf 1 in leaf 2. Detaching leaf 1 at intervals after inoculation demonstrated that leaf 1 must remain attached for only 4 hours after inoculation to result in the systemic accumulation of salicylic acid. Because the levels of salicylic acid in phloem exudates from leaf 1 did not increase to detectable levels until at least 8 hours after inoculation with P. s. pathovar syringae, the induction of increased levels of salicylic acid throughout the plant are presumably the result of another chemical signal generated from leaf 1 within 4 hours after inoculation. Injection of salicylic acid into tissues at concentrations found in the exudates induced resistance to disease and increased peroxidase activity. Our results support a role for salicylic acid as an endogenous inducer of resistance, but our data also suggest that salicylic acid is not the primary systemic signal of induced resistance in cucumber.
ABSTRACT
Calcium and strontium ions enhanced rishitin but not lubimin accumulation in tuber tissue of potato (Solanum tuberosum cv Kennebec) treated with arachidonic acid (AA). The same cations in the presence of poly-l-lysine (PL) enhanced the accumulation of lubimin more than rishitin. In contrast, Mg(2+) did not affect AA-elicited rishitin and lubimin accumulation and inhibited the accumulation of these compounds following application of PL. AA-elicited potato tuber tissue remained sensitive to the stimulatory effects of Ca(2+) and Sr(2+) up to 24 h after application of AA, but PL-elicited tuber tissue was sensitive to Ca(2+) and Sr(2+) for only 6 hours after PL application. Ethyleneglycol-bis (beta-aminoethyl ether)-N,N'-tetraacetic acid and La(3+) both inhibited rishitin and lubimin accumulation elicited by AA. The inhibition by either agent was overcome by the addition of Ca(2+). Calcium was more effective in overcoming lanthanum inhibition when applied simultaneously than when applied 12 hours later. Lanthanum was only effective in inhibiting rishitin and lubimin accumulation when applied within 3 hours of the application of AA. Inhibition of phytoalexin accumulation was greater when La(3+) was applied simultaneously with AA compared to La(3+) application after AA application to discs. These observations suggest that the mobilization of calcium may play a central regulatory role in the expression of phytoalexin accumulation following elicitation in potato tissue.
