The transport of salicylic acid (SA) was studied in cucumber (Cucumis
sativus L.) using C-14-labeled benzoic acid that was injected in the c
otyledons at the time of inoculation. Primary inoculation with tobacco
necrosis virus (TNV) on the cotyledons led to an induction of systemi
c resistance of the first primary leaf above the cotyledon against Col
letotrichum lagenarium as early as 3 d after inoculation. [C-14]SA was
detected in the phloem or in the first leaf 2 d after TNV inoculation
, whereas [C-14]benzoic acid was not detected in the phloem during the
first 3 d after TNV inoculation of the cotyledons, indicating phloem
transport of [C-14]SA from cotyledon. In leaf 1, the specific activity
of [C-14]SA decreased between 1.7 and 8.6 times compared with the cot
yledons, indicating that, in addition to transport, leaf 1 also produc
ed more SA. The amount of SA transported after TNV infection of the co
tyledon was 9 to 160 times higher than in uninfected control plants. T
hus, SA can be transported to leaf 1 before the development of systemi
c acquired resistance, and SA accumulation in leaf 1 results both from
transport from the cotyledon and from synthesis in leaf 1.