Translocation in Phloem Necrosis-Diseased American Elm Seedlings

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Abstract
Seedlings of Ulmus americana in their 2nd season of growth were exposed to 14CO2 and allowed to transport assimilated materials for periods of 4-24 h. At the end of the transport period, samples consisting of the treated leaf, serial stem segments, and the root system were extracted in boiling 80% ethanol; extracts were analyzed for 14C. Healthy appearing leaves on seedlings previously graft inoculated with the agent of phloem necrosis (PN) exported assimilate less rapidly than did leaves on healthy trees. Also, transport of 14C in stems of infected seedlings was impaired significantly prior to development of histopathologic symptoms in the secondary phloem. Translocation occurred, although slowly, in secondary phloem exhibiting histologic aberrations. Callose deposition and sieve tube collapse, which are early symptoms of PN, are not solely responsible for the impairment of translocation in PN-diseased elms.