Seasonal variation in cytokinin activity and content in two species of dwarf mistletoes and their hosts

Cytokinin levels were determined by bioassay on a seasonal basis, for two species of dwarf mistletoes (Arceuthobium douglasii and Arceuthobium tsugense) and their respective hosts, Douglas-fir (pseudotsuga menziesii) and western hemlock (Tsuga heterophylla). The highest cytokinin levels occurred in early Spring for all of the samples studied. However, with the exception of the first growth segments of Douglas-fir, the levels of cytokinins detected were significantly greater in infected tissue and aerial shoots of the mistletoe than in the healthy tissue. Further studies were carried out on Spring and Winter samples to determine the kinds of cytokinins present. High performance thin layer chromatography and gas liquid chromatography results showed that zeatin riboside (ZR) was the only detectable cytokinin in Winter samples. ZR was also detected in all Spring samples excepting the aerial shoots of A. douglasii. Two other cytokinins were also detectable in Spring samples. Zeatin was present in aerial shoots from both dwarf mistletoes and in the infected fifth growth segments from Douglas-fir and in infected western hemlock tissue. Zeatin occurred in the healthy tissue only in first growth segments of Douglas-fir. The 2iPA was detectable in the aerial shoots of the dwarf mistletoe, in the fifth grmvth segments of Douglas-fir, and in the infected tissue from western hemlock. These results are discussed with particular reference to the role of cytokinins in the physiological interactions between the host and the parasite