Effect of dormancy on the development of phloem fiber clusters

The development of phloem fibers was investigated in poplars grown under artificially fixed conditions and under an artificially shortened annual cycle system that includes a dormancy phase. The phloem tissues in these trees formed two- or three-layer fiber clusters; however, clusters in trees that underwent periodic dormancy were larger than those in trees that did not. Individual fibers were also larger in the former than in the latter, as assessed on transverse sections. Differences were most pronounced in the radial diameter of fibers in the layer nearest the cambium. These results suggest that dormancy affects both the size of fiber clusters and the diameter of individual fibers. Furthermore, these results imply that dormancy affects the development of phloem fibers, and that the shortened annual cycle system is a useful model for investigating phloem development caused by dormancy repetition.