Condition of phloem of sour orange tree trunk in winter

Since the sieve tubes of the functioning phloem of sour orange stock of sweet orange trees become necrotic when the tree is affected by quick decline, the condition of the functioning phloem of the trunk of the healthy sour orange, both as a rootstock and as a seedling tree, especially in winter, was investigated.

Throughout the winter months trunks of healthy sour orange trees, and of sour orange rootstock under sweet orange tops, maintained a band of functioning phloem averaging about 500 microns in width. A ring of degenerating phloem external to this was either absent or as much as 100 microns wide. Occasionally, there were abnormally wide bands of degenerating phloem or bands of necrotic sieve tubes within the functioning phloem.