Seasonal Production of Phloem Tissue in HLB-affected Citrus Trees

Citrus trees affected by Huanglongbing (HLB or citrus greening) exhibit a progressive degeneration of the phloem tissue that culminates phloem collapse. The physical collapse of the phloem conduits obstructs photoassimilate transport to heterotrophic tissues ultimately resulting in tree death. However, despite the apparent collapse of phloem elements in HLB affected trees, new vegetative growth continues to develop for several additional seasons. In fact, HLB-affected branches with evident HLB symptoms and seemingly obstructed phloem are capable of maintaining fruit production for some time, albeit progressively smaller fruit sizes and crop yields. We therefore, hypothesize the existence of temporary or ancillary (albeit less efficient) route for photoassimilate transport that can sustain some level of growth. We examined samples of petioles, new flush stem tissue, mature stems, and fruit pedicels after termination of the 2012 summer flush. In petioles and mature stems, a distinct ring of new and apparent functional phloem was evident inside a circle of collapsed protophloem cells. In fruit pedicels, a larger functional layer of phloem cells was present. These exhibited a more gradual transition between functional and damaged phloem elements. In new vegetative growth, vascular tissue showed no evidence of damage, the only signs of HLB presence was the abundance of starch on cortex parenchyma. Our observations demonstrate the seasonal production of healthy phloem which provides for temporary passage of photoassimilates capable of sustaining limited growth and development.