Citrus Section Phloem Anatomy of Citrus Trees: Healthy vs. Greening-affected

Phloem cells from HLB-affected trees become obstructed with callose and P-protein plugs. The presence of these plugs is believed to hinder the translocation of photoassimilates (nitrogenous and reduced carbon compounds) to the root system. However, even with seemingly collapsed phloem tissue, citrus trees remain viable and produce fruit for some time, suggesting either incomplete plugging of phloem elements or the existence of alternative routes for photoassimilate transport. In this study, we examined the basic structure of phloem tissue from HLB-free and HLB-affected trees under light and scanning electron microscopy. To avoid any possible interference with callose induced by injury during sampling, we employed a freeze substitution technique. Sieve elements from HLB-free trees show sizable lateral pores to phloem and ray parenchyma. Early stage HLB-affected phloem cells contain dark electron dense material and jagged appearing cell walls. Eventually, these cells totally collapse into almost a solid cell-wall barrier. The large number of wall depressions all along the cortex, ray and vascular parenchyma with abundant pit fields, could be an essential anatomical feature necessary to support symplastic transport of photoassimlates in an HLB-compromised phloem system.