The 2012 ASHS Annual Conference

Phloem cells from HLB-affected trees become obstructed with callose and P-protein plugs. The presence of these plugs is believed to hinder the transport of photoassimilates (nitrogenous and reduced carbon compounds) to the root system. However, even with a 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-unaffected and HLB-affected trees under light and scanning electron microscopy. To avoid any possible interference with callose induced by injury during sampling, we employed freeze substitution technique. Sieve elements from HLB-unaffected trees show sizable lateral pores to phloem and ray parenchyma. The cells have very angular sieve plates and appear relatively clean from cellular components. HLB-affected phloem cells contain massive amounts of amorphous material clearly traversing sieve plates and lateral pores. Eventually, these cells totally collapse into almost a solid cell wall barrier. Occasionally, wound phloem appeared along the petiole cortex in HLB-affected trees.  Most notably is the large number of wall perforation all along the cortex parenchyma with abundant pit fields.  These anatomical features will be discussed in terms of alternate routes for photoassimilates.