Beyond the Wood Log: Relationships Among Bark Anatomy, Stem Diameter, and Tolerance to Eucalypt Physiological Disorder (EPD) in Cultivated Clones of Eucalyptus grandis Hill Ex Maiden and E. urophylla T. Blake

Eucalyptus plantation forests play an important role in the global trade balance, and have been challenged with the Eucalypt Physiological Disorder (EPD) exhibiting symptoms on barks. Despite of that, there is little information on the anatomical features of phloem and periderm associated with this disorder. Although tolerant and susceptible commercial clones exhibited similar anatomical structures, they differed in the proportions of conducting and total phloem tissue and the amount of phloem containing Calcium oxalate (CaOx) crystals. The frequency and diameter of sieve tube elements (STEs) also varied among the tested clones. The increased area of phloem with non-collapsed STE and CaOx crystals were linked to the EPD tolerant phenotype. Bark, secondary phloem, and periderm thickness were correlated with EPD scores. Structural characteristics of phloem cells is correlated with increasing stem diameter. Bark and phloem thickness exhibited significant and positive associations with EPD-tolerant clones and stem diameter, while negative correlations with EPD scores. These connections corroborate the positive impact of increasing the proportion of total phloem thickness on stem diameter growth and EPD tolerance. The present results were based on restricted, yet commercially important, Eucalyptus species (E. grandis, E. urophylla and E. grandis × E. urophylla hybrids) highlighting bark and phloem traits linked to plant growth and EPD tolerance.