Xylem morphology influences lemon susceptibility to mal secco disease

In plants, xylem is directly involved in conveyance of water and dissolved minerals, mechanical support of the plant, and tolerance to drought stress. Moreover, for several fruit crops affected by vascular diseases, an association between the morphology of xylem vessels and susceptibility was described. In fact, compartmentalization represents a key determinant mechanism of plant resistance to vascular infections. Mal secco is a severe tracheomycosis affecting many citrus species of relevant economic importance. Currently, both chemical and agronomic measures are not sufficient to contain the diffusion of the pathogen raising the interest for the elucidation of the host tolerance mechanism against mal secco. This study investigated the constitutive morphology of xylem tissue, in terms of vessel diameter and vessel density in 28 citrus genotypes, all characterized by a different degree of tolerance/susceptibility towards the disease. One-year-old stems, in three replicates per each genotype, were cut into 50-μm sections and observed under an optical microscope after staining in safranine-O. Analysis revealed a positive correlation between xylem vessel density and susceptibility to mal secco disease. These findings suggest that the constitutive morphology of xylem tissue could play a role in tolerance to mal secco, even though other mechanisms, both at histological and biochemical level, need to be unlocked and better elucidated.