Nectary structure of pear cultivars and its relation to fire blight susceptibility

Morphology and histology of the floral nectary was studied between 1999 and 2003 in 10 pear cultivars, susceptible to Erwinia amylovora to various degree. Samples were collected in a Hungarian commercial pear orchard and a cultivar collection. The nectary was receptaculo-ovarial in each pear cultivar studied, the gland was located on the adaxial side of the plate-like hypanthium and on the apical part of the ovary. The nectary was automorphic, protruding out of the hypanthial tissue on the apical, and in most cultivars on the basal part, too. In most cultivars the nectariferous tissue continued along the style, too, where the accumulating nectar could stay in the flower for a longer time, thus increasing the possibility of infection by E. amylovora. The size of the nectary was bigger in non-/less susceptible cultivars than in highly susceptible ones. No relationship was found, however, between the thickness of the nectary, the thickness of the glandular tissue and the degree of susceptibility. In most cultivars studied the nectary stomata were sunken below the level of epidermal cells, on the basis of which they belong to the xeromorphic ecological type. Thus nectar can accumulate in the small cavity above the guard cells, ensuring a favourable environment for the growth of bacteria, provided that both sugar composition and concentration of nectar is optimal. The size of nectar chambers, located under nectary stomata, was usually larger in tolerant cultivars than in susceptible ones.