Differences in cellular abnormalities induced by the probing behavior of Empoasca kraemeri (Homoptera: Cicadellidae) on tolerant and susceptible common beans

Cellular abnormalities induced by the feeding behavior of Empoasca kraemeri Ross & Moore were studied in 2 tolerant lines ('EMP 84' and 'EMP 392'), an intermediate ('Porrillo Sintetico') and a susceptible ('Calima') cultivar of Phaseolus vulgaris L. Groups of two 3- to 4-d-old female leafhoppers were placed for 3 h in 1 clip cage per plant, each covering 1 cm(2) of fully expanded primary leaves, 15 d after planting. Leaf tissues were fixed 48 h after this access period and prepared for light and confocal microscopy. Damage to the leaf lamina in all genotypes included cell emptying and breakage of cell walls, as well as enlarged intercellular spaces in the palisade and spongy parenchyma. In the main vascular bundle, hypertrophied parenchyma cells plus cells with enlarged nuclei and stained nucleoli were observed. In general, cellular damage was less severe in the tolerant lines than in the susceptible genotypes, although more so for EMP 84 than for EMP 392. Morphometry of autofluorescent tracheary elements showed what seems to be a compensatory response to leafhopper attack in the tolerant lines. Although control leaves from tolerant lines had fewer tracheary elements per vascular bundle after leafhopper feeding EMP 84 had more tracheary elements in damaged tissues. Both tolerant genotypes had tracheary elements with larger internal radii, which increased their estimated relative flow rates. The implications of cellular damage for the onset of hopperburn, in bean as well as other host plants such as alfalfa and potato, and the mechanisms of tolerance are discussed.