Pulvinus structure and leaf abscission in cadmium-treated bean plants (Phaseolus vulgaris)

The effects of a phytotoxic cadmium concentration (4.45 × 10−5 M) on the structure of primary and secondary pulvini of bean (Phaseolus vulgaris L.) leaves were studied by light and scanning electron microscopy. Pulvini were also analysed for dry weight, water content, and cadmium, calcium and potassium concentrations. Relatively high amounts of Cd accumulated in the pulvini. The Cd treatment affected Ca more intensively than K concentrations, resulting in increased K/Ca molar ratios. Increase in the density of covering, glandular, and hydathode trichomes on the pulvini and internodes was observed in Cd-treated plants. Cadmium caused premature leaf abscission. Within both primary and secondary pulvini of unifoliolate and first trifoliolate leaves, Cd induced formation of a secondary abscission zone, which involved cortical cells parallel to the vascular cylinder. Cytological alterations associated with this zone were similar to those of normal plants in which the abscission zone lies at the pulvinus–petiole junction. Results suggest that Cd probably indirectly induces the conversion to cells with the competence of abscission zone cells in the cortex of pulvini.