Effect of light intensity on abnormal swelling of peduncle in 'Muscat of Alexandria' grape

'Muscat of Alexandria' grapevines, grown under heated greenhouses, show abnormal swelling on the peduncles and rachises during an early stage of berry growth. The cause of this abnormality was investigated by controlling the light intensity to the clusters. Complete shading of the peduncular nodes for 4 weeks after full bloom caused a noticeable swelling in all the treated peduncles. Fifty % shading caused swelling in half the number of the treated peduncles. The rate and the degree of swelling were reduced by the exposure of clusters to sunlight or by using a sunlight reflector film. In vitro culture of excised disks from swollen peduncles revealed active division of callus cells even on hormone-free MS medium. The division was stimulated when the cultures were kept in the dark. These results suggest that accumulation of some light-decomposable factors at the peduncular nodes causes this abnormality.