Mor, Y., Department of Horticulture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot, 76 100, Israel
Zieslin, N., Department of Horticulture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot, 76 100, Israel

Blackening of petals is common in roses cv. Mercedes during the winter in unheated greenhouses clad with polyethylene films which are transparent to solar radiation of 300 nm wavelength and longer. Petal blackening was prevented when the rose plants were kept at 18°C or a higher temperature. Blackening was also prevented when the flower buds were covered with aluminum foil from dawn to dusk, or when flower buds were covered with polyethylene or PVC films, opaque to solar radiation in a u.v. range shorter than 350 nm even when they were exposed to outdoor conditions with minimum temperatures as low as 5°C. Blackening was not prevented when the flower buds were covered with aluminum foil only during the night, or when covered with polyethylene film transparent to u.v. radiation. Measurements of the spectral transmission of the various plastic materials showed that PVC film and the IR/VR-type of polyethylene were opaque to u.v. radiation. High levels of u.v. transmission were measured in the IR-type polyethylene and low levels of u.v. transmission were present under horticultural glass. The involvement of u.v.-B radiation in the phenomenon of rose petal blackening and its interaction with low temperature is discussed. © 1990.