Effect of irradiance and spectral composition of radiation on in vitro shoot proliferation in Malus domestica Borkh.

Four clones of Malus domestica Borkh. cv. Golden Delicious - namely Smoothee, Crielaard, Reinders and Golden B - were cultured in vitro from single-node microcuttings placed on solid medium under irradiance (PPFD) of 50 µmol m-2 s-1. After 9 months an average shoot proliferation of 5.3 was achieved; Crielaard showed the highest rate (7.1), followed by Golden B(5.4), Smoothee and Reinders (4.4). Proliferating shoots were then exposed to higher PPFD (80 µmol m-2 s-1) and different spectral composition of radiation using PMMA-B and PMMA-R/FR filters. High PPFD decreased the average proliferation rate to 4.5, in particular in Crielaard and Golden B, while it increased proliferation in Reinders. When a PMMA-R/FR filter was interposed, the mean proliferation rate slightly increased. PMMA-B filters decreased the overall proliferation rate to 3.0; only in Crielaard it was increased, but shoots were very small. Thus PPFD and spectral composition influenced in vitro shoot proliferation and growth and the responses were different among the clones.