Rageau, R., U. A. Bioclimatologie, PIAF, INRA Site de Crouelle, 234, avenue du Brezet, 63039 Clermont-Ferrand Cedex 2, France
Bonhomme, M., U. A. Bioclimatologie, PIAF, INRA Site de Crouelle, 234, avenue du Brezet, 63039 Clermont-Ferrand Cedex 2, France
Richard, J.P., U. A. Bioclimatologie, PIAF, INRA Site de Crouelle, 234, avenue du Brezet, 63039 Clermont-Ferrand Cedex 2, France
Erez, A., Institute of Horticulture, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel

The influence of the climatic factors on bud break in trees deprived from chilling is still poorly documented, due mainly to lack of experiments under well controlled conditions. Using a greenhouse (the temperature was maintained above 15°C) and chambers with well controlled temperature and photoperiod, we applied 5 treatments to peach trees: they all included the same chilling deprivation from October onwards. From the end of March onwards the treatments differed in the temperature, photoperiod and light intensity conditions. Outdoor conditions were a supplementary treatment. The data confirmed some previous results (break of endodormant buds can be induced without any chilling) and gave much additional information. Candidate conditions for triggering off bud burst included the following: 1-temperature rising above a critical value (of 27°C to 35°C) 2-heat units accumulated from October 11 exceeding 3650 degree-days 3-photoperiod becoming longer than a critical value (of 12 to 16 hours) 4-light intensity becoming higher than a critical value (of 250 to 600 μmol.m-2.s-1), None of them proved necessary individually. Only certain combinations of conditions (1+3, 1+4, 2+4, 3+4) proved effective; the other combinations by pairs and the conditions considered alone proved insufficient (2+3, 2, 3, 4) or could not be achieved through the treatments (1+2, 1). However, further experiments under controlled conditions are needed for improving knowledge on the determinism of bud burst without chilling (precise quantification of the effects of temperature and light, other factors being taken into account).