VARIOUS FREEZING STRATEGIES OF FLOWER-BUD HARDINESS IN PRUNUS

Flower buds of 20 Prunus species showed quite different strategies to cope with low temperatures. Buds of most species deep supercooled. The two hardiest species, both from the subgenus Padus (P. padus L. and P . virginiana L.), did not supercool and survived -33C with no bud kill . Prunus serotina J.F. Ehrh., also in Padus, did supercool. Prunus nig ra Ait., P. americana Marsh, P. fruticosa Pall., and P. besseyi L.H. B ailey had a low minimum hardiness level (MHL), small buds, and a low w ater content. Exotherms were no longer detectable from the buds of the se species after 2 days at -7C and some buds survived -33C. Prunus tri loba Lindl. and P. japonica Thunb. were similar to that group, but no buds survived -33C. Prunus davidiana (Carriere) Franch., P. avium L., and P. domestica L. had a relatively high MHL but hardened rapidly whe n the buds were frozen. Prunus persica (L.) Batsch., P. subhirtella Mi q., P. dulcis (Mill) D. A. Webb, and P. emarginata (Dougl. ex Hook) Wa lp. deep supercooled, had large flower buds and a high MHL, and were k illed in the Dec. 1990 freeze. Prunus salicina Lindl., P. hortulana L. H. Bailey, P. armeniaca L., and P. tomentosa Thunb. were in an interme diate group with a moderately low MHL and a moderate rate of hardiness increase while frozen. Prunus dulcis and P. davidiana had a low chill ing requirement and bloomed early, whereas P. virginiana, P. fruticosa , P. nigra, and P. domestica had high chilling requirements and bloome d late.