Bud dormancy evolution in apple genotypes with contrasting chilling requirements

This work evaluates how thermal regimes with constant or oscillating temperatures affect the onset and overcome of cndodormancy in apple buds with low ('Caste! Gala') and high ('Royal Gala') chilling requirements. Apple twigs were collected in Papanduva-SC orchards during the autumn/winter of 2010 and 2011, and submitted to constant (3 degrees C) or cycling (3/15 degrees C for 6/18h, 12/12h or 18/6h) temperatures for up to 1466 chilling hours (CH). Periodically, a portion of twigs in each treatment was transferred to 25 degrees C, for budburst evaluation. An environmental stress in 2010 caused premature leaf fall on the field, reduced initial budburst in 'Royal Gala' samples and interfered with dormancy onset in both cultivars. Cycling temperature treatments with few CH/day decreased partially this effect, allowing buds to enter a dormancy state more efficiently. No significant stress was observed in the orchard in 2011, when dormancy evolution was similar in all thermal treatments, being induced with 54 and 123 CH and overcome with 363 and 662 CH for 'Castel Gala' and 'Royal Gala', respectively. Apparently, the manner in which apple bud dormancy is induced affects its depth and consequently the total number of chilling hours required to overcome it, making the onset phase fundamental in dormancy studies.