Winter apple tree buds

Photo Beau Vista: orchard

Apple bud scales are modified leaves that develop during the growing season and harden to protect the bud inside through the winter. 

The formation of the scales is an integral part of the annual growth cycle and is influenced by environmental cues and plant hormones. Bud development begins within the mid growing season, usually in the later summer months. The apical meristems stop vegetative growth and  begins to form protective structures instead. This transitions the branch from vegetative leaf primordia growth into thick, tough scales in preparation for dormancy - endodormancy. External factors such as Short day (SD) photoperiods and cooling temperatures in later summer are the primary external signals affecting key regulatory genes, particularly the DAM - Dormancy Associated MADS box genes which are highly expressed during this period. These genes act as promoters of dormancy by suppressing growth-promoting pathways and activating Abscisic acid (ABA) related pathways. Abscisic acid is involved in signalling the cessation of active growth and the onset of protective scale formation. Once fully formed, the tight, overlapping scales enclose and protect the delicate inner meristem through winter.  Internally physical separation through  the closure of plasmodesmata (intercellular channels) close to the bud meristem effected through the deposition of callose, restricts the movement of growth signals to the bud. After winter chilling requirements are met and spring temperatures rise, the buds absorb moisture and swell. The scales begin to separate and break open, exposing the new growth (e.g., 'silver tip', 'green tip' stages) as the tree resumes active growth. The scales eventually fall away as the new shoot and leaves expand. 

Bud scales beginning to be shed.