Xylem differentiation and microsporogenesis during dormancy of apricot flower buds (Prunus armeniaca L.)

The purpose of this research was to know whether xylem differentiation and micro sporogenesis processes were synchronised with the dormancy cycle in apricot (Prunus armeniaca L.) flower buds. These processes were related to the dormancy breaking in three apricot cultivars with a different environmental adaptability. Cultivars were selected on the basis of their different flowering and fruiting pattern: 'San Castrese', 'Canino' and 'Orange Red'. Biological and anatomical observations were carried out during an autumn-winter season in central Italy. The three cultivars showed a great difference in flower bud endodormancy overcoming and their chilling requirement varied from low to very high. During the different dormancy phases, anatomical observations showed a continuous and regular acropetal progression of primary xylem differentiation along the flower bud axis up to the ovary. in the cultivar with a low chilling requirement ('San Castrese'), the appearance of xylem 'Stage 3' (xylem vessels up to 3/4 of bud axis) and tetrads coincided with the end of endodormancy. In cultivars with a medium and high chilling requirement ('Canino' and 'Orange Red', respectively), a desynchronism between different events (endodormancy release, xylematic differentiation and microsporogenesis) was observed. Xylem differentiation and microsporogenesis appeared in advance with respect to endodormancy breaking. This state could be one of the causes determining the low or inconstant rate of blooming of certain apricot cultivars under non native environmental conditions.