HISTOLOGICAL STUDIES ON FLOWER BUDS OF CULTIVAR "STARK EARLY ORANGE".

Cultivar "Stark Early Orange" is very interesting for its remarkable Sharka (Plum Pox Virus) resistance. However, its diffusion has been limited by inconstant and poor fruit-set. In coastal environments of Tuscany, this cultivar consistently has a low fertility index, evolution through the normal phenologic stages is absent, and flowering consequently fails to occur. The aim of this research was to investigate the possible causes of the lack of flower bud swelling, focusing in particular on anatomical observations during the bud growth period. The development of vascular connections, microsporogenesis and flower bud growth were studied from the end of floral differentiation to blooming. The same observations were made on cv. "S. Castrese", used as control on account of its regular and elevated productivity in all cultivation environments. Anatomical observations showed that in "Stark Early Orange", xylem vessel elements were located only at the base of the bud axis, without reaching the ovary. The early presence of an abscission region was detected, localized between the base of the flower bud axis and the twig. This region is characterized by bands of smaller cells with intercellular spaces containing organized crystalline inclusions. In addition, the microsporogenesis process was anomalous and pollen tetrad formation appeared only rarely.