Pistillate flower abscission (PFA) severely affects walnut ( Juglans regia
L.) production, mainly of the cultivar 'Serr'. A study on symptoms of flower abortion in walnut 'Serr', induced by excess or lack of pollen, was carried out following the evolution of flowers and peduncles. Hand pollinated flowers, with different pollen concentrations: 0, 1, 5, 25, 50 and 100% pollen diluted (w/w) in talcum powder, showed external and internal abortion symptoms. External symptoms were recorded directly from the tree, while for internal symptoms, flower excisions stained with blue Toluidin-O at 0.05%, were analyzed under an optical microscope. Excess pollen abortion produced flowers with rapid damage. The growth rate was very low, involucre brightness was lost and the stigma turned necrotic, in contrast to the flower aborted from lack of pollen. Normal flowers continued growing to fruit harvest, without losing brightness. Excess pollen abortion induced flower drop, with or without attached peduncle. Two separation zones can be present: the distal and proximal areas of the peduncle. Peduncle scars allowed for differentiating abortion induced from excess or lack of pollen, or other reasons.