The obtainment of mutants from Persian walnut, which is considered to have little mutability, is a reference for future breeding. Pollen characteristics and chromosome pairings at pro-, meta- and anaphase II of 'Mitsuru', a diploid Shinano walnut cultivar ('2x-Mitsuru', hybrid of Teuchi and Persian walnut), and tetraploid 'Mitsuru' ('4x- Mitsuru'), derived from 'Mitsuru' by colchicine treatment, were investigated. Moreover, the possible application as breeding material was also studied. In the control '2x-Mitsuru', the pairing was 16II valence (n=16II) for all investigated cells. However, for '4x-Mitsuru', out of 45 nuclear plates, 35.6% were 5IV + 9III + 411 + 9I, and 6.7% were 32II. In the pairing at prophase of '4x-Mitsuru', multivalent chromosomes above IV valence were not observed, and IV valence were at most five. Furthermore, in both meta- and anaphase II, 32 chromosomes were comparatively well distributed to both poles. From the distribution of pollen size it can be deduced the mitotic process of the pollen mother cell. The pollen of '4x-Mitsuru' was 1.2 times bigger than that of the control '2x-Mitsuru', the distribution of pollen size was 17.5 μm and 50 μm, respectively, and for the tetraploid the size was uniform. Results concerning to pollen germination rate and fertility, and their relation to storage, and to pollen tube length: freshly harvested pollen germination rate was 7.0%-7.3% and 10.2%-12.1%, respectively for '4x-Mitsuru' and the control: and the germination rate of pollen stored at 5°C for 10 days was 1.4%-1.9% and 7.0%-8.5%, respectively. Therefore, for '4x- Mitsuru', the decrease in pollen germination rate between freshly harvested and cold stored pollen, was 3.5 times the '2x-Mitsuru'. The pollen fertility for both cases had no significant difference. Pollen tube of '4x-Mitsuru' grew poorly, showing no effect of humidity during cultivation, and with few deviation. As conclusion, the pollen of tree result of colchicine treatment ('4x-Mitsuru') can maintain its viability, and its possible application as breeding material has been shown.
