effect of sampling time, explant type, culture orientation and antioxidant type on in-vitro explant establishment and growth of persian walnut

four separate experiments were conducted to study the effects of factors controlling the establishment of explants from two walnut genotypes z60 and z63 (damavand). factors were explant type, sampling time, explant orientation on the growth medium and antioxidants type. the explants were collected from the collection garden of tooyserkan walnuts research station. four explant types (apical meristem, single node, petiole and leaf), five sampling times (april, may, june, july and august), three modes of explant orientation on growth media (vertical, inverted vertical, and horizontal) as well as six antioxidant agents (ascorbic acid, citric acid, cysteine, asparagine, polyvinyl pyrolidone, and fluroglucinol) were studied. culture medium was dkw supplemented with 0.2 mg iba. for either one of the genotypes, apical meristem and single node explants were collected in april and may. due to the existence of plant growth hormones in meristematic tissue, the highest growth and establishment was observed for apical meristem type of explants, and with significant differences as compared with the other types of explants. also explants oriented vertically and upwards on the growth medium, due to normal stem polarity direction and probably better absorption and transfer of nutrients, had greater growth, exhibiting significant differences in comparison with other treatments. as for z63 genotype, after a passage of six weeks time, results indicated that polyvinyl pirolidone, and fluroglucinol increased the rate of explants establishment, reduced the browning of explants and significantly increased callus fresh weight and diameter. for z60 genotype, cysteine antioxidant had significant effects on increasing the explants' establishment, reducing the rate of browning as well as increasing callus fresh weight.