Production of cloned trees of Populus deltoides through in vitro regeneration of shoots from leaf, stem and root explants and their field cultivation

Amongst a large number of Populus species, micropropagation of P. deltoides has not been possible. The authors now report a reproducible protocol for mass production of cloned plants of 2 important clones of P. deltoides, viz., G3 and G48 through induced shoot bud differentiation in the in vitro cultured segments of leaf, stem and root of adult tree origin. Establishment of experimental plants in aseptic culture was possible only when nodal stem segments from fresh flush of growth were initially cultured in a liquid nutrient medium, a modification of Murashige and Skoog (1962) medium (MS) supplemented with 0.25 mg l each of BAP and IAA and 15 mg l AdS. Axillary shoot growth from nodal stem segments was sustained in an agarified medium of the same composition following at least 2 subcultures in the liquid medium. Rate of multiplication got enhanced many-fold when the segments of leaf, stem and root of regenerated shoots were induced to differentiate multiple shoot buds in the same medium instead of obtaining a single plantlet per regenerated shoot. The maximum number of shoot buds, i.e., 28.8 was differentiated in leaf segments followed by 22.7 and 7.8 in stem and root segments, respectively. However, the regenerated shoots appeared similar both morphologically and in vigour irrespective of their origin from any of the explants. Explants with differentiated shoot buds on being subcultured in the same morphogenic medium repeatedly produced crops of proliferated shoots. Developed isolated shoots, measuring about 3 cm in length, rooted 100% in 20-25 days, in a liquid rooting medium having 0.25 mg l IAA. The corresponding cultures on agarified medium of the same composition not only rooted poorly, but also necrosed and died. The rooted shoots required precision during their hardening in a purely inorganic salt solution following the procedure developed earlier. It took about.12 months to obtain hardened plants of about 30 cm in height in potted soil from the time of establishment of aseptic cultures using nodal stem segments of an adult tree. The properly hardened plants survived more than 80% on their transplantation to soil and grew luxuriantly under field conditions. The in vitro-raised trees attained an average height of 5 m with straight boles of an average diameter of 12 cm in 2 yrs.