The effects of in vitro and ex vitro root initiation on subsequent microcutting root quality in three woody plants

Abstract

In vitro- and ex vitro-rooted microcuttings of Acer rubrum L. ‘Red Sunset’, Betula nigra L., and Malux x- domestica Borkh ‘McIntosh’ were distinguished by several important anatomical and morphological properties which continued to regulate both root system and whole plant quality in later stages of production. In vitro microcuttings formed adventitious roots in greater number and more quickly than ex vitro microcuttings. Roots produced in vitro were characterized by extremely enlarged cortical cells and, consequently, had a much greater diameter than ex vitro roots. However, the vascular system of in vitro roots was underdeveloped (primary vascular tissues only) as compared to ex vitro roots, which produced vascular cambium and secondary growth during the same early stage of production. At least 50% of the post-transplant in vitro adventitious roots either died immediately, or temporarily persisted during acclimatization without producing any further growth. For the surviving in vitro-produced roots, the cortex partially collapsed after transplant, and new root extensions with ex vitro-like structure were produced. Only then did the in vitro portion of the root begin to form secondary vascular tissues. Shoots from in vitro treatments continued to grow vigorously during adventitious root initiation and during acclimatization, so that the plants were significantly taller and had a greater shoot area than those receiving comparable ex vitro rooting treatment. In vitro rooting led to a horizontal root morphology which continued to distinguish these treatments from ex vitro rooted plants during later stages of production, when anatomical differences in the roots could no longer be detected.