A MORPHOLOGICAL AND HISTOLOGICAL COMPARISON OF THE INITIATION AND DEVELOPMENT OF PECAN (CARYA-ILLINOINENSIS) SOMATIC EMBRYOGENIC CULTURES INDUCED WITH NAPHTHALENEACETIC ACID OR 2,4-DICHLOROPHENOXYACETIC ACID

Somatic embryos produced in vitro may exhibit structural abnormalities that affect their subsequent germination and conversion into plants. To assess the influence of auxin type on embryo initiation and develop ment, a morphological and histological comparison was made of pecan (C arya illinoinensis) somatic embryogenic cultures induced on media with naphthaleneacetic acid or 2,4-dichrorophenoxyacetic acid (2,4-D), usi ng light and scanning electron microscopy. Both auxins promoted enhanc ed cell division, particularly in subepidermal cell layers. However, n otable differences were observed in mitotic activity, location of embr yogenic cell proliferation, epidermal continuity, callus growth, and e mbryo morphology. Cultures induced on naphthaleneacetic acid had embry ogenic regions composed of homogeneous, isodiametric, meristematic cel ls. Embryos derived from these cultures generally had a normal morphol ogy, were single, and had a discrete apical meristem. In contrast, tis sues induced on media with 2,4-D had more intense and heterogeneous re gions of cell division. Proliferating cell regions were composed of me ristematic cells interspersed with callus and involved more extensive regions of the mesophyll. Marked callus proliferation caused epidermal rupture in some areas. Embryos induced on medium with 2,4-D had a hig her incidence of abnormalities that included fasciated, fan-shaped, an d tubular embryos. Defined apical meristems were often lacking or part ially obliterated due to callus proliferation. The heterogeneous, ofte n intensive proliferation of cells in cultures induced with 2,4-D may interfere with normal patterns of embryo development.