THE APOPLASTIC PERMEABILITY OF ROOT APICES

The apoplastic permeability of regions near the root tips of two nonexodermal species (Vicia faba L. and Pisum sativum L.) and three exodermal species (Zea mays L., Allium cepa L., and Helianthus annuus L.) was investigated with a berberine-thiocyanate tracer procedure. In regions of nonexodermal roots where the endodermis was mature, the walls of the epidermis and cortex were permeable; inward diffusion of the tracers was arrested at the endodermis. In regions of exodermal roots where the exodermis was mature, the apoplast was permeable only up to the anticlinal walls of the exodermis; in regions where the exodermis was immature but the endodermis was mature, the cortex was permeable up to die Casparian band of the endodermis. In root tips of both exodermal and nonexodermal species, berberine penetration into the meristem was greatly restricted. In some species there were additional areas adjacent to the meristem into which berberine movement was similarly restricted. It is concluded that even though root apices lack suberized structures, these zones are nevertheless partially apoplastically isolated from the external environment.