Apoplastic barrier development and water transport in Zea mays seedling roots under salt and osmotic stresses

The development of apoplastic barriers was studied in Zea mays seedling roots grown in hydroculture solution supplemented with 0-200 mM NaCl or 20% polyethylene glycol (PEG). Casparian bands in the endodermis of both NaCl- and PEG-treated roots were observed closer to the root tip in comparison with those of control roots, but the cell wall modifications in the endodermis and exodermis induced by salt and osmotic stresses differed. High salinity induced the formation of a multiseriate exodermis, which ranged from several cell layers to the entire cortex tissue but did not noticeably influence cell wall suberization in the endodermis. In contrast, osmotic stress accelerated suberization in both the endodermis and exodermis, but the exodermis induced by osmotic stress was limited to several cell layers in the outer cortex adjacent to the epidermis. The hydrostatic hydraulic conductivity (Lp) had decreased significantly after 1 day of PEG treatment, whereas in NaCl-treated roots, Lp decreased to a similar level after 5 days of treatment. Peroxidase activity in the roots increased significantly in response to NaCl and PEG treatments. These data indicate that salt stress and osmotic stress have different effects on the development of apoplastic barriers and water transport in Z. mays seedling roots.