Effect of water stress on root meristems in woody and herbaceous plants during the first stage of development

We investigated the effect of water stress on the root system architecture of pine saplings and pea seedlings during the first stage of development. A ttention was focused on meristematic tissue situated at the root tip becaus e of the leading role played by the tissue in the planning of root system a rchitecture. The data showed that both species are extremely sensitive and that plants arrest their growth immediately during water stress treatment. When stress treatment was not intense, both species recovered growth but pr esented modifications in the root system architecture. In pine saplings, th e modification in root system architecture was the consequence of fine root meristems not recovering from water stress. The saplings survived by produ cing new lateral meristems from the cortical tannin zone above the fine roo t tip. In the case of pea seedlings, the meristematic tissues in the primar y root arrested proliferation during water stress although they recovered w hen the event occurred during the first hours of germination. The response was different when water stress was enforced on older seedlings. In this ca se, root meristems never completely recovered their proliferation despite t he increase in proline content observed in the cells. The modification of r oot system architecture in pea seedlings depended on the arrest of primary root elongation and the formation of new root laterals. As regards the prim ary roots, water stress treatment induced along the axis the formation of i rregular 'swellings' in the cortical zone above the meristematic zone. Anat omical investigations suggested that such swellings may have derived from t he changes in elongation direction of derivatives. The formation of new lat erals was observed in hydroponic cultures when water stress treatment was e nforced slowly and prolonged for a long time. The production of new lateral meristems may have been a similar response of woody and herbaceous plants to water stress conditions. It is not known whether these new meristems pre sent characteristics of resistance to water stress.