Growth Responses of Excised Roots of Red Clover

Sucrose was the only sugar which supported a high growth rate of the excised root clone 1 of the Dorset Marlgrass strain of red clover (Trifolium pratense L.). D-fructose, D-glucose, and maltose supported a lower level of growth. The other sugars tested were inactive as carbon sources. Additions of D-xylose, D-mannose, and D-arabinose strongly inhibited the growth of roots when simultaneously supplied with sucrose. 2. Nitrate was the most effective source of nitrogen tested. Ammonium, urea, and L-glutamine as nitrogen sources supported a lower level of growth. The optimum pH range for growth was the same whether nitrate or ammonium was used as nitrogen source. 3. The omission of any constituent from White's vitamin mixture limited growth. The most marked growth depression followed the omission of niacin. Niacin plus thiamine was more effective than any other combination of two factors. There was no evidence that pyridoxine and niacin could partially replace one another. 4. Indoleacetic acid (IAA), according to its concentration, is either without effect or is an inhibitor of growth. The anti-auxins α-(1-naphthylmethyl-sulfide)-propionic acid (NMSP) and 1-naphthoxy-acetic acid (1-NOA) do not stimulate main-axis growth or prolong meristematic activity of root apexes repeatedly subcultured in standard medium. Under appropriate conditions NMSP stimulates lateral-root extension. 5. Growth responses of red clover roots are compared with those of excised tomato roots, particularattention being paid to differences in their responses to IAA and the anti-auxins. The data are considered to support the view that different hormone factors control the duration of meristematic activity in the apexes of excised roots of the two species.