The Carbohydrate Nutrition of Tomato Roots

Iron is only consistently present in an available form in White's root culture medium if the inorganic salts are autoclaved with the sugar. The substitution of ferric ethylenediamine-tetra-acetate for the inorganic ferric salt of White's medium ensures iron availability when the carbon source of the medium is rendered sterile by ether treatment and subsequently added to the remainder of the constituents which have been sterilized by autoclaving. The biological activity of sugars, and particularly of dextrose and laevulose, is altered by autoclaving them in presence of the inorganic salt solution of White's medium. The only sugar which supports a considerable growth of excised tomato roots is sucrose. The activity of this sugar is not affected by alcohol-precipitation nor is it dependent upon the simultaneous presence of traces of its constituent mono-saccharides. Dextrose or laevulose or a mixture of the two sugars support a low but sustained level of excised-root growth. All other sugars and sugar alcohols tested were inactive as carbon sources. The addition of sucrose at low concentration (0–2 per cent.) to a medium containing dextrose as the main carbon compound does not make possible a level of growth comparable with that obtained with an adequate sucrose supply. It has not been possible to enhance the growth-rate of excised roots supplied with dextrose by previous presentation of this sugar under conditions permitting active growth. Using media containing 'etherized' sucrose and dextrose, no evidence was obtained of any competitive inhibition of sucrose utilization by dextrose. Certain sugars when added to a medium, containing the optimum concentration of sucrose1, markedly inhibited excised root growth. Thus l-sorbose, l- and d-arabinose, and d-xylose caused not less than 80 per cent, inhibition at a concentration of 0-5 per cent. d-mannose and d-galactose completely inhibited growth at o-1 per cent. The oligosaccharides, dextrose, laevulose, and the sugar alcohols tested had, by contrast, very low inhibitory activity.