Sugar Absorption and Sucrose Inversion by Excised Tomato Roots

The growth of excised tomato roots in sucrose is accompanied by the appearance in the medium of glucose and fructose. The quantitative relations between this appearance of glucose and fructose in the medium, total sugar absorption by the roots, and the decrease in the sucrose concentration of the medium do not suggest any causal relationship between sucrose uptake and glucose and fructose appearance in the medium. Excised tomato roots exhibit surface invertase activity with a pH optimum at 4.0–4.4. Alterations of external pH, which did not affect sucrose absorption, drastically altered the levels of glucose and fructose appearing in the medium. Glucose is preferentially absorbed from mixtures of glucose and fructose, and by adjustment of the ratio of the two sugars, a mixture can be obtained from which equimolar absorption of the two sugars occurs. Root growth in this mixture is, however, very poor compared with that occurring in presence of sucrose. The results are discussed in the light of earlier studies on sucrose uptake by cultured tomato roots.