Sugar Absorption and Sucrose Inversion by Excised Tomato Roots

The growth of excised tomato roots in sucrose is accompaniedby the appearance in the medium of glucose and fructose. Thequantitative relations between this appearance of glucose andfructose in the medium, total sugar absorption by the roots,and the decrease in the sucrose concentration of the mediumdo not suggest any causal relationship between sucrose uptakeand glucose and fructose appearance in the medium. Excised tomato roots exhibit surface invertase activity witha pH optimum at 4.0–4.4. Alterations of external pH, whichdid not affect sucrose absorption, drastically altered the levelsof glucose and fructose appearing in the medium. Glucose is preferentially absorbed from mixtures of glucoseand fructose, and by adjustment of the ratio of the two sugars,a mixture can be obtained from which equimolar absorption ofthe 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 onsucrose uptake by cultured tomato roots.