HETEROTROPHY IN THE LEMNACEAE

Lemna minor L. and Wolffia brasiliensis Weddell can use sucrose to support heterotrophic growth in darkness and photomixotrophic growth in the light, but each is killed by galactose in the medium. Spirodela punctata (G. F. W. Meyer) Thompson growth on sucrose and galactose was indistinguishable. L. minor, Wolffia, and Spirodela plants could not utilize lactose or sorbitol; Callitriche stagnalis Scopoli could not utilize galactose or sorbitol and made only marginal use of sucrose to drive growth in light or darkness. L. minor callus and Callitriche callus, on the other hand, utilized galactose effectively, and L. minor callus also utilized sorbitol. The specific reducing sugar content was lowered in Lemna callus during heterotrophic growth on galactose, but not in Spirodela plants. The specific starch and sucrose contents were reduced in both when growing on galactose. Callitriche plants appeared to absorb galactose, but to not utilize it to drive growth, whereas the callus that did utilize galactose showed the Lemnacean pattern of diminished specific starch and sucrose pools. Catabolic sucrose synthase (EC 2.4.1.13) activity diminished during growth in darkness in all cases, consistent with an increase in the specific sucrose content. Invertase (EC 3.2.1.26) activity, alkaline and acidic, diminished in darkness (excepting heterotrophic Spirodela plants), consistent with the large increases in specific reducing sugar content being due to diminished alkaline invertase activity. L. minor callus not only showed lowered invertase and sucrose synthetase activities when growing on galactose or sorbitol, but also had lowered reducing sugar, sucrose, and starch contents. The enzymology of exogenous lactose, galactose, and sorbitol utilization is open to study with respect to primary carbon partitioning.