CELL-WALL AUTOHYDROLYSIS IN ISOLATED ENDOSPERMS OF LETTUCE (LACTUCA-SATIVA L)

Cell walls prepared from the endosperm tissue of hydrated lettuce (Lactuca sativa L.) seeds undergo autohydrolysis. Release of carbohydrates is most rapid (0.4-0.6 mu g per endosperm) within the 1st h of incubation in buffer, but substantial autolysis is sustained for at least 10 h. Autolysis is temperature sensitive, and the optimum rate occurs at pH 5. The rate of autolysis increases markedly in the period just prior to radicle emergence. The cell-wall polysaccharide composition in micropylar and lateral endosperm regions differs significantly; the micropylar walls are rich in arabinose and glucose with substantially lower amounts of mannose. Although walls prepared from both micropylar and lateral regions undergo autolysis, micropylar walls release carbohydrates at a higher rate than lateral walls. Autolysis products elute as large polymers when subjected to size-exclusion chromatography, suggesting that endo-enzyme activity is responsible for release of fragments containing arabinose, galactose, mannose, and uronic acids. Arabinose, galactose, mannose, and glucose are also released as monomers. As a function of time, the ratio of polymers to monomers decreases, indicating that exo-enzyme activity is also present. Thermoinhibition or treatment with abscisic acid suppresses germination and reduces the rates of autolysis of walls isolated from the endosperm by about 25%. Treatments that alleviate thermoinhibition (kinetin and gibberellic acid) increase the rates of autolysis by 20 to 30% when compared to thermoinhibited controls.