RELATIONSHIP OF ENDO-BETA-D-MANNANASE ACTIVITY AND CELL-WALL HYDROLYSIS IN TOMATO ENDOSPERM TO GERMINATION RATES

The endosperm tissue enclosing the radicle tip (endosperm cap) governs radicle emergence in tomato (Lycopersicon esculentum Mill.) seeds. We akening of the endosperm cap has been attributed to hydrolysis of its mannan-rich cell walls by endo-beta-D-mannanase. To test this hypothes is, we measured mannanase activity in tomato endosperm caps from seeds allowed to imbibe under conditions of varying germination rates. Over a range of suboptimal temperatures, mannanase activity prior to radic le emergence increased in accordance with accumulated thermal time. Re duced water potential delayed or prevented radicle emergence but enhan ced mannanase activity in the endosperm caps. Abscisic acid did not pr event the initial increase in mannanase activity, although radicle eme rgence was markedly delayed. Sugar composition and percent mannose (Ma n) content of endosperm cap cell walls did not change prior to radicle emergence under any condition. Man, glucose, and other sugars were re leased into the incubation solution by endosperm caps isolated from in tact seeds during imbibition. Pregerminative release of Man was suppre ssed and the release of glucose was enhanced when seeds were incubated in osmoticum or abscisic acid; the opposite occurred in the presence of gibberellin. Thus, whereas sugar release patterns were sensitive to environmental and hormonal factors affecting germination, neither ass ayable endo-beta-D-mannanase activity nor changes in cell wall sugar c omposition of endosperm caps correlated well with tomato seed germinat ion rates under all conditions.