A SINGLE-SEED ASSAY FOR ENDO-BETA-MANNANASE ACTIVITY FROM TOMATO ENDOSPERM AND RADICLE TISSUES

Completion of germination (radicle emergence) is an all-or-none develo pmental event for an individual seed. Variation in germination timing among seeds in a population therefore reflects variation among seeds i n the rates or extents of physiological or biochemical processes prior to radicle emergence. For tomato (Lycopersicon esculentum Mill.) seed s, correlative evidence suggests that endo-beta-mannanase activity wea kens the endosperm cap tissue opposite the radicle tip to permit radic le emergence. To test whether endo-beta-mannanase activity is causally related to germination rates, we have developed a sensitive assay sui table for use with individual radicle tips or endosperm caps. We show that endo-beta-mannanase activity varies at least 100-fold and often m ove than 1000-fold among individual inbred tomato seeds prior to radic le emergence. Other sources of variation (tissue size and experimental error) were evaluated and cannot account for this range of activity. Endo-beta-mannanase activity was generally 10-fold greater in leachate s from endosperm caps than from radicle tips. Release of reducing suga rs from individual endosperm caps also varied over a considerable (9-f old) range. These extreme biochemical differences among individual tom ato seeds prior to radicle emergence indicate that results obtained fr om bulk samples could be misleading if it is assumed that all seeds ex hibit the ''average'' behavior.