ABSTRACT
Does the endosperm pose a mechanical resistance on
embryonic axis (radicle) growth for lettuce seed (achene) germination? To aid
answering this question, the cell wall
degrading enzyme, carboxymethylcellulase
(CMCase) was extracted and assayed from lettuce seeds imbibed
for 0 to 12 h, prior to germination. Measuring the loss of viscosity of carboxymethylcellulose,
CMCase activity was high in dry seeds, low after 6 h of imbibition, high after
9 and 10 h, and then reduced again after 12 h. Fractions from Sephadex columns
showed CMCase activity in three peaks labeled
E1, E2, E3. The greatest change in CMCase activity during imbibition was
with E3 (molecular weight of about 40,000 Daltons) and some reduction in E2
(molecular weight about 280,000). The RNA
synthesis inhibitor, 6-methyl purine, eliminated CMCase activity when
present from 4.5 to 7 h of imbibition and the protein synthesis inhibitor,
cycloheximide, eliminated CMCase activity when present between 5.5 and 9 h.
Imbibition in darkness lowered CMCase activity while 15 min of light at 3.5 h
restored it and 30 min of far-red light at 3 h eliminated it. Increasing the
imbibition temperature to 35°C under light
reduced activity while under darkness, activity was eliminated under 24°C and 35°C. CMCase activity was localized in the endosperm surrounding the
embryonic axis (micropylar end) of 9 h imbibed
seeds. These observations showed that CMCase was active in degrading the
cell wall in the endosperm surrounding the radicle, weakening it, prior to
radicle protrusion so that the radicle remains undamaged.