Enzyme activities during imbibition and germination of seeds of tamarack (Larix laricina)

Activities of six enzymes from extracts of separated embryos and gametophytes of tamarack Larix laricina (Du Roi) K. Koch] seeds were assayed at various stages of imbibition and germination. On a per seed part basis, activities of 6-phosphogluconate dehydrogenase (6-PGD, EC 1.1.1.44), glucose-6-phosphate dehydrogenase (G-6-PD, EC 1.1.1.49), malate dehydrogenase (NAD⁺–MDH, EC 1.1.1.37), isocitrate dehydrogenase (NADP⁺–IDH, EC 1.1.1.42), soluble peroxidase (PER, EC 1.11.1.7), and acid phosphatase (ACP, EC 3.1.3.2) from both the embryo and gametophyte tissues generally increased slowly, following cold stratification for 30 days and imbibition under germinating conditions for 5 days, but then increased at a faster rate with emergence of the radicle and subsequent growth of the seedling. The rate of increase of enzyme activity was highest for PER. Soluble protein levels also increased with imbibition and germination, with about 3 times greater levels present in the gametophyte than in the embryo. Heat inactivation experiments showed that, except for G-6-PD, activities were stable up to 40°C. Inactivation occurred at lower temperatures for G-6-PD, while higher temperatures were required for PER. Incubation of extracts for 7 days at 4°C indicated that loss of enzyme activity was greatest for G-6-PD (3.9% remaining) and least for PER and ACP (94 and 95% remaining, respectively).