Seed Dormancy and Germination of Jeffersonia dubia in Relation to Temperature, Hormone Levels, and Cell Wall Polysaccharides

Abstract

Germination ecophysiology and assay of cell wall sugar content and endo--mannanase were conducted in seeds of Jeffersonia dubia. We also analyzed the levels of ABA during dormancy breaking and hormone interactions in relation to embryo growth and germination. The ripe seed of J. dubia contained an underdeveloped embryo and was permeable to water. In nature, seeds were dispersed in May, and while embryos began to grow in September and were fully elongated by late November. Germination started in March the next year and seedlings emerged soon after germination. In the laboratory experiments, incubation at high temperatures (constant 25°C

25/15°C day/night) for at least 8 weeks was required to initiate embryo growth, while a transfer to moderate temperatures (20/10°C

15/6°C) was needed for the completion of embryo growth. Minimum 8 weeks at 5°C was effective in overcoming the physiological dormancy and seed germination after the embryos were fully elongated. Gibberellic acid (GA3) treatment could substitute for the high temperature requirement, but not for the low temperature requirement. Based on the dormancy-breaking requirements, it is confirmed that the seeds have deep simple morphophysiological dormancy (MPD). Although seeds require 10–11 months from seed dispersal to germination in nature, under controlled conditions they required only 3 months by treatment with 1,000 mg•L-1 GA3 followed by incubation at 15/6°C. This represents practical knowledge for the propagation of plants from seeds. Chemical hydrolysis of endosperm cell walls of seeds showed that they were mainly composed of mannose, and smaller quantities of glucose and galactose. The endo--mannanase was not detected for the first three months when underdeveloped embryos hardly grew at all. The embryo started to grow after September and the activity of endo--mannanase increased in micropylar endosperm regions of the seeds. The erosion of the endosperm cell wall was observed at the lateral side around the embryo, while the micropylar endosperm showed to be no obvious signs of being collapsed or damaged. The increase of enzyme activity coincided with the increase in length of the embryo under the moderate temperature condition. The embryos stopped growing during the winter season for about 2-3 months and enzyme activity was also in a low state. The enzyme activity resumed to increase again when during germination occurred in early March. Then the micropylar endosperm rupture occurred and germination was completed. The seasonal pattern of endo--mannanase activity was unique in seeds with a deep simple morphophysiological dormancy (MPD) and different from that of physiological dormancy (PD), morphological dormancy (MD), or non-deep MPD. Exogenously-applied GA or fluridone, an ABA biosynthesis inhibitor, promoted embryo growth in seeds of J. dubia, but exogenously-applied ABA restrained the growth of embryos. Embryos of the seeds exhumed in November were fully grown, but dormant. Although these seeds required cold stratification, exogenously-applied GA or fluridone could substitute cold stratification and promoted the radicle protrusion. However, treatments with either ABA or paclobutrazol, a GA biosynthesis inhibitor, treatments suppressed seed germination even in those subjected to cold stratification for 8 weeks. The data support a GA and ABA balance mechanism that could be applied for germination of seeds with a deep simple MPD as well as a non-deep PD or non-dormancy.