After-Ripening and Germination of Juniperus Seeds

Abstract

1. The germination of non-after-ripened juniper seeds under ordinary conditions is very low, amounting to 1 per cent. 2. These seeds are protected by a semipermeable and thick coat which makes up 75 per cent by weight of the entire seed. Acids enter very slowly, while bases, silver and mercury salts, enter rapidly. While the coat serves as a protection against fungal attack and prevents water-imbibed seeds from expanding and rupturing the tissues before after-ripening is accomplished, it takes little or no part in the dormancy or after-ripening of the seed. 3. Food material in the resting seed is stored in the form of fats and proteins, with traces of glucose but no starch. The resting seed endosperm has a PH value of about 5, while that of the embryo is about 8. 4. Although some forcing agents changed the respiration and catalase activity of seeds, it was not possible to force the germination of non-after-ripening juniper seeds by high temperature, alternating temperature, wounding, warm bath, dry air, removal of coats, treatment with hydrogen peroxide, mercuric chloride, ether, carbon dioxide, oxygen, light, soil, dilute acids, dilute bases, nitrates, suphates, or strong acids. 5. Freezing and thawing as such has no forcing action on the germination of juniper seeds, neither does it hasten after-ripening. Freezing and thawing produces marked chemical changes in this seed, but these changes, as has been outlined, are quite different from those occurring during after-ripening. Seeds ready to germinate (after the coat is cracked and their water content increased to 52 per cent) are killed by an exposure to -5⚬ C. 6. The juniper seed has a dormant embryo that must after-ripen before germination. After-ripening occurs at temperatures between 0±1⚬ C. and 10⚬ C., although fastest at about 5⚬ C. 7. The changes that accompany after-ripening of the juniper seed at 5⚬ C. were found to be as follows: (1) rather rapid and complete imbibition, followed by a steady slow decrease in water content during after-ripening or until near germination; (2) increased H+ ion concentration, especially of the embryo; (3) an increment of titratable acid; (4) a steady and enormous increase in the degree of dispersion of the stored fat; (5) decrease in the amount of stored fat and protein, with an increase of sugar content and the first appearance of starch; (6) the translocation of food in the form of fat or fatty acids from endosperm to embryo; (7) a seven-fold increase in the amino acid content, and a complete disappearance of histidine from the endosperm; (8) an increase of soluble proteins, with a marked hydrolysis of the stored proteins: (9) slight growth of embryo; (10) very slight increase of the respiration intensity; (11) increased respiratory quotient; (12) decreased intramolecular respiration; (13) a doubling of the catalase activity; and (14) the rise in vigor of seeds as shown by their resistance to fungal attack. 8. In conjunction with after-ripening at 5⚬ C., desiccation seems to be the only promising means of shortening this after-ripening period. 9. The time at which the hypocotyl breaks through the nucellus was fixed as the end of after-ripening and the beginning of germination. 10. Neither the resting nor the after-ripened juniper seeds yield more than about 1 per cent germination at temperatures above 15⚬ C. Seeds after-ripened at 5⚬ C., then placed at 10⚬ C., germinate slower than those left at 5⚬ C. When after-ripened seeds are transferred from 5⚬ C. to temperatures above 15⚬ C. they are thrown into a state of secondary dormancy. Hence these seeds require a low temperature for germination as well as for after-ripening, and therefore no seed should be transferred to higher temperatures until germination has started. If these seeds are given sufficient time they will germinate, even at 0 ±1⚬ C. 11. Subsequent to after-ripening and germination at 5⚬ C., the best temperature for seedling development is 15⚬ C. 12. The development of chlorophyll in the juniper seed and seedling was found to be independent of light, but conditioned by the temperature range. Seedlings grown at temperatures of 0 ±1⚬ C. or 30⚬ C. never developed chlorophyll. Anthocyanin development in seedlings seems to depend upon relative temperature and carbohydrate supply. 13. A more complete chemical analysis of these seeds at different stages of development will be given in a later paper.