Dormancy in seeds of Hibbertia cuneiformis and H. huegelii (Dilleniaceae)

Abstract

Dormancy delays the germination of seeds in two species of the primitive angiosperm genus Hibbertia (H. cuneiformis and H. huegelii, family Dilleniaceae). After seed coat removal, germination increased in 18-month-old seeds of H. cuneiformis and 6- to 8-month-old seeds of H. huegelii. Hence, seeds of the two species exhibit seed coat dormancy. The removal of the seed coat may stimulate germination, as the result of increased water uptake, and/or the removal of mechanical and chemical inhibition. However, the occurrence of imbibitional injury and a reduced percentage of vigorous seedlings in decoated seeds suggest that embryo dormancy, as a second type of dormancy, impedes germination in H. cuneiformis. Embryo dormancy also delays the germination of seeds of H. huegelii, since a high percentage of seeds did not germinate after removal of the seed coat. Embryo dormancy appears to vary among individual seeds and between species. The germination experiments suggest a high percentage of non-dormant and weakly dormant embryos for 18-month-old seeds of H. cuneiformis. By contrast, many seeds of H. huegelii appear to contain deeply dormant embryos. In H. cuneiformis, the depth of the seed dormancy varied with the age of the seeds. Freshly harvested seeds did not germinate for 3 months. Treatment with gibberellic acid (GA₃) (120 mg L^-1) significantly enhanced germination of freshly harvested and 18-month-old, intact seeds of H. cuneiformis and the germination of decoated 18-month-old seeds of H. huegelii. The plant hormones kinetin and abscisic acid did not affect the final germination percentage in 18-month-old seeds of H. cuneiformis. For H. huegelii, germination was reduced in decoated seeds and seeds with cracked coats exposed to an 8 h day: 16 h night diurnal cycle compared with complete darkness.