Factors affecting the germination of doum palm (Hyphaene thebaica Mart.) seeds from the semi-arid zone of Niger, West Africa

Hard-seeded tropical tree species such as doum palm (Hyphaene thebaica Mart.), a common component of the traditional agroforestry systems practiced in semi-arid West Africa, often present considerable problems to nursery managers because their hard, impermeable seed coats (pericarps) restrict the entry of both water and oxygen and offer high physical resistance to the growth of the embryo. Such seeds often require pretreatments in order to obtain rapid and uniform germination. To develop recommendations for obtaining high rates of seed germination for doum palm, three 100-day long experiments were conducted following application of different pretreatments. For freshly-collected seeds, the effects of 3 ecozones (northern, central and southern), 3 mechanical scarification treatments, and 10 different types and durations of water-soaking treatments were determined. In the second experiment, 13-month-old seeds from the southern ecozone were used to assess the effects of the same mechanical scarification and water-soaking treatments used in the first experiment. In the third experiment, mechanical scarification of 13-month-old seeds from the southern ecozone was combined with chemical treatments using sulfuric acid, gibberellic acid, or ethylene. For all three ecozones together, intact fresh seeds had much lower (0.6% to 2.5%) germination than either seeds with their pericarp removed (78% to 82%) or bare nuts (pericarp, and endocarp removed; 73% to 85%). Percentage germination increased significantly from the northern to the southern ecozone, particularly for bare nuts (P < 0.002), Soaked seeds had higher germination than did unsoaked seeds (P < 0.0001) for both the fresh and the older seeds. The average for the treatments yielding the highest germination of the 13-month-old southern ecozone seeds (66%) was 25% lower than it was for the freshly-collected southern ecozone seeds (91%). The highest germination (85%) for freshly-collected seeds tall ecozones) was obtained with mechanically-scarified seeds soaked in water for 3 days. For intact 13-month-old seeds, the sulfuric acid treatment resulted in 28.3% germination and was the only treatment which resulted in any germination of intact seeds. The 13-month-old mechanically scarified seeds treated with either gibberellic acid or ethylene had higher germination (57% for both) than mechanically scarified seeds treated either with sulfuric acid or not chemically treated (40% and 26.5%, respectively). Thus, it appears that the impermeability of the pericarp to water and oxygen, the need to physiologically activate the embryo with a water-soaking treatment and, in older seeds, a lowered physiological capacity of the embryo all constrained germination rates in older doum palm seeds. (C) 1998 Elsevier Science B.V.