Seed dormancy and persistence of Acacia berlandieri andLeucaena pulverulenta in a semi-arid environment

Seed longevity of the leguminous shrubs Acacia berlandieri and Leucaenapulverulenta was evaluated under field conditions at the soil surface and at 3-5 cm depth. Abiotic influences on seed longevity and dormancy were investigated by comparing seeds maintained at 5°C with seeds experiencing fluctuating day/night temperatures (40°C/20°C vs. 50°C/30°C) under dry and moist conditions. Acacia seed demonstrated no dormancy and initial germinability was > 82% in the laboratory. Viability of Leucaena seed was >97%, but seed coat dormancy limited germination to <7%. Persistence of Acacia seeds in the soil seed bank varied with depth. After 42 days the viable seed population of Acacia was reduced by 10 and 80% for surface and buried seeds, respectively, and no viable seeds were present after 3·5 months of burial. In contrast, Leucaena seeds on the soil surface exhibited no significant decline in viability over 60 days and attrition of buried seed was < 11 %. After 3 · 5, 6 and 12 months of burial, 86, 81 and 71 % of the Leucaena seeds dispensed remained viable, but the hard seed coat limited germination to 5, 2 and 0%, respectively. Loss of dormancy of Leucaena seeds stored in the laboratory over 7, 30 and 78 days was 7,12 and 19%. Thus, the 29% attrition after 1 year of burial was probably associated with a gradual loss of seed coat dormancy. Light levels (full sun and 25% full sun) manipulated with neutral density shade cloth had no signficant influence on surface seed viability for either species. Temperature and moisture interacted to influence Leucaena seed coatdormancy and embryo viability, but fluctuating temperatures under dry conditions affected neither viability nor germination. However, when moisture was available, germinability of seeds in the 40°C/20°C regime was higher (41%) than that of the control group maintained at 30°C (7%). All seeds in the 50°C/30°C regime imbibed water within 50 days, but none germinated. Low germination of buried seeds (29%) suggest that temperature/moisture combinations capable of disrupting seed coat dormancy or inducing embryo mortality were infrequent during the year of the field experiment.