SEED DORMANCY AND PERSISTENCE OF ACACIA-BERLANDIERI AND LEUCAENA-PULVERULENTA IN A SEMIARID ENVIRONMENT

Seed longevity of the leguminous shrubs Acacia berlandieri and Leucaen a pulverulenta was evaluated under field conditions at the soil surfac e and at 3-5 cm depth. Abiotic influences on seed longevity and dorman cy were investigated by comparing seeds maintained at 5 degrees C with seeds experiencing fluctuating day/night temperatures (40 degrees C/2 0 degrees vs. 50 degrees C/30 degrees C) under dry and moist condition s. Acacia seed demonstrated no dormancy and initial germinability was >82% in the laboratory. Variability of Leucaena seed was >97%, but see d coat dormancy limited germination to <7%. Persistence of Acacia seed s in the soil seed bank varied with depth. After 42 days the viable se ed population of Acacia was reduced by 10 and 80% for surface and buri ed seeds, respectively, and no viable seeds were present after 3.5 mon ths of burial. In contrast, Leucaena seeds on the soil surface exhibit ed no significant decline in viability over 60 days and attrition of b uried 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 dorman cy of Leucaena seeds stored in the laboratory over 7, 30 and 78 days w as 7, 12 and 19%. Thus, the 29% attrition after 1 year of burial was p robably associated with a gradual loss of seed coat dormancy. Light le vels (full sun and 25% full sun) manipulated with neural density shade cloth had no significant influence on surface seed viability for eith er species. Temperature and moisture interacted to influence Leucaena seed coat dormancy and embryo viability, but fluctuating temperatures under dry conditions affected neither viability nor germination. Howev er, when moisture was available, germinability of seeds in the 40 degr ees C/20 degrees C regime was higher (41%) than that of the control gr oup maintained at 30 degrees C(7%). All seeds in the 50 degrees C/30 d egrees C regime imbibed water within 50 days, but non germinated. Low germination of buried seeds (29%) suggest that temperature/moisture co mbinations capable of disrupting seed coat dormancy or inducing embryo mortality were infrequent during the year of the field experiment.