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Black locust (Robinia pseudoacacia) is a tree in the subfamily Faboideae, native to North America, that has been naturalized and widely planted in temperate Europe and Asia. Black locust has important ecological and economic value, but its quality needs improvement. Hybridization programs are important for black locust breeding, but the low rate of fruit set after controlled pollination limits both its breeding and that of other monoclinous plant species that share this problem. In this study, we investigated gene expression in emasculated black locust flowers using the cDNA-amplified fragment length polymorphism technique to determine why the rate of fruit set is low after controlled pollination. Flowers that were emasculated after being frozen in liquid nitrogen were used as controls. Changes in the flower transcriptome were more dramatic at 5 h after emasculation than at 48 h. Injury caused by emasculation decreased the expression levels of genes associated with metabolism, growth regulation, signal transduction, and photosynthesis, and it increased the expression of genes related to stress-response metabolism, signal transduction, and promotion of senescence. The changes in the expression levels of these genes had negative effects on sugar metabolism, protein metabolism, lipid metabolism, energy metabolism, matter transport, signal transduction, osmotic regulation, pH regulation, and photosynthesis. Thus, emasculation accelerated flower senescence, resulting in low fruit set.

The magnitude of inbreeding depression within populations is important for the evolution and maintenance of mixed mating systems. However, data are sparse on the magnitude of inbreeding depression in Robinia pseudoacacia. In this study, we compared differences in the mature seed set per fruit, seed mass, germination success, and seedling growth between self- and cross-pollination treatments and estimated the inbreeding depression at 3 stages: seed maturation, seedling emergence, and seedling growth at 10 and 20 weeks. We found that progenies resulting from cross-pollination treatments showed significantly higher fitness than progenies resulting from self-pollination, causing high levels of inbreeding depression. Inbreeding depression was not uniformly manifested, however, over the 3 stages. Inbreeding depression was the greatest between fertilization and seed maturation stage (δ = 0.5419), and the seedling emergence (0.3654) stage was second. No significant differences in seedling growth were observed between selfed and crossed progenies. The cumulative inbreeding depression (δ) across all 3 stages averaged 0.7452. Inbreeding depression may promote outcrossing in R. pseudoacacia by acting as a post-pollination barrier to selfing. The large difference in the seed set between self- and cross-pollination that we detected indicated that inbreeding depression would probably be a reasonable explanation for the high abortion and low seed set in R. pseudoacacia.