| Summary: | Mixed cropping might be seen as an alternative to monocultures by better protecting biodiversity and improving ecosystem services and resources. In the presented study, we tested the genetic and ecological effects of pure and mixed propagation of different poplar hybrids planted together with black locust trees. Poplar (<i>Populus</i>) hybrids are widely used for bioenergy in monoculture systems due to their rapid and high biomass production. Black locust (<i>Robinia</i><i> </i><i>pseudoacacia</i> L.) is a species with the ability to fix nitrogen and seen as a promising candidate for mixed cultivation. Eight different poplar hybrids and black locust trees from three provenances planted in two study sites with different environmental conditions were tested in varying combinations in pure and mixed stands to observe effects of the different hybrids and genotypes, site conditions and the mixed growing on the performance of poplar and its gene expression. Transcriptome analyses of leaves from four poplar clones selected according to their divergent growth performance were conducted to study differential gene expression that can be an important indicator of differences in growing conditions and success. Differences in gene expression were most pronounced among hybrids and different genotypes of the same hybrid, followed by the study site influence, and were least pronounced between mixed and pure stands. The genotypes of the same hybrid were clearly separated from each other. Clear separation between the study sites for all clones was also observed. Only a few genes were differently expressed in pure vs. mixed stand comparisons for each clone, but there were no common genes that were differently expressed in pure vs. mixed stands in all clones. In total, 199 genes showed differential expression between the study sites regardless of poplar clone or type of stands. The analysis suggested that plant genotypes and environmental conditions were more important at the early stage of stand development than pure or mixed cultivation.
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