This preprint (Bouffier et al, 2023) analyses different biological (tree genotype, age, flowering phenology) and environmental factors (vicinity with external pollen sources, orchard structure, soil type, climatic conditions) with influence on the of seed lots in seed orchards of an important forest tree species (Pinus pinaster Ait.).  The analysis is based on an optimized set of 60 SNP markers that constitute a new tool for characterizing improved material in the breeding program of the species.

One of the main questions when managing seed orchard is to obtain a precise estimation of pollen contamination, as it causes major losses to genetic improvement from selection and breeding (Di Giovanni and Kevan, 19911) but also will determine the adaptive potential of the species (Kremer et al. 2012). The results indicate that contamination rates were highly variable between seed lots (from 20 to 96%), with a mean value of 50%). The main factors determining these rates include the distance between the seed orchard and external pollen sources, rain during the pollination period, seed orchard age, soil conditions and seed parent identity. 

A second point of interest in this paper is the determination of the overall self-fertilization rate. This factor also determines the quality of the seed-lots and was estimated as 5.4%, with high variability between genotypes (from 0% to 26%). The overall value is of the same order of magnitude than in other species. 

These results are used to define some recommendations for managing seed orchards in the French breeding program, but that can be generalized to other species (eg. Mullin and Lee, 2013). As an example, they recommend that sampling 100 seeds annually should be sufficient to estimate pollen contamination (with a standard error of 5%). Also, they suggest that one of the main measures to reduce pollen contamination is carefully selecting the location of the orchard, in terms of its distance from external pollen sources and soil conditions, and not collecting seeds from young trees (below 8 years old). 

The present preprint revisits an important topic of research with interest for the biology of tree species, but also with great implications in applied breeding activities. The main conclusions are essential to understand the importance of different factors in managing seed orchards and in the future performance of the reproductive material. 

In conclusion, this paper stresses the need for more studies, taking advantage of new genomic tools, to advance the knowledge of factors influencing the success of breeding programs.

REFERENCES

Bouffier L, Debille S, Alazard P, Raffin A, Pastuszka P, Trontin JF (2023). Pollen contamination and mating structure in maritime pine (Pinus pinaster Ait.) clonal seed orchards revealed by SNP markers. bioRxiv, 2022.09.27.509769, ver. 2 peer-reviewed and recommended by Peer Community in Forest and Wood Science. https://doi.org/10.1101/2022.09.27.509769

Di-Giovanni F, Kevan PG (1991) Factors affecting pollen dynamics and its importance to pollen contamination: a review. Can J For Res 21(8):1155-1170.
https://doi.org/10.1139/x91-163
 
Kremer A, Ronce O, Robledo-Arnuncio JJ, Guillaume F, Bohrer G, Nathan R, Bridle JR, Gomulkiewicz R, Klein EK, Ritland K, Kuparinen A, Gerber S, Schueler S (2012) Long-distance gene flow and adaptation of forest trees to rapid climate change. Ecol Lett 15(4):378-92.
https://doi.org/10.1111/j.1461-0248.2012.01746.x

Mullin TJ, Lee SJ (2013) Best practice for tree breeding in Europe. Skogforsk, Uppsala, Sweden. ISBN: 530 978-91-977649-6-4. https://www.skogforsk.se/contentassets/42acda01f83843bf925f690bd0a6ed37/best-practice-hela-low.pdf

Seed physiology, which was a little forgotten in forest ecology since several decades, needs be revitalized as a research area given the many open questions about population dynamics and demography in rapidly changing environments (and not only for endangered species). 

Indeed, seed physiology was long mobilized mainly to optimize seed conservation and germination for the production of plant material in a range of tree species used for afforestation/plantation. In the case of back poplar (which by the way is the male genitor of the multiple hybrid Populus x euramerica poplar cultivars), the focus is rather on the conservation ecology of this riparian species, and mainly on in situ conservation (Lefèvre et al, 1998). Indeed, the protection of populations of Populus nigra L requires an improved understanding of the ecology of this species with a focus on reproduction. Indeed, black poplar seeds need to be rapidly disseminated, to germinate as soon as the conditions are favourable (with rather small time windows) and establish seedlings with access to water in the rather harsh environment of mobile and sandy river banks submitted to alternating periods of flooding and of severe water deficits during low river flows in summer (Imbert and Lefèvre, 2003; Corenblit et al, 2014; Tinschert et al, 2020).

This process is therefore central to the propagation/maintenance of these populations that are threatened by the destruction of river banks and by introgression by either genes from the widespread “Italica” cultivar of black poplar, of from other poplar species leading to a variety of natural hybrids (Smulders et al, 2008).

Many questions remain open about seeds of black poplar (Michalak et al, 2015). One of the most intriguing one is to what extent seed properties and physiology differ within and among local populations from different river catchments. This question was addressed in this preprint by Lefebvre et al. (2021) that provides a very detailed and comparative analysis of two populations from central and southern France, each represented by 10 half sib families (i.e., seed collected separately from 10 adult individuals after open pollination).

Investigated properties were mainly seed biomass, anatomy, germination rate, root growth, lipid and sugar contents, protein content (with identification of some major protein families).

The within populations variability was indeed quite large, but nevertheless there were significant differences between the two populations in several traits, like seed weight, lipid content, and starch content. Storage proteins differed among families, but only slightly between the two populations. However, the main conclusion was that intrinsic qualities of the seeds were not critical for early stage establishment in the two populations, despite some significant differences in mean seed biomass, in lipid and in soluble sugars contents.

The preprint nicely analyses these differences, brings a large set of new observations about the seed physiology of Populus nigra. The referees found the data produced during this research quite important and original. This is why, despite the fact that the number of tested groups of populations remains rather small and the link with seedling establishment remains rather weak, this study is an important contribution to conservation ecology. This research (and that of many other groups) needs be further developed with an emphasis on inter and intra population variation and on demogenetics of forest tree species.

References

Corenblit D., Steiger J., González, E et al. (2014), The biogeomorphological life cycle of poplars during the fluvial biogeomorphological succession: a special focus on Populus nigra L.. Earth Surf. Process. Landforms, 39: 546-563. doi: https://doi.org/10.1002/esp.3515

Imbert E. and Lefèvre F. (2003) Dispersal and geneflow of Populus nigra (Salicaceae) along a dynamic river system. Journal of Ecology  91: 447-456. doi: https://doi.org/10.1046/j.1365-2745.2003.00772.x

Lefebvre M., Villar M., Boizot N., Delile A., Dimouro B., Lomelech A.-M. and Teyssier, C. (2021) Variability in seeds’ physicochemical characteristics, germination and seedling growth within and between two French Populus nigra populations. arXiv, 2008.05744, ver 3 peer-reviewed and recommended by Peer community in Forest and Wood Sciences. https://arxiv.org/abs/2008.05744

Lefèvre F., Légionnet A., de Vries S. and Turok J. (1998) Strategies for the conservation of a pioneer tree species, Populus nigra L., in Europe. Genetics, Selection, Evolution 30, S181-196. doi: https://doi.org/10.1186/1297-9686-30-S1-S181

Michalak M., Plitta B.P., Tylkowski T. et al. (2015) Desiccation tolerance and cryopreservation of seeds of black poplar (Populus nigra L.), a disappearing tree species in Europe. European Journal of Forest Research 134, 53–60. doi: https://doi.org/10.1007/s10342-014-0832-4

Smulder M.J.M., Beringen R., Volosyanchuk R. et al. (2008) Natural hybridisation between Populus nigra L. and P. x canadensis Moench. Hybrid offspring competes for niches along the Rhine river in the Netherlands. Tree Genetics & Genomes 4, 663–675. doi: https://doi.org/10.1007/s11295-008-0141-5

Tinschert E., Egger G., Wendelgass J. et al. (2020) Alternate reproductive strategies of Populus nigra influence diversity, structure and successional processes within riparian woodlands along the Allier River, France. Journal of Hydro-environment research 30, 100-108. doi: https://doi.org/10.1016/j.jher.2020.03.004