Does the choice of rootstock impact the resilience to drought of grafted clones in <i>Larix</i> ?

Abstract Grafting is a common technique used by breeders to propagate selected trees vegetatively for establishment of clonal archives and seed orchards, but also for clonal evaluation. The root-stock can have a beneficial impact, improving tree development and reproduction in the context of seed orchards, but may also be detrimental in the case of genetic evaluation if uncontrolled. We studied the impact of rootstock on radial growth and resilience to drought in relation to two large-scale grafting experiments, repeatedly subjected to periods with low summer precipitation: one with homo-plastic grafting (12 x 12 grafting diallel design; hybrid larch) and one associating different rootstock species (progeny-clonal trial of European, Japanese and hybrid larches). Our hypothesis was that more vigorous rootstock could enhance growth and resilience to drought. Overall, whatever the grafting systems used and traits considered, both rootstock and scion had a significant effect. Scion effect, though, was dominant and significant rootstock x scion interactions were rare. Across years, less influence of rootstock and scion on the growth of grafted trees was observed during the drier years, but the effect increased during the recovery periods. Our hypothesis was only partly validated when using hybrid larch in hetero-plastic grafting: rootstocks of the more vigorous hybrid larch significantly improved radial growth compared to Japanese larch, whatever the grafted species clones and the climatic years (dry and humid). Hybrid larch rootstocks also improved resilience to drought of European larch but had no effect on Japanese larch. In contrast, when hybrid larch clones were grafted onto hybrid larch rootstocks (homo-grafting), the vigour of the rootstocks had no influence on growth and resilience but well that of the scions. More broadly, the performance of individual clones (evaluated by self-grafting) was only rarely modified when grafted on any of the other 10 rootstocks. The use of hetero-grafting with hybrid larch rootstock appears beneficial to obtain seed orchards of European larch that are more robust and resilient to drought (but only more vigourous for Japanese larch), but it will be a source of bias in the genetic evaluation of individuals as in clonal testing. For this purpose, homo-grafting should be preferred with less concern for rootstock vigour.