Perspectives

La formation de greffes racinaires demeure une curiosité naturelle. Bien que communes chez certaines espèces, les mécanismes de la formation des greffes et leur portée écologique demeurent largement incompris. Puisque les greffes racinaires existent en proportion relativement importante, les résultats ouvrent des perspectives considérables sur les recherches au niveau des espèces utilisées en foresterie de courte rotation ou les plantations à croissance rapide. Des recherches approfondies devront être menées sur la signification précise des connexions racinaires pour comprendre ce phénomène naturel chez les espèces ligneuses. Sont-elles bénéfiques pour l’amélioration de la productivité en plantations forestières et donc faudra-t-il les favoriser en plantant les arbres plus proches les uns des autres? Ou au contraire, le transfert de maladies est plus important et faudra-t-il éviter que le contact racinaire ne s’établisse?

La question à savoir si la formation de greffes racinaires naturelles est un trait adaptatif chez une espèce ou si elle est de nature accidentelle demeure toujours aussi. Bien que nos résultats aient montré que les facteurs amenant les racines à être en contact les favorisent, l’hypothèse de la nature accidentelle de la formation des greffes racinaires semble donc mieux être appuyée par cette étude. Cependant, il pourrait s’agir de substances chimiques ou phytohormones envoyées par les arbres qui initient la formation de greffes. Des métabolites ou exsudats racinaires pourraient être sécrétés par les racines et déclencher leur fusion. Durant les différentes étapes de sélection, de tests et de croisements génétiques, pourrait-on alors envisager de considérer les greffes racinaires comme un caractère à favoriser et à exploiter?

Cette étude a permis de mettre en exergue l’étendue de la présence des greffes racinaires et du fait qu’elles existent tôt chez le peuplier hybride. En lumière de ces différentes implications, les arbres ne devront plus uniquement être considérés en termes d’individus en compétition pour les ressources, mais plus comme un groupe fonctionnel où des partages de ressources et de transmissions de maladies peuvent être facilités par les greffes racinaires.

ANNEXE A

Tableau A.1 Belowground characteristics of the two hybrid poplar clones (915319: Populus maximowiczii x Populus balsamifera, 747215: Populus balsamifera x Populus trichocarpa) under two spacings, after 15 growing seasons. Numbers between brackets are standard error of the mean.

Plots Clone 915319 Clone 747215

1 x 1 m 3 x 3 m 1 x 1 m 3 x 3 m Mean root diameter (cm) 4.4 (0.2) a _{5.6 (0.2)} b _{4.2 (0.2)} a _{5.7 (0.3)} b

Mean CSA (cm2_{) 15.9 (1.6)} a _{25.0 (1.5)} b _{14.9 (1.5)} a _{25.9 (2.4)} b

Mean height-root ratio 28.8 (4.2) a _{15.8 (0.9)} b _{33.2 (2.0)} a _{17.8 (0.8)} b

Mean number of roots (>2

cm) 13 (0.8) c 15 (0.8) c 8 (0.5) b 9 (0.6) b Mean root age (years) 12 (0.2) a _{11.2 (0.2)} a _{10.5 (0.2)} b _{10.7 (0.3)} b

Mean root surface (cm) 55.9 (4.1) a _{86.6 (6.1)} b _{34.0 (1.9)} c _{52.9 (3.6)} a

Notes: CSA represents the mean cross-sectional area of the roots. Different letters within a row indicate significant statistical differences using Tukey-Kramer adjustment tests

Tableau A.2 Two-way Analysis of variance giving sources of variation, degrees of freedom (df), F values, and associated probabilities for the belowground parameters: root diameter, CSA, height-root ratio, Number of roots, root surface and root age.

Sources of variation df F P-value

Root diameter (cm) Spacing 1 24.9 <0.001 Clone 1 0.2 0.62 Spacing x Clone 1 0.12 0.68 CSA (cm2₎ Spacing 1 23.6 <0.001 Clone 1 0.09 0.76 Spacing x Clone 1 0.2 0.68 Height-root ratio Spacing 1 17.0 <0.001 Clone 1 1.5 0.23 Spacing x Clone 1 0.1 0.73 Number of roots >2cm Spacing 1 7.3 0.008 Clone 1 41.6 <0.001 Spacing x Clone 1 0.8 0.31 Root surface (cm) Spacing 1 46.3 <0.001 Clone 1 50.8 <0.001 Spacing x Clone 1 2.2 0.14

Root age (years)

Spacing 1 0.2 0.65

Clone 1 29.3 <0.001

Spacing x Clone 1 3.9 0.05

Notes: Significant p-values are indicated in bold, CSA represents the mean cross- sectional area of the roots

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