Evolution du potentiel hydrique et du systeme racinaire de jeunes plants de cedre, pin laricio de Corse et pin noir plantes a l'automne et au printemps

Les auteurs ont suivi, après plantations à racines nues en automne et au printemps, l'évolution du potentiel hydrique de base et l’état de régénération du système racinaire de plants de cèdre, de pin laricio de Corse et de pin noir d’Autriche. Deux traitements ont été considérés : traitement I (témoin) : les plants ont conservé leurs extrémités racinaires blanches, traitement II : toutes les racines blanches ont été excisées. Les observations effectuées montrent que quel que soit le traitement, les plants pourvus de racines blanches au moment de la mesure, ont un potentiel hydrique de base significativement plus élevé que ceux qui en sont dépourvus. Lorsque le potentiel de base était inférieur à - 1,5 MPa, les plants ne présentaient pas de racines blanches. L’étude de la répartition des plants, par classe de potentiel hydrique, montre que le pin laricio de Corse, contrairement aux deux autres espèces, a des difficultés à maintenir un état hydrique élevé malgré une plantation préservant les extrémités racinaires. L’excision des racines blanches a eu, pour conséquence, des taux de débourrement faibles pour le cèdre et le pin noir d’Autriche, et nuls pour le pin laricio de Corse. Globalement, des trois espèces étudiées, le pin laricio de Corse a été celle qui a été la plus perturbée par la plantation à racines nues.

This study deals with the evolution of predawn water potential of cedar (Cedrus atlantica Manetti), Corsican pine (Pinus nigra Arn ssp. laricio poir. var. corsicana) and Austrian black pine (Pinus nigra ssp. nigricans Host) seedlings together with their root development after out-planting with bare-rooted nursery stock in the fall and spring (tab]. 1). The experiment consisted of two different treatments : - treatment I : plantations were realised with seedlings having their white root tips intact ; - treatment II : where all white roots were pruned and seedlings were replanted immediately under field conditions together with the seedlings of treatment I. The results obtained in both treatments showed a direct relationship between seedlings water potential after plantation and the existence of white root extremites. Seedlings presenting white roots at the moment of measurement, had higher (less negative) water potential than seedlings without white roots (fig. 1, 2, 3 ; tab. 4 and 5). In general, it was observed that, in plants of both treatments with water potential inferior to - 1.5 MPa did not present white roots. The assessment of the seedlings water potential (tabl. 3) showed clear differences between the three species. In plants subjected to treatment I, 5.1 p. 100 of Corsican pine seedlings had a water potential between zero and - 0.5 MPa against 45.1 p. 100 and 50 p. 100 respectively for cedar and Austrian black pine seedling. For seedlings subjected to treatment II, all Corsican pine seedlings showed a water potential inferior to - 0.5 MPa whereas 43.1 p. 100 of cedar and 11.9 p. 100 of Austrian black pine were found with a water potential superior to - 0.5 MPa. However, seedling survival was excellent for the different species. On the otherhand, the removal of white roots (treatment II) from seedlings of cedar and Austrian black pine at plantation in autumn, lowered their budbreak rate in the following spring compared with seedlings of treatment I. Budburst did not occur in Corsican pine seedlings subjected to treatment II in autumn (tabl. 2).The apparent differences between the tree species with Corsican pine seedlings being most deleteriously, affected by pruning of white roots at plantation, reveal that, seedlings of this specie are less efficient in maintaining high levels of hydration as a consequence to their failure in regulating their water loss, in low ability of their subrized roots to absorb water and/or to their less tolerance to dryness.