Dépérissement du pin maritime en Vendée. Résistance au chlorure de sodium de 3 provenances géographiques dans différentes conditions édaphiques

Résumé - Le travail présenté vise à tester la résistance de différentes provenances de pins maritimes au NaCl dans des conditions édaphiques proches du terrain et dans un environnement minéral le plus favorable possible. Le stress nutritionnel imposé par l'alcalinité et la pauvreté des sables _ littoraux de Vendée s'est avéré extrêmement péjoratif à la croissance des semis de pin en serre (1- 25 g) MS/2 ans). Le jaunissement des aiguilles et la mortalité observés sur le sol de «Pays de Monts» est bien en relation avec le caractère dépérissant du peuplement installé sur ce site et fait apparaître le caractère résistant de la provenance Tamjoute. En revanche, en ce qui concerne la production de biomasse pour le sol d'Oléron (placette saine), la différence de potentiel de croissance entre provenances ne s'exprime pas. L'effet de l'augmentation des teneurs en NaCl de la rhizosphère sur la croissance est peu sensible pour les pins cultivés dans ces conditions, et affecte en priorité le système racinaire. Les conditions d'alimentation minérale non limitantes (solutions nutritives coulantes) ont permis d'exprimer très nettement la toxicité au NaCl avec une sensibilité décroissante : «Ibérique», «Landais», «Tamjoute». Compte tenu des différences de croissance entre provenances : «Landais» > «Ibérique» > «Tamjoute» établies avec la solution nutritive témoin, les «Landais» conservent le meilleur taux de croissance pour toutes les doses, alors que les «Ibériques» sont supplantés en terme d'accroissement par les «Tamjoute» pour les doses de NaCl les plus fortes.

Abstract - Maritime pine dieback on the West coast of France. Growth response to sodium chloride of 3 geographic races in various edaphic conditions. The responses of 3 geographic races of maritime pine, French ("Landais"), Iberian ("lbérique") and Moroccan ("Tamjoute") to salt stress were studied on original soils and hydroponic culture. Landais and Ibérique pine seeds were collected from pine trees from west France and the genetic origin was determined by terpene shoot analysis on trees. Tamjoute seeds were collected in the Tamjoute mountains (Morocco). Homogenous seedlings were planted in calcareous sand from forest site at the "Pays de Monts" and "Oléron" (table I) and salt stress was applied to the Oleron soil after 12 months of culture and maintained for 60 d (experiment 1). In the quartz (2-4 mm) culture system, plants were supplied with fresh nutrient solution twice per h. Four stress levels were obtained with the addition of 0, 50, 150, 250 mM NaCl to the basic solution. Total heights were measured nearly twice per month (fig 1). The 3 curves showed well-synchronized height growth. The winter dormancy was clearly marked and the NaCl stress was installed at the maximum and linear growth stage. The final inflexion of the curve was due to the NaCl toxicity. Growth in the Pays de Mont treatment was completely absent and dieback symptoms appeared at the end of the first growing season, showing that the Tamjoute pines were more resistant (table II). Dry weights of roots, stems, and needles were evaluated after the stress period for the Oléron soil (tables III, IV). Genotypic response was not significant and the salt stress slowly reduced final biomass and particulary root weight. Analyses of variance showed significant differences in final harvest weights among geographic races in the second experiment (table V). The most vigorous race was found to be the Landais with 22% gain compared with the lbérique, and 36% gain compared with the Tamjoute race. Dry matter allocation was very similar between Landais and Ibérique, but conversely, Tamjoute showed stronger root allocation. Toxic symptoms of NaCl on plant organs were not detected after 60 d culture at 250 mM MaCl concentrations but the final biomasses were reduced substantially 40% less than the control (table VI). Growth (fig 2) was calculated by substracting the final weights from the initial weights of the same plant and estimated by linear regression on the total height. The regressions were established from data of the first harvest for each geographic race (table VII). Tamjout appeared to be more resistant to salt stress than Landais on Tamjoute.