© 2001 Heron Publishing—Victoria, Canada
Competition for water between walnut seedlings (Juglans regia) and rye grass (Lolium perenne) assessed by carbon isotope discrimination and δ18O enrichment
Catherine Picon-Cochard (1, 2), Amelie Nsourou-Obame (1), Catherine Collet (3), Jean-Marc Guehl (1) and Andre Ferhi (4)
1. Unité de Recherche en Ecophysiologie, Equipe Bioclimatologie-Ecophysiologie, INRA Nancy, 54280 Champenoux, France / 2. Unité de Recherche en Agronomie, Equipe FGEP, INRA Clermont-Ferrand, 234 avenue du Brézet, 63039 Clermont-Ferrand Cédex, France / 3. Unité de Recherche en Croissance, Production et Qualité des Bois, Equipe Croissance, INRA Nancy, 54280 Champenoux, France / 4. Université Pierre et Marie Curie, Centre de Recherches Geodynamiques, 47 avenue de Corzent - BP 510, 74203 Thonon-Les-Bains
Cédex, France / Received December 21, 1999
Summary
Container-grown walnut seedlings (Juglans regia L.) were subjected to competition with rye grass (Lolium perenne L.) and to a 2-week soil drying cycle. One and 2 weeks after the beginning of the drought treatment, H218O (δ ≈ +100‰) was added to the bottom layer of soil in the plant containers to create a vertical H218O gradient. Rye grass competition reduced aboveground and belowground biomass of the walnut seedlings by 60%, whereas drought
had no effect. The presence of rye grass reduced the dry weight of walnut roots in the upper soil layer and caused a 50% reduction
in lateral root length. Rye grass competition combined with the drought treatment reduced walnut leaf CO2 assimilation rate (A) and leaf conductance (gw) by 20 and 39%, respectively. Transpiration rates in rye grass, both at the leaf level and at the plant or tiller level,
were higher than in walnut seedlings. Leaf intrinsic water-use efficiency (A/gw) of walnut seedlings increased in response to drought and no differences were observed between the single-species and mixed-species
treatments, as confirmed by leaf carbon isotope discrimination measurements. Measurement of δ18O in soil and in plant xylem sap indicated that the presence of rye grass did not affect the vertical profile of soil water
uptake by walnut seedlings. Walnut seedlings and rye grass withdrew water from the top and middle soil layers in well-watered
conditions, whereas during the drought treatment, walnut seedlings obtained water from all soil layers, but rye grass took
up water from the bottom soil layer only.
Keywords:
growth, H218O, leaf gas-exchange, roots, soil drought, tree–grass competition, water acquisition.
