© 1996 Heron Publishing—Victoria, Canada
Carbon and nitrogen allocation in ectomycorrhizal and non-mycorrhizal Pinus sylvestris L. seedlings
Jan V. Colpaert (1), André van Laere (1) and Jozef A. van Assche (2)
1. Laboratory of Developmental Biology, Institute of Botany, Katholieke Universiteit Leuven, K. Mercierlaan, 92, B-3001 Leuven,
Belgium / 2. Laboratory of Ecology, Institute of Botany, Katholieke Universiteit Leuven, K. Mercierlaan, 92, B-3001 Leuven, Belgium / Received November 29, 1995
Summary
We studied carbon and nitrogen allocation in mycorrhizal and non-mycorrhizal Scots pine (Pinus sylvestris L.) seedlings grown in a semi-hydroponic system with nitrogen as the growth limiting factor. Three ectomycorrhizal fungi
were compared: one pioneer species (Thelephora terrestris Ehrh.: Fr.) and two late-stage fungi (Suillus bovinus (L.: Fr.) O. Kuntze, and Scleroderma citrinum Pers.). By giving all plants in each treatment the same amount of readily available nitrogen, we ensured that the external
mycelium could not increase the total nitrogen content of the plants, thereby guaranteeing that any change in carbon or nitrogen
partitioning was a direct effect of the mycorrhizal infection itself. Carbon and nitrogen partitioning were measured at an
early and a late stage of mycorrhizal development, and at a low and a high N addition rate.
Although mycorrhizal seedlings had a higher net assimilation rate and a higher shoot/root ratio than non-mycorrhizal seedlings,
they had a lower rate of shoot growth. The high carbon demand of the mycobionts was consistent with the large biomass of external
mycelia and the increased belowground respiration of the mycorrhizal plants. The carbon cost to the host was similar for pioneer
and late-stage fungi. Above- and belowground partitioning of nitrogen was also affected by mycorrhizal infection. The external
mycelia of Scleroderma citrinum retained 32% of the nitrogen supplied to the plants, thus significantly reducing nitrogen assimilation by the host plants
and consequently reducing their growth rate. By contrast, the external mycelia of T. terrestris and Suillus bovinus retained less nitrogen than the mycelia of Scleroderma citrinum, hence we attributed the decreased growth rates of their host plants to a carbon drain rather than a nitrogen deficiency.
Keywords:
carbon allocation, nitrogen partitioning, root respiration, Scleroderma citrinum, shoot/root ratio, Suillus bovinus, Thelephora terrestris.
