Interactive effects of mycorrhization and elevated atmospheric CO2 on sulphur nutrition of young pedunculate oak (Quercus robur L.) trees
Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO 2 concentration and 650 μmol mol -1 CO 2 in the presence and absence of the ectomycorrhizal fungus Laccaria laccata for a total of 22 weeks under non-limiting nutrient conditions. Sulphate uptake, xylem loading and exudation were analysed in excised roots. Despite a relatively high affinity for sulphate (K M = 1.6 mmol m -3 ), the rates of sulphate uptake by excised lateral roots of mycorrhizal oak trees were low as compared to herbaceous plants. Rates of sulphate uptake were similar in mycorrhizal and non-mycorrhizal roots and were not affected by growth of the trees at elevated CO 2 . However, the total uptake of sulphate per plant was enhanced by elevated CO 2 and further enhanced by elevated CO 2 and mycorrhization. Sulphate uptake seemed to be closely correlated with biomass accumulation under the conditions applied. The percentage of the sulphate taken up by mycorrhizal oak roots that was loaded into the xylem was an order of magnitude lower than previously observed for herbaceous plants. The rate of xylem loading was enhanced by mycorrhization and, in roots of mycorrhizal trees only, by growth at elevated CO 2 . On a whole-plant basis this increase in xylem loading could only partially be explained by the increased growth of the trees. Elevated CO 2 and mycorrhization appeared to increase greatly the sulphate supply of the shoot at the level of xylem loading. For all treatments, calculated rates of sulphate exudation were significantly lower than the corresponding rates of xylem loading of sulphate. Radiolabelled sulphate loaded into the xylem therefore seems to be readily diluted by unlabelled sulphate during xylem transport. Allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH to mature oak leaves. The rate of export of radioactivity from the fed leaves was 4.5 times higher in mycorrhizal oak trees grown at elevated CO 2 than in those grown at ambient CO 2 . Export of radiolabel proceeded almost exclusively in a basipetal direction to the roots. From these experiments it can be concluded that, in mycorrhizal oak trees grown at elevated CO 2 , the transport of sulphate to the shoot is increased at the level of xylem loading to enable increased sulphate reduction in the leaves. Increased sulphate reduction seems to be required for the enhanced allocation of reduced sulphur to the roots which is observed in trees grown at elevated CO 2 . These changes in sulphate and reduced sulphur allocation may be a prerequisite for the positive effect of elevated CO 2 on growth of oak trees previously observed.