INTERACTIVE EFFECTS OF MYCORRHIZATION AND ELEVATED ATMOSPHERIC CO2 ONSULFUR NUTRITION OF YOUNG PEDUNCULATE OAK (QUERCUS-ROBUR L) TREES

Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO2 concentration and 650 mu mol mol(-1) CO2 in the presence and abse nce of the ectomycorrhizal fungus Laccaria laccata for a total of 22 w eeks under non-limiting nutrient conditions. Sulphate uptake, xylem lo ading and exudation were analysed in excised roots. Despite a relative ly high affinity for sulphate (K-M = 1.6 mmol m(-3)), the rates of sul phate uptake by excised lateral roots of mycorrhizal oak trees were lo w as compared to herbaceous plants. Rates of sulphate uptake were simi lar in mycorrhizal and non-mycorrhizal roots and were not affected by growth of the trees at elevated CO2. However, the total uptake of sulp hate per plant was enhanced by elevated CO2 and further enhanced by el evated CO2 and mycorrhization. Sulphate uptake seemed to be closely co rrelated with biomass accumulation under the conditions applied. The p ercentage of the sulphate taken up by mycorrhizal oak roots that was l oaded into the xylem was an order of magnitude lower than previously o bserved for herbaceous plants. The rate of xylem loading was enhanced by mycorrhization and, in roots of mycorrhizal trees only, by growth a t elevated CO2. On a whole-plant basis this increase in xylem loading could only partially be explained by the increased growth of the trees . Elevated CO2 and mycorrhization appeared to increase greatly the sul phate supply of the shoot at the level of xylem loading. For all treat ments, calculated rates of sulphate exudation were significantly lower than the corresponding rates of xylem loading of sulphate. Radiolabel led sulphate loaded into the xylem therefore seems to be readily dilut ed by unlabelled sulphate during xylem transport. Allocation of reduce d sulphur from oak leaves was studied by flap-feeding radiolabelled GS H to mature oak leaves. The rate of export of radioactivity from the f ed leaves was 4.5 times higher in mycorrhizal oak trees grown at eleva ted CO2 than in those grown at ambient CO2. Export of radiolabel proce eded almost exclusively in a basipetal direction to the roots. From th ese experiments it can be concluded that, in mycorrhizal oak trees gro wn at elevated CO2, the transport of sulphate to the shoot is increase d 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 obser ved in trees grown at elevated CO2. These changes in sulphate and redu ced sulphur allocation may be a prerequisite for the positive effect o f elevated CO2 on growth of oak trees previously observed.