Long-term drought stress alters nitrogenase activity and carbon translocation in split-root cultured Alnus incana

Split-root cultured grey alder, Alnus incana (L.) Moench., was grown in sand in cuvettes with a continuous supply of nutrient solution. During the drought treatment for up to 9 days the supply of solution was withheld from one of the split-root halves. After 2–3 days of treatment, soil water became depleted and the unwatered root halves were at a constant drought stress, water potential (Ψ_nodules) = -1.1 to -1.6 MPa. Nitrogenase activity in the dry half decreased to about 70% of the initial value during the first 2–3 days and then stayed at this level. The water supply to the shoot from the wet root half was high and only a temporary slight decrease in photosynthesis and stomatal conductance was found in drought-stressed split-root plants. Labelling studies showed a reduced translocation of photoassimilates to the dry nodules. The fixation of CO₂ in the nodules seemed to be more tolerant to drought than nitrogenase activity. During the drought treatment there was an osmotic adjustment from -0.9 to -1.7 MPa, but no change in the storage of starch in the nodules. In alders where parts of the root system is kept dry these roots acclimate and continue a persistent nitrogenase activity.