Effects of elevated [CO2] and nitrogen nutrition on cytokinins in the xylem sap and leaves of cotton

We measured the level of xylem-derived cytokinins (CKs) entering a cotton l eaf, and the CK levels in the same leaf, thus enabling xylem sap and foliar CKs to be compared concurrently. Although zeatin was the dominant CK in xy lem sap, zeatin, dihydrozeatin, and N-6-(2-isopentenyl) adenine were presen t in approximately equimolar levels in leaves. Elevated [CO2] (EC) has an e ffect on the levels of cytokinins in sap and leaf tissues. This effect was modulated by the two levels of root nitrogen nutrition (2 and 12 mM nitrate ). Growth enhancement (70%) in EC over plants in ambient [CO2] (AC) was obs erved for both nitrogen nutrition treatments. Low-nitrogen leaves growing i n EC exhibited photosynthetic acclimation, whereas there was no sign of pho tosynthetic acclimation in high-nitrogen grown leaves. Under these prevaili ng conditions, xylem sap and leaf tissues were obtained for CK analysis. Hi gher nitrogen nutrition increased the delivery per unit leaf area of CKs to the leaf at AC. EC caused a greater increase in CK delivery to the leaf at low nitrogen conditions (106%) than at high nitrogen conditions (17%). EC induced a significant increase in CK content in low-nitrogen leaves, wherea s CK content in leaf tissues was similar for high-nitrogen leaves growing i n AC and EC.