Distribution and partitioning of newly acquired boron (B) in a mature sunflower (Helianthus annuus L., cv. 3101) plant was investigated. In leaf blades of sunflower plants grown under 0.93, 2.8, and 9.3 mmol B m(-3), the level of cell-wall-bound B was rather uniform, irrespective of leaf position and B concentration. Boron concentration gradients among leaf positions were produced mainly by different levels of water-soluble B. To determine the distribution of newly taken-up B in plant parts, B-10-labeled boric acid at a concentration of 2.8 mmol B m(-3) was applied. The majority of newly acquired B was delivered to the younger leaves, however, approximately one-fourth of the B in the top and second leaves was the older B which was taken up before the 6 d treatment period. In the root tissues, two-fifth of the water-soluble B was new B taken up in the last 6 d, however, within 6 h of the application new B contributed to approximately 80% of the xylem sap B, suggesting that newly taken-up B is preferentially transported to the shoots. When B was withdrawn from the culture solution, the B concentration per leaf area of the lower leaves decreased slightly over 9 d. However, there was an abrupt decrease in the younger leaves, even when taking into account the rapid expansion of the leaf blade, suggesting that B moves more rapidly from the younger leaves than from the older leaves.