Nitrogen Uptake and Mobilization by Hydrangea Leaves from Foliar-sprayed Urea in Fall Depend on Plant Nitrogen Status

Rooted liners of Hydrangea macrophylla (Thumb.) Ser. 'Berlin' were fertigated with different rates of nitrogen (N) from July to Sept. 2007 and leaves were sprayed with N-15-labeled urea in late October to evaluate urea uptake and N-15 translocation by hydrangea leaves in relation to plant N status. Four plants from. each N fertigation rate were harvested before they were sprayed with urea and 2, 5, 10, and 15 days after urea spray. Increasing rate of N fertigation increased plant N content in October before being sprayed with urea. Leaves rapidly absorbed N-15 from urea spray. The highest rate of N-15 uptake occurred during the first 2 days after urea spray and then decreased. Export of N-15 from leaves occurred rapidly after uptake and the highest rate of N-15 export occurred (luring the first 2 days after urea spray and then decreased. During the first 5 days after urea spray, the rate of N-15 uptake by leaves and export from leaves decreased with increasing rate of N fertigation. On a whole plant basis, the total amount of N-15 from foliar N-15-urea spray increased with increasing rate of N fertigation; however, the percentage of N-15 exported from leaves and the percentage of N that derived from foliar N-15-urea spray decreased with increasing rate of N fertigation. Results suggest that hydrangea plants with lower N status in the fall are more efficient in absorbing and translocating N from foliar urea than plants with higher N status.