Uptake, transport and assimilation of 15N-nitrate and 15N-ammonium in tea (Camellia sinensis L.) plants

N-15 studies were conducted using hydroponically grown tea (Camellia sinensis L.) plants to clarify the characteristics of uptake, transport and assimilation of nitrate and ammonium. From the culture solution containing 50 mg L-1 NO3-N and 50 mg L-1 NH4-N, the uptake of NH4-N after 24 h was twice as high as that of NO3-N, while the uptake of NO3-N from the culture solution containing 90 mg NO3-N and 10 mg NH4-N was twice that of NH4-N. The presence of 0.4 mM Al had no significant effect on the NO3-N and NH4-N uptake from the culture solutions containing 50 mg L-1 NO3-N and 50 mg L-1 NH4-N, 90 mg L-1 NO3-N and 10 mg L-1 NH4-N or 99 mg L-1 NO3-N and 1 mg L-1 NH4-N. Transport of NO3-derived N to young leaves was much more rapid than that of NH4-derived N, and NH4-derived N was largely retained in the roots and lower stem. Young and mature shoots separated from the roots absorbed more NO3-N than intact plants. Nitrate assimilation occurred in both, roots and young as well as mature leaves. Internal cycling of NO3-derived N and NH4-derived N from one root part to another part was not appreciable after 28 h, suggesting that a longer of time is required for cycling in woody plants.