Partitioning and Remobilization of Nitrogen during Regrowth in Nitrogen‐Deficient Ryegrass

Little is known about the relative importance of different sources of N during crop regrowth after defoliation. This study was conducted to determine the effect of N deficiency on N uptake and redistribution throughout plants during a 14‐d period of regrowth following cutting in ryegrass (Lolium perenne L.). Eight‐week‐old plants grown in hydroponic culture, were labeled for 5 d with 15 N prior to cutting treatment. Regrowth occurred during 14 d with a low or high N supply in the growth medium and 15 N amounts recovered in plants were used to estimate both N uptake from the medium and N remobilization from roots and stubble to regrowing leaves. Comparison of clipped plants regrowing under nonlimited and limited N conditions showed that N deficiency reduces the growth of leaves. Nitrogen starvation decreases N uptake from the medium and protein N remobilization of roots, while N remobilization from stubble remains unaffected. After 14 d on N‐starved medium, about 69% of N in new regrowing leaves was obtained from endogenous compounds accumulated in roots and stubble, while only 40% was remobilized from these tissues in plants grown on a nonlimited N medium. Consequently, there is an important deficit between net N outflow and net N inflow, particularly in stubble of N limited plants. The overall results demonstrated that stubble tissues contribute to remobilization of N compounds during regrowth after defoliation in proportion to the level of N deficiency in the medium, while remobilization of protein N of roots was decreased with a low supply of N in the medium.