Nitrate reductase activity and nitrogen compounds in xylem exudate of Juglans nigra seedlings: relation to nitrogen source and supply

摘要

Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms is limited. We fertilized 1-year-old, half-sib black walnut (Juglans nigra L.) seedlings with ammonium (NH4 +) [as (NH4)2SO4], nitrate (NO3 ?) (as NaNO3), or a mixed N source (NH4NO3) at 0, 800, or 1,600 mg N plant?1 season?1. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed. Specific leaf NRA was higher in NO3 ?-fed and NH4NO3-fed plants compared to observed responses in NH4 +-fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO3 ?-fed than in NH4 +-fed plants. Exogenous NO3 ? was assimilated in roots or stored, so no difference was observed in NO3 ? levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over NO3 ? or NH4 + alone, suggesting NH4NO3 is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms.