Original Paper

Trees

, Volume 22, Issue 5, pp 685-695

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

  • Michael A. NicodemusAffiliated withDepartment of Forestry and Natural Resources, Hardwood Tree Improvement and Regeneration Center, Purdue University
  • , K. Francis SalifuAffiliated withDepartment of Forestry and Natural Resources, Hardwood Tree Improvement and Regeneration Center, Purdue University
  • , Douglass F. JacobsAffiliated withDepartment of Forestry and Natural Resources, Hardwood Tree Improvement and Regeneration Center, Purdue University Email author 

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Abstract

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.

Keywords

Black walnut Nitrate Nitrate reductase Ammonium Xylem exudate