Tomato growth, nitrogen fraction and mineral composition in response to nitrate and ammonium foliar sprays1

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Abstract
Tomato plants were grown in sand culture with NO3- or NH4-N at 2 levels of light. Foliar sprays at 3 levels of N and combinations of foliar and root feeding were used. Shade increased NH4 toxicity in plants sprayed with NH4 but decreased the toxicity in plants receiving NH4 through the roots. NH4-N greatly reduced growth and cation uptake when supplied through the roots but not with foliar application. Plants sprayed with NH4 showed better growth, higher K, Ca and Mg content, and lower free NH4 in shoot, compared to plants receiving NH4 through the roots. The overall free amino acid contents of shoots was higher for NH4-fed plants regardless of how the N was applied. Plants sprayed with NH4 incorporated a greater amount of N into insoluble compounds compared with NO3 nutrition. The N uptake per unit of leaf area was higher for plants grown under full sun light whereas N content was higher for plants grown under shade. N content in tissue increased with N concentration in foliar spray, although plants supplied with N through the roots had higher levels of free amino acids and total N.