TOMATO RESPONSE TO NITRATE AND AMMONIUM AS INFLUENCED BY LIGHT AND NUTRIENT MEDIA

Tomato growth, nitrogen fractions, and nutrient uptake responses to NO(,3)* and NH(,4)* nutrition as influenced by light intensity, nutrient media, nitrogen feeding (roots or foliage), and stage of development were studied. Plants grown at full sun light with NH(,4) in solution or sand culture exhibited NH(,4) toxicity symptoms while in shade growth appeared normal. Dry weight was reduced by both shade and NH(,4) nutrition. NH(,4) nutrition reduced total K, Ca, and Mg accumulation in tissue and cation uptake per unit of root surface. P, NH(,4), and free amino acid contents were increased with NH(,4)-N compared to that of NO(,3)-N plants. Shading greatly decreased uncomplexed NH(,4) and increased free amino acids, mainly asparagine and glutamine, contents in the shoot of NH(,4)-fed plants. It is hypothesized that ammonia released in photorespiration contributed to higher toxicity of NH(,4) under high light intensity. The NO(,3) contents in shoots increased with shade in NO(,3)-fed plants. Growth of plants in sand, vermiculite, and solution culture was much better with NO(,3)-N, but in peat, growth of NH(,4)-fed plants was slightly better. The total dry weight of NH(,4)-treated plants grown in peat was more than twice that of plants growing in vermiculite, sand or solution culture. The K and Ca contents in tissue of plants grown in peat were not greatly affected by N form. Content of uncomplexed NH(,4) was reduced in NH(,4)-fed plants grown in peat compared with other media. A low total amino acid:free NH(,4) ratio was obtained for NH(,4)-fed plants displaying NH(,4) toxicity symptoms, when grown in sand, vermiculite, and solution culture, whereas in peat with no NH(,4) toxicity symptoms, it was higher, similar to NO(,3)-treated plants. Foliar spray with NH(,4) increased growth, K, Ca and Mg content and reduced free NH(,4) in shoot compared to root feeding NH(,4). Plants sprayed with NH(,4) incorporated greater amounts of N into insoluble compounds compared with NO(,3)-N. The N uptake per unit of leaf area was higher for plants grown under full sunlight compared to shade. *Wherever NO(,3) or NH(,4) appears it refers to NO(,3)(\u27-) or NH(,4)(\u27+) ion