Ammonium toxicity development in tomato plants relative to nitrogen form and light intensity1

Abstract

Tomato plants were grown in sand culture with NH₄ ⁺ and NO₃ ^– forms of N and three levels of light established by using shade cloth. Plants supplied with NH₄ ⁺ nutrition under high light intensity showed symptoms of stunting, leaf‐roll, wilting, interveinal chlorosis of the older leaves, and produced one third the dry weight of NO₃ ^– fed plants. In contrast, plants receiving NH₄ ⁺ nutrition with light reduced over 50% appeared normal. NH₄‐N increased NH⁴ and free amino acid contents of the tissue as compared to NO₃‐N nutrition. With shading there was a greatly decreased level of free NH₄ and increased levels of free amino acids, mainly asparagine and glutamine, in shoots of NH₄‐treated plants. The NO₃ content in shoots of NO₃‐treated plants increased sharply with shade. The results suggest that the increased NH₄ concentration in leaves from photorespiration can he a major factor in the ammonium toxicity expression under high light intensity.