The differing responses of two <Emphasis Type="Italic">Arabidopsis</Emphasis> ecotypes to ammonium are modulated by the photoperiod regime

Responses to excessive ammonium (NH₄ ⁺) were compared between two Arabidopsis ecotypes (Col-0, JA22) with respect to different photoperiods in hydroponics. In this study, we showed that external extra NH₄ ⁺ led to severe growth suppression, accumulations of free NH₄ ⁺ and amino acids and increased the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in shoots of the two Arabidopsis ecotypes. However, the levels of free NH₄ ⁺ and total amino acids increased, whereas the activities of GS, NADH-dependent glutamate synthase and GDH decreased under the continuous light when compared with the light (16 h)–dark (8 h) cycle photoperiod. Statistical analyses suggested that strong correlations exist among the growth reduction, accumulations of free NH₄ ⁺, total amino acids and levels of GS activity in shoots under the high NH₄ ⁺ stress regardless of the photoperiod regimes. Interestingly, under the continuous light, Col-0 showed more resistant to such growth reduction and maintained about onefold higher capability of converting excess free NH₄ ⁺ into amino acids, with onefold higher GS activity induced by the external NH₄ ⁺ when compared with JA22. In contrast, these differences were abolished between Col-0 and JA22 under the light–dark cycle condition. Taken together, our results conclude that the sensitivity to NH₄ ⁺ of Col-0 and JA22 is changed between the continuous light and the light–dark cycle photoperiod, which is correlative to the alteration of the GS activity in shoots.