The differing responses of two Arabidopsis 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.