Arabidopsis thaliana seedlings were grown in hydroponic culture under controlled conditions in mediums with different NO 3 − /NH 4 + ratios. The mediums were not buffered, against pH change, but their pH was reset at pH 5.5 each 2 days. When ammoniacal N source was predominant, the whole plant biomass deposition was diminished, parallely to several parameters which revealed degraded leaf physiological status, such as leaf chlorosis associated with diminished chlorophyll content, lowered K + , Ca 2+ , and water content, along with restricted leaf expansion, and apparition of cellular stress symptoms. However, leaf functioning for biomass production did not seem affected, since biomass production rate per unit leaf surface area remained unchanged. Indeed, the main factor of decrease biomass production was leaf elimination and/or repression of leaf initiation. The results are discussed in the context of literature data on the effects of NH 4 + on ion fluxes at the root level and properties of K + transport systems in Arabidopsis. Disturbance of mineral (cationic) nutrition by NH 4 + seemed to be a major cause of the detrimental effect of ammoniacal nutrition.