CO2 enrichment modulates ammonium nutrition in tomato adjusting carbon and nitrogen metabolism to stomatal conductance

Ammonium (NH4+) toxicity typically occurs in plants exposed to high environmental NH4+ concentration. NH4+ assimilating capacity may act as a biochemical mechanism avoiding its toxic accumulation but requires a fine tuning between nitrogen assimilating enzymes and carbon anaplerotic routes. In this work, we hypothesized that extra C supply, exposing tomato plants cv. Agora Hybrid F1 to elevated atmospheric CO2, could improve photosynthetic process and thus ameliorate NH4+ assimilation and tolerance. Plants were grown under nitrate (NO3−) or NH4+ as N source (5-15 mM), under two atmospheric CO2 levels, 400 and 800 ppm. Growth and gas exchange parameters, 15N isotopic signature, C and N metabolites and enzymatic activities were determined. Plants under 7.5 mM N equally grew independently of the N source, while higher ammonium supply resulted toxic for growth. However, specific stomatal closure occurred in 7.5 mM NH4+-fed plants under elevated CO2 improving water use efficiency (WUE) but compromising plant N status. Elevated CO2 annulled the induction of TCA anaplerotic enzymes observed at non-toxic NH4+ nutrition under ambient CO2. Finally, CO2 enrichment benefited tomato growth under both nutritions, and although it did not alleviate tomato NH4+ tolerance it did differentially regulate plant metabolism in N-source and -dose dependent manner.