Does nitric oxide alleviate the effects of ammonium toxicity on root growth of Atlantic forest tree species?

Nitrate and ammonium are the main sources through which plants obtain nitrogen from the soil. Nevertheless, several plant species exhibit symptoms of toxicity when grown with ammonium, including reduced root growth. As nitrite derived from nitrate is the primary pathway for nitric oxide (NO) synthesis, environments containing ammonium as the sole nitrogen source have lower concentrations of this signaling molecule. Application of NO can enhance plant tolerance to stresses. In our study, the effect of NO application on seedlings of two tree species from the Atlantic Forest with different nitrogen utilization strategies and contrasting tolerances to ammonium was evaluated. The tolerant species Cariniana estrellensis did not show a significant difference in root growth under nitrate or ammonium. However, the non-tolerant species Cecropia pachystachya showed low growth when supplied with ammonium. Malondialdehyde did not accumulate in both species, suggesting that ammonium toxicity is not related to oxidative stress. As expected, C. pachystachya roots exhibited higher concentration of NO when grown with nitrate but C. estrellensis displayed higher endogenous concentration of NO when supplied with ammonium, suggesting a predominance of NO synthesis through oxidative pathways. NO application increased root growth in C. pachystachya seedlings grown in ammonium but had no effect on C. estrellensis. Together, these results suggest that greater tolerance to ammonium may be related to higher concentrations of NO and its modulating role in anti-stress responses. Further investigation with a broader range of species is necessary to identify the mechanisms underlying ammonium tolerance and NO production.