Light-emitting diode improves tomato growth under nitrate-N but exacerbates ammonium toxicity under ammonium-N nutrition

High irradiation can enhance the availability of carbon (C) skeletons for ammonium (NH4+ assimilation, thus preventing NH4+ toxicity, but it can also have a detrimental effect. The objective was to verify the physiological, nutritional and growth effects of light source (fluorescent or Light-emitting diode, LED) and N form (NO3− and NH4+) on tomato cv. Micro-Tom (MT). A randomized block design with 5 replications in a 2x2 factorial scheme was used, comprising lamp fluorescent (70 µmol m−2 s−1) or LED (300 µmol m−2 s−1) and 5.9 mmol L−1 N as NO3− or NH4+. For types of lamps, plants supplied with NH4+ exhibited decreased total chlorophyll and carotenoid contents, increased anthocyanin content and electrolyte leakage, and leaf epinasty. Plant shoot and root dry mass were higher under LED than under fluorescent lamp in both N treatments. However, symptoms of NH4+ toxicity, especially lower plant growth, leaf epinasty, and chlorosis and necrosis in old leaves, were more evident in the plants grown under LED. In conclusion, NH4+ was toxic to tomatoes, regardless of the lamp type, i.e. fluorescent or LED, as it reduces total chlorophyll, carotenoids and increases the electrolyte leakage index. Tomato grown in NO3− under LED had an increase in biomass due to the greater number and length of lateral branches. NH4+ toxicity was more severe in the plants grown under LED than in the plants grown under fluorescent lamp.