## 4 Light Intensity- and Spectrum-Associated Adjustment of Metabolism
### Light Regulation of Nitrogen Assimilation and Amino Acid Levels
#### 4.2.1 Light Intensity-Associated Changes in Nitrogen Assimilation and Amino Acid Levels  
A coordinated upregulation of carbon assimilation and nitrate reduction was induced by high PPFD in cotton, which supports the regulatory model shown in Figure 5[147]. The effect of light on nitrate reduction was also confirmed in maize, in which nitrate reductase activity exhibited a diurnal rhythm with 2-3-fold higher activity during the light period compared to the dark (Figure 5) [148]. The light-dependent transcriptional regulation of this enzyme was observed in _Arabidopsis_[149]. In _Arabidopsis_ loss-of-function mutants, it was shown that the HY5 and the ELONGATED HYPOCTYL 5-HOMOLOG (HYH) transcription factors are involved in this process [7; 97; 150; 151]. In addition, the light signalling-related COP1 and PIF transcription factors, promoting the degradation of HY5 and HYH, also participate in the regulation of nitrate reductase activity in _Arabidopsis_ (Figure 5) [150]. HY5 may have a similar function in the light-dependent control of nitrate reduction in the monocot crop species, such as maize, wheat, and rice, but this hypothesis should be corroborated in future studies. Besides HY5, the NAD(P)H/NAD(P)\({}^{+}\) redox couple is also involved in the adjustment of nitrate reduction to the changes in the light intensity through its effect on the redox state of the plant tissues [152]. The activation of nitrate reduction by HL allows an increased incorporation of nitrogen into Gln and other amino acids. Accordingly, HL resulted in a coordinated strong increase in several free amino acids (Asp, Pro, Val, Ala, Thr, Tyr, Ile, Phe, Lys, Cys) in _Chlamydomonas reinhardtii_, as shown by the high positive correlations between the changes in their levels (Figure 5) [153]. Simultaneous increase in free amino acid and GSH levels was observed during the light period compared to the dark in transgenic poplar overexpressing \(\gamma\)GluCys synthetase (rate-limiting in GSH synthesis) in chloroplasts which indicates the participation of the redox system in the mediation of the effect of light on amino acid synthesis [154]. A positive relationship between the amount of free amino acids and light intensity was also observed in wheat seedlings except for Pro, Met, Thr, ornithine, and cystathionine [67]. Light intensity regulates free amino acid concentration at the transcriptional level, as shown by the increased expression of the genes related to Arg, Glu, Asp, and Ser metabolism. Since Arg is the precursor of polyamines, its light-induced accumulation activates their synthesis, as demonstrated with elevated putrescine and spermidine contents at higher PPFDs in leaves of wheat [155]. Polyamines, together with certain free amino acids (Pro, Glu, Cys), may have a role in the adaptation to changes in light conditions through their associations with the redox system. The light-induced increase in free amino acid levels was not accompanied by a change in the soluble protein content, indicating that free amino acid accumulation was not a result of protein degradation in _Arabidopsis_[113]. Figure 5: Light intensity- and spectrum-associated coordination of carbon, nitrate, and sulphate assimilation. The control of the three metabolic pathways is interconnected through the effect of light on the redox system and the HY5 transcription factor. The redox system regulates them through the NAD(P)H/NAD(P)\({}^{+}\) and GSH/GSSG redox couples and HY5 through its binding to the promoter of the genes related to these processes. White, blue, red, and dark red arrows indicate the effect of white, blue, red, and far-red light, respectively. Upwards arrows: induction, downwards arrows: repression, horizontal line: no effect. Continuous lines: direct connection, dashed lines: indirect connections with additional intermediates.