The effects of various light intensities and spectral compositions on glutathione and amino acid metabolism were compared in wheat. Increase of light intensity (low—normal—high) was accompanied by a simultaneous increase in the shoot fresh weight, photosynthetic activity and glutathione content. These parameters were also affected by the modification of the ratios of blue, red and far-red components (referred to as blue, pink and far-red lights) compared to normal white light. The photosynthetic activity and the glutathione content decreased to 50% and the percentage of glutathione disulfide (characterising the redox state of the tissues) in the total glutathione pool doubled in far-red light. The alterations in the level and redox state of the antioxidant glutathione resulted from the effect of light on its synthesis as it could be concluded from the changes in the transcription of the related genes. Modification of the light conditions also greatly affected both the amount and the ratio of free amino acids. The total free amino acid content was greatly induced by the increase of light intensity and was greatly reduced in pink light compared to the normal intensity white light. The concentrations of most amino acids were similarly affected by the light conditions as described for the total free amino acid content but Pro, Met, Thr, ornithine and cystathionine showed unique response to light. As observed for the amino acid levels, the expression of several genes involved in their metabolism also enhanced due to increased light intensity. Interestingly, the modification of the spectrum greatly inhibited the expression of most of these genes. Correlation analysis of the investigated parameters indicates that changes in the light conditions may affect growth through the adjustment of photosynthesis and the glutathione-dependent redox state of the tissues. This process modifies the metabolism of glutathione and amino acids at transcriptional level.

Light-intensity-dependent (low, normal and high) differences in thiol and amino acid metabolism during drought were compared in wheat seedlings. The drought-tolerant genotype (Plainsman) recovered better than the sensitive one (Cappelle Desprez) after the stress as shown by growth and photosynthetic parameters, the levels of which were greater in low and high light, respectively. Glutathione as an antioxidant contributed to this difference, since its level was twofold greater in Plainsman throughout the experiment. In addition, the accumulation of most amino acids even increased in normal light during drought in Plainsman, while such change occurred in Cappelle Desprez only in high light. The higher contents of proline, glutamate and γ-aminobutyrate are especially important because of their involvement in the protection against drought.The transcription of certain genes related to amino acid and glutathione metabolism and various antioxidants was even induced by higher light intensities before drought, which can contribute to the subsequent increase in the amount of the corresponding metabolites during stress. Increase in light intensity activated various protective mechanisms including greater accumulation of glutathione, proline and other amino acids during drought, which contributed to the efficient recovery of wheat after stress. K E Y W O R D S drought, free amino acids, glutathione, light intensity, photosynthesis, wheatThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The proposed spectral control of the adaptive metabolic responses to drought was tested by the comparison of two wheat genotypes with different stress tolerance. White light was used as the reference to see the effects of the three altered ratios of blue, red, and far-red spectral components, which conditions are referred to as blue, pink, and far-red lights. Under all spectral conditions, drought greatly reduced the growth of both genotypes. However, the glutathione content decreased and the proline level increased, independently of the spectrum, only in the sensitive genotype. Far-red light greatly decreased the amount of cystine and glutathione disulphide during the stress which resulted in their lower ratios compared to the reduced forms only in the tolerant genotype. The maintained more reducing redox environment contributes to its better stress tolerance. In far-red light, drought induced a greater accumulation of several free amino acids (mHis, Val, Ile, Leu, Asn, His, Tyr, Lys, Arg) in the tolerant genotype, while a smaller one in the sensitive genotype compared to the other spectral conditions. The transcript level of the genes related to amino acid and glutathione metabolism was also different between the two genotypes under this condition. The present results indicate the adaptive adjustment of glutathione and amino acid levels by far-red light during drought which observation can serve as a basis for the spectrum-dependent modification of the protective metabolites (glutathione, proline) of crops to reduce the stress-induced damages.

Spectral quality, intensity and period of light modify many regulatory and stress signaling pathways in plants. Both nitrate and sulfate assimilations must be synchronized with photosynthesis, which ensures energy and reductants for these pathways. However, photosynthesis is also a source of reactive oxygen species, whose levels are controlled by glutathione and other antioxidants. In this study, we investigated the effect of supplemental far-red (735 nm) and blue (450 nm) lights on the diurnal expression of the genes related to photoreceptors, the circadian clock, nitrate reduction, glutathione metabolism and various antioxidants in barley. The maximum expression of the investigated four photoreceptor and three clock-associated genes during the light period was followed by the peaking of the transcripts of the three redox-responsive transcription factors during the dark phase, while most of the nitrate and sulfate reduction, glutathione metabolism and antioxidant-enzyme-related genes exhibited high expression during light exposure in plants grown in light/dark cycles for two days. These oscillations changed or disappeared in constant white light during the subsequent two days. Supplemental far-red light induced the activation of most of the studied genes, while supplemental blue light did not affect or inhibited them during light/dark cycles. However, in constant light, several genes exhibited greater expression in blue light than in white and far-red lights. Based on a correlation analysis of the gene expression data, we propose a major role of far-red light in the coordinated transcriptional adjustment of nitrate reduction, glutathione metabolism and antioxidant enzymes to changes of the light spectrum.