Effects of nitrogen and/or sulphur deprivation on the regulation of photosynthesis in barley seedlings.

The present study investigates the effects of nitrogen and/or sulphur deprivation on photosynthetic apparatus in barley seedlings grown in hydroponics culture. For this reason four different growth conditions were induced at 100 PFD and 25°C: Control, -N, -S, -N-S and simultaneous measurements of gas exchange and chlorophyll a fluorescence were performed as well as total protein content, glutathione and cysteine levels in leaves were determined. No difference was observed in AN, qP and ETR/AN among -N, -S and control (C) plants, differently from NPQ that increased significantly in –S compared to C and –N. In -N-S plants a reduction of AN and qP as well as a significant increase of NPQ and ETR/AN were found compared to other plant groups; moreover high levels of glutathione and cysteine were detected. Among the different growth conditions,–N and –N-S plants, exhibited the lowest total protein content Nevertheless, despite the strong AN decline in –N–S condition, the maximum photochemical PSII efficiency (Fv/Fm) resulted the same for all plant groups. Data indicate that the simultaneous starvation of N and S leads to an alteration of plant metabolism that affects photosynthesis. However, since no difference in maximum photochemical efficiency was found, it may be hypothesized that in –N–S seedlings processes other than photosynthesis and thermal dissipation are effective in photoprotection at low light too. In -N-S plants, the highest glutathione content could represent a mean to face oxidative stress triggered within photosynthetic membranes by carbon fixation decline.