• nitrate
• ammonium
• arabidopsis
• NR
• light
• sugar
• nitrate reductase
• nitrogen
• C/N

Carbon (C) and nitrogen (N), among many others, are essential elements for plant metabolism and growth. The environment conditions such as CO2, light availability, diurnal cycles, seasonal effect and biotic or abiotic stresses and many others, modulate the availability of carbon and nitrogen. So, the crosstalk named as ‘C/N balance’ is central for the regulation of plant growth and development. From previous experiments it is known that stressful C/N condition affects a large amount of aspects of plant metabolism. In the work from Huarancca Reyes et al., 2016, several physiological aspects resulted affected by unbalanced C/N ratio. Light dissipation was different under C/N stressful condition depending on the timing if compared with time zero. Change of C/N balance could also modulate the phosphorylation status of many enzymes which may modify their activity, subcellular localization, stability or even signal transduction. In addition, also total soluble sugar level was modulated depending on change of C/N balance.
Since differences in C/N affect these parameters, in this project we checked if different sources of N had different effects when stressful condition was due to an unbalanced C/N ratio. In the previous work NH4NO3 was used as N source, and measurement of fluorescence, soluble sugar and enzymes activity were conducted on seedlings after 0,5; 2 and 24 hours after transfer in C/N medium. Here, since plant can assimilate N through different sources, NO3- and NH4+ were separately provided in C/N medium analyzing the same time course. In this work, we investigated on light energy dissipation, total soluble sugars level and Nitrate Reductase (NR) activity under stressful C/N condition (200 mM glucose/0,3 mM NO3- or NH4+) compared with control condition (100 mM glucose/30 mM NO3- or NH4+).
We found that treated plants accumulated more total soluble sugars (TSS) if compared with control and, in the long period (after 24 hours), TSS was higher with NH4+ in the medium. Light energy dissipation in control plants increased, while in treated plants decreased with no significant differences between the two sources of nitrogen, except at 30 minutes after treatment where in control condition dissipation was higher, but then returned similar.
Nitrate reductase actual activity was the result of an interaction between activity, activation state and protein level. This activity decreased starting from time zero in both treated and control samples, even if NH4+ as nitrogen source in control medium seemed to delay the reduction of NR total activity; by contrast, NR total activity showed a peak at 2 hours after treatment with NO3- , and at 30 minutes with NH4+.
In conclusion, our data shows that low concentration of N affects NR that remains active to work as more as possible to use the low N available, but in different time depending on the source of N. When nitrate is applied, NR actual activity is rapidly decreased, but after 2 h of treatment it has a peak due to its low availability. By contrast, with ammonium in the medium the actual activity of NR is not affected, since nitrate is missing. NR is a dissipation channel of energy in excess derived from photosynthesis, therefore its activity when nitrate is missing could affect photosynthesis when N is low in the medium.