Influence of Elevated CO2 on kinetics and expression of high affinity nitrate transport systems in wheat

The rate of nitrate uptake was studied in ambient (AC) and elevated (EC, 600 ± 50 μmol mol−1) carbon dioxide conditions in wheat seedlings adapted to low nitrogen (uninduced) or high nitrogen (induced) conditions. Twenty five days old seedlings grown in climate controlled growth chambers were incubated in a range of nitrate concentrations (0.01–1 mM). Rate of uptake observed in low N adapted seedlings was used to calculate kinetics of constitutive high affinity transport system (CHATS). The difference in uptake observed between induced and uninduced seedlings were indicative of inducible high affinity transport system (IHATS). In both the CO2 levels, the nitrate uptake was biphasic in induced as well as uninduced seedlings, i.e. the rate of nitrate uptake saturated at about 0.08–0.1 mM and then a sharp increase in the rate of nitrate uptake was noticed in seedlings incubated in solutions of 0.5 mM nitrate and the uptake increased linearly in both induced as well as uninduced seedlings in the concentration beyond 0.5 mM. Hence, uninduced seedlings growing under EC took up nitrate more efficiently as compared to the seedlings growing under AC suggestive of efficient CHATS in EC grown plants. Growth under EC decreased both the affinity and rate of nitrate uptake in induced seedlings. However, the expression of TaNRT2.1, TaNRT2.2, TaNRT2.3 genes were highly induced by EC when the N supply level was low. Recent evidences suggest the involvement of ECO2-triggred synthesis of nitric oxide (NO) as a mediator of high affinity nitrate transporter activity in plants. The observed negative impact of EC on IHATS might also be an after effect of NO perturbation. EC could modify the inorganic nitrogen uptake by altering the access to or accessibility of nutrients in soil or by altering the kinetics of CHATS.