Comparative uptake and assimilation of nitrate by excised roots and leaves from seedlings of C₃ (barley) and C₄ (corn) plants: Effect of light, ambient CO₂ and exogenously supplied sucrose

Effect of light, ambient CO₂ and exogenously supplied sucrose on in vivo NO₃⁻ assimilation was studied in excised roots and leaves from 9-day-old C₃ (barley) and C₄ (corn) seedlings. The seedlings used were uninduced for NO₃⁻ uptake and assimilation (i.e. had never seen nitrogen during germination and growth) and were exposed to continuous light for three days to avoid any diurnal variation, followed by 24 h dark pre-treatment to make the seedlings carbohydrate deficient. Excised roots of such carbohydrate deficient barley seedlings took up 5 times higher NO₃⁻ than that of corn and assimilated 7 μmol g⁻¹fw 6 h⁻¹ as compared to 2 μmol g⁻¹fw 6 h⁻¹ assimilated by corn roots. Addition of 1% sucrose in the uptake medium increased NO₃⁻ uptake by 140% and in vivo assimilation by 203% in corn roots. The corresponding increase in barley roots were 26% in uptake and 73% in assimilation. In absolute terms, increase in uptake in response to sucrose feeding was more than increase in assimilation in both corn and barley roots.

Excised leaves of shuch barley seedlings failed to show significant in vivo NO₃⁻ assimilation in darkness while corn leaves assimilated 10 μmol g⁻¹fw 24h⁻¹ constituting about 18% of uptake. Addition of 1% sucrose increased the NO₃⁻ assimilation to 36 μmol g⁻¹fw 24h⁻¹ constituting 90% of uptake in corn and to 7 μmol g⁻¹fw 24h⁻¹ constituting 22% of uptake in barley. In light and absence of CO₂ also, barley leaves failed to show significant assimilation and corn leaves assimilated 15 μmol g⁻¹fw constituting about 15% of uptake. Addition of 1% sucrose under these conditions increased the assimilation to 57% of uptake in corn and 41% of uptake in barley. The data presented here and the discussion provide evidence that (i) Light per se is not obligatory for NO₃⁻ assimilation and that the carbohydrate supply may mimic light. (ii) Limiting factor for lower NO₃⁻ uptake by the roots during darkness may not be low assimilation/translocation but lower availability of carbohydrates to the roots. (iii) C₃ and C₄ plants behave differently in terms of efficiency of NO₃⁻ uptake and assimilation. While roots of C₃ plants are efficient in NO₃⁻ uptake, C₄ leaf metabolism appears to have a built-in control that maintains an efficient assimilation of NO₃⁻ at all times, light as well as darkness.