Inorganic nitrogen uptake kinetics of sugarcane (Saccharum spp.) varieties under in vitro conditions with varying N supply

Abstract

An in vitro system was established for the characterisation of inorganic nitrogen uptake by sugarcane plantlets of variety NCo376. After multiplication and rooting, plantlets (0.27–0.3 g fresh mass) were placed on N-free medium for 4 days, and then supplied with 2–20 mM N as NO3 −-N only, NH4 +-N only or NO3 −-N + NH4 +-N (as 1:1). With few exceptions, on all the tested N media, the in vitro plants always had a higher Vmax for NH4 +-N (28.69–66.51 μmol g−1 h−1) than for NO3 −-N uptake (10.24–30.19 μmol g−1 h−1) and the Km indicated a higher affinity for NO3 −-N (0.02–7.38 mM) than for NH4 +-N (0.06–9.15 mM). When N was applied as 4 and 20 mM to varieties N12, N19 and N36, the interaction between variety, N form and concentration resulted in differences in the Vmax and Km. The high N-use efficient varieties (N12 and N19), as determined in previous pot and field trials, behaved similarly under all tested conditions and displayed a lower Vmax and Km than the low N-use efficient ones (NCo376 and N36). Based on this finding, it was suggested that the N-use efficient designation (from pot and field trials) may not be ascribed solely to N uptake. Assessment of the relative preference index (RPI) for NO3 −-N and NH4 +-N uptake revealed that, at present, the RPI has no application in sugarcane due to its preferential uptake of NH4 +-N.