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

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 V-max for NH4+-N (28.69-66.51 mu mol g(-1) h(-1)) than for NO3--N uptake (10.24-30.19 mu mol g(-1) h(-1)) and the K-m 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 V-max and K-m. 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 V-max and K-m 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.