Nitrogen use efficiency of sugarcane (Saccharum spp.) varieties under in vitro conditions with varied N supply

The nitrogen use efficiency (NUE) and parameters contributing to it, of four sugarcane varieties (NCo376, N36, N12 and N19) were determined in vitro at 4 and 20 mM nitrogen (N), supplied as NO3 (-)-N or NH4 (+)-N. Significant differences amongst varieties were apparent at the lower N supply (4 mM) in the sub-components of NUE (N uptake efficiency and N utilisation efficiency) and overall NUE. All tested varieties had a higher NUE on medium with NO3 (-)-N than NH4 (+)-N. A concentration of 20 mM N was deemed too high to resolve differences amongst genotypes. Evidence for different mechanisms of NUE amongst varieties was provided under 4 mM NO3 (-)-N supply where NCo376 and N12 displayed the highest NUE but this was attributed to superior NUpE in the former and NUtE in the latter. The NUE response of NCo376, N12 and N19 were similar in vitro, while N36 behaved differently. To our knowledge, this is the first report on the contribution of NUpE and NUtE to NUE in sugarcane in vitro. In terms of biomass production, in all cases but one, in vitro plants accumulated more biomass on medium with NO3 (-)-N than NH4 (+)-N. Furthermore, there was an indication that the distribution of NO3 (-)-N between shoots and roots might affect NUE since varieties with a high in vitro NUE stored less NO3 (-)-N in roots (compared with shoots) or exhibited better translocation of NO3 (-)-N from roots to shoots than those with low NUE. In addition, evidence of luxury N uptake even at 4 mM N was obtained.