Nitrogen Nutrition Index and Leaf:Stem Ratio of Maize Crops in Early and Late Sowings.

Nutritional N status of maize crops can be established by N nutrition index (NNI). This index is estimated from actual N concentration (%Nact) of aerial biomass (B) and the critical N concentration (%Ncrit) which is derived from the N dilution curve, i.e. the function between %Ncrit and B. Actual N concentration depends on N allocated both in metabolic and structural pools, which may be affected by agronomical practices such as genotype, sowing date, plant density and N rate. We investigated the effect of these factors on the NNI at flowering and the leaf:stem ratio (L/S, i.e. a rough estimation of N allocation) of two maize hybrids (DK70-10VT3PRO and DK73-10VT3PRO) cultivated in two sowing dates (early and late), at three plant densities (5, 7 and 9 plants m^-2) and with three N rates (0, 90 and 270 kg N ha^-1) in Paraná (31.8°S), Argentina.  The NNI was affected (p<0.0001) by N rate, plant density and sowing date, whereas L/S ratio was affected by genotype (P<0.007) and sowing date (P<0.0001). The NNI was reduced as plant density was increased whereas the opposite effect occurred when N rate was increased. The L/S ratio was higher in early than in late sowing date, and was significantly increased by N rate in early sowing. The NNI was positively associated with the L/S ratio in early sowing, whereas both variables were negatively associated in late sowing. Moreover, both NNI and L/S ratio were positively associated with grain yield in early sowing but not in late sowing. These results suggest no benefits of late sowing in terms of a higher N allocation in metabolic pool.