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A better understanding of the physiological processes related to nitrogen (N) metabolism in maize (Zea mays L.) inbred lines is important for increasing the efficiency of breeding programs targeting low-N environments. This study analyzed the response to contrasting N availability of morphophysiological traits in a set of 12 maize inbred lines, from different origins (USA and Argentina) and breeding eras (from 1952 onward). Traits included in the analysis were related to canopy structure, light interception, shoot biomass production, and grain yield. Our results indicate that (i) the start of N effects on canopy size was more related to a threshold crop leaf area index (about 2) than to a given leaf stage (i.e., V n ), (ii) the light attenuation coefficient value was not affected by N availability, (iii) variations in kernel number per plant were explained by prolificacy (r 2 5 0.59), and (iv) differences in harvest index were related to kernel number per plant (r 2 5 0.77). The most important finding of our research was the detection in some inbreds of a particular response of kernel number to plant growth rate around silking, different from the general model established for hybrids. In these inbreds an additional effect of N availability was detected as reduced kernel set at a given plant growth rate under N deficient conditions (i.e., reduced reproductive efficiency). This result highlights the need of more research on reproductive sink development in this type of germplasm.

Maize {Zea mays L.) hybrid vigor for plant grain yield (PGY) is associated with heterosis for plant biomass at maturity (aboveground biomass at physiological maturity [BiomasSpJ), kernel number per plant (KNP), and harvest index (HI); however, no evidence of the effects of nitrogen (N) availability or combination of abiotic stresses on heterosis for physiological components of PGY has been reported. The objective of this study was to determine the response of heterosis for ecophysiological traits related to PGY at contrasting N supply levels in a set of six inbred lines and 12 derived hybrids. Field experiments were conducted in five growing seasons at low nitrogen (LN; no N added) and high nitrogen (HN) supply (200 or 400 kg N ha-^) under irrigation and dryland farming. Increased PGY (65% for hybrids and 30% for inbreds) was ascribed to similar increase in BiomasSp^^ as no increase in HI was found. Heterosis for PGY was higher under HN (137%) than LN (87%). A similar response was observed for traits related to light capture and biomass accumulation. Heterosis for HI did not differ between HN (31%) and LN (28%). Heterosis for PGY was associated (p < 0.01) with heterosis for KNP, BiomasSp,^, radiation use efficiency (RUE) during grain filling, HI, and traits related to maximum light capture. Heterosis for PGY at LN was also correlated with heterosis for RUE at critical period (i.e., 30 d bracketing silking) and kernel weight. Under the combined effect of N and drought, PGY heterosis was reduced and more affected at HN (59%) than at LN (70%).