LAUSR

Abstract In maize (Zea mays, L.) genotypic variability in the relationship between total kernel number per plant (KNP) and plant growth rate (PGR) during the period bracketing silking (R1 ± 15 d), and in kernel set efficiency (KNP PGR−1, KSE) have been widely documented. Nitrogen supply (Ns) affects PGR and hence KNP (i.e. indirect effect on KNP), but reports on a direct N effect on KNP PGR−1 are still contradictories. Moreover, recent studies have documented that prolificacy (the number of ears per plant) was cancelled out in N-limited plants despite of their high PGR. Additionally, PGR can be differentially affected by environmental conditions (e.g. sowing dates) through indirect effects on plant size or directly, which could determine changes on KSE. Field experiments were carried-out in Parana (31° 48' S, 60° 32' W), Argentina, during two growing seasons (2014−15 and 2015−16). Two single cross hybrids were sown in two sowing dates (early: September and late: December), with three N fertilizer rates (0, 90, and 270 kg N ha−1) at three plant densities (5, 7 and 9 pl m−2). The effects of plant N status and plant size on KNP, KSE and prolificacy were study for a wide range of PGR and relative PGR (rPGR). Plant N status was estimated by using SPAD measurements on the ear-leaf blade at R1. Ranges of lower, mid and higher PGR (absolute and relative values) and SPAD units were established using 33rd and 66th percentiles. Curvilinear functions were fitted to kernel number at the apical ear (KNE1) vs. PGR and KNP vs. PGR and boundary functions were also fitted to calculate the unexplained variance (i.e. residuals) of these functions, indicative of direct plant N status effects on KSE. Residuals of KNE1 vs. PGR relationship decreased with increases in plant N status up to a threshold value of 46.8 SPAD units (R2 = 0.75, P