Kernel setting at the apical and sub-apical ear of older and newer Argentinean maize hybrids

Rainfed maize (Zea mays, L.) crops in temperate semi-arid regions with high inter-annual variation of summer precipitation, are commonly cultivated at low population densities. During seasons with favorable conditions for plant growth (e.g., summer rainfalls above normal records), the number of kernels of sub-apical ear can contribute significantly to total kernel number per plant (TKN). However, there is no information of the determinant traits of kernel setting at sub-apical ear, or the effects of breeding on these traits. We used a crop physiology model with an individual plant approach in attempt to describe genotypic differences in those traits related to kernel setting at the apical (KNE1) and the sub-apical ear (KNE2) of older and newer Argentinean maize hybrids. Four representative hybrids of the decades of 80', 90', 00' and 10' were cultivated during two growing seasons at three densities (4, 8 and 12 pl m(-2)). Non-destructive techniques were used to estimate the growth rate of individual plant (PGR(PC)), apical (E(1)GR(CP)) and sub-apical (E(2)GR(CP)) ear during the period bracketing silking (critical period), biomass partitioning to reproductive sinks (E(1)GR(CP) PGR(CP)(-1) and E(2)GR(CP) PGR(CP)(-1)) and the efficiency to set kernels of the plant (TKN PGR(CP)(-1)) and the ears (KNE1 E(1)GR(CP)(-1) and KNE2 E(2)GR(CP)(-1)). Differences among tested hybrids in KNE1 (ca. 400-600 k ear(-1)) were mainly determined by KNE1 E(1)GR(CP)(-1) (ca. 230-280k d(-1) g(-1)) while newer hybrids had the highest E(1)GR(CP) PGR(CP)(-1) (>0.50). By contrast, hybrids had similar KNE2 (ca. 124 k ear(-1)) due to their similar E(2)GR(CP) PGR(CP)(-1) (ca. 0.08) and KNE2 E(2)GR(CP)(-1) (ca. 93 k d(-1) g(-1)). These results suggest that breeding effect on TKN was mainly determined by KNE1 E(1)GR(CP)(-1) and for the newest hybrid this trait did not interact with plant density. Hence, the newest hybrid exhibited a positive linear response of TKN to PGR(PC) (r(2) = 0.82) reaching the highest TKN among hybrids at low, mid and high density. A better performance of maize crops at low densities could be obtained by breeding for E(2)GR(CP) PGR(CP)(-1). 2016 Published by Elsevier B.V.