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greater kernel growth rate. Kernel growth rate has been shown to be closely associated with
the potential kernel weight. However, the ear demand was increased to a greater extent than
the source (i.e. plant growth during the grain filling period), and stresses during the grain
filling period reduced kernel weight of newer hybrids more than that of the older hybrids.
Thus, future breeding efforts for yield improvement would need to focus also on an increase
in source capacity during the grain filling period. Kernel chemical quality was also modified
with the year of the hybrid release; as such, protein concentration in kernels was lower in
newer hybrids at moderate soil N availability. This change was attributed to both, the
genotypes used in the selection programs as well as the increased carbon fluxes to the
kernels without proportional increments in the flux of nitrogen. When N luxury
consumption occurred, protein concentration in kernels was similar among hybrids released
in different decades. Oil concentration also decreased in newer hybrids released in 1993.
Resource use efficiency increments, rather than greater resource capture, concomitantly
increased with grain yield of Argentinean maize hybrids released in different decades. In
non-limiting environments, radiation use efficiency for grain production (i.e. grain yield per
unit of intercepted radiation) was a consistently increased mechanism contributing to
explain the greater grain yield of newer maize hybrids. This was associated mainly with the
greater partitioning of assimilates to the ear and/or a greater potential kernel number per ear
that allowed for an increased harvest index. An improved light distribution within the
canopy was not evident. In soil N-limited environments, greater yield of newer maize
hybrids were associated with greater nitrogen use efficiency for grain production; which
was largely explained by a greater nitrogen internal efficiency (i.e. the ratio of grain yield to
whole plant N uptake at physiological maturity). Similarly, in water limited environments,
water use efficiency for grain production was greater in a newer than in an older maize
hybrid. The lower thresholds PGRs for kernel set in newer compared with older maize
hybrids might have resulted in a lower frequency of barren plants or with low number of
kernels and thus in a greater kernel number at low N or low water availability. Resource
capture was not consistently increased with the year of the hybrid release indicating that
stressful conditions are not more frequent in current maize hybrids than before.
Author details
Laura Echarte, Lujan Nagore, Javier Di Matteo and Mariana Robles
Research Council of Argentina (CONICET), Argentina
Laura Echarte, Matías Cambareri and Aída Della Maggiora
INTA Balcarce - Facultad de Ciencias Agrarias, Universidad Nacional de Mar del Plata. CC 276,
7620 Balcarce, Argentina
Acknowledgement
This work was supported by the Research Council of Argentina (CONICET), Agencia
Nacional de Promoción Científica y Tecnológica (ANPCyT) and INTA.
