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in kernel number at low resource availability per plant [29]; which might reflect a strong
apical dominance [24; 30]. Using contrasting plant densities along with individuals instead
of plot means provide a wide range of values for PGRs and KNP; and it is possible to obtain
more precise estimations of the threshold PGRs for kernel set [28; 29]. Allometric models are
fitted to the relationship between shoot biomass and morphometric measurements (i.e. stem
diameter, ear length, ear diameter) and are used to estimate the growth during the critical
period for kernel set of individuals that remains in the field from sowing to physiological
maturity (i.e. individual plant methodology, 29). The regression between estimated shoot
biomass using allometric models and the actual shoot biomass of plants before silking is
depicted in Figure 2 and it shows an example of the reliability of the individual plant
methodology.
180
120
60
older hybrid
newer hybrid
1:1 line
0
0
60
120
180
Observed biomass (g pl
-1
)
Figure 2.
Relationship between estimated and actual shoot biomass at the beginning of the critical
period for kernel set, for an older (DKF880) and a newer (DK752) maize hybrid. Shoot biomass was
estimated using allometric models. The dotted line shows the 1:1 ratio and the solid lines show the
fitted model for the older (gray) and the newer (black) maize hybrids. Fitted linear equations were y =
0.89 x + 7.1, R
2
= 0.87, n=71 for the older hybrid, and y = 0.94 x + 4.2, R
2
= 0.91, n=72 for the newer hybrid
(Adapted from Echarte et al. (10)).
A comparison of the KNP-PGRs relationship among 5 Argentinean hybrids released
between 1965 and 1993 established that newer hybrids set more kernels per unit PGRs than
older hybrids as was indicated by (i) the lower threshold PGRs for kernel set and (ii) the
greater potential kernel number at high availability of resources per plant, for newer than
for older hybrids (10; Figure 3). Plant growth rate during the critical period for kernel set at
each plant density did not show a clear trend with the year of release. The lower threshold
PGRs for kernel set contributed to reduce the number of sterile plants in modern than in
older maize hybrids and thus to a higher kernel number per plant as resource availability
per plant decreases. Other authors also found less % of barren plants in newer than in older
hybrids [31; 32; 18]. The lower threshold PGRs for kernel set could have probably resulted
from indirect selection of genotypes under progressively higher plant densities and from a
wide testing area that includes low-yield environments [33-38]. The determination of the
