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thresholds of plant growth rate for kernel set were recently suggested as a phenotyping trait
in breeding programs (39). However, the individual plant methodology [29; 10] seems more
suitable for a reliable estimation of PGRs thresholds for kernel set than the mean PGRs per
plot calculated in other works [18; 40]. At high resource availability per plant, the greater
potential kernel number in the topmost ear contributed to a high KNP [9; 10]. Although
differences were found among hybrids, there was not a clear trend with the year of hybrid
release in threshold PGRs for prolificacy, nor in percentage of prolific plants beyond that
threshold [9]. Also, no significant changes in ears per plant for US maize genotypes released
between 1930 and 1980 were evident [32]. However, an increase in prolificacy with the year
of hybrid release was reported in other works [3; 18].
1600
1600
Low plant
density
Intermediate
plant density
High plant
density
Total KN of
prolific plants
Older
hybrid
(1965)
Newer
hybrid
(1993)
1200
1200
800
800
400
400
0
0
0369 2
0369 2
Plant growth rate (g d
-1
)
Figure 3.
Relationship between kernel number per uppermost ear or per plant and plant growth rate
during a period bracketing silking (PGRs) in an older (DKF880) and a newer (DK752) maize hybrid
released in Argentina in different decades (year of release between brackets). Triangles represent kernel
number of prolific plants (kernel number of the topmost plus the second ear). Other symbols represent
KN of the topmost ear at low (2-4 plants m
-2
; solid circles); intermediate (8 plants m
-2
; squares), and high
plant densities (16-30 plants m
-2
; white circles). Adapted from Echarte et al. (10).
A greater dry matter partitioning to the ear (i.e. ear growth rate per unit PGRs) and/or a
greater grain efficiency factor (i.e. kernel set per unit of ear growth rate during the critical
period for kernel set) are physiological processes contributing to a greater KNP per unit
PGRs (41; Figure 1). It has been stated that kernel set improvements with the year of the
hybrid release were attributable to (i) increased partitioning of dry matter to the ear during
the critical period for kernel set at low and intermediate resource availability per plant; and
to (ii) greater kernel set per unit of ear growth rate at high resource availability per plant (10;
Figure 4). Previous works have shown dry matter partitioning to the ear increments as a
result of a reduction in tassel size or tassel removal [24; 42; 43]. Greater dry matter
partitioning to the ear in newer compared with older maize hybrids is in agreement with the
declined tassel size of US hybrids from the 1930s to the 1990s [15]. Tassel branch number
and dry weight were reduced over the years in US hybrids [2; 3]. At high resource
availability, the greater kernel set per unit ear growth rate was mainly attributable to the
greater potential kernel number per ear [10]. Other processes contributing to elucidate
differences among hybrids in grain efficiency factor, like a lower assimilate requirement per
