Chemistry Reference
In-Depth Information
Argentinean maize hybrids released between 1980 and 2004 has been associated with
increased water use efficiency for grain production and not with water uptake, which has
remained relatively stable [98]. This is in contrast with previous reports suggesting that
water capture increased with the year of the hybrid release in US hybrids [99]. However, the
consistently increased total shoot biomass with the year of the hybrid release in US hybrids
and not in Argentinean maize hybrids may contribute to explain discrepancies between
works. Although seasonal water uptake was similar among Argentinean maize hybrids
released in different decades, soil water uptake during the critical period for kernel set was
greater in newer than in older maize hybrids when soil available water was low [100]. In
agreement, a modern Canadian hybrid was able to maintain higher leaf photosynthesis and
transpiration during short periods of low water availability at silking than an older hybrid
in a greenhouse study [55]. Water use efficiency for grain production was consistently
higher in a newer than in an older Argentinean maize hybrid, and differences were greater
at low water availability [101].
2. Conclusions
Greater grain yield of newer Argentinean maize hybrids was mainly related to an increased
harvest index; whereas shoot biomass did not consistently increased with the year of the
hybrid release. Kernel number was the main yield numerical component contributing to
explain grain yield increments. Processes influencing kernel number determination in
hybrids released in different decades were analyzed using as a framework the relationship
between kernel number per plant (KNP) and plant growth rate during the critical period for
kernel set (PGRs); and it was evident that features of the relationship were changed through
the years. As such, threshold PGRs for kernel set was lower and maximum kernel number
per plant was higher in newer than in older hybrids. The lower threshold PGRs for kernel
set contributed to explain the greater tolerance of newer hybrids to high plant densities, and
it probably contributed to a greater tolerance to other stresses like low water availability or
low soil N. The lower threshold PGRs for kernel set was associated with a greater assimilate
partitioning to the ear at low resource availability per plant; which was probably related to a
lower apical dominance in newer than in older maize hybrids. The higher maximum kernel
number per plant at high resource availability was associated with morphogenetic changes
leading to a greater potential kernel number per ear; whereas prolificacy was not
consistently improved. This response of kernel number to an increased resource availability
contributed to explain the greater grain yield of newer hybrids at low plant densities. As
such, harvest index of newer maize hybrids was not only greater but it was also more stable
at different resource availability than that of older maize hybrids.
Kernel weight did not show a clear trend with the year of the hybrid release; but it was
evident that kernel weight of newer hybrids was more susceptible to stresses during the
grain filling period than that of the older hybrids. Kernel weight response to resource
availability during the grain filling period was analyzed in terms of the source-sink ratio.
The sink or the ear demand for assimilates during the grain filling period was greatly
increased in newer maize hybrids; as a result of either a greater kernel number and/or a
