4.1 Different responses of Te, Tr, and Te×Tr maize to high
temperature around flowering and early grain filling
Temperate (Te), Tropical (Tr), and Te×Tr (TeTr) maize inbred lines
responded differently to high temperature around flowering and
KDD during the entire growth
season, with Tr maize producing higher kernel number and kernel weight
(Figure 1 and 5), supporting previous evidences that Tr maize had an
enhanced tolerance to heating (Giauffret, Lothrop, Dorvillez, Gouesnard,
& Derieux, 2000; Suwa et al., 2010; Rattalino Edreira et al., 2011,
2014; Rattalino Edreira & Otegui, 2012, 2013). The higher heat
tolerance of Tr maize is related to the higher stable flowering pattern
in response to high temperature than Te and TeTr maize (Figure 6). A
higher biomass production and a faster plant growth rate during the
period bracketing silking can also explain the high heat tolerance of Tr
maize than Te maize (Rattalino Edreira & Otegui, 2012; Cerrudo,
Hernandez, Tollenaar, Vega, & Echarte, 2020). In addition, tropical
maize has a large tassel and heavy husks (Campos & Caligari, 2017); the
large tassel can contribute more pollens and the heavy husks protect
kernel formation and growth from biotic and abiotic stresses
(Uribelarrea, Carcova, Otegui, & Westgate, 2002; Banziger, Setimela,
Hodson, & Vivek, 2006). Hence, tropical germplasm was incorporated into
inbred lines to increase yield and improve tolerance to abiotic stresses
in maize (Lewis & Goodman, 2003; Goodman, 2004). Maize hybrids that are
developed from crossing tropical with temperate inbred lines are able to
produce high yield stably and adapt to broad environments (Mushayi et
al., 2020).
Despite Tr maize lines overall showed a lower kernel number reduction
(KNR) based on comparison between two sowing dates, KNR varied largely
for Tr lines in both experimental years (Figure 3), indicating some Tr
lines with higher KNR values are sensitive to high temperature. In
contrast, there are also some Te and TeTr lines that have low KNR
values, exhibiting a higher tolerance to heat stress. These results
suggest that it is unreliable to determine heat tolerance of a given
maize line or hybrid only based on genetic background. Instead,
flowering traits of Tr, Te, and TeTr lines all respond strongly to high
temperature (Figure 6, Rattalino Edreira et al., 2011; Lizaso et al.,
2018; Wang et al., 2019). Hence, flowering – time plasticity (i.e.,
pollen shedding, silking, and ASI) should be indexes that are more
precise for evaluating heat tolerance of maize lines or hybrids.
Flowering – time plasticity has already been used to evaluate the
adaption of many plant species to abiotic stresses and environmental
changes (Richardson, Chaney, Shaw, & Still, 2017; Schmid, Stocklin,
Hamann, & Kesselring, 2017)