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)