4.2 qEMF3 enhances heat-resilience at flowering by stabilizing percentage of filled grains under hot field conditions
Central Myanmar is projected as one of the heat-vulnerable regions for spikelet sterility and yield reduction in the future (Horie, 2019; Wassmann et al., 2009). By employing staggered sowings in different dry seasons, wide range of average daily maximum temperatures during heading was obtained (Table 5). IR64+qEMF3 was tested under such field conditions to examine if an EMF trait could mitigate the yield loss under heat stress at flowering.
PFG was similar between genotypes or slightly higher in IR64+qEMF3 under normal temperature at flowering (Table 4; Table S3), whereas a total of seven trials in the central Myanmar revealed that PFG greatly varied depending on the seasons (Table 5). Genetic and environmental interaction were not observed in other yield components such as PN, SN, and 1000GW (Table 5). A clear difference in response of PFG to average daily maximum temperature at heading between genotypes was observed (Figure 2). PFG steadily dropped in IR64 as average daily maximum temperature at heading increased above 36.5 oC (Figure 2). On the other hand, high PFG of greater than 80% was maintained in IR64+qEMF3 even when average daily maximum temperature at heading was around 38 oC (Figure 2). Among yield components, PFG was the only parameter that significantly contributed to the yield advantage of IR64+qEMF3 (Figure 1). Flowering pattern observation revealed less frequency of spikelet sterility in IR64+qEMF3 than in IR64 on a single day due to the earlier FOT in IR64+qEMF3 (Figure 3a, 3b). These results indicate the yield advantage that qEMF3 conferred under high temperature conditions at heading by significant advancement of FOT in IR64 genetic background. Notably, an approximately one-hour difference in FOT50 and FOT 90 in Crop 1 of 2018 (Table S2) made a difference in spikelet sterility between IR64 and IR64+qEMF3 (Figure 3b), supporting that one-hour advancement of FOT is sufficient to avoid heat-induced spikelet sterility as shown in previous chamber experiments (Ishimaru et al. 2010; Satake & Yoshida 1978).
Horie (2019) documented that EMF is one of the key traits that could retain rice grain yield by mitigating heat-induced spikelet sterility during flowering under future hotter temperatures in inland of continental South East Asia. Our study clearly demonstrated that EMF trait could diminish heat stress damage at flowering on grain yield through stabilization of PFG in the hot dry season of central Myanmar. IR64 is a moderately heat-tolerant cultivar (Shi, Ishimaru, Gannaban, Oane, & Jagadish, 2015). Some of Myanmar cultivars could be reaching critical limits of heat tolerance as described by Wassmann et al., (2009). Assessment of heat tolerance among Myanmar local cultivars helps to identify the target cultivars to be conferred with EMF trait. Heat-vulnerable regions, in terms of heat-induced spikelet sterility, are estimated to be located in different geographic locations including China, South-East Asia, South Asia, and West Africa (Laborte et al., 2012). Since IR64+qEMF3 can significantly advance FOT in diverse environmental conditions (Table 2; Table S2), qEMF3 may potentially minimize reduction in heat-induced spikelet sterility across regions. It should be noted, however, microclimate including humidity (Matsui et al., 2014; Tian et al., 2011), wind velocity Ishimaru, Hirabayashi, Kuwagata, Ogawa, & Kondo, 2012; Matsui et al., 1997), and solar radiation (Ishimaru et al., 2016), which are not inclusive in analysis in the present study, have a complex interaction effect on spikelet sterility under field heat stress through changes in panicle temperature (Yoshimoto et al., 2011). Whether IR64+qEMF3 can mitigate yield loss in other heat-vulnerable regions with different microclimate needs to be further tested.