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.