Abstract (189-words)
An early-morning flowering (EMF) trait is supposed to be effective in
enhancing grain yield due to mitigation of heat-induced spikelet
sterility at flowering in rice. This study evaluated (i) phenotypic
differences between a near-isogenic line carrying a QTL for EMF trait,
designated as IR64+qEMF3 , and a recurrent parent, IR64, under
wide variation in climates and (ii) whether an EMF trait can enhance
grain yield under heat stress at flowering. IR64+qEMF3 had
significant earlier flower opening time (FOT) in diverse environmental
conditions including temperate, subtropical, and tropical
regions. Under normal temperatures
at flowering, IR64+qEMF3 had similar grain yield to IR64 with
some significant changes in agronomic traits and yield components. Field
trials in heat-vulnerable regions of central Myanmar for seven crop
seasons showed that higher percentage of filled grains contributed to
the significantly higher grain yield in IR64+qEMF3 among yield
components when plants were exposed to daily maximum air temperatures
around 36.5 oC or higher. Lower spikelet sterility in
IR64+qEMF3 was attributed to the earlier FOT during cooler early
morning hours. This is the first field study that clearly demonstrates
the enhancement of heat-resilience due to EMF trait at flowering.
Keywords: early-morning flowering; flower opening time; heat-resilience;
percentage of filled grains; rice; yield-related traits
Abbreviations:
EMF; early-morning flowering
FOT; flower opening time
NIL; near-isogenic line
PFG; percentage of filled grains
PN; panicle number per hill
SN; spikelet number per panicle
1000GW; one thousand grain weight
INTRODUCTION
Rice (Oryza sativa L.) is one of the most important staples
feeding half the world’s population (Carriger & Vallee, 2007). While
there is a need to produce more rice in order to feed the increasing
population, challenges for increasing rice production have been rising
due to progressive global warming (IPCC, 2013). Consequently,
development of climate-smart rice cultivars is a pressing issue today.
Genetic improvement of heat-resilience is one of the key requirements
for enhancing global food security to tackle the increasing episodes of
heat stress damage on crop production.
Major cereal crops show extreme sensitivity to heat during anthesis
(Prasad, Bheemanahalli, & Jagadish, 2017). Chamber experiments
conducted previously revealed that high proportion of sterility occurs
in rice when spikelets are exposed to temperatures around 35oC (Matsui, Omasa, & Horie, 1997; Satake & Yoshida,
1978) even for a short period of time (Jagadish, Craufurd, & Wheeler,
2007). Actual increase in spikelet sterility by heat stress at flowering
was reported in hot dry seasons of tropical and subtropical regions
(Ishimaru et al., 2016; Matsushima, Ikewada, Maeda, Honma, & Niki,
1982; Osada, Sasiprapa, Rahong, Dhammanuvong, & Chakrabandhu, 1973).
Rice crop is considered to be already at critical limits of heat stress
vulnerability in central Thailand and Myanmar in dry season (Wassmann et
al., 2009). Prediction models have identified high risk areas for
reduction in rice grain yield by global warming in many rice-growing
regions of Asia, mostly due to heat-induced spikelet sterility (See
review, Horie, 2019). Central Myanmar has been projected as one among
the heat-vulnerable regions for spikelet sterility and yield reduction
in the future (Horie, 2019; Wassmann et al., 2009).
The EMF trait is proposed to be effective in escaping heat-induced
spikelet sterility at flowering by shifting FOT to the cooler early
morning times (Satake & Yoshida, 1978). Flower opening of modern
cultivars predominantly occurs between 3.5–5.5 hours after dawn in the
natural conditions of tropics (Bheemanahalli et al., 2017; Hirabayashi
et al., 2015). Under elevated high temperature greenhouse conditions,
spikelets that flower at later hours tend to be sterile with higher
proportion; hence, shifting flower opening time to the cooler early
morning, even a one-hour advancement, is effective in mitigating
heat-induced spikelet sterility at flowering (Ishimaru et al., 2010). We
previously developed a near-isogenic line (NIL) carrying a QTL for EMF
(qEMF3 ) derived from a wild rice, O. officinalis , in the
genetic backgrounds of IR64 (Hirabayashi et al., 2015). The NIL was
designated as IR64+qEMF3 .qEMF3 advanced FOT by approximately 1.5–2.0 h in the tropical
Philippines during both wet and dry seasons (Hirabayashi et al., 2015).
Another field experiment also showed earlier FOT and lower sterility in
IR64+qEMF3 compared to a genetically diverse panel of germplasm
in the dry rice growing season in Tamil Nadu, India (Bheemanahalli et
al., 2017). However, whether the EMF trait has yield advantage under
different heat stress levels on the field during flowering is yet to be
ascertained. Whether qEMF3 itself influences grain yield and
other phenotypes of IR64 under normal temperatures during flowering
stage is still not clear.
This study consists of two major objectives: the first is to evaluate
advancement of FOT, agronomic traits, and yield performance of
IR64+qEMF3 in a wide range of field environments, including
temperate, subtropical, and tropical regions, under normal rice-growing
temperatures at flowering, and the second objective is to investigate if
the EMF trait enhances heat-resilience at flowering in the field of
heat-vulnerable region of central Myanmar.
MATERIALS AND METHODS