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