Abstract
Higher spikelet sterility due to heat stress exposure during flowering in rice is becoming a major threat for sustaining productivity in tropical and sub-tropical regions. Therefore, exploiting and incorporating early morning flowering (EMF) trait into ongoing breeding pipelines could be an effective strategy to minimize the damage. In this study, we have focused on quantifying the time of day of flowering traits such as first spikelet opening time (FSOT) and peak spikelet opening time (PSOT) in a diverse set of cultivars (n=289) representing major rice growing regions (13 tropical and 20 subtropical countries) over three years (wet season; WS-2012, dry season; DS-2013 and 2014), under field conditions. EMF traits (FSOT and PSOT) and spikelet sterility displayed significant (p<0.001) variations among cultivars, both within and between seasons (WS and DS). Averaged across two dry seasons, the FSOT ranged between 2.35h and 5.08h after dawn compared to 3.05h and 5.50h during the WS, while, PSOT varied from 3.32 to 6.27h in DS and from 3.50 to 7.05h in WS. On average, PSOT was strongly associated with FSOT both in WS (R2=0.78) and DS (R2=0.77). A near-isogenic line (IR64+qEMF3) effectively minimized the spikelet sterility by 71% during dry seasons under field conditions compared to 289 tropical and subtropical cultivars. None of the tropical and subtropical originated cultivars possess EMF trait including the popular IR64, thus indicating the usefulness of incorporating this trait to reduce heat stress damage under hotter climate. Our findings provide evidence for the effectiveness of the EMF trait in overcoming heat stress induced sterility under field conditions. Hence, it sounds logical to introgress EMF trait into currently growing popular rice cultivars for improving their resilience to heat stress episodes coinciding with flowering.
Highlights
Rice is extremely sensitive to short duration heat stress episodes (>35 ◦C for ≥1 h) coinciding with the reproductive stage, anthesis (Prasad et al, 2006; Jagadish et al, 2007, 2008, 2010; Sathishraj et al, 2015)
These results provide first evidence of the effectiveness and stable expression of the early morning flowering (EMF) trait in maintaining spikelet fertility across different rice growing environments under field conditions, highlighting the need to introgress the trait for mitigating heat stress induced spikelet sterility
This is the first attempt to record key flowering pattern traits involving 289 cultivars originating from 33 countries (13 tropical and 20 subtropical) over three years under fully irrigated field conditions (Supplementary Fig. S1 and Fig. 2)
Summary
Rice is extremely sensitive to short duration heat stress episodes (>35 ◦C for ≥1 h) coinciding with the reproductive stage, anthesis (Prasad et al, 2006; Jagadish et al, 2007, 2008, 2010; Sathishraj et al, 2015). With greater certainty indicated increased frequency of hotter days with temperatures above known critical threshold (>33 ◦C; Jagadish et al, 2007) to coincide with the flowering stage in major rice growing regions (Wassmann et al, 2009; Gourdji et al, 2013; Teixeira et al, 2013; Bheemanahalli et al, 2016). Rice plants possess different mechanisms such as true tolerance (Jagadish et al, 2010) and avoidance through transpiration cooling (Julia and Dingkuhn, 2013) to overcome heat stress inducing damage under field conditions. The time of day of flowering (Julia and Dingkuhn, 2012) or the early morning flowering (EMF) trait has been demonstrated to significantly reduce heat stress damage by employing an escaping mechanism (Ishimaru et al, 2010; Hirabayashi et al, 2014). During early 1990s, randomly selected rice accessions representing different latitudes and altitudes showed significant variation in start of time of flowering, ranging between 0530 h (Oryza. eichingeri) to 2300 h (O. alta)
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