Is early morning flowering an effective trait to minimize heat stress damage during flowering in rice?

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 2.35 h and 5.08 h after dawn compared to 3.05 h and 5.50 h during the WS, while, PSOT varied from 3.32 to 6.27 h in DS and from 3.50 to 7.05 h 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.

Keywords: Early morning flowering; Heat stress; Rice (Oryza sativa); Spikelet sterility.

Fig. 1

Temporal spread in first spikelet opening time (FSOT) among 289 cultivars in any given day under normal (wet season-WS) and hotter climatic (dry season-DS) conditions in Tamil Nadu, India. FSOT and peak spikelet opening time (PSOT), were observed visually from 6.30 to 14.00 (Indian Standard Time, IST) at 15 min interval by focusing on tagged main tiller panicles (for details see materials and methods).

Fig. 2

Genetic variability in flowering traits including first spikelet opening time (FSOT; A) peak spikelet opening time (PSOT; B) and percent spikelet sterility (C) among rice cultivars originated from tropical and sub-tropical regions and tested under normal (wet season-WS) and hotter climate (dry season-DS) conditions in Tamil Nadu, India. Cumulative average of air temperature (AT) and relative humidity (RH, %) from dawn to six hours after dawn on the day flowering pattern was measured (D). Countries in and outside the box on Y axis are tropical and sub-tropical regions of origin of the cultivars, respectively. IR64, a popular cultivar is used as a reference and data presented is the average across all cultivars a country, with response of individual cultivar provided in Supplementary Table 1.

Fig. 3

Time of day of flowering (TDF) in IR64 + qEMF3 and recurent parent (IR64) under field conditions in hot tropical climate. A) To estimate TDF, number of opened spikelets per panicle (five panicles from different plants) were counted at every 15 min interval following Jagadish et al., 2007 procedure. Closed black circles and open circles represent cumulative average number of spikelet’s opened per panicle over five days. B) Effectiveness of early morning flowering in overcoming high temperature inducing spikelet sterility was accounted in DS-2013 and DS-2014 (tropical and subtropical cultivars) and DS-2015 (IR64 + qEMF3 and recurrent parent, IR64). Black circle symbols is the PSOT. Bars indicate the mean ± standard error. Alphabet with the same letter are non-signifcant at p < 0.05.