Popular Rice (Oryza sativa L.) Cultivars Show Contrasting Responses to Heat Stress at Gametogenesis and Anthesis

Abstract

Studies on heat tolerance in rice (Oryza sativa L.) have focused on a range of accessions that are currently not grown or cultivated in small localized geographic regions, while popular rice cultivars occupying millions of hectares have not been tested for their response to heat stress. Hence, 18 popular rice cultivars grown across tropical and subtropical regions were subjected to 38°C for 6 h of heat stress at anthesis and five selected contrasting entries were exposed to six consecutive days (6 h each day) of stress coinciding with both gametogenesis and anthesis. Among the tested entries, Ciherang was the most tolerant, after both 6 h at anthesis and 6 d of stress across both gametogenesis and anthesis, whereas Moroberekan and KDML‐105 were the most susceptible. Using a quick and easy phenotyping protocol and by considering all the sterile spikelets across target panicles, the crucial physiological window “spikelets with exserted anthers but with no ovule enlargement” was identified as the most sensitive stage with exposure to heat stress at both critical reproductive stages. Geographic origin of rice varieties was not clearly related to the degree of tolerance or susceptibility to heat stress because both conditions were observed in cultivars from each target region. Our findings complement other ongoing efforts that are using heat‐tolerant landraces and wild rice as donors to further increase tolerance to sustain global rice production under future warmer climate.

Breeding efforts to mitigate damage by heat stress to spikelet sterility and grain quality

“…In the chamber experiments, heat tolerance at flowering is often tested at 37.5-38.0 °C (relative humidity around 60-70%) to have a great contrast in spikelet fertility between susceptible and tolerant genotypes (Kobayasi et al, 2011;Mackill et al, 1982;Matsui & omasa, 2002;Matsui et al, 2001;Satake & Yoshida, 1978;Shi et al, 2015). n22, an Indian aus-type landrace, was identified as one of the most heat-tolerant genotypes in both chamber and open field experiments (Jagadish et al, 2010;Mackill et al, 1982;Manigbas et al, 2014;Poli et al, 2013;Ye et al, 2012), while Moroberekan, an African Japonica upland cultivar, is known as one of the most heat-susceptible cultivars (Jagadish et al, 2008).…”

“…n22, an Indian aus-type landrace, was identified as one of the most heat-tolerant genotypes in both chamber and open field experiments (Jagadish et al, 2010;Mackill et al, 1982;Manigbas et al, 2014;Poli et al, 2013;Ye et al, 2012), while Moroberekan, an African Japonica upland cultivar, is known as one of the most heat-susceptible cultivars (Jagadish et al, 2008). These cultivars can be used as check cultivars in heat tolerance tests (Shi et al, 2015). In the Japonica cultivars, Akita-komachi and nipponbare (Maruyama et al, 2013;Matsui et al, 2001), Hitomebore (Maruyama et al, 2013), and Todorokiwase (Tenorio et al, 2013) are classified as considerably heat-tolerant genotypes.…”

“…The panicle temperature of Chinese hybrid rice exceeded the ambient air temperature by 4.0 °C under humid and low wind conditions, and also caused a severe reduction in spikelet fertility (Tian et al, 2010). In the record hot summer of 2007, the percentages of spikelet sterility rose to 25% when the maximum daily temperature was around 38 °C and BG90-2 (Shi et al, 2015), and dular (Tenorio et al, 2013) are known as heat-tolerant genotypes. It is notable that Giza178, an Egyptian cultivar developed from a japonica-indica cross, has considerable heat tolerance at booting stage as well as flowering stage (Tenorio et al, 2013).…”

Field level evaluation of rice introgression lines for heat tolerance and validation of markers linked to spikelet fertility

“…Heat stress during flowering stage negatively impacts both percent seed set and total grain yield (Bui et al 2014;Morita et al 2005;Peng et al 2004). The critical physiological parameter "spikelet with exerted anthers but with no ovule enlargement" was considered as the most sensitive stage with exposure to heat stress at important reproductive stages (Shi et al 2015b). Later, the milky stage (6-16 d after heading) during grain filling is also a period sensitive to high temperature as it prevents the photosynthate transportation to grain, shortens the effective grain filling stage and thus reduces grain weight (Liao et al 2011;Shi et al 2015b).…”

“…Experiments on heat stress under field conditions are very few. Geographic origin of rice varieties was not clearly related to the degree of tolerance or susceptibility to heat stress because both conditions were observed in cultivars from each target region (Shi et al 2015b). N22 seedlings showed higher basal thermo tolerance level than Nipponbare but results were opposite when the long term acquired thermo tolerance level was considered.…”