A genetic resource for early-morning flowering trait of wild rice Oryza officinalis to mitigate high temperature-induced spikelet sterility at anthesis

Abstract

Background and aims: High temperatures over 32-36 degrees C at anthesis induce spikelet sterility in rice. The use of a germplasm with an early-morning flowering (EMF) trait has been hypothesized as a way of avoiding this problem. In this study, the effect of the EMF trait on avoiding high temperature-induced sterility at anthesis by flowering at a cooler temperature in the early morning was evaluated.

Methods: The EMF trait was introgressed from wild rice (Oryza officinalis) into the rice cultivar 'Koshihikari' (O. sativa). First, spikelets of the EMF line and Koshihikari were subjected to rising temperatures during the daytime in the greenhouse to test for differences in spikelet sterility. Secondly, spikelets of both plants were exposed to 26, 34 and 38 degrees C at anthesis and to 38 degrees C beginning at least 1 h after flowering, in the growth chambers at 70 % relative humidity, to test for differences in tolerance to high temperatures.

Key results: Spikelets of the EMF line started and completed flowering a few hours earlier than Koshihikari. In a greenhouse experiment, spikelets of Koshihikari opened after the air temperature reached 35 degrees C, but those of the EMF line could open at cooler temperatures. Under these conditions, spikelet sterility significantly increased in Koshihikari, but did not in the EMF line. The number of sterile spikelets increased as their flowering time was delayed in Koshihikari. Furthermore, the chamber experiments revealed that 60 % of the spikelets from both lines were sterile when exposed to 38 degrees C at anthesis, indicating that tolerance of high temperature was similar in both genotypes.

Conclusions: Reduced sterility in the EMF line subjected to rising temperatures at anthesis in the greenhouse was attributed to an earlier flowering time compared with Koshihikari. The EMF trait of wild rice is effective in mitigating anticipated yield loss due to global warming by escaping high-temperature stress at anthesis during the daytime.

Fig. 1.

Percentages of sterile spikelets in a panicle. Entire, total number of spikelets per panicle; Primary, spikelets on primary rachis branches; Secondary, spikelets on secondary rachis branches. **, Significant at the 1 % level; ns, non-significant at the 5 % level by t-test. Bars indicate s.e.

Fig. 2.

Hourly changes in percentages of opened spikelets in a single day.

Fig. 3.

Hourly changes in percentages of sterile spikelets in a single day. The spikelets used in this experiment were the same as for Fig. 2. The values were calculated for the hour when the percentage of opened spikelets was >10 % in a single day: 1100–1400 h and 1000–1300 h for Koshihikari under control and greenhouse conditions, 0700–1100 h and 0700–1000 h for the EMF line under control and greenhouse conditions, respectively. The difference of mean values in a single day was analysed by Tukey's test. Bars indicate the s.e. Different letters above the bars indicate the significant difference at the 1 % level. The experiment was repeated on another day with a similar result.

Fig. 4.

Percentages of sterile spikelets after temperature treatment at anthesis or beginning at least 1 h after flowering in growth chambers. Temperature treatments at anthesis, or treatment applied at least 1 h after flowering, as indicated on axis. Bars indicate the s.e.; ns, non-significant at the 5 % level by t-test.