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Correlation between reliability of pollination and length of basal dehiscence of the thecae in rice under a hot and humid condition

Tsutomu Matsui1, Kazuhiko Kobayasi2, Hisashi Kagata1 and Takeshi Horie3

1Experimental Farm, Kyoto University, Hatchonawate 12-1, Takatsuki, Osaka 569-0096, Japan.
Email matsuit@kais.kyoto-u.ac.jp
2
Faculty of Life and Environmental Science, Shimane University, Matsue 690-8504, Japan
3
Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.

Abstract

Anticipated global warming may increase the floret sterility of rice. For selection of genotypes tolerant to high temperatures during the flowering period, it is important to identify morphological traits associated with tolerance to temperature stress. This study investigated relationship between the length of dehiscence at the basal part of thecae and the reliability of pollination in 18 cultivars of rice subjected to a hot and humid condition (37/25 ℃, day/night, >90% relative humidity) for three days at flowering. Control plants were left under the ambient conditions in a semi-cylindrical house covered with cheesecloth (30% shading; temperature range: 24-35℃). The length of basal dehiscence of thecae and the number of pollen grains on the stigmata were examined with a light microscope after flowering. Under the hot condition, the length was strongly correlated with % florets having more than 20 pollen grains (r = 0.93, p < 0.001), and basal dehiscence was shorter with lower percentage of florets in the non-japonica-type than in many of the japonica-type. We concluded that the low pollination in the non-japonica-type is associated with their small basal dehiscence on the thecae, and the length of basal dehiscence can be used as a selection marker for high temperature tolerance of pollination.

Media summary

Large basal dehiscence on the rice thecae increased pollination viability under hot and humid condition and can be used as a marker for heat tolerance.

Key Words

High-temperature tolerance, morphological marker.

Introduction

The effects of increasing temperature and of high CO2 concentration in the atmosphere on the rice yield have been analyzed using crop simulation models (Jin et al. 1995; Horie et al. 1996; Matthews et al. 1997) and under controlled environments (Collins et al. 1995; Horie et al. 1995; Kim et al. 1996; Matsui et al. 1997; Ziska et al. 1997). An anticipated high temperature (daily maximum temperature > 34 at flowering period) may induce floret sterility and decrease rice yield even in temperate regions such as West Japan (Kim et al. 1996; Horie et al. 1996).

For efficient selection in a breeding program, visible markers of high-temperature tolerance are required. Matsui and Kagata (2003) reported that, using japonica rice cultivars in Japan, long basal dehiscence of thecae for pollen dispersal increased the stability and sureness of pollen transport to the stigmata under normal conditions. The objective of this study was to determine if the length of dehiscence at the basal part of the thecae can be used as a potential marker of pollination reliability for selection of genotypes tolerant to high temperatures. We examined the correlation between the length of the dehiscence at the basal part of the thecae and the reliability of pollination in a wide range of cultivars. The humid condition was used to avoid the drying damage of the plants under the high-temperature condition.

Materials and Methods

Plant Materials and Treatments

The experiment was conducted at the Experimental Farm of Kyoto University (3451’ N, 13538’ E, 10 m above sea level) located in Takatsuki, Japan. Twelve japonica type cultivars and six non-japonica type cultivars of rice were used. Seedlings at the 5th leaf stage were transplanted in a circular pattern into pots (20 cm in height and 15 cm in diameter), 20 seedlings per pot, and grown in water-logged soil. Plants were grown in a semi-cylindrical house covered with cheesecloth (30% shading) throughout the experimental period to avoid wind damage.

The experiment was a 18 by 2 (18 cultivars and two temperature conditions) factorial design. When 50 % of the plants headed, six pots with uniformly grown plants were selected from each cultivar, and were then randomly divided into two groups. One group was exposed to a high air temperature of 37 for 6-h (1000h – 1600h) for 3-d in a growth chamber with a 10-h (0800h – 1800h) photoperiod. Light was provided from white fluorescent tubes (270 μmol m-2 s-1). Night temperature (1800h – 0800h) was controlled to 25.0. From 0800h to 1000h and from 1600h to 1800h, the air temperature was changed stepwise. The relative humidity was controlled over 90 % during the high temperature treatment. The other group was left in the semi-cylindrical house covered with cheesecloth (temperature range 24-35 ).

Sampling and Measurements

Seventeen florets that completed flowering were sampled from the primary branches on panicles of each cultivar every day at 1600h during the treatment under the high temperature condition, and at 1300h for more than 3-d during the flowering periods under the ambient condition.

Anthers from five florets randomly selected from 17 florets of each cultivar were used for counting the number of undehisced thecae, and in the dehisced thecae, the length of the dehiscence at basal parts of the thecae was also measured. The stigmata from all 17 florets were then stained with cotton blue and pollen grains on the stigmata were counted.

Results and Discussion

The length of basal dehiscence strongly correlated with the percentage of florets having more than 20 pollen grains on the stigmata after anthesis (>20-pollen-grain florets) under the high-temperature condition (Fig. 1). Our data suggest that the long basal dehiscence increases the percentage of sufficiently pollinated florets under a high temperature condition and thus increases the high-temperature tolerance of the pollination. Matsui and Kagata (2003) reported a strong correlation between the length of basal dehiscence and the percentage of florets with a large number of pollen grains under normal condition in old japonica cultivars. They assumed that since the basal dehiscence is located just above the stigmata and opens at the floret opening, the pollen grains in thecae with a long basal dehiscence fall easily onto the stigmata, and thus a long basal dehiscence ensures pollination (Matsui and Kagata, 2003). The present results indicate that the correlations hold under a hot and humid condition in a wide range of genotypes including non-japonica type.

Both the length of basal dehiscence and the percentage of >20-pollen-grain florets under the high-temperature condition in the non-japonica-type cultivars were smaller than in many of the japonica-type cultivars (Fig. 1). These results indicate that pollination of the non-japonica type cultivars was susceptible to high temperatures and suggest that the susceptibility was caused by the small dehiscence at the basal part of thecae. Genetic improvement for a large basal dehiscence would increase the high-temperature tolerance of these cultivars. Some japonica-type cultivars with long basal dehiscence may be a useful genetic resource for the improvement of heat tolerance in rice.

The length of basal dehiscence under the ambient condition was positively correlated with that under the high-temperature condition (Fig. 2). This correlation shows that character of the long dehiscence expresses stably both under the high temperature and normal environments at the flowering. Therefore, we can estimate the length of the basal dehiscence under a high-temperature condition from that under a normal condition.

Conclusion

The long basal dehiscence of the thecae raised the reliability of the pollination under the hot and humid condition in a wide range of cultivars, and the length of the basal dehiscence under the ambient condition was parallel to that under the high-temperature condition. Since the length of dehiscence is a visible trait, the length under normal conditions may be useful as an appropriate morphological marker for selection of a high-temperature tolerant genotype.

1 Shanguichao
2 WAB450-1-B-P-38-HB
3 Takanari
4 IR72
5 IR65564-44-2-2
6 Banten
7 Somewake
8 Homura3
9 Koshihikari
10 Nipponbare
11 Kokuryomiyako
12 Ginbouzu
13 Husakushirazu
14 Takenari
15 Tairaippon
16 Magatama
17 Kameji
18 Kinmaze

Figure 1. Relationship between the length of dehiscence at the basal part of thecae and the percentage of florets with more than 20 pollen grains on the stigmata under the high-temperature condition (day temperature = 37, > 90 % R. H.). ***P < 0.001. ○, japonica-type cultivars; ●, non-japonica-type cultivar.

Figure 2. Relationship between the dehiscence at basal part of thecae under the ambient condition (in a semi-cylindrical house covered with cheesecloth) and that under the high-temperature condition (day temperature = 37, > 90% R.H.). ***P < 0.001. Symbols are the same as those in Figure 1.

References

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