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Screening rice genetic resources for cold tolerance at different growth stages

Changrong Ye1, Shu Fukai1, Russell Reinke2, Ian Godwin1, Peter Snell2 and Jaya Basnayake1

1 School of Land, Crop and Food Sciences. The University of Queensland. St Lucia, QLD 4072. Email c.ye@uq.edu.au
2
Yanco Agricultural Institute. NSW Department of Primary Industries. Yanco, NSW 2703.

Abstract

To investigate the potential genetic resources for stabilising future Australian rice production, we have evaluated 17 rice varieties from several countries for cold tolerance at the germination, seedling, booting and flowering stages. Results showed that the seeds from varieties B55, Banjiemang, HSC55, and Lijiangheigu could germinate quickly at low temperature. At seedling stage, the seedling growth was severely delayed by low temperature stress. Changes in leaf number, leaf elongation rate, tiller number, days to 1st tiller, plant height, and shoot dry weight were useful measurements for evaluating the cold tolerance of rice seedlings. The seedling growth of Amaroo, B55, Banjiemang, HSC55, Jyoudeki, Lijiangheigu and Quest were less affected by low temperature. The cold tolerance at germination and seedling stage were correlated. After low temperature treating at booting and flowering stage, plant height, panicle neck length, spikelet fertility and grain weight were significantly reduced. Varieties differed in spikelet fertility in response to low temperature treatment at booting and flowering stage. After low temperature treatment at booting stage, anther length, anther width, number of fertile pollen grains, and pollen area decreased significantly, and the reduction of pollen grains was correlated with spikelet fertility. Rice varieties B55, Bangjiemang, HSC55 and Lijiangheigu were consistently tolerant to low temperature at germination, seedling, booting and flowering stages, and should prove useful genetic resources for improving the cold tolerance of Australian commercial rice varieties.

Key Words

rice, genotype, low temperature, cold tolerance

Introduction

Climate change is likely to have a significant impact on Australian agriculture, especially through the increased likelihood of extreme temperatures and drought. Low temperature damage has long been a problem in temperate rice growing areas of the world. Low temperature damage has caused serious yield losses in more than 20 countries, such as Japan, China, Korea, Italy, Hungary, Australia, and USA. In Australia, the frequency of yield loss due to low temperature damage equates to 1 t ha-1 every four years and greater than 2 t ha-1 every 10 years (Farrell et al. 2001). To protect the panicles from low temperature damage, a significant amount of water is used to increase the water depth from 10cm to 20-25cm for two weeks following the panicle initiation stage. However, due to continued conditions of drought experienced over the past 6 years, the Australian rice industry, an $800 million industry with around 2500 rice farms, has been severely reduced. To stabilise future Australian rice production, new rice varieties with superior cold tolerance are urgently needed. Using such varieties may reduce the low temperature damage and provide the foundations for future reduction in water use.

In temperate zones and high-elevation regions, low temperature may occur at any growing stage through rice cultivation. Satake (1976) indicated that the most sensitive stage of cold injury in rice was booting stage, especially the early pollen microspore stage. However, rice plants are susceptible to low temperature at different stages of development, all the growing stages are critical for the final grain yield. This study was conducted to evaluate the cold tolerance of different rice varieties at different growing stages, to identify the relationships between agronomic characters and cold tolerance, and to identify germplasm with cold tolerance at some critical growing stages for potential use in breeding and genetic studies.

Methods

Experiment 1: Germination under low temperature

A total of 17 rice varieties from the Australian rice breeding program were used for this study. These varieties were selected from various origins and represented a range of susceptible, moderate and tolerant genotypes in response to low temperature at booting stage. Rice seeds were soaked in water in Petri-dishes (50 seeds per dish, 6 dishes per variety) for 24 h under room temperature (19-21C). Then, three Petri dishes (control) were moved into a 28C incubator, another 3 Petri dishes (treatment) were moved into a 15C incubator. The germinated seeds were counted everyday for control, and every 2-day for cold treatment. The germination rate (GR), mean germination time (MGT) and germination index (GI) were calculated according to Ellis and Roberts (1981).

Experiment 2: Seedling growth under low temperature

Seeds from 17 rice varieties were germinated and sown in 20 cm plastic pots (6 seeds per pot, 6 pots per variety). Three pots (control) were grown in a glasshouse with non-limiting temperatures (21-36C), and another 3 pots (treatment) were grown in a cool glasshouse (mean temperature 18.51.0C, range 15-22C). The emerging date and length of each leaf were recorded. The leaf elongation rate of the 5th leaf was measured daily for 4 days. Thirty-five days after sowing, the plant height, shoot fresh weight and dry weight were measured. The reduction rate for all the measured characters were calculated as: RR = [(Mean value of control – Mean value of cold treatment)/Mean value of control]100%.

Experiment 3: Cold tolerance at booting and flowering stages

Seeds from 17 rice varieties were sown and grown in 20cm plastic pots (6 seeds per pot, 9 pots per variety) in a general glasshouse with non-limiting temperatures. At the booting stage (occurring when the auricle distance of most of the plants were between -5 to +5 cm (Haque 1988)), three pots of each variety were moved into a cool glasshouse (mean temperature 18.40.8C, range 14-25C). After 14 days, the pots were moved back to the general greenhouse. When the main stem began heading, the 3rd and 4th spikelet from the top 3 primary branches were sampled for measuring anther length and width, and counting the pollen grains per anther using an image analysis system described by Farrell et al. (2006).

For the treatment at flowering stage, three pots of each variety were moved into the cool glasshouse as the panicles emerged. After 14 days, the un-flowered spikelets on the main stem were removed to assure the retained spikelets were pollinated during the treatment, and the pots were moved back to general glasshouse to grow till harvest. Another three pots were always grown in the general glasshouse as control. At physiological maturity, the final tiller, leaf number plant height, panicle neck length, panicle length, flag leaf length, grain weight (100 seed weight), filled and unfilled grains of the main stem were investigated.

Results

Germination under low temperature

Under low temperature (15C), the germination was slowed, no germination was observed till 4th day after treatment commencement. The fastest variety, HSC55, reached peak germination on 5th day, followed by Banjiemang, B55, Lijiangheigu, M103 and WAB38. The latest varieties were Jyoudeki and YRL39. On 14th day after the treatment commencement, some seeds from Amaroo, Azucena, Langi, Pelde, WAB160, WAB38, YRL39 and YRM64 were still not germinated. But most of these seeds germinated within 3 days after moving into favourable temperature (28C). Using cluster analysis of GR, MGT and GI under low temperature, the cold tolerance of these 17 rice varieties were classified into tolerant, moderate, susceptible and very susceptible groups (Table 1). Among them, B55, Banjiemang, HSC55, and Lijiangheigu were cold tolerant varieties which could germinate quickly under low temperature. Jyoudeki also showed a high germination under low temperature, but had a slower speed of germination. The varieties Azucena and Pelde showed a different germination pattern to other varieties, with both having a low of germination rate and low germination speed.

Cold tolerance at the seedling stage

Under low temperature, the rice seedling growth was slower. At the end of the treatment, the seedlings under low temperature showed an average reduction of 3.1 leaves, 2 tillers, 35.7 cm plant height, 6.71 g shoot fresh weight; and 1.23g shoot dry weight less than those under the more favorable (control) conditions. At the same time, the leaf elongation rate decreased by 4.2 cm/d (61.6%). The reduction rates of most measured characters were significantly correlated except the days from sowing to full expansion of the 5th leaf (L5). The time to L5 was delayed 14.7 days (89.2% of the control) under low temperature, however, there was no significant difference among 17 varieties (F=0.122, n=17, p=1.000). By cluster analysis using the reduction rate of leaf number, leaf elongation rate, tiller number, days to 1st tiller, plant height, and shoot dry weight, the 17 rice varieties could be classified into very tolerant, tolerant, moderate, and susceptible groups (Table 1). The cold tolerance of these 17 rice varieties at germination and seedling stage were correlated (rs=0.515, n=17, p=0.035). For most varieties, those having higher germination index under low temperature also showed higher seedling growth under low temperature.

Table 1. Cold tolerance at different stages of growth and some agronomic characters of 17 rice varieties.

Variety

Agronomic characters (control)

Cold toleranceA

Origin

Heading dateB

Plant height (cm)

Panicle length (cm)

Grain number

Germination stage

Seedling stage

Booting stage

Flowering stage

Amaroo

Australia

63

86

18

80

S

T

S

S

Azucena

Philippine

108

132

23

75

SS

T

S

S

B55

China

71

87

15

53

T

T

T

T

Banjiemang

China

78

166

23

107

T

TT

T

T

HSC55

Hungary

57

106

16

76

T

T

T

T

Jyoudeki

Japan

70

88

15

53

S

T

M

M

Langi

Australia

76

82

19

92

M

M

M

S

Lijiangheigu

China

64

135

20

84

T

TT

T

T

M103

USA

59

87

17

95

M

M

M

T

M104

USA

66

85

17

83

M

M

M

T

Pelde

Australia

80

109

22

118

SS

S

S

S

Quest

Australia

62

87

17

84

M

T

M

S

Reiziq

Australia

67

85

16

81

M

M

S

S

WAB160

Africa

78

110

21

111

S

S

S

S

WAB38

Africa

71

100

20

89

M

S

SS

T

YRL 39

Australia

78

88

21

92

S

S

S

T

YRM 64

Australia

74

85

18

87

S

S

M

S

A. Cold tolerance: SS = very susceptible, S = susceptible, M = moderate, T = tolerant, and TT = very tolerant.
B. Heading date is the days to heading after sowing; grain number is the number of grains per panicle.

Cold tolerance at the booting stage

After treating at booting stage, final leaf number, tiller number, flag leaf length and 2nd leaf length were not significantly different between control and treatment. However, plant height, panicle neck length, panicle length, grain number, spikelet fertility and grain weight were significantly reduced. Low temperature at booting stage reduced plant height, panicle neck length and panicle length. The low temperature treatment further reduced the grain yield by reducing the grain number, spikelet fertility and grain weight. After low temperature treatment at booting stage, anther length, anther width, number of fertile pollen grains, percentage of sterile pollen grains and pollen area were all significantly decreased. Both the number of fertile pollen grains and its reduction rate were correlated with the spikelet fertility. The varieties with more engorged pollen grains showed higher spikelet fertility. After low temperature treatment, the spikelet fertility decreased and varied from 13.2% (WAB38) to 90.3% (B55). Since plant height, panicle length, grain number and spikelet fertility were significantly affected by low temperature treatment, the reduction rates of these characters were used for cluster analysis. The results indicated that these 17 varieties could be classified into tolerant, moderate, susceptible and very susceptible groups (Table 1).

Cold tolerance at the flowering stage

After low temperature treatment at the flowering stage, plant height, panicle neck length, spikelet fertility and grain weight were significantly reduced. The spikelet fertility was varied from 16.9% (Pelde) to 80.6% (HSC55). Low temperature at flowering stage reduced the plant height by reducing the elongation of the panicle node (shorter panicle neck length), and further reduced the grain yield by reducing the spikelet fertility and grain weight. The results of cluster analysis using the reduction rate of plant height, panicle length, grain number and spikelet fertility also indicated that these 17 varieties could be classified into tolerant, moderate and susceptible groups with different cold tolerance (Table 1). The spikelet fertility (or sterility) is a more powerful indicator for evaluating the cold tolerance at booting and flowering stage than other single agronomic character.

Conclusion

Some agronomic characters, such as germination index (for germination), reduction rate of shoot dry weight (for seedling stage) and spikelet fertility (for booting and flowering stages) are useful for evaluating the cold tolerance of rice plant at different growth stages. Significant variation in cold tolerance exists among rice varieties and development stages. Rice varietiesy B55, Bangjiemang, HSC55 and Lijiangheigu were consistently tolerant to low temperature at germination, seedling, booting and flowering stages. These varieties are useful genetic resources for improving the cold tolerance of new rice varieties and for genetic studies. The cold tolerance at germination stage was correlated with those at seedling, booting and flowering stages, and the cold tolerance at seedling stage was also correlated with those at booting stage. This suggests that selection at germination and early seedling stage may be useful in rice breeding programs focused on cold tolerance. The efficiency of selection at earlier growing stages should be further evaluated.

References

Ellis RH and Roberts EH (1981) The quantification of ageing and survival in orthodox seeds. Seed Science and Technology 9, 373-409.

Farrell TC, Williams RL, Fukai S (2001) The cost of low temperature to the NSW rice industry. Proceeding of the 10th Australian Agronomy Conference, Hobart, Australia, 28 January 28- 1 February 2001.

Farrell TC, Lewin LG. (2006) Minimising cold damage during reproductive development among temperate rice genotypes. II. Genotypic variation and flowering traits related to cold tolerance screening. Australian Journal of Agricultural Research 57, 89-100.

Haque MZ (1988) Effect of nitrogen, phosphorus and potassium on spikelet sterility induced by low temperature at the reproductive stage of rice. Plant and Soil 109, 31–36.

Satake T (1976) Determination of the most sensitive stage to sterile-type cool injury in rice plants, Research Bulletin of Hokkaido National Agriculture Experiment Station 113, 1–33.

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