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Responses to boron (B) levels in F2 and F3 populations derived from B-efficient and B-inefficient wheat parents

Supawadee Ngorian, Sansanee Jamjod, Suthat Julsrigivul, Dumnern Kaladee and Benjavan Rerkasem

Agronomy Department, Faculty of Agriculture chiang Mai University, Chiang Mai, 50200, Thailand
Email agosjmjd@chiangmai.ac.th

Abstract

Responses of an F2 population derived from a cross between boron (B) efficient (Fang 60) and B inefficient (Bonza) wheat (Triticum aestivum L.) parents to levels of B were studied. The first experiment was conducted on a low boron soil in a split plot design with three replications. The three B levels were main plots, including nil (B0), limed at the rate 1 t ha-1 (BL) and boron at the rate 10 kg borax ha-1 (B+), applied to the soil and the two parents and their F2 population, were assigned to sub plots. Response to B, measured as grain set index (GSI) and grain yield, was found to differ among populations when grown in low B (BL and B0). The mean grain set of the F2 in low B was between the two parents but closer to the more efficient parent, Fang 60. This suggests that B efficiency is expressed as a dominant trait. Twenty-four F2 plants from each B level with the highest grain set and grain yield were selected and their F3 progenies were evaluated in sand culture without B in the second experiment. Segregation for response to B was found within F2-derived F3 families selected from all B treatments. Populations selected from low B (BL and B0) displayed higher proportion of B efficient genotypes than that from sufficient B (B+). The results suggested that F2 populations selected from BL and B0 comprised of homozygous and heterozygous efficient genotypes, whereas no B inefficient genotype was found. In contrast, both B efficient and inefficient genotypes were found within populations selected under B+ condition.

Media summary

The Fang 60 wheat variety, selected from Thailand, can be used as a source of boron efficiency for breeding and will boost yields in low boron regions.

Key Words

Boron (B) deficiency, Boron (B) efficiency, wheat

Introduction

Low boron (B) soils are widespread in many wheat growing, subtropical areas (Sillanpaa, 1982). These include the northern and northeastern regions of Thailand (Rerkasem et al., 1988; Keerati-Kasikorn et al., 1987), where wheat is being promoted, as well as major wheat producing countries in Asia from China, India, Bangladesh and Nepal (Li et al., 1978; Kataki et al., 2002; Sakal and Singh, 1995, Subedi et al., 1996). Boron deficiency causes yield reduction by inducing male sterility, resulting in grain set failure (Cheng and Rerkasem, 1993). A wide range of genotypic variation for response to low B has been identified and genotypes were classified into four distinct groups, namely, efficient, moderately efficient, moderately inefficient and inefficient (Rerkasem and Jamjod, 1997). At low B, the inefficient genotypes were completely sterile and set only a few or no grain, while the efficient genotypes set grain normally. Evidence of genotypic variation offers a solution to B deficiency through selection and breeding for B efficient cultivars. In this study, the B efficient (Fang 60) and inefficient (Bonza) genotypes were used as parents to produce an F2 generation. The response of the F2 population to B levels and the genetic control of the response are described. Understanding the response of segregating populations and genetic control will facilitate selection and breeding for B-efficiency.

Methods

Genetic materials

Fang 60 (B efficient, Jamjod et al., 1992) and Bonza (B inefficient, Rerkasem and Jamjod, 1997) were used as parents. F1 plants were sown in B sufficient soil to produce the F2 generation. The F2 population was tested in 3 levels of B in the soil in Experiment 1. Seventy-two F2 plants were selected and multiplied. Selected F3 families were tested in sand culture in Experiment 2.

Experiment 1: Evaluation of F2 population

The parents and F2 were sown in a low B soil at Chiang Mai University. The experiment was arranged as split plot design with 3 replications. Three B levels were randomized in main plots, including nil (B0), limed (Ca(OH)2) at the rate 1 t ha-1 (BL) and boron at the rate 10 kg borax (Na2B4O7.10H2O) ha-1 (B+), applied to the soil. The three populations, two parents (Fang 60 and Bonza) and their F2, were assigned to sub plots. In each plot, 10 plants of Fang 60 and Bonza each, and 120 of F2 plants were sown. At ear emergence, the first two ears from each plant were bagged to prevent outcrossing. At maturity, the bagged ears were harvested and analysed for Grain Set Index (GSI, percentage of the 20 basal florets from 10 central spikelets that contained grain; Rerkasem and Loneragan, 1994). All ears from each plant were counted, pooled, threshed and determined for grain yield. Responses to B of the parents and F2 population were compared by LSD.

Experiment 2: Evaluation of selected F2-derived F3 families

Twenty-four F2 plants which had the highest grain yield and gain set were selected from each B level, and seeds harvested from these plants represented F2-derived F3 families. All families were grown in trays containing washed river quartz sand. Families were grown in rows, 12 plant row -1, with a spacing of 6 cm between plant and 6 cm between rows. Grid rows of Fang 60 and SW 41 parents were included every 6 rows. Trays were watered with a complete nutrient solution without added B. At booting, two ears from each plant were bagged and at maturity the bagged ears were harvested and GSI determined. Family mean and variance within family were calculated and compared to both parents.

Results and Discussion

The parents used in this study displayed the expected differences for response to B (Table 1). Boron had no effect on number of ear plant-1 and spikelets ear-1 of both parents and their F2 (data not shown). Severe reduction in grain yield and GSI were found in Bonza while those of Fang 60 parent were not affected, compared to B+.

Table 1. Grain yield and grain set index (GSI) of Fang 60, Bonza wheat varities and their F2 grown in 3 levels of boron.

Characters/population

B treatment

 

BL

B0

B+

Grain yield (g plant-1)

     

Bonza

0.2

0.8

6.6

F2

6.3

4.6

6.0

Fang 60

5.6

8.8

8.1

LSD P0.05 B x G = 3.2

GSI (%)

Bonza

2.1

8.6

89.9

F2

69.9

79.1

87.6

Fang 60

96.8

97.0

99.3

LSD P0.05 B x G = 8.3

At B+, means grain yield of the F2 population and both parents were similar. Although GSI of Fang 60 was slightly higher, the GSI of Bonza and the F2 population approached 90%. In low B, the F2 population means were intermediate between the two parents but closer to Fang 60 (Table 1). The F2 distribution for GSI was used to study segregating pattern of F2 compared to parents in response to low B. Segregation in the F2 population was not evident when grown in B+. However, when grown in low B, most of F2 plants fell into the range of the efficient parent, Fang 60. This distribution suggested that B efficiency was controlled by dominant gene action. However, selection for B efficiency based on phenotype will not be effective because it is not possible to distinguish between heterozygous and homozygous efficient genotypes. In addition, low B treatments in this study allow genetic variability to be expressed and could be used to screen other segregating populations.

The value of each F2 individual was the expression of phenotype, which included both genotypic and environmental effects. The measurement of B response of F2 derived F3 families allowed the genotype of the F2 plants to be identified. Twenty-four F2 plants with the highest grain yield and grain set were selected from each B treatments. Their F3 progenies were tested in low B and assigned to homozygous efficient, segregating or homozygous inefficient to B categories by comparing the family mean and variance to those of the parents. Bonza parental check set no grain while means GSI of Fang 60 were between 80-90% and variances were <1000 (Figure 1). Although all selected F2 plants had GSI >95%, most of their progenies were segregating when grown in low B. The segregation within the F3 families selected from BL and B0 confirmed the dominant gene action of B efficiency, while segregation of families selected from B+ resulted from random selection for B efficient genes. It is suggested that progeny testing should be employed in selection for B efficiency.

Figure 1. Mean and variance of F2-derived F3 families selected from BL, B0 and B+, and of Fang 60 parent grown in sand culture without B. Note: Mean GSI and variance of Bonza parent were both 0 and obscured by F3 family data. The range of all selected F2 plants was 95-100%.

A high proportion of B efficient alleles was found in populations selected under low B, compared to those selected under B sufficient conditions (Table 2). Four families or 8% of selected populations from BL and B0 were homozygous efficient and none of homozygous inefficient was found from BL selection. In contrast, in B+ treatment where B efficient genes were randomly selected, none of homozygous efficient and 8% homozygous inefficient were found. This indicates that low B in this study acted as selection pressure for B efficiency and high yield. Moreover, the population selected under non B stress conditions was likely to contain a high proportion of inefficient genotypes (for example, 46% compared to 29% of those selected from BL, Table 2). In breeding programmes, genotypes or breeding materials should be screened for response to B prior to release or promotion in low B areas.

Most cereal breeding programmes devote a considerable proportion of their resources to selection for grain yield per se. This involves sowing yield trials at several contrasting locations within the one target region and retaining those selections with a comparatively high mean yield. Advanced lines from CIMMYT germplasm, #144, #1510 and #1015, which are all sister lines, outyielded other varieties in National trials during early 1980s (Mann, 1991). Line #1015 was released as Fang 60 from Fang Horticultural Research Station, Chiang Mai, one of the low B site, in 1987, and has shown outstanding yield performance on soils of Northern Thailand that have been largely low in B. A high correlation (r = 0.81) between GSI and grain yield was found when wheat and barley genotypes were grown in low B, but no correlation was found when plants were grown in B sufficient conditions (Jamjod and Rerkasem, 1999). Thus, B efficient genes in Fang 60 might have been unconsciously selected by selecting for grain yield on low B soils.

The results of this study demonstrated that B efficiency in Fang 60 and Bonza wheat varieties is controlled by dominant gene action. A progeny testing method should be employed in selection for B efficient genotype.

Table 2. GSI (%) of selected F3 families grown in sand culture without added B.

 

Number of family (%)

 
 

Variance

 

Mean

0

1-500

>500-1000

>1000-1500

>1500

Total

BL selection

0-25

   

17

13

 

30

>25-50

     

17

21

38

>50-75

     

13

12

25

>75-100

 

8

     

8

Total

0

8

17

43

33

 

B0 selection

0-25

 

4

13

17

 

34

>25-50

     

8

25

33

>50-75

     

8

17

25

>75-100

 

4

4

   

8

Total

0

8

17

33

42

 

B+ selection

0-25

8

4

25

8

 

45

>25-50

     

17

21

38

>50-75

     

4

13

17

>75-100

         

0

Total

8

4

25

29

34

 

Acknowledgments

This work was supported by a grant from the Thailand Research Fund (TRF).

References

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