Stem strength in the core collection of Pisum germplasm

McPhee, K.E. and Muehlbauer, F.J.

USDA-ARS and the Department of Crop and Soils
Washington State University, Pullman, Washington, 99164-6434

An erect growth habit has many advantages in pea production including decreased incidence of foliar diseases, ease of harvest, increased seed quality and decreased economic losses. The semi-leafless or afila leaf morphology (afafStStTlTl) has been incorporated into numerous cultivars for the past 20 years to improve standing ability. Although the tendrils intertwine with each other to provide mutual support, it is obvious from field observations that leaf morphology alone is not sufficient for upright growth. Certain unknown stem characteristics are also required for upright plant architecture. Investigations into stem structural properties as they relate to upright growth are limited in Pisum.

Materials and Methods

Four hundred eighteen PI accessions from the core collection of Pisum germplasm were obtained from the USDA-ARS Western Regional Plant Introduction Station, Pullman, Washington. The accessions included 9 Pisum fulvum, 2 P. sativum spp. arvense, 8 P.s. ssp. abyssinicum, 13 P.s. ssp elatius, and 386 P. sativum accessions. Fifty seeds from each accession were planted.

Six controls were included in the experiment (Table 1). Four were cultivars grown locally, and two were experimental lines, PS010603 and PS110028, from the USDA-ARS breeding program in Pullman, Washington.

Table 1. Summary of pea lines used as controls

Line

Cotyledon Color

Internode Length

Leaf Morphology

Alaska 81

green

tall

normal

Radley

green

dwarf

afila

PS110028

green

tall

normal

Latah

yellow

tall

normal

Rex

yellow

dwarf

normal

PS010603

yellow

tall

normal

The field trial was grown at the Washington State University Spillman Research Farm 3 km south of Pullman, Washington (46o 72’N, 117o 18’ W) during the 1996 field season. The experimental design was completely randomized with the PI accessions present only once. The six control lines were replicated seven times and randomly spaced throughout the plot area. Each plot consisted of two rows 1.5 meters long and spaced 0.3 m apart. A single border row of ‘Dark Skin Perfection’ was planted on either side of the plot rows to reduce border effects. Twenty-five seeds were sown per row resulting in a plant density of 55 plants/m2. Stem samples were collected from 402 of the 418 PI accessions included in the experiment. Sixteen accessions, including many P.s. ssp. elatius and P. fulvum, did not emerge. Stem samples from all the control entries were collected. The eighth internode from four randomly chosen plants was tested for resistance to shearing forces on a Stable Microsystems TAXT2 texture analyzer. The texture analyzer was fitted with a blunt blade and slotted platform. The internodes were dried in an oven at 50oC for 24 hours to an equal and constant moisture content prior to testing. The length and diameter of each internode was measured and recorded using a metric ruler and caliper, respectively. Forces were recorded digitally on a personal computer for analysis. Statistical analyses were performed using the SAS Statistical Package ver. 6.12 (SAS Institute, Inc., Cary, North Carolina, 1996).

Results and Discussion

Armstrong and Pate (1) reported variation in stem strength between internodes of single plants sampled prior to maturity and significant differences between cultivar means. Generally, the internodes toward the middle of the stem were stronger than those at either the top or bottom. Unpublished results in our lab using internodes collected at harvest maturity concur with the results of Armstrong and Pate (1).

The data reported in this contribution were collected on mature stems sampled at harvest maturity. Two forces were recorded as the internodes were broken. The first force was that required to crush the internode (crushing force) and was weaker than the second, which was the force necessary to break the internode perpendicular to the main axis (shearing force). The resistance to crushing and shearing forces for the PI accessions ranged from 3.5 to 36.8 and from 3.6 to 64.9 newtons, respectively (Table 2). Individual internodes resisted shearing forces as high as 70.0 newtons. Resistance to crushing and shearing forces for the control cultivars ranged from 12.4 to 18.5 and from 22.1 to 40.7 N (Table 3). Individual PI accessions possessed 50% greater stem strength than the controls.

Table 2. Average internode length, diameter, crushing force, and shearing force for the eighth internode from 402 PI accessions in the Pisum core collection ranked by average shearing force

Accession

Mean
Length
(cm)

Mean
Diameter
(mm)

Mean
Crushing
Force
(N)

Mean
Shearing
Force
(N)

Accession

Mean
Length
(cm)

Mean
Diameter
(mm)

Mean
Crushing
Force
(N)

Mean
Shearing
Force
(N)

PI 324695

4.0

3.3

30.0

64.9

PI 344012

1.8

1.7

3.5

9.7

PI 103709

5.2

3.6

17.4

61.8

PI 134271

2.3

1.5

8.5

9.7

PI 180696

3.6

3.6

24.7

58.7

PI 347281

3.4

2.1

9.3

9.7

PI 124595

4.1

3.4

18.5

55.2

PI 162909

2.5

1.5

7.4

9.4

PI 120630

6.1

3.2

19.5

54.5

PI 358611

2.2

1.5

—

9.1

PI 285739

3.9

3.4

22.3

54.5

PI 358613

2.6

2.2

—

8.9

PI 264623

4.1

3.3

31.8

54.4

PI 164779

2.9

1.5

7.5

8.6

PI 273605

3.3

3.1

28.3

53.3

PI 358610

2.1

1.5

6.7

7.9

PI 477371

2.7

3.5

26.1

52.0

PI 280609

1.8

1.4

4.5

7.2

PI 180699

4.2

3.8

21.4

52.0

PI 269810

1.7

1.3

—

3.6

 

Table 3. Mean internode length, diameter, crushing force, and shearing force for the eighth internode of six adapted check entries

Check Entry

Mean Length (cm)

Mean Diameter (mm)

Mean Crushing Force (N)

Mean Shearing Force (N)

Alaska 81

4.4

3.4

16.7

33.2

Latah

3.6

2.9

15.2

28.8

PS010603

3.1

2.7

14.5

22.8

PS110028

4.1

3.2

18.5

31.2

Radley

1.8

2.9

12.4

22.1

Rex

2.9

3.4

18.5

40.7

Stem strength was positively correlated with internode diameter (r = 0.68, p < 0.001) and internode length (r = 0.36, p < 0.001). Shearing force was positively correlated with the yield production by the accessions. Table 4 summarizes the mean resistance to shearing force for ten PI accessions with the greatest and least total biomass, seed and straw production and harvest index. On average, the accessions with the greatest yield potential resisted shearing forces twice that of accessions with lower yield potential.

Table 4. Mean shearing force for the ten accessions ranked at the top and bottom for total biomass, seed yield, straw yield, and harvest index

Production Variable
Top 10		 Shearing Forse (N)
Bottom 10
Total biomass 30.8 16.5
Seed yeld 29.8 17.2
Straw yeld 28.0 16.6
Harvest index 30.2 15.8

Significant variation for stem strength within Pisum germplasm greater than current cultivars indicates that improvements in stem structure and strength can be realized. Future research in determining genetic, physiological, and environmental effects on stem strength and structure is needed to improve the ideotype for the pea crop.

 

1. Armstrong, E. and Pate, J. 1997. In International Food Legume Research Conference III. Adelaide, Australia. September 22-26, 1997. p. 130.
2. Goldenburg, J.B. 1965. Boletin Genetico, I:27-28.
3. SASTAT Handbook. 1996. SAS Institute, Inc., Cary, North Carolina.