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Stem nodes – the central building blocks of growth and yield in field pea.

EL Armstrong1, PW Matthews2, LG Gaynor1, ID Menz1 and BC Armstrong1

1NSW Department of Primary Industries, Wagga Wagga Research Institute, Wagga Wagga, NSW
2
NSW Department of Primary Industries, Temora Agricultural Research & Advisory Station, Temora, NSW

Abstract

Node production for 22 modern field pea genotypes was measured during 2007 in field trials covering a range of sowing dates from the 26th April through to the 19th July across a range of conditions from full irrigation to drought. Under these environments, comparisons were made for vegetative and reproductive node production, nodes to first flower, stem height and pod set and yield on all reproductive nodes for each variety. This paper presents some preliminary findings for three genotypes and discusses these results in light of modern breeding objectives for the pea crop.

Key Words

Growth and development, phenology, yield components, pod mapping, flowering, grain legume

Introduction

Stem nodes can be viewed as basic building units for all growth, development and yield in the field pea crop since each node supports stipules, leaflets, tendrils and defines an internode. Nodes also produce flowers and pods and provide the sites for initiation of basal and aerial branches. Nodes evolve from the growing point soon after the shoot emerges from the seed and develop in a sequential upwards pattern. Anatomically, the first two nodes are designated as scale nodes because they often do not fully develop leaflets, stipules and tendrils. The length and strength of the internode is determined genetically, thereby allowing the height and standing ability of the variety to be manipulated through breeding. Nodes, because of their sequential pattern of development, provide an accurate, easily measured and simple key to identify specific stages of growth and development of the crop throughout its life cycle (Knott 1987).

Environmental factors (largely temperature) and genetic factors (the cultivar) trigger nodes to change from producing vegetative buds to reproductive buds. The node at which this conversion occurs is labelled node to first flower and this marks the date of flowering for a particular cultivar. This node number appears to remain relatively constant for a given cultivar (personal observations). Thus, earlier flowering varieties flower at a lower node number (10-12) and later varieties at a higher node number (18-22). From this point onwards, reproductive nodes are produced sequentially upward until terminated either by senescence or by overriding terminal weather conditions. It is this number of reproductive nodes that sets a variety’s flower number, pod number and eventual yield potential for given season.

Understanding growth, development and yield partitioning in crops provides an insight into the functioning of different cultivars and what makes some higher yielding compared to others. This knowledge can help breeders identify critical physiological yield attributes that can be incorporated to improve performance of current cultivars (French 1990; Walton 1991; Armstrong 1994). In this paper, pod mapping studies conducted across a range of field pea genotypes, sowing times and moisture conditions are used to help identify some of these critical physiological attributes for field pea.

Methods

Phenology Trial

Ten field pea varieties were hand sown in 2007 on 26th April, 17th May, 7th June, 28th June and 19th July. The trial was located at the Wagga Wagga Agricultural Institute, NSW and was watered on demand using a dripper irrigation system. Plots consisted of two rows 20 cm apart with a plant spacing of 7.5 cm. The lines chosen for this study were Morgan, Parafield, Kaspa, Yarrum, PSL4-RRSEL-1, OZP0602, OZP0610, OZP0703, OZP0704 and OZP0705. Plots were trellised by running bird netting (supported by steel posts and plain wire) along the outside of the two rows. This allowed easy management, access and assessment, and more uniform illumination and growth across all entries. Very poor emergence in the fourth sowing, particularly for Kaspa and Yarrum, resulted in changed growth patterns and pod production due to the low plant numbers.

At maturity, ten plants were randomly sampled from each plot. Care was taken to keep part of the root intact to allow easy and accurate identification of the main stem, its first two scale nodes and the first true stem node. In the laboratory, each of these stems was mapped for height and number of nodes to the first flower, first pod, last pod and top node. Starting from the lowest (first) flowering node across all ten stems, pods were removed and bulked separately for each node in a sequential order up the stem. Then for each reproductive node, pod number, seed number and seed weight (oven dry) were recorded. This data allowed subsequent calculations of pods/node, seeds/pod, seed size (100 seed weight) and % yield from each node up the reproductive canopy.

Table 1. The effects of sowing time, irrigation and drought on stem height and node production of field pea.

Variety

Moisture

Sowing

Stem

Nodes

Total

Seed Wt/

Pods/

Seeds

100 Sd

Regime

Time

Ht (cm)

to FF

Nodes

Stem(g)

Node

/Pod

Wt (g)

Kaspa

Irrigated

26 Apr

134

26

34

16.9

1.96

5.8

21.5

 

Irrigated

17 May

137

25

35

14.3

1.79

5.7

17.0

 

Irrigated

7 Jun

98

23

30

9.2

1.76

5.0

16.7

 

Irrigated

28 Jun

82

22

31

3.2

0.50

4.3

19.0

 

Irrigated

19 Jul

83

22

32

8.8

1.43

3.5

19.5

 

IRRIGATED

MEAN

107

23

32

10.5

1.49

4.9

18.7

Drought

18 Jun

48

21

23

1.2

1.34

2.8

16.5

Yarrum

Irrigated

26 Apr

113

22

30

22.5

2.25

6.5

21.8

 

Irrigated

17 May

115

22

28

14.5

2.37

6.0

20.8

 

Irrigated

7 Jun

72

21

25

10.6

1.95

6.3

20.4

 

Irrigated

28 Jun

73

18

27

3.9

0.52

6.3

20.1

 

Irrigated

19 Jul

69

19

24

9.4

1.90

4.8

20.5

 

IRRIGATED

MEAN

89

20

27

12.2

1.80

6.0

20.7

Drought

18 Jun

36

18

19

1.3

1.36

3.8

14.4

OZP0703

Irrigated

26 Apr

129

18

28

12.6

1.71

5.1

18.9

 

Irrigated

17 May

132

17

29

18.0

1.73

5.5

16.4

 

Irrigated

7 Jun

102

16

24

12.0

1.72

4.5

20.8

 

Irrigated

28 Jun

96

16

26

7.3

1.28

3.9

17.8

 

Irrigated

19 Jul

99

16

28

11.6

1.85

3.4

23.7

 

IRRIGATED

MEAN

111

17

27

12.3

1.66

4.5

19.5

Drought

18 Jun

40

15

17

1.4

1.45

3.3

16.9

Dryland Trial

Twenty two entries of field pea from a NSW Stage 4 variety trial were sown with a cone seeder on the 18th June 2007 at Wagga Wagga Agricultural Institute. Plots were 22 m2 in size (12.2m long by 6 rows wide) and replicated 3 times. Unlike the phenology trial, this experiment was not watered and suffered severe moisture stress with Jun-Oct rainfall only 40% of the long term average (100 mm c.f. 249 mm). Pod mapping and yield analysis were conducted on all genotypes in this experiment using identical procedures outlined in the phenology trial above.

Results

Drought severely reduced growth, development and yield of all field pea varieties (Table 1). While both drought-affected and well-watered plants from equivalent sowing dates flowered at similar node number, droughted plants were severely stunted and produced far fewer reproductive and total plant nodes. Drought also resulted in large reductions in pods per node, seeds per pod and seed size (100 seed weight).

Delays in sowing (from 26th April to 19th July) resulted in similar but less severe trends, despite ample irrigation. Stem height declined, as did pods per node, seeds per pod, seed number per stem and seed weight per stem. However delays in sowing had little or no effect on the number of flowering nodes, fertile podding nodes, total stem nodes and seed size (100 seed weight). Node number to first flower declined as sowing was delayed, contrary to earlier observations that suggested this to be relatively constant for a given cultivar.

Figure 1. Mapping seed weight per node (1a), 100 seed weight (1b), seed number per pod (1c) and pods per node (1d) for Kaspa, Yarrum and OZP0703 field pea at Wagga Wagga in 2007. Each genotype was sown at five times under an irrigated regime (26 April, 17 May, 7 June, 28 June and 19 July) and one time under drought conditions (18 June). The histograms map the particular character from the lowest reproductive node (left) in a sequential pattern to the top (right), reaching in some instances a maximum of 16 reproductive nodes.

Production of individual nodes within the pod canopy is depicted in Figure 1, showing a progressive decline in performance from the first (lowest) reproductive node to the top (up to 16 flowering nodes). This is particularly evident in the number of pods produced at each node, seeds per pod and seed weight per node. The drought-affected plants failed to produce seed beyond their second reproductive node and performance at these nodes was well below equivalent nodes on irrigated plants.

Cultivars differed in several key areas. There were consistent differences in nodes to first flower across all sowing dates and moisture regimes, with Kaspa having the greatest number (mean 23) and OZP0703 the least (mean 17) out of the three varieties reported here. Total stem nodes followed the same ranking. OZP0703 had the greatest number of flowering nodes and fertile nodes of the three varieties. Yarrum consistently had the highest number of fertile pods per node and seeds per pod across the sowing dates.

Discussion and Conclusion

This work clearly demonstrated the huge losses in productivity of field pea crops from severe drought and delays in sowing. These losses can be traced to a reproductive node level showing the number of reproductive nodes per stem, the number of fertile pods per node and the number of seeds per pod became increasingly sensitive to moisture and temperature stresses.

These studies highlight the importance of the lower reproductive nodes. The greatest productivity of the field pea crop occurs at these nodes, and the proportion of yield produced at these nodes increases as stresses from drought and delays in sowing intensify. These studies also identified genetic variability for pods per node and seeds per pod in current cultivars and breeding lines. This suggests hardiness and adaptability of the crop in an Australian environment (where heat and moisture stress frequently occur) could be readily enhanced by putting greater emphasis on productivity of lower reproductive nodes, particularly pods per node and seeds per pod. Upper reproductive nodes provide the plant with plasticity, enabling it to respond to extended favourable conditions, or some insurance against damage at lower nodes from frost, moisture, insects and/or disease.

Similarly, selecting for these same attributes could greatly enhance productivity in more favourable seasons - it is these lower nodes that are best placed to take advantages of the milder, longer and more favourable conditions during the earlier phase of grain fill. These pods are also best positioned to utilise photosynthates mobilised from nearby vegetative structures.

The variety Yarrum displayed some of these characteristics in these experiments, producing more pods per node and seeds per pod, particularly evident at the lower nodes. This may help explain Yarrum’s 6% yield advantage over Kaspa and Parafield in recent yield trials in southern NSW (Armstrong et al 2008).

This work (supported by current more detailed analysis and data interpretation across the full compliment of genotypes studied) suggests field pea breeding based more on nodes to first flower, pods per node, seeds per pod and seed size at lower reproductive nodes could deliver better adapted and higher, more stable yielding varieties for Australian farming systems.

Acknowledgements

The authors would like to acknowledge Peter Shephard for his work in managing these trials.

References

Armstrong E and Pate JS (1994). The field pea crop in SW Australia. II Effects of contrasting morphology and environment on reproductive performance. Aust. J. Agric. Research 45, 1363-78.

Armstrong EL, Matthews PW, Haskins B, Schwinghamer M, Jenkins L and van Leur J (2008). Field pea. In. Winter Crop Variety Sowing Guide 2008, NSW Department of Primary Industries, 81. Ed. McRae FJ, McCaffery DW & Matthews PW.

French RJ (1990). The contribution of pod numbers to field pea (Pisum sativum L.) yields in a short growing season environment. Aust. J. Agric. Research 41, 853-62.

Knott CM (1987). A key for stages of development of the pea. Annals Applied Biology, 111, 233-44.

Walton G H (1991). Morphological influences on the yield of field pea. Aust. J. Agric. Research 42, 79-94.

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