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Response of mustard and canola genotypes to soil moisture stress during the post-flowering period

C.P. Gunasekera1, 2, L.D. Martin1, R.J. French3, 4 and K.H.M. Siddique4

1 Muresk Institute, Curtin university of Technology, Northam, WA 6401, Australia www.curtin.edu.au
Email Lionel.Martin@curtin.edu.au
2 Faculty of Agricultural Sciences, Wayamba University, Makandura, via Gonawila, NWP, Sri Lanka.
Email: gunasekp@hotmail.com
3 Dryland Research Institute, Department of Agriculture, Merredin, WA 6415, Australia, www.agric.wa.gov.au Email bfrench@agric.wa.gov.au
4 Centre for Legumes in Mediterranean Agriculture (CLIMA), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia www.clima.uwa.edu.au Email: ksiddiqu@fnas.uwa.edu.au

Abstract

Indian mustard has been identified as a potential and profitable alternative oilseed crop in Australia. Recent field studies at several sites in the low rainfall grain belt of WA suggest that mustard has superior adaptation to canola in the short season environments of southwestern Australia. The responses of selected mustard and canola genotypes to varying levels of post-flowering water stress was studied at Merredin. Detailed morphological and physiological measurements suggest that dry matter production of mustard was higher than canola under severe water stress. This was related to their superior osmotic adjustment and leaf water potential. Poor ability of mustard to convert its dry matter into seed yield (low harvest index) was related to the lower seed yield in mustard when compared to canola under post-flowering water stress. Future breeding effort in mustard should be directed to improving its harvest index through modification of its morphology and yield components.

Media summary

Mustard has many agronomic advantages over canola in the low rainfall grain belt of WA. Developing better adapted mustard genotypes will enhance the adoption of this crop as an alternate oil seed in the low rain fall cropping environments.

Key words

Canola, mustard, water stress, osmotic adjustment, dry land, adaptation

Introduction

Indian mustard (Brassica juncea L.) is reputed to be more drought tolerant than canola and therefore considered to be better adapted than canola to the Mediterranean-type short season environments of southern Australia. Recent field studies at Merredin, Mullewa and Newdegate suggests that mustard genotypes have better adaptation than canola in these environments (Gunasekera, 2003). The present study was conducted in order to understand the morphological and physiological basis of adaptation of mustard compared to canola under post-anthesis soil moisture stress conditions.

Methods

Experimental design and trial management

A field experiment was conducted at Merredin, WA (31o29’S, 118o18’ E) in the 2001 growing season. Two mustard breeding lines, Muscon (early maturing, short, condiment line) and 887.1.6.1 (early maturing, short, near canola quality line), and an early maturing commercial canola variety, Monty, were tested for their response to water stress after flowering. Crops were sown in three adjacent blocks with three stress treatments: (i) severe water stress (all rain excluded by covering with a rainout shelter during rainfall events after flowering), (ii) mild water stress (rainfed) and (iii) control (trickle irrigated), which were imposed from flowering (ie., 80 days after sowing when 50 % of plants produced their first open flower in all plots) to physiological maturity. Treatments were arranged in a randomized complete block design with four replicates. Plots 1.8 m wide (10 rows, 18 cm apart) and 4 m long were hand seeded on 6 June 2001 at the rate of 6 kg/ha. The soil was a sandy loam with a pH of 4.7 and 5.2 (in CaCl2) at 10 and 30 cm respectively. Standard weed and pest control practices were adopted. Post-flowering irrigation equivalent to the pan evaporation occurring since the previous irrigation was applied twice each week. In total, 228 mm of water was applied over a 53-day period.

Measurements

The leaf water potential (Ψl), osmotic potential (π) and relative water content (RWC) of leaves were measured in all plots of the rainout shelter and irrigated blocks weekly using the methods described by Turner (1988). Measurements were made from the date of 50% flowering until leaf senescence (78 to 142 days after sowing (DAS)) . Osmotic potential at 100% RWC was calculated (πsat) and osmotic adjustment was estimated by the difference in πsat between severe stress and irrigated treatments. Four quadrats, 1 m2 each, were harvested randomly from each plot at physiological maturity to determine the above ground dry matter and seed yield. All data collected and derived were statistically analysed using Genstat Statistical Software Package.

Results

Weather

Total annual rainfall in 2001 (364.5 mm) was higher than the long-term average (315.8 mm). Rainfall during the growing period (May to November) was 200 mm, which was lower than the long-term average of 224 mm for this period. June was exceptionally dry, but the July rainfall was 40 % higher than the long-term average. All plots received 111.7 mm rainfall from sowing until flowering. Plots in the rainfed and irrigated blocks received 48.4 mm rainfall after flowering, but this rainfall was excluded from the rainout shelter block. Plots in the irrigated block received 228 mm of water from 24 August to 16 October.

Leaf Water Potential

Leaf water potential (Ψl) of all genotypes did not differ significantly between treatments until after water stress treatments were imposed. Ψl of all genotypes under severe stress treatment decreased compared to that of the irrigated treatment two weeks after the treatments were imposed (Figure 1). Ψl of Muscon in severe stress treatment was significantly lower than that of irrigated treatment from 98 DAS until leaf senescence.

Figure 1. Leaf Water Potential (Ψl) in two mustard genotypes (887.1.6.1 and Muscon) and one canola variety (Monty) from severe water stress (solid symbols) and irrigated (open symbols) treatments at Merredin, WA in 2001. Bars indicate +/- LSD (P = 0.05) for selected dates and stress treatments when means are significantly different. Arrows indicate the date on which treatments were first imposed/ flowering.

Lead water potential of 887.1.6.1 and Monty in the severe stress treatment was significantly lower than that of irrigated treatment from 105 DAS until leaf senescence. There was no difference in Ψl between genotypes in the irrigated treatment at any date of measurement. However, Ψl of Monty was significantly higher than that of Muscon and 887.1.6.1 from 112 DAS until leaf senescence in severe stress treatment by 30 and 25% respectively (Figure 1). Although Ψl of Muscon and 887.1.6.1 were not significantly different until 126 DAS, Ψl of 887.1.6.1 decreased significantly faster compared to that of Muscon after 126 DAS until the leaf senescence. Therefore, at the leaf senescence, Ψl of 887.1.6.1 was significantly higher than that of Muscon.

Osmotic adjustment

Osmotic adjustment in Monty was low compared to the two mustard genotypes during the period of measurement (Table 1). In the two mustard genotypes, osmotic adjustment varied during the post-flowering period with the highest osmotic adjustment occurring at 126 DAS in 887.1.6.1 and 119 DAS in Muscon. Osmotic adjustment tended to be higher in 887.1.6.1 than in Muscon. Leaf water potential was correlated with osmotic adjustment in mustard but not in Monty (Figure 2). The coefficient of determination between Ψl and osmotic adjustment in 887.1.6.1, Muscon and Monty was 0.7, 0.3 and 0.1 respectively.

Table 1.Osmotic adjustment of two mustard genotypes (887.1.6.1 and Muscon) and one canola variety (Monty) under severe water stress at Merredin, WA in 2001. Measurements werer made between 105 and 133 days after sowing (DAS)

Genotype

Osmotic adjustment (MPa)

 

105 DAS

112 DAS

119 DAS

126 DAS

133 DAS

887.1.6.1

0.01

0.04

0.14

0.35

0.19

Muscon

0.00

0.16

0.30

0.04

0.00

Monty

0.02

0.06

0.12

0.17

0.08

LSD (P = 0.05) genotypes at each date = 0.19

Figure 2. Association between leaf water potential and osmotic adjustment in two mustard genotypes (887.1.6.1 - ♦ and Muscon - ■) and a canola variety (Monty) under severe water stress at Merredin, WA in 2001.

Dry matter and seed Yield

Final dry matter yield (FDMY) decreased significantly with increasing water stress (Table 2). FDMY was significantly higher in the irrigated treatment than in the severe stress or mild stress treatments in Muscon and 887.1.6.1 and that of Monty was significantly higher in the irrigated treatment than in the severe stress treatment. FDMY did not differ significantly between severe and mild stress treatments in any genotype (Table 2). 887.1.6.1 produced significantly more FDMY than Monty in all stress treatments. Mean FDMY in Muscon was significantly lower compared to that of 887.1.6.1, but significantly higher compared to that of Monty. Mean seed yields of 887.1.6.1. and Monty were significantly greater than that of Muscon (Table 2). Mean seed yield of the severe stress treatment was significantly lower than that of the mild stress treatment which in turn was significantly lower than that of the irrigated treatment (Table 2). Seed yield of the mild stress and irrigated treatments were not significantly different in any genotype.

Harvest Index (HI) was significantly lower under severe stress compared to the mild stress and irrigated treatments (Table 2). HI of 887.1.6.1 and Muscon was not affected significantly by the water stress treatments but that of Monty was significantly lower in severe stress treatment than in mild stress or irrigated treatments (Table 2). HI of Monty was significantly higher than that of 887.1.6.1 and Muscon regardless of the stress treatment. HI did not differ significantly between 887.1.6.1 and Muscon in any stress treatment.

Conclusion

Increased water stress in the post-flowering period significantly reduced dry matter production and seed yields in both canola and mustard in this study. However mustard showed superior adaptation to post-flowering water stress at Merredin. Mustard genotypes generated lower Ψl and maintained higher RWC (data not presented) due to their greater osmotic adjustment than canola. In general mustard produced greater dry

Table 2.The effect of genotype, water stress and their interaction on seed yield (t/ha), final above ground dry matter (t/ha) and harvest index (%) in three genotypes of mustard and canola under three water stress treatments at Merredin in 2001.

Genotype

Water stress treatment

  
 

Severe stress

Mild stress

Irrigated

Mean

Seed yield (t/ha)

887.1.6.1

1.2

1.6

1.9

1.6

Muscon

1.0

1.2

1.5

1.2

Monty

1.1

1.6

1.9

1.5

Mean

1.1

1.4

1.8

1.4

LSD (P=0.05)

Stress = 0.2; Genotype = 0.2; Stress x Genotype = 0.4

Final above ground dry matter (t/ha)

887.1.6.1

7.6

9.1

10.9

9.2

Muscon

6.8

7.9

9.9

8.2

Monty

5.6

7.0

8.4

7.0

Mean

6.6

8.0

9.7

8.1

LSD (P=0.05)

Stress = 1.1; Genotype = 1.0; Stress x Genotype = 1.7

Harvest Index (%)

887.1.6.1

15.6

17.6

17.4

16.9

Muscon

14.7

15.2

15.2

15.0

Monty

19.6

22.9

22.6

21.7

Mean

16.6

18.6

18.4

17.9

LSD (P=0.05)

Stress = 1.5; Genotype = 1.5; Stress x Genotype = 2.6

matter than canola under post-flowering water stress. Despite their superior adaptation to post-flowering water stress mustard had a poorer ability to convert dry matter into seed yield (low HI). Further breeding is required in mustard to modify its morphology and yield component structure.

Acknowledgements

CPG acknowledges the financial support from Curtin University of Technology for a Curtin International Student Scholarship and the Sri Lankan Government for a Presidential Scholarship. Ms Tammi Short, Muresk Institute, Curtin University of Technology provided the technical assistance.

References

Gunasekera, CP. (2003). Adaptation of Indian Mustard (Brassica juncea L.) to Short Season Dryland Mediterranean-type Environments. PhD Thesis, Muresk Institute, Curtin University of Technology, 168p.

Turner, NC (1988). Measurement of plant water status by pressure chamber technique. Irrigation Science, 9, 289-308.

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