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2001 10th AAC
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Pasture Physiology And Management
Lucerne Roots Growth As A Function Of Summer Watering 1

Lucerne Roots Growth as a Function of Summer Watering

J. Hoffmann¹, P. Eberbach¹, J. Virgona² and A. Katupitiya¹

¹School of Agriculture Charles Sturt University Wagga Wagga NSW.²School of Wine and Food Sciences, Charles Sturt University Wagga Wagga NSW.

Abstract

The reaction of the root system of lucerne (Medicago sativa) plants subjected to water stress was studied in a 24-core lysimeter. Plants were subject to summer soil moisture stress or given one irrigation to simulate a summer rainfall. Roots were observed through minirhizotron tubes. Root development in the irrigated cores was significantly greater than that in the dry cores with plant roots growing into the deeper zones as the soil dried. Ongoing drought failed to promote root extension. Root development was not significantly influenced by soil type or cultivar dormancy characteristic. Some development of the root system was observed at the 2 metres which was independent of the irrigation, soil type or cultivar. We conclude that a growth period to accumulate carbohydrate reserves may be necessary for a lucerne plant to extend roots in response to drought. Threshold root extension rates relative to soil water availability and transpiration requirement may determine the root response to drought.

Key Words

Lucerne, roots, minirhizotron, lysimeter.

Introduction

Dryland salinity threatens the productivity of the Murray Darling basin. In appropriate agro-climatic zones, lucerne pastures have the potential to alleviate that threat by developing deep root systems that can recover soil moisture from beneath the root zone of the annual crops and pastures (1). Extensive areas of the winter crop lands in southern Australia are farmed with a system that involves several years of annual winter crops followed by legume-based pastures. Lucerne can be included in the pasture phase to utilise unused soil moisture and reduce accessions to groundwater. However to maximise the ability of lucerne to dry subsoils a sound knowledge of the stimulii for the development of the root system is required. The results reported are part of a broader study examining the development and function of lucerne roots in a rainfed environment. This study examines the effect of summer rainfall on the root development of juvenile lucerne plants.

Materials and Methods

24 undisturbed soil monoliths (74 cm diameter * 250 cm deep), representing the two principal soil types of the region (kandasol and sodosol) were collected during the summer of 1998 and installed in the drainage lysimeter complex at Charles Sturt University Wagga Wagga. Two cultivars of lucerne (winter dormant Pioneer L34, and winter active Pioneer L90) were planted in May 1999. 125 mm of water was applied to half the cores on 18th February to simulate the effect of summer rain. Root development was assessed through 74 cm long horizontal minirhizotron tubes. A borescope was used to count roots crossing a graduated intersect line. Data shown are the means of three replicates.

Results and Discussion

Watered cores showed significantly more (P<0.05) root development in the 60-145cm zone. No significant differences were observed between soil types or cultivars. Some root development was observed at the 200cm level, however root growth at this depth was not affected by summer watering not related to soil type or cultivar (Table 1).

Table 1. Root intersections 74cm^-1 6 weeks after simulated summer rain (30th March).

*Depth*	W^a90K	W90S	W34K	W34S	D90K	D90S	D34K	D34S
*60cm*	22	22	31	26	3	0	1	2
85cm	7	14	16	16	0	1	0	11
145cm	22	26	7	10	7	2	0	3
200cm	0	0	0	0	1	0	1	0

^aW=Watered, D=Dry. 90=Pioneer L90 winter active, 34=Pionereer L34 winter dormant. K=Kandasol, S=Sodosol

Table 2. Root intersections 74cm^-1 10weeks after simulated summer rain (28th April).

*Depth*	W90K	W90S	W34K	W34S	D90K	D90S	D34K	D34S
*60cm*	22	5	9	21	21	1	1	2
85cm	10	20	18	16	4	0	0	13
145cm	43	42	23	39	12	1	1	11
200cm	0	0	8	0	2	8	2	0

Comparison of the root development behaviour over the two observation periods (Tables 1 & 2) showed that in the summer watering treatment, as soil moisture was depleted in the upper soil layers (65cm) root numbers declined, but simultaneously developed at further depth. These observations led us to the conclusion that plants respond to drying, or a decrease in the availability of soil moisture by developing roots further down in the soil where soil moisture and strength permit. The principal soil water extraction zone moved deeper into the soil over time. Plants were able to follow a drying zone downwards by extending roots, a finding which is in keeping with the observations of Lolicato (3) who found the extraction zone of lucerne progressing down the profile in response to surface water depletion.

Plants that are droughted for a period of time, as in the dry treatment in the present study, may lack the carbohydrate and water resources to grow roots (4,5). In contrast, plants growing during a drying period that were active photosynthetically maintained the transpiration stream by continually extending roots. Passioura (4) suggested that the water required to produce drymatter in the plant roots is probably double that required for shoot production and that roots consume assimilate more rapidly than shoots. Hence the ability of the plant to produce new roots to acquire water will be dependent on the rate of root extension in relation to the transpiration requirement, and the extent of available carbohydrate resources. Extending roots in an endeavour to meet a water deficit may only be feasible down to a threshold soil water potential, beyond which root extension may be uneconomic for the plant. Root extension to meet a water deficit or drought has been reported, for example (5,6). Extension due to increased water supply has also been reported (2), and is shown in this example. The inability of the plant to meet root extension and water extraction thresholds may account for apparently conflicting results.

Conclusion

Plants that were subject to ongoing soil moisture deficit produced significantly fewer roots than those where the deficit was temporarily relieved. Plants were able to follow a drying front downward to meet a soil moisture deficit. Other research has indicated that drought promotes root growth. These results suggest that drought will not promote growth of roots into water bearing soil unless the plant has previously been able to accumulate the required carbohydrate resources, and threshold extraction rates can be met.

Acknowledgments

This research is funded by Grains Research and Development Corporation.

References

1. Brun, L. J. & Worchster, B. K. (1975). Agron. J. 67, pp. 586-588.

2. Janson, C. G. (1975). New Zealand Journal of Experimental Agriculture, 3, pp. 223-229.

3. Lolicato, S. J. (2000). Aus. J. Exp. Agric. 40, pp. 37-45.

4. Passioura, J. B. (1983). Agricultural Water Management, 7, pp. 265-280.

5. Pietola, L. M. & Smucker, A. J. M. (1995). Agron. J., 87, pp. 1161-1169.

6. Sheafer, C. C., Tanner, C. B. & Kirkham, M. B. (1988) In: Alfalfa and Alfalfa Improvement, (Eds. A.A. Hanson, D.K. Barnes, R.R. Hill.) American Society of Agronomy: Madison, Wis. pp373-402

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Introduction

Materials and Methods

Results and Discussion

Depth

Wa 90K

60cm