GENETIC IMPROVEMENT OF ALFALFA TO CONSERVE WATER

D. Claypool¹, R. Delaney¹, R. Ditterline² and R. Lockerman²

¹Department of Plant, Soil, and Insect Sciences
University of Wyoming

²Department of Plant, Soil, and Environmental Sciences
Montana State University

NOTE: Presented as a poster at Wyoming Water 1997 Conference, April 21-23, Casper, WY.

ABSTRACT: Irrigated alfalfa (Medicago sativa L.) is a high user of water and has received limited research to improve its water-use efficiency (WUE). Alfalfa is grown on approximately one-half of the cultivated land and has the highest consumptive water use of all crops in Wyoming. Alfalfa lines were developed which expressed lighter colored and larger leaves. The water relations and productivity of these lines were evaluated under a series of soil water levels in a controlled environment and under field environments in Wyoming and Montana. In the controlled environment the WUE of the pale and dark leaf genetic lines were 1.47 and 1.22 g dry forage kg^-1water, respectively, when averaged across seven soil moisture levels. Large-leaved alfalfa plants produced 1.43 and small-leaved 1.29 g dry forage kg^-1 water. Leaf transpiration per unit leaf area of pale leaves in irrigated field studies ranged from 12 to 17% lower than dark (normal) leaves. Transpiration of large leaves was 10 to 16% lower than small (normal) leaves. In field studies, the forage yield of pale leaves was 25 to 29% higher than dark leaves. The yield of large-leaved plants was 17 to 22% higher than small-leaved. These studies indicate the WUE of alfalfa can be improved through plant breeding to conserve water in the western U.S.

INTRODUCTION

Irrigated alfalfa (Medicago sativa L.) is a high user of water and has received limited research to improve its water-use efficiency (WUE). Alfalfa is grown on approximately one-half of the cultivated land and has the highest consumptive irrigation requirement (CIR) of all crops in Wyoming. A comparison of some Wyoming irrigated crops is shown in Table 1. Alfalfa breeding lines were developed which expressed lighter colored and larger leaves. The water relations and productivity of these lines were evaluated under a series of soil water levels in a controlled environment and under field conditions in Wyoming and Montana. These studies indicate the WUE of alfalfa can be improved through plant breeding to conserve water in the western U.S. A 15% reduction in consumptive water use of irrigated alfalfa in the 11 western states would reduce the agricultural water demand for this crop by 24,700 ha-m yr^-1 (2 million ac-ft yr^-1). Water consumption for alfalfa is also very important in Wyoming as the 175,000 ha of irrigated alfalfa represents greater hectarage than all other irrigated cash crops combined. A 15% reduction appears to be a realistic goal for an approach using physiological and morphological criteria with a plant breeding program. The objectives of these studies were to evaluate the transpiration and yield response of alfalfa breeding lines selected for reduced chlorophyll (lighter-colored leaves) and larger leaves.

Table 1. Consumptive irrigation requirements
(CIR) for selected crops at Wheatland, WY.

Crop	Growing season	CIR (mm)
Alfalfa	Apr.-Oct.	658
Lawn grass	Apr.-Oct.	574
Corn	May-Oct.	472
Dry beans	June-Sept.	343
Sugarbeets	Apr.-Sept.	511

(Pochop et al., 1992)

METHODS

Populations divergently selected for dark- and pale- leaflet and large- and small-leaflet characteristics were developed from 'Ladak 65' alfalfa according to methods described by Estill et al. (1991 and 1993). For controlled environment and field studies at Laramie, WY, twelve hundred plants of each population were established from Cycle-2 seed in the greenhouse. Plants were selected which expressed the traits for transplanting to complete the studies.

Controlled-Environment Studies

A commercial root medium (Metro-mix 220, W.R. Grace, Inc., Cambridge, MA) was used for plant culture. The root medium had a high water-holding capacity (wet density approximately three times dry density). Plants were inoculated with commercial Rhizobium meliloti Danegard. Controlled-environment chamber conditions were 21°C day (16h) and 18°C night (8h) at 40% relative humidity. Light intensity at the top of the plant canopy was 640 µmol m^-2 s^-1 for the dark/pale study and 840 µmol m^-2 s^-1 for the large/small study.

Water-use efficiency (WUE = grams of dry matter per kilogram of water transpired) was calculated from four replications of pure stands of each leaf type. A steady-state porometer was used to measure instantaneous transpiration. Plants were harvested at the early bloom stage and dried (80°C for 48h) to determine dry matter yield.

Field studies

Field studies were conducted at Bozeman, MT. A line-source sprinkler system was used to provide a decreasing soil water gradient at right angles to the source (Hanks et al., 1976). For each growth cycle (2 harvests per year) precipitation plus irrigation application averaged 284, 249, 178, 109, and 72 mm for soil water levels 1 through 5 (high, medium high, intermediate, low, zero applied water), respectively. Populations were uniformly watered once before the sprinkler gradient treatments were applied. Irrigation occurred when approximately 50% of available soil water was depleted in water level 2.

RESULTS AND DISCUSSION

Pale vs Dark Leaves

Total chlorophyll concentration for pale and dark leaves was 2.17 and 2.48 mg g^-1respectively for irrigated alfalfa and 2.43 and 3.04 mg g^-1 respectively for nonirrigated alfalfa.

Water-use efficiency of the pale-leaved population was superior to the dark-leaved population for five of seven moisture regimes in the controlled environment (Fig. 1 of Appendices). Averaged across the seven moisture regimes, pale leaf types produced 20% (P0.05) more dry matter per unit of water use. In this alfalfa study, the greater WUE observed for the pale types was primarily due to their superior forage yield, which averaged 43% (P0.05) more than the dark types across moisture regimes (yield data not shown). The observed relationship between alfalfa yield and WUE is in agreement with previous studies by Dobrenz et al. (1971). Reduced forage yield observed for both leaf types at the highest moisture regime did not adversely affect WUE. The reduced forage yield of alfalfa at moisture levels near field capacity is frequently observed in alfalfa (Peterschmidt et al., 1979). Chlorophyll concentration did not affect this response, although the pale plants maintained superior WUE. Reduced forage yield at the two lowest moisture regimes did reduce WUE for both leaf types. Carter and Sheaffer (1983) reported a similar WUE response to stress in alfalfa. The pale variants, however, maintained a small superior WUE at the lowest moisture level.

In Montana field studies, transpiration rate evaluated on a leaf-area basis was significantly higher for the dark-leaved population than the pale-leaved population at the two highest water levels (Fig. 2 of Appendices). The pale-leaved population expressed 11 and 16% lower transpiration than the dark-leaved population for the two highest water levels, respectively. Transpiration did not differ among populations at the three lowest water levels. The pale-leaved population yielded 15 and 29% more than the dark population for water levels 1 and 2, respectively (Fig. 3 of Appendices). Therefore, under normal soil water (irrigation) levels, both reduced transpiration and increased yield of pale leaves contribute to improved WUE.

Large vs Small Leaves

Leaf sizes varied with environment. Depending on the study and leaf node sampled, leaves of the large-leaved population were 35 to 75% larger than the small-leaved population. In all studies leaf size decreased for both populations as moisture stress increased.

Water-use efficiency of both leaf size populations decreased as moisture stress increased (Fig. 4 of Appendices). The large-leaved population had significantly higher WUE compared to the small-leaved population at the two driest moisture regimes. Across the four moisture regimes the large-leaved population averaged about 11% more forage production per unit of water. This higher efficiency resulted from the large-leaved population use of 51% more water but a 63% higher yield (data not shown) than the small-leaved population. Cole et al. (1970) observed that WUE was positively correlated with leaf size and forage yield in seedling plants grown under non-stressed moisture conditions. The higher WUE of the large-leaved population can partially be explained by the higher forage yield of this population. Instantaneous transpiration per unit leaf area was also lower for large leaves (data not shown).

Transpiration rate for the small-leaved population in the field at the Montana location was also significantly higher at the medium-high and intermediate water levels (Fig. 5 of Appendices). Forage yields of the large-leaved population were greater than small-leaved population at all five water levels (Fig. 6 of Appendices).

CONCLUSIONS

Leaf Color

Chlorophyll concentration can be genetically changed in alfalfa.

Chlorophyll concentration appears to be excessive in alfalfa.

Reducing chlorophyll decreases transpiration.

Reducing chlorophyll increases forage yield.

Reducing chlorophyll improves water-use efficiency.

Leaf Size

Leaf size can be genetically changed in alfalfa.

Larger leaves decrease transpiration.

Larger leaves increase forage yield.

Larger leaves improve water-use efficiency.

THEREFORE: OF THE IRRIGATION WATER APPLIED TO ALFALFA:

24,700 ha-m yr^-1 (2 million ac-ft yr^-1) IN THE 11 WESTERN STATES

OR

1,951 ha-m yr^-1 (158,000 ac-ft yr^-1) IN WYOMING

COULD BE SAVED ANNUALLY THROUGH PLANT BREEDING WITHOUT REDUCING FORAGE YIELD.

REFERENCES

Carter, P.R., and C.C. Sheaffer. 1983. Alfalfa response to soil water deficits: II. Plant water potential, leaf conductance, and canopy temperature relationships. Crop Sci. 23:676-680.

Cole, D.F., A.K. Dobrenz, M.A. Massengale, and L.N. Wright. 1970. Water requirement and its association with growth components and protein content of alfalfa (Medicago sativaL.). Crop Sci. 10:237-240.

Estill, K., R.H. Delaney, W.K. Smith, and R.L. Ditterline. 1991. Water relations and productivity of alfalfa leaf chlorophyll variants. Crop Sci. 31:1229-1233.

Estill, K., R.H. Delaney, W.K. Smith, and R.L. Ditterline. 1993. Water relations and productivity of alfalfa populations divergently selected for leaflet size. Field Crops Res. 33:423-434.

Dobrenz, A.K., D.F. Cole, and M.A. Massengale. 1971. Yield components and leaf characteristics associated with the water requirement of alfalfa. Crop Sci. 11:124-125.

Hanks, R.J., J. Keller, V.P. Rasmussen, and G.D. Wilson. 1976. Line source sprinkler for continuous variable irrigation: Crop production studies. Soil Sci. Soc. Am. J. 40:426-429.

Peterschmidt, N.A., R.H. Delaney, and M.C. Greene. 1979. Effects of overirrigation on growth and quality of alfalfa. Agron. J. 71:752-754.

Pochop, L., T. Teegarden, G. Kerr, R. Delaney, and V. Hasfurther. 1992. Consumptive use and consumptive irrigation requirements in Wyoming. Cooperative Extension Service and Wyoming Water Research Center Bull. MP-87.

APPENDICE OF FIGURES

Figure 1: Water use efficiency (WUE) of pale and dark leaf alfalfa variants grown in pure stands in seven moisture regimes in a controlled environment. Asterik indicates leaf type within a moisture regime is significantly different at P<0.05 (Estill, et al., 1991)

Figure 2: Instantaneous transpiration rates of pale and dark alfalfa variants grown at five soil water levels in the field at Bozeman, MT. Asterik indicates leaf type within a water level is significantly different at P<0.05 (Estill, et al., 1991)

Figure 3: Forage dry matter yields of pale and dark leaf alfalfa variants grown at five soil water levels in the field at Bozeman, MT. Asterik indicates leaf type within a water level is significantly different at P<0.05 (Estill et al., 1991)

Figure 4: Water use efficiency (WUE) of large and small leaf alfalfa variants grown in pure stands in four moisture regimes in a controlled environment. A water content of 3.2 kg water kg-1 root media represented 98% moisture-holding capacity and the 0.5 to 0.7 kg water kg-1 root media treatment exhibited midday wilting of some plants. Asterik indicates leaf type within a moisture regime is significantly different at P<0.05 (Estill, et al., 1991)

Figure 5: Instantaneous transpiration rates of large and small leaf alfalfa variants grown at five soil water levels in the field at Bozeman, MT. Asterik indicates leaf type within a water level is significantly different at P<0.05 (Estill, et al., 1991)

Figure 6: Forage dry matter yields of large and small leaf alfalfa variants grown at five soil water levels in the field at Bozeman, MT. Asterik indicates leaf type within a water level is significantly different at P<0.05 (Estill et al., 1991)