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The response of temperate pasture legumes to salinity and to saline-waterlogging

M.E. Rogers, D.W. West, C.L. Noble and D.M. Whitfield

Institute for Sustainable Agriculture, Dept of Agriculture, Ferguson Road, Tatura VIC 3616

Summary. The response of six commercial cultivars of white clover to root zone soil salinity was evaluated in the field over two irrigation seasons. Yield of all six cultivars decreased with increasing levels of soil salinity but the reduction was less in cvv. Haifa and Irrigation than in the other four cultivars. Photosynthetic efficiency rates in plots irrigated with saline water of 5.0 dS/m were significantly greater (P<0.05) in cv. Haifa than in cv. Tamar. A separate experiment evaluated the tolerance of five species of Trifolium, including white clover, to the combined effects of saline-waterlogging in the root zone. Balansa and strawberry clover were more tolerant than subterranean clover to these adverse conditions (in terms of yield and lower tissue ion concentrations, P<0.05) and adapted by producing adventitious roots and forming air spaces within the root tissue.

Introduction

In the irrigation areas of northern Victoria and in discharge areas of dryland regions, salinity and/or waterlogging affect more than 400,000 ha of agricultural land. This has resulted in widespread reduction in pasture growth particularly in Trifolium species which are the most salt and waterlogging sensitive components of pastures. Many Trifolium species are outcrossing and polyploid, and there is evidence that there is considerable inter- and intra-specific variation for both salinity and waterlogging tolerance (1, 2, 5).

This paper describes the results of two separate experiments at the Tatura, Victoria. The first, a field experiment, assessed the salt tolerance of commercial cultivars of white clover, 7'. repens, and examined the effect of salinity on some physiological growth mechanisms. This experiment forms part of an overall program studying the physiological and genetic mechanisms influencing salt tolerance in white clover. The second experiment, performed in the greenhouse, examined the response of five Trifolium species, including white clover, to the combined effects of salinity and waterlogging in the root zone.

Materials

Experiment 1

Field plots of six commercial cultivars of white clover (Aran, Haifa, Irrigation, Kopu, Pitau, and Tamar) were established in April 1989 with natural rainfall. Saline irrigation water at levels of 0.1, 2.5 and 5.0 dS/m was applied in December 1989 and continued throughout the 1989-90 and 1990-91 irrigation seasons. Each plot had an area of 10 m2 and there were five replicates of each treatment.

The plots were harvested and soil sampled to a depth of 90 cm at approximately four week intervals over the irrigation season. Soil samples were analysed for saturation extract electrical conductivity (ECe) and Na, Cl, K, Mg and Ca were analysed on both soil and plant samples since tolerance to high salinity in legumes is linked with the ability to maintain low leaf tissue concentrations of Na and Cl. Plant densities were measured at the beginning and end of the irrigation season. During the 1990-91 season, measurements of photosynthesis, stomatal conductance and transpiration were made on the canopies of selected plots of cv. Haifa and cv. Tamar using a portable photosynthesis system (Licor 6200) with a field chamber attached. Leaf water potentials and leaf area were also measured to examine how high external NaC1 concentrations may affect these physiological processes.

Experiment 2

Seedlings of five Trifolium species, subterranean clover, T. subterraneum cv. Clare, purple clover, T. isthmocarpum, T. purpureum cv. Paratta, balansa clover, T. michelianum cv. Paradana, and strawberry clover, T. fragiferum cv. Palestine, were transplanted into 10 cm pots containing washed river sand and placed into stainless steel tanks in a greenhouse. The NaC1 treatments (0, 20, 40 and 60 mol/m3NaC1), attained in increments of 20 mol/m3NaCI per day, were imposed after a first harvest at three weeks and plants were then subject to 0, 5, 10 or 15 days flooding. The experiment was a randomised block design with split plots containing six replicates each consisting of five plants of each species.

All plants were harvested on day 15 after the longest waterlogging treatment had been imposed. Plants were separated into roots and shoots, weighed, and the shoots were analysed for Na, Cl, K, Ca and Mg.

A follow-up experiment (without purple clover but including white clover) was undertaken to examine in more detail the adaptive root characters that enabled some Trifolium species to withstand periods of saline-waterlogging. Plants were subject to saline-waterlogging conditions similar to those described in the earlier experiment but with a restricted set of treatments (either 0 or 60 mol/m3 NaC1 and 0 or 15 days flooding). The experiment was conducted in a growth room at 17/23C and a 14 hour photoperiod and there were four replicates.

After yield and tissue ion analysis, one plant from each replicate was retained to measure the amount of new roots formed and for pycnometer measurement of root air spaces (4). A further subsample of root tissue was taken for viewing under a Hitachi 570 Scanning Electron Microscope at 10 kV.

Results and discussion

Experiment I

Shoot yield and plant density decreased significantly P<0.001) with increasing soil salinity levels in all cultivars of white clover. The results shown in Figure 1 are for plant yield in December 1990 after six irrigations with saline water over the 1990/91 season. The only significant differences in the intercepts occurred between cv. Aran (1.9320.141) and cv. Tamar (1.4240.206). Based on the soil ECe levels at which a 50% yield loss occurred, the salt tolerance rankings for the six cultivars were Irrigation (9.07 dS/m), Haifa (8.53 dS/m), Kopu (7.46 dS/m), Aran (5.86 dS/m). Tamar (5.32 dS/m) and Pitau (5.28 dS/m). The concentration of sodium and chloride in the shoots of all cultivars increased significantly (P<0.001) with increasing levels of soil salinity. The ability to limit the entry and distribution of Na and Cl into the shoot was related to the salt tolerance rankings observed in the yield measurements (e.g., at 5.0 dS/m, the concentration of chloride in the shoots of cv. Haifa was 1.44 mol/m3/kg dry weight compared with 1.78 mol/m3/kg dry weight for cv. Pitau).

Measurements of canopy photosynthesis revealed some significant salinity cultivar responses. At ambient air temperatures of less than 31C, the photosynthetic efficiency for cv. Haifa did not decrease with increasing soil salinity levels (P<0.05), however the photosynthetic efficiency rates for cv. Tamar were reduced by increasing levels of soil salinity (P<0.05), (viz 3.4 mole CO2/J in plots irrigated with 0.1 dS/m compared with 2.7 mole CO2/J at 5.0 dS/m). The photosynthetic rates for cv. Tamar at 5.0 dS/m were significantly less than those of cv. Haifa (P<0.05) and this may have been caused by the lowerrates of stomatal conductance in cv. Tamar (viz. 3.8 mol/m2/s compared with 4.5 mol/m2/s for cv. Haifa) or to the higher concentrations of Na and Cl that occurred in the shoots of cv. Tamar plants. At higher temperatures (i.e. >31C), photosynthetic efficiency rates for both cultivars were significantly lower in the plots irrigated with 2.5 and 5.0 dS/m than for the control plots for both cultivars (P<0.05). There was no significant difference in the shoot water potential between cultivars (P<0.05).

Figure 1. The effect of NaCl on the yield of six cultivars of white clover over a six week period. Equations for regression lines:
Tamar Y=1.424(0.206)-0.165(0.042)X
Other cv v. Y=1.932(0.141)-0.165(0.042)X.

Assessment of these field plots is continuing over the 1991/92 irrigation season. The evidence to date suggests that cv. Haifa has superior salt tolerance to the other cultivars and this information is supported by yield and tissue ion trends and by the photosynthesis data.

Experiment 2

In the first part of this experiment, yield in all five Trifolium species decreased significantly (P<0.05) with increasing levels of saline-waterlogging (Fig. 2), however individual species did vary in their response. strawberry and balansa clovers were significantly (P<0.05) less sensitive to periods of saline-waterlogging than were plants of subterranean and purple clover. The concentration of Na and Cl in the shoots of all species increased steadily and significantly (P<0.001) with increasing NaCI concentrations but there was no clear trend in concentrations of Na or Cl in response to increasing waterlogging levels. The shoots of purple clover (the species which was most sensitive to the saline-waterlogged conditions) consistently contained the greatest concentrations of Na and Cl but although concentrations reached 2.2 mol/m3/kg dwt and 2.1 mol/m3/kg dwt for CI and Na respectively, these levels did not produce leaf damage.

The ability to develop air spaces or aerenchyma in root tissue varied significantly (P<0.05) between species (Table 1) and was related to tolerance to saline-waterlogging. Roots of subterranean clover plants developed significantly less (P<0.05) new roots or aerenchyma than did roots of strawberry or white clover (viz under saline-waterlogged conditions new root development in subterranean clover was 1.1% of the total roots formed compared with 17.9% and 24.1% for strawberry and white clovers respectively).

While the relative tolerances of Trifolium species to waterlogging alone have been reported previously (2, 3), there has been no research which examines the effects of both salinity and waterlogging on plant growth in Trifolium species. Our research clearly shows that there are significant species differences and suggests that adaptation to these conditions is related to the ability to develop aerenchyma and adventitious roots.

Figure 2. The effect of salinity and waterlogging on the pooled yield of five species of Trifolium grown in the greenhouse.

Table 1. Percentage of air spaces in the total root tissue in four Trifolium species subject to periods of saline-waterlogging.

Acknowledgments

We thank Ian Treacy for technical assistance. This research was funded by the Victorian State Salinity Program.

References

1. Ab-Shukor, N.A., Kay, Q. 0. N., Stevens, D.P. and Skibinski, D.O.F. 1988. New Phytol. 109, 483-490.

2. Francis, C.M. and Devitt, A.C. 1969. Aust. J. Agric. Res. 20, 819-825.

3. Hoveland, C.S. and Webster, H.L. 1965. Agron. J. 57, 3-4.

4. Jenson, C.R., Luxmoore, R.J., Van Gundy, S.D. and Stolzy, L H. 1969. Agron. J. 61, 474-475.

5. Winter, E. and Lauchli, A. 1982. Aust. J. Plant Physiol. 9, 221-226.

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