Salt tolerance in warm-season turfgrasses

As part of a wider project, Amenity Grasses for Salt-Affected Parks in Coastal Australia, glasshouse trials were carried out to determine the relative salt tolerance of 41 different turfgrasses.

The research was conducted by the then Department of Primary Industries and Fisheries (now part of the Department of Employment, Economic Development and Innovation) turf research team at Redlands Research Station, South East Queensland, with financial support from Horticulture Australia Ltd and Redland Shire Council.

The effects of salinity on growth

Salinity can affect growth via changed water relations, hormonal balance, or carbon supply – the relative importance of each of these processes changes depending on the time scale involved in measuring the response.

The initial effect of salinity is largely an osmotic one, reducing the ability of plants to take up water and inducing effects identical to water stress. This rapidly causes reductions in growth rate.

As exposure to salinity continues, more salt-specific effects on growth can be seen but take time to develop. If excessive amounts of salt enter the plant and cannot be partitioned within plant cells, toxic levels will eventually develop in the older transpiring leaves. The resultant internal injury reduces the plant’s photosynthetic capacity, which in salt-sensitive species continues to decline.

The salinity tolerance of turfgrasses (and other plants) is quantified in terms of their growth response to increasing levels of salinity. This is usually defined by the salt level that equates to a 20%, a 50%, or an 80% reduction in shoot yield, or alternatively the threshold salinity (at which point shoot yield starts to decline) together with the rate of yield reduction beyond that point. Because of the uncontrolled variation in salinity levels even over very short distances in the field, critical determinations of salt tolerance are invariably conducted in controlled pot experiments in the glasshouse, and later related to the field.

The experiments

A flood-and-drain hydroponic system was used to impose salt treatments (6 levels x 6 replicates) on a range of grasses, beginning in Experiment 1 with what were thought to be the most salt-tolerant group and then progressively screening less and less salt-tolerant grass groups in Experiments 2 and 3.

The different grass cultivars in each experiment received complete nutrients in solution, as well as variable amounts of salt to impose the six different treatment levels, through a flood-and-drain hydroponic system operating once a day to avoid waterlogging.

Each experiment involved three phases:

• a salt-free settling-in phase following planting of the pots (up to 4-6 weeks)
• a transitional phase in which the different treatments were gradually applied by progressively raising salt levels (at 3 weeks)
• a 12 to 14-week experimental phase during which measurements were taken on the grasses in the different salt treatments.

Sixteen grasses were accommodated in each of the three experiments, including three ‘standards’ which were present in all three experiments and a fourth standard which was dropped after the first two experiments.

Species (with numbers of cultivars in brackets) that have been screened:

• Digitaria didactyla (blue couch) (1 cultivar)
• Eremochloa ophiuroides (centipede grass) (1 cultivar)
• Distichlis spicata (saltgrass) (1 cultivar)
• Paspalum vaginatum (seashore paspalum) (4 cultivars)
• Sporobolus virginicus (marine couch) (3 diverse genotypes)
• Zoysia matrella (Manila grass) (5 cultivars)
• Pennisetum clandestinum (kikuyu grass) (3 cultivars/accessions)
• Stenotaphrum secundatum (buffalo grass) (9 cultivars)
• Cynodon dactylon (green couch) (14 cultivars).

The six salt treatments in the first experiment (0, 8, 16, 24, 32, 40 dS/m) covered a range to around 75% of seawater salinity levels. The second and third experiments, however, involved less salt-tolerant turfgrasses (nine Stenotaphrum secundatum, three Pennisetum clandestinum, and 14 Cynodon dactylon varieties), so the range of salinity covered by the six treatments was reduced in the two later experiments (0, 6, 12, 18, 24, 30 dS/m).

All grasses were clipped to a constant height at the start of the experimental phase. Subsequent fortnightly clippings were collected and oven dried to quantify dry matter production. Leaf height was measured prior to each cut. Per cent leaf firing was visually assessed fortnightly. At completion of both the second and third experiments, the grasses were removed from pots and crown and root material separated and washed prior to being oven dried at 60-70°C and weighed.

Some results

Salinity threshold levels for each of the species taken from the three screening experiments are shown below. As salinity tolerance rankings vary with exposure time, these rankings are not absolute and the discussion of species and cultivar tolerance draws on productivity and leaf firing (death) charts not presented here.

Table 1. 80%, 50%, and 20% threshold electrical conductivity (EC) levels in dS/m for each species of turfgrass screened for salinity tolerance. Species and cultivars are loosely ranked for salt tolerance, with the most salt-tolerant grasses listed first, through to the least tolerant species and cultivars.

• ¹ Used as a standard. Results averaged over three experiments.
• ² Used as a standard. Results averaged over two experiments.

Salt tolerant species

During the project the Redland Shire Council used seashore paspalum to good effect to revegetate salt-affected parkland. However, Table 1 shows that three other species from saline areas, Distichlis spicata, Sporobolus virginicus, and Zoysia matrella, are also highly salt tolerant. This provides additional choices for saline turf sites, depending on the presence or absence of other issues impacting on the growth of turf, such as shade or waterlogging.

It is difficult to define or quantify differences in salt tolerance among species, because any measured reduction in growth is also related to the time during which the plants have been grown under saline conditions.

In assessing the relative salt tolerance of the 41 cultivars screened, emphasis was placed on growth data obtained after extended (at least 2-3 months) exposure to salinity and on trends (such as whether or not leaf deaths – firing levels – were stabilising or continuing to increase).

The four most salt-tolerant species in our trials were:

• Sporobolus virginicus
• Distichlis spicata
• Paspalum vaginatum
• Zoysia matrella.

Collectively, these provide a range of options for turfgrass managers on salt-affected sites, because each is adapted to different environments (e.g. shade, waterlogging, and soil type) and management regimes.

There was some variation among the five Z. matrella lines screened, with Diamond the most tolerant and Royal the least tolerant. This may reflect different origins (coastal versus sub-coastal) for these lines. Nevertheless, all five Z. matrella lines were highly salt tolerant, and much better in this regard than the few plants of the single Z. japonica cultivar used to check the possible treatment range for that species in the future.

Although also coming from coastal origins, the Stenotaphrum secundatum cultivars were not as salt tolerant as the four species above. There was also variation among the nine cultivars tested in their level of salt tolerance, with Shademaster and Palmetto at the high end and Sapphire and ST-91 at the low end. Although Sapphire was still productive at 24 dS/m, it was the first to die off at higher salt levels.

The 14 Cynodon dactylon cultivars screened were only slightly less salt tolerant than the S. secundatum group, and the more salt tolerant C. dactylon varieties overlapped with the least salt tolerant S. secundatum cultivars.

FLoraTeX is rated by American authorities as the most salt-tolerant of the US C. dactylon varieties. In our experiment, Hatfield and Oz-E-Green were at least equivalent to FLoraTeX in terms of tolerance. As a tropically adapted Cynodon cultivar, Mountain Green also showed a usefully high level of salt tolerance, based on leaf firing, within this group. At the other end of the scale, Wintergreen and CT-2 (both extensively used commercially) were the least salt-tolerant Cynodon lines tested.

All three Pennisetum clandestinum lines showed poor salt tolerance. It would therefore appear that the frequent occurrence of P. clandestinum close to the beach along the east coast south of about the Gold Coast is probably restricted to low salinity niches, perhaps maintained by leaching with natural rainfall through the often sandy profile.

The two remaining standards, Digitaria didactyla Aussiblue and Eremochloa ophiuroides TifBlair, were sensitive to even quite low salt levels. Both also developed yellow foliage in the hydroponic system, which was corrected by foliar sprays with ferrous sulphate in the third trial.

Conclusion

As with other plants, turfgrasses range from extremely salt sensitive to highly salt tolerant. For turf managers, the results of glasshouse trials provide valuable information for matching turfgrass varieties to the range of soil salinity environments encountered at coastal and inland sites. The use of salt tolerant grasses is an important part of a range of practices that must be applied to effectively manage salinity problems. They are not however a substitute for sustainable long-term management practices.

Further information

• Copies of the full report ´Amenity Grasses for Salt-Affected Parks in Coastal Australia, TU02005´ (2006), DS Loch, RE Poulter, MB Roche, CJ Carson, TW Lees, L O´Brien and CR Durant, published by Horticulture Australia Limited, are available for download or purchase from Horticulture Australia Ltd
• Contact the DEEDI Customer Service Centre