Abstract
Two-year-old seedlings of seven woody ornamental species ranging in reported salt tolerance from good to poor were salinized with 250 ml of 0.25 N NaCI applied daily to the soil. Elaeagnus angustifolia and Rosa rugosa were not injured and contained the lowest leaf levels of Na and Cl. Elaeagnus umbellatus and Prunus tomentos showed the greatest injury while Acer platanoides, Prunus besseyl, and Rhus glabra were intermediate in tolerance. Leaf Na contents did not reflect the degree of injury among Elaeagnus umbellatus, Prunus tomentosa, Acer platanoides, Prunus besseyi, and Rhus glabra. However, leaf C| content in all injured species was greater than 2.5 percent and more accurately reflected the severity of injury although the most seriously injured species did not contain the greatest leaf Cl levels.
Evaluations of woody plant salt tolerance have been primarily serendipitous in nature (2, 7, 8, 10, 11, 14, 15). Controlled testing for plant salt tolerance has been slow in developing (1, 3, 4, 12) but offers the only logical approach for accurately determining the degree of resistance or susceptibility (5). Several authors (2, 7, 8, 9, 10, 11, 12) have formulated lists of salt tolerant plants and categorized them as showing good, moderate, or poor tolerance. Unfortunately, a particular species was often listed under all three categories. A plant’s degree of tolerance to soil salts may be different than its aerial salt tolerance and, therefore, woody plants may have been incorrectly categorized as displaying complete resistance when, in fact, only one type of resistance was actually evaluated.
Evaluations of salt-induced injury must be based on salts, application methods (aerial-versus soil-applied), osmotic effects, shoot or leaf Cl levels, and perhaps tissue Na levels (4, 5). This study examined only the effects of soil salts on woody plant species ranging in reported tolerance (9, 12) from good to poor and attempted to further elucidate the relationship between leaf Cl and/or Na content and appearance.
Materials and Methods
Two-year-old seedlings of Elaeagnus angustifolia (Russian olive), Rosa rugosa (rugosa rose), Elaeagnus umbellatus, (autumn-olive), Prunus tomentosa (manchu cherry), Acer platanoides (Norway maple), Prunus besseyi (western sand cherry), and Rhus glabra (smooth sumac) were treated daily with 250 ml of soil-applied 0.25 N NaCI dissolved in deionized water. All plants were thoroughly established at the time of salinization and were growing in a soil:peat;perlite (v:v:v) medium (pH 5,0 to 5.5) in 15 cm plastic pots. The plants were fertilized with Hoagland’s solution (6) 3 times per week prior to salinity treatments and twice per week during the experiment, which was initiated on April 12 and terminated on April 24, 1977. Plants were maintained on a 15-hour photoperiod at 24 deg. C (day) and 20 deg. C (night). All species, except Prunus tomentosa, were evaluated (0=dead; 5=vigorous, no necrosis, dark green foliage) and leaves harvested on April 24. Prunus tomentosa was harvested and evaluated on April 19, since the majority of the leaves had abscised.
All leaf tissue was thoroughly washed in distilled water and oven-dried for 72 hours at 65 deg. C. The tissue was then ground through a 40-mesh screen in a Wiley mill. Na and Cl were determined as previously described (3). The experiment was a completely randomized design with ten single plant replicates per treatment.
Results and Discussion
Appearance
Soil salinity treatments resulted in marked visual differences among the seven species (Table 1). Elaeagnus angustifola and Rosa rugosa were the only species not affected by the salinity treatments. These species in most instances were rated among the most salt tolerant (5, 9, 12); however, Rosa rugosa has been rated as resistant and susceptible to aerial salts (9). Elaeagnus umbellatus, although taxonomically closely related to Elaeagnus angustifolia, was severely injured as was Prunus tomentosa. Our results with soil-salt tolerance of Elaeagnus umbellatus agreed closely with those of Buschboom (1) who rated the species as susceptible to aerial-applied salts. Generalizations concerning the salt tolerance of members of the same genus or family are a mistake and have led to erroneous evaluations (14, 15). Acer platanoides, Prunus besseyi, and Rhus glabra were injured but not as seriously as Prunus tomentosa or Elaeagnus umbellatus. There are conflicting evaluations of the degree of salt tolerance displayed by Acer platanoides and Rhus glabra. Several authors (9, 12) have ascribed good soil and spray tolerance to Acer platanoides; however, based on the results of this study, moderate tolerance to soil salts appeared a more realistic evaluation. Rhus glabra (9) was rated as tolerant to soil and spray salts but again a moderate rating appeared more accurate. Prunus besseyi should be rated as moderate to poor in degree of soil-salt tolerance while Prunus tomentosa and Elaeagnus umbellatus were severely susceptible.
Effect of soil-applied NaCI on the appearance of seven woody plant species.
Prunus tomentosa developed significant leaf necrosis after 3 days, and after 7 days all leaves were completely necrotic. Acer platanoides leaves developed a marginal necrosis that covered about one-half of the leaf area at the end of the 12-day period. Elaeagnus umbellatus leaves were completely necrotic and most had abscised after 12 days. Rhus glabra and Prunus besseyi exhibited no leaf necrosis, but the leaves showed wilting, pronounced fall coloration, and abscised from the basal portion of the plant. Among all injured species, injury progressed in an acropetal pattern. Elaeagnus angustifolia and Rosa rugosa appeared similar to control plants in all respects. These symptomatic differences in plant response to soil-applied salts illustrate the problem with using appearance (leaf characteristic response) as the sole criterion for assessing salt damage. If control plants are not included, accurate assessment of salt damage to plants is difficult, For example, Prunus tomentosa showed extensive foliar necrosis but Rhus glabra did not. Only comparison with the control-treated Rhus glabra permitted an accurate assessment of the soil-salt treatment effects. Growth data (fresh weight, dry weight) were not determined in this experiment since the treatment period was so short (12 days) and negligible growth would have occurred.
Sodium Content
Leaf Na content did not vary among the control-treated species; however, pronounced differences occurred among NaCI treated plants (Table 2). The two species which exhibited the greatest visual tolerance (Elaeagnus angustifolia and Rosa rugosa) contained the lowest leaf Na levels. In fact, there were no significant differences in leaf Na content between control and NaCI-treated plants for either species. Leaves of Prunus besseyi contained the greatest levels of Na followed by Prunus tomentosa, Elaeagnus umbellatus, Acer platanoides, and Rhus glabra. The leaf Na levels did not reflect the degree of visual injury except for the two resistant species. Acer platanoides, Elaeagnus umbellatus, and Prunus tomentosa, although similar in Na content, varied markedly in appearance (Table 1). Prunus besseyi did not differ in appearance index from Acer platanoides or Rhus glabra although the leaf Na content was greater. Previous work (3, 4) with honey locust and English ivy has shown that leaf Na content was not a consistent indicator of the degree of soil-applied NaCI-induced injury.
Leaf Na content of seven woody plant species.
Chloride Content
Leaf tissue Cl corresponded closely with the visual indices of soil-salt induced damage (Table 3). Although there were no significant differences in leaf Cl content among the control-treated species, Rosa rugosa contained lower leaf Cl levels than other species. Leaf Cl contents of Elaeagnus angustifolia or Rosa rugosa control and NaCI-treated plants were not significantly different. Prunus besseyi showed the greatest leaf Cl pattern while the other four species were similar. The severely damaged species showed significantly greater leaf Cl levels than resistant species. Previous investigations (5, 12, 13) have shown that Cl is more abundantly accumulated in leaf tissue than Na and more accurately reflected the degree of injury. Although Prunus besseyi accumulated the greatest levels of Na and Cl, it was not the most severely injured species.
Leaf Cl content of seven woody plant species.
Apparently, specific plants are able to accumulate greater levels of Cl (possibly Na) and pool the ions in non-metabolic areas. The critical Cl level (that which induces necrosis or some other form of visual injury) often ranges from 2 to 3 percent of dry weight (5, 13). When the Cl levels of the 5 severely injured species progressed beyond 2 percent, severe injury resulted. Na leaf content did not exhibit a consistent critical range among the injured species. The species which exhibit the greatest salt resistance are those which effectively preclude Cl and Na (5).
Future work will attempt to determine the biochemical and anatomical factors responsible for the pronounced salt-resistance of Elaeagnus angustifolia and Rosa Rugosa compared to the other species. Presently, selected conifers (Abies, Cedrus, Juniperus, Picea, Pinus, Pseudotsuga, Taxus, Tsuga) and Ilex species and cultivars are being screened for their tolerance to soil-and aerial-applied salts.
Footnotes
↵1 The author wishes to acknowledge the ISA for the grant that partially supported this research.
- © 1978, International Society of Arboriculture. All rights reserved.
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