Abstract
Rhododendron obtusum Planch. ‘Delaware Valley’ (Delaware Valley Azalea), Buxus sempervirens L. (common boxwood) and Cotoneaster divaricata Rehd. and Wils, (spreading cotoneaster) grown in sand culture received a complete nutrient solution containing Cu concns ranging from 0.032 ppm to 100 ppm. Development of Cu toxicity symptoms were recorded for 8 weeks, then Cu, Fe, and Mn levels in roots, stems, and leaves were determined. Chronic Cu toxicity symptoms were interveinal chlorosis and stunted growth. Acute Cu toxicity caused wilting, desiccation, and death of all affected plants. Tissue Fe and Mn levels were not consistently correlated to the Cu concn of the treatment solution. Only tissue Cu levels proved useful as an index of Cu toxicity.
Copper toxicity symptoms have been recorded for a number of fruit and agronomic crops (3), but not for ornamentals. Ornamentals may also be subjected to high Cu levels in several ways. Some industries regularly emit smoke containing Cu which can be deposited on foliage and soil. Repeated applications of Cu fungicides, such as Bordeaux mixture may result in toxic Cu levels in soil (5). Finally, many woody ornamentals are sold balled and burlapped with Cu treated burlap, which may be toxic to enclosed plants if CuSO4 was the Cu source (2).
Diagnosis of a Cu toxicity problem is difficult because soil test results are hard to interpret, the symptoms are almost indistinguishable from other nutrient and physiological disorders, and little information exists regarding nutrient levels in Cu toxic plants. Soil test results are difficult to interpret because Cu is one of the most tightly bound cations, so it may be very localized in the soil. Its availability is also hard to determine since soil pH, cation exchange capacity, and organic matter content all influence the level at which Cu will be toxic (3).
Above ground symptoms of Cu toxicity are generally stunted growth and interveinal chlorosis (3), conditions which mimic both Fe and Mn deficiency. In fact, interrelationships between Cu, Fe, and Mn uptake have been found, with the levels of each affecting the uptake of the others (1). Foliar analysis has shown that leaves of Cupoisoned plants may be Fe deficient. This deficiency may be overcome by supplying the plant with Fe-EDTA, but it does not reduce the toxic effect of Cu on the roots, which either accompanies or precedes leaf chlorosis (6).
To aid nurserymen and landscape contractors in identifying Cu toxicity of woody ornamentals, an experiment was carried out to obtain visual symptoms of Cu toxicity and tissue analysis data which would be useful in pinpointing a Cu toxicity problem.
Materials and Methods
Softwood cuttings of ‘Delaware Valley’ Azalea, common boxwood, and spreading cotoneaster were rooted under mist in unsterilized sand, transplanted into acid washed silica sand (particle size 0.3 to 1.0 mm) in 454 ml (16 oz.) containers, and watered with a complete nutrient solution. Iron was supplied in the chelated (EDTA) form to prevent its precipitation from solution. After plant establishment the Cu concn in the nutrient solution was adjusted with CuSO4 to vary from 0.032 ppm (control) to 100 ppm. Plants were observed for visual symptoms of Cu toxicity. After 8 weeks roots, stems, and leaves were dry ashed and analyzed for Cu, Fe, and Mn using a Perkin Elmer 303 atomic absorption spectrophotometer.
Four replications of cotoneaster and boxwood and 3 replications of azalea were grown. However there was not enough dry weight produced for individual analysis of boxwood or all of the cotoneasters. Analyses of all boxwood, and cotoneasters receiving 5 ppm Cu or above, were done on a composite of the 4 replications.
Results and Discussion
The Cu concns used caused 2 types of injury to the plants. At concns of 50 ppm Cu and below, chronic injury was induced resulting in the gradual decline of the plants. At 100 ppm Cu acute injury was induced, resulting in the sudden death of all affected plants.
Chronic injury is the type injury which would most commonly be found in ornamentals. It was induced in azalea and cotoneaster by Cu concns between 1 and 50 ppm, and in boxwood between 5 and 50 ppm. Generally, chronic symptoms were interveinal chlorosis and stunted growth (Figures 1 and 2), except for boxwood which did not become chlorotic but was simply stunted. The location of the chlorosis in azalea and cotoneaster was dependent on concn. With Cu concns between 1 and 5 ppm chlorosis began on the new growth, while at 50 ppm bottom leaves became chlorotic first. The higher Cu concns resulted in faster symptom expression. Chlorosis may have developed at lower Cu levels if chelated Fe had not been used, as it has been shown to alleviate the effects of high Cu concns (6). Surviving cotoneasters were pruned during the experiment, and this seemed to accentuate the problem. New growth was very severely stunted and chlorotic with 5 ppm causing all but terminal leaves to drop.
Chlorotic Delaware Valley White Azalea following 7 weeks treatment with 5 ppm copper.
Comparison of the effects of copper treatment on spreading cotoneaster. The plant on the left received 0.032 ppm copper and the plant on the right 5.0 ppm copper.
Chronic injury of the roots resulted in thicker main roots and fewer lateral roots (Figure 3). Dark stubs present on the root system were apparently lateral roots which were killed before elongating (Figure 4). Necrotic lesions were common on affected roots, which were also darker than healthy roots.
Effect of copper concentration on azalea roots. Lateral branching is reduced by chronic copper levels.
Chronic copper toxicity injury on boxwood roots. Necrotic lesions and undeveloped laterals are characteristic.
Copper at 100 ppm caused acute injury, but far exceeds the amount of Cu normally available to a plant. General symptoms of acute injury were wilting (Figure 5), dessication, and death of all affected plants. The youngest leaves were affected last, but newly expanded leaves remained small and poorly developed.
Wilting is a symptom of acute copper injury. Silver halrs ware also prominent on young azalea leaves.
Other symptoms of acute injury were species related. Azaleas showed a gray discoloration of new leaves; and, as the wilting occurred, the hairs on the new growth became very prominent and appeared silver in color (Fig. 5). On cotoneaster and boxwood there occurred either a brown discoloration or chlorosis beginning at the base of the leaf and spreading outward (Figures 6 & 7), until the leaves abscised. With Cu toxicity, as opposed to water stress symptoms, the leaf margin is affected last. Lower leaves curled upward toward the stem, especially on boxwood, and some rosetting of terminal growth was evident. With boxwood only, leaves and stems became chlorotic, then leaf veins turned charcoal gray, beginning with the midvein and proceeding toward the margins.
Basal chlorosis of cotoneaster caused by treatment for 10 days with 100 ppm copper.
Basal necrosis of cotoneaster caused by treatment for 10 days with 100 ppm copper
Acute injury was hard to define on the roots because the plants declined so quickly. However, darkened root tips, necrotic lesions, and some desiccation were apparent when the plant roots were washed (Figure 8).
Comparison of chronic and acute levels of copper on boxwood roots.
No consistent correlation was found between the Cu level in the nutrient solution and either Fe or Mn levels in roots, stems, or leaves (Table 1). Again, the FeEDTA in the nutrient solution probably accounts for the normal Fe levels in plants supplied high amounts of Cu. The exceedingly high Fe concn found in azalea roots suggests Fe was accumulating in the azalea roots instead of being translocated. This was not the case though, as the stems and leaves contained normal Fe levels. The Mn concn of cotoneaster roots and stems appeared to be abnormally low, yet the leaves contained adequate levels, precluding the appearance of foliar Mn deficiency symptoms.
Copper, Fe, and Mn concentrations in roots, stems, and leaves of Delaware Valley Azalea, common boxwood, and spreading cotoneaster grown under varying copper regimes.Z
The Cu concn of all plant parts of the 3 species tested was highly correlated to the Cu concn of the treatment solution (r = 0.9 or greater). Azaleas, boxwoods, and cotoneasters exhibiting Cu toxicity symptoms had leaf Cu levels of 11, 23, and 22 ppm or higher, stem Cu levels of 40, 61 and 37 ppm or higher, and root Cu levels of 257, 673, and 641 ppm or higher, respectively.
Copper toxicity symptoms are very hard to distinguish from other nutrient, and some physiological disorders. Soil tests are generally unsatisfactory in determining a Cu toxicity problem, as interpretation of the results depends on the analytical method and soil characteristics. Copper is toxic at much lower levels in a light sandy soil than in a soil high in organic matter or clay. High Cu levels resulting from Cu treated burlap are confined to the area around the burlap and would probably be missed by a soil test. Tissue analysis seems to be the only way to positively identify a Cu toxicity problem, and because Cu was the only one of the three elements analyzed that was consistently correlated to the Cu concn of the treatment solution, it is the only one which can be used as an index of Cu toxicity. Roots, stems, and leaves can all be used as indicators of Cu toxicity, but because of the magnitude of the difference between normal and toxic Cu levels in the roots, root Cu levels would seem to be the best indicator. In a related experiment though, Cu was found to be toxic at much lower levels (200 ppm) to roots of spreading cotoneasters grown in a media containing soil. Because of this and because the Cu concn which induced toxicity varied so much between both tissue and species, application of this data to other species must be done with caution.
Footnotes
↵1 Approved for publication as Journal Article No. 90-75 of the Ohio Agricultural Research and Development Center, Wooster, Ohio 44691. Taken from a thesis submitted by the senior author in partial fulfillment of the Master of Science Degree. The study was supported in part by the Chadwick Fund of the Ohio Nurserymen’s Association. The authors thank Connie Cobbs for technical assistance.
↵2 Graduate student and assistant professor, respectively, Department of Horticulture, Mailing address: The Ohio State University, Department of Horticulture, 2001 Fyffe Court, Columbus, Ohio 43210.
- © 1976, International Society of Arboriculture. All rights reserved.
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