Responses to ultraviolet-B radiation by purely symbiotic and NO3-fed nodulated tree and shrub legumes indigenous to southern Africa

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Tree Physiology, 24:181–192

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Responses to ultraviolet-B radiation by purely symbiotic and NO₃-fed nodulated tree and shrub legumes indigenous to southern Africa

Samson B. M. Chimphango (1), Charles F. Musil (2) and Felix D. Dakora (3, 4)

1. Botany Department, University of Cape Town, Private Bag, Rhondebosch 7701, South Africa / 2. Research and Scientific Services, National Botanic Institute, Private Bag X7, Claremont 7735, Cape Town, South Africa / 3. Research Development and Technology Promotion, Cape Technikon, P.O. Box 652, Cape Town 8000, South Africa / 4. Author to whom correspondence should be addressed (dakora@ctech.ac.za) / Received April 15, 2003; accepted July 11, 2003; published online December 15, 2003

Summary

Purely symbiotic and NO₃-fed nodulated seedlings of Virgilia oroboides (Bergius) T.M. Salter, Cyclopia maculata (L.) Vent and Podalyria calyptrata Willd. were exposed to biologically effective ultraviolet-B radiation (UV-B) to assess the effects of above- and below-ambient UV-B on growth, symbiotic function and metabolite concentrations. Seedlings were grown outdoors either on tables under ambient or 34 or 66% above-ambient UV-B conditions (UV-B₁₀₀ control, UV-B₁₃₄ and UV-B₁₆₆, respectively), or in chambers providing below-ambient (22% of ambient) UV-B (UV-B₂₂) along with a UV-A control and a photosynthetically active radiation (PAR) control. Exposure of seedlings to UV-B₁₆₆ radiation reduced (P ≤ 0.05) leaf and stem dry mass by 34 and 39%, respectively, in C. maculata, and reduced leaf nitrogen concentration (%N) by 12% in V. oroboides. Nodule %N in C. maculata and stem %N in P. calyptrata also decreased (P ≤ 0.05) in response to UV-B₂₂ radiation compared with the UV-A control, but not compared with the PAR control. Concentrations of flavonoids, soluble sugars and starch were unaltered by the UV-B treatments. Application of 1 mM NO₃ to UV-B₁₆₆-treated seedlings increased whole-plant dry mass of V. oroboides and P. calyptrata by 47 and 52%, respectively. Dry mass of organs, nodule %N and total N concentration of these species also increased with NO₃ application. However, NO₃ supply decreased (P ≤ 0.05) nodule dry mass, stem %N and leaf %N as well as root and leaf anthocyanin concentrations in C. maculata. In terms of UV-B × N interactions, dry mass of stems, roots, nodules and total biomass of NO₃-fed C. maculata seedlings were reduced, and nodule %N, total N and leaf anthocyanins were depressed by the UV-B₁₃₄ and UV-B₁₆₆ treatments relative to UV-B₁₀₀-treated seedlings. Although we found that above-ambient UV-B had no effects on growth and symbiotic function of V. oroboides and P. calyptrata seedlings, feeding NO₃ to these species increased (P ≤ 0.05) seedling growth. In contrast, purely symbiotic C. maculata seedlings were sensitive to the UV-B₁₆₆ radiation treatment, and adding NO₃ further increased their sensitivity to both the UV-B₁₃₄ and UV-B₁₆₆ treatments.

Keywords: Cyclopia maculata, flavonoids, nodulation and N₂ fixation, NO₃ supply, Podalyria calyptrata, soluble sugars, starch, Virgilia oroboides .