Extraordinarily high leaf selenium to sulfur ratios define 'Se-accumulator' plants

Abstract

Background and aims: Selenium (Se) and sulfur (S) exhibit similar chemical properties. In flowering plants (angiosperms) selenate and sulfate are acquired and assimilated by common transport and metabolic pathways. It is hypothesized that most angiosperm species show little or no discrimination in the accumulation of Se and S in leaves when their roots are supplied a mixture of selenate and sulfate, but some, termed Se-accumulator plants, selectively accumulate Se in preference to S under these conditions.

Methods: This paper surveys Se and S accumulation in leaves of 39 angiosperm species, chosen to represent the range of plant Se accumulation phenotypes, grown hydroponically under identical conditions.

Results: The data show that, when supplied a mixture of selenate and sulfate: (1) plant species differ in both their leaf Se ([Se](leaf)) and leaf S ([S](leaf)) concentrations; (2) most angiosperms show little discrimination for the accumulation of Se and S in their leaves and, in non-accumulator plants, [Se](leaf) and [S](leaf) are highly correlated; (3) [Se](leaf) in Se-accumulator plants is significantly greater than in other angiosperms, but [S](leaf), although high, is within the range expected for angiosperms in general; and (4) the Se/S quotient in leaves of Se-accumulator plants is significantly higher than in leaves of other angiosperms.

Conclusion: The traits of extraordinarily high [Se](leaf) and leaf Se/S quotients define the distinct elemental composition of Se-accumulator plants.

Fig. 1.

Leaf Se concentrations of 36 angiosperm species grown hydroponically in a complete mineral nutrient solution containing 910 µm sulfate and 0·63 µm selenate, ranked according to their relative shoot Se concentrations estimated from a meta-analysis of literature data (White et al., 2004). The species numbers are: (1) Sorghum bicolor, (5) Bromopsis inermis, (8) Panicum miliaceum, (14) Atriplex hortensis, (19) Trifolium subterraneum, (23) Lolium multiflorum, (30) Dactylis glomerata, (34) Lycopersicon pennellii, (40) Medicago sativa, (43) Medicago lupulina, (50) Trifolium pratense, (56) Bouteloua gracilis, (60) Solanum tuberosum, (65) Hordeum vulgare, (70) Holcus lanatus, (80) Cynodon dactylon, (84) Sinapis alba, (86) Beta vulgaris, (93) Astragalus sinicus, (104) Astragalus glycyphyllos, (112) Agrostis stolonifera, (123) Sporobolus airoides, (135) Solanum melongena, (142) Raphanus sativa, (153) Cucumis sativa, (163) Brassica oleracea, (168) Helianthus annuus, (169) Oryzopsis hymenoides, (171) Linum usitatissimum, (172) Brassica juncea, (174) Oryza sativa, (175) Stanleya pinnata, (176) Brassica nigra, (179) Trifolium repens, (180) Brassica carinata, (181) Astragalus racemosus.

Fig. 2.

Frequency distributions of (A) the natural log of leaf Se concentrations (mg Se kg⁻¹ d. wt), (B) the natural log of leaf S concentrations (g Se kg⁻¹ d. wt) and (C) leaf Se/S quotients (mg Se g⁻¹ S) in 39 angiosperm species grown hydroponically in a complete mineral nutrient solution containing 910 µm sulfate and 0·63 µm selenate. Lines indicate the log-normal (A, mean = 2·35, s.d. = 0·496, n = 37; B, mean = 1·87, s.d. = 0·520, n = 39) or normal (C, mean = 1·72, s.d. = 0·240, n = 37) distributions fitted to data from either all species or the 37 species with the lowest leaf Se concentrations.

Fig. 3.

(A) The relationship between leaf Se concentration and leaf S concentration in 39 angiosperm species, including two species with extreme leaf Se concentration, grown hydroponically in a complete mineral nutrient solution containing 910 µm sulfate and 0·63 µm selenate. The line indicates a leaf S concentration of 20 g kg⁻¹ d. wt. (B) The relationship between leaf Se concentration and leaf Se/S quotient in these 39 angiosperm species. The line indicates the Se/S in the nutrient solution supplied to the plants (1·71 mg S g⁻¹ Se). (C) The relationships between leaf Se concentration and leaf S concentration (closed circles) or leaf Se/S quotient (open circles) in a subset of 37 of these species. The horizontal line indicates the Se/S quotient in the nutrient solution supplied to the plants. The sloping line indicates the regression between leaf Se concentration (mg kg⁻¹ d. wt) and leaf S concentration (% d. wt) in these 37 species (y = 0·0567x + 0·053, R² = 0·946).