Dose-response-a challenge for allelopathy?

Abstract

The response of an organism to a chemical depends, among other things, on the dose. Nonlinear dose-response relationships occur across a broad range of research fields, and are a well established tool to describe the basic mechanisms of phytotoxicity. The responses of plants to allelochemicals as biosynthesized phytotoxins, relate as well to nonlinearity and, thus, allelopathic effects can be adequately quantified by nonlinear mathematical modeling. The current paper applies the concept of nonlinearity to assorted aspects of allelopathy within several bioassays and reveals their analysis by nonlinear regression models. Procedures for a valid comparison of effective doses between different allelopathic interactions are presented for both, inhibitory and stimulatory effects. The dose-response applications measure and compare the responses produced by pure allelochemicals [scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one); DIBOA (2,4-dihydroxy-2H-1,4-benzoxaxin-3(4H)-one); BOA (benzoxazolin-2(3H)-one); MBOA (6-methoxy-benzoxazolin-2(3H)-one)], involved in allelopathy of grain crops, to demonstrate how some general principles of dose responses also relate to allelopathy. Hereupon, dose-response applications with living donor plants demonstrate the validity of these principles for density-dependent phytotoxicity of allelochemicals produced and released by living plants (Avena sativa L., Secale cereale L., Triticum L. spp.), and reveal the use of such experiments for initial considerations about basic principles of allelopathy. Results confirm that nonlinearity applies to allelopathy, and the study of allelopathic effects in dose-response experiments allows for new and challenging insights into allelopathic interactions.

Keywords: benzoxazinoids; hormesis; log-logistic model; scopoletin.

FIGURE 1

Stimulation of root growth of Medicago sativa at low doses of benzoxazolin-2(3H)-one and its description by the peaked logistic model [Schabenberger et al. (1999)]. ED₅₀ = effective dose causing 50% inhibition in root growth; LDS = limited dose for stimulation; M = dose giving maximum response; R² = 1 – residual SS / corrected SS.

FIGURE 2

Influence of test medium (silica sand, vermiculite, loam) on the effect of benzoxazolin-2(3H)-one on the root growth of Lactuca sativa (a) and the interdependence with mean potential cation exchange capacity [CEC_pot according to Scheffer and Schachtschabel (2002), Kuntze et al. (1994), LfU (2002)] (b). Response-curves normalized by the D parameter with C = 0; ED₅₀ = effective dose causing 50% inhibition in root length; asymptotic 95 % confidence interval in parentheses; nonlinear R² = 1 – residual SS / corrected SS; linear R² = coefficient of determination; r = Pearson correlation coefficient (significant at P = 0.01).

FIGURE 3

Effect of DIBOA (2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one), BOA (benzoxazolin-2(3H)-one), MBOA (6-methoxy-benzoxazolin-2(3H)-one), and scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one) on the relative increase in leaf area of Lemna paucicostata. Response curves normalized by the D parameter with C = 0; B = slope; ED₅₀ = plant density causing 50% inhibition; asymptotic 95% confidence interval in parentheses; R² = 1 – residual SS / corrected SS.

FIGURE 4

Effect of scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one) on the leaf area of Lemna paucicostata depending on the time after exposure (a) and the chronology of the interdependence between ED₅₀, slope B, and upper asymptote D for scopoletin (b). ED₅₀ = effective dose causing 50% inhibition.

FIGURE 5

Increase in amount of allelochemicals in hydroponic culture with plant density. (a) Density-dependent fluorescence (254 nm) mainly due to scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one) in aliquots from root exudates of Avena sativa cv. Jumbo (x plants/290 ml/6 d). (b) Density-dependent increase in amount of benzoxazinoids [DIBOA (2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one), DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one)] in root exudates of Secale cereale cv. Amilo (x plants/100 ml/2 hr; 13-d-old plants).

FIGURE 6

Effect of root exudates of Secale cereale cv. Amilo on fresh weight and phenylalanine ammonia-lyase activity (PAL) of leaves and roots of Sinapis alba.

FIGURE 7

Stimulatory allelopathy at low densities of Triticum aestivum (TRZAX) cv. Primus compared to Secale cereale (SECCE) cv. Amilo on Sinapis alba (a) and the amount of DIBOA (2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one) and DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one) in root exudates of both cultivars (30 plants/100 ml/2 hr; 12-d-old plants) (b). ED₅₀ = plant density causing 50% inhibition in root growth; asymptotic 95% confidence interval in parentheses; R² = 1 – residual SS / corrected SS.

FIGURE 8

DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one) content in root exudates (30 plants/100 ml/2 hr; 12-d-old plants) of two cultivars of Triticum aestivum, cultivated under two different light intensities [100% (300 μE/m²/s), 50% (150 μE/m²/s)] and the interdependence with their allelopathic activity on Sinapis alba 2. ED₅₀ = plant density causing 50% inhibition of root growth; R² = 1 – residual SS / corrected SS; r = Pearson correlation coefficient (significant at P = 0.05).

FIGURE 9

Growth inhibition of Sinapis alba by Triticum aestivum cv. Moldau in mixed cultivation versus monoculture (a) and the concentration of DIMBOA (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one) in the test solution (30 plants/x ml/6 d) and directly exuded (30 plants/100 ml/2 hr; 12-d-old plants) (b). ED₅₀ = plant density causing 50% inhibition in root growth; asymptotic 95% confidence interval in parentheses; R² = 1 – residual SS / corrected SS; small letters indicate significant differences (Tukey-test, P = 0.05).

FIGURE 10

Chronological development of the dose-response relationship for the interference of Triticum aestivum cv. Pegassos with Sinapis alba, displayed as generalized dose-response growth surface.

FIGURE 11

Comparison between the effects of root exudates of different plant species and cultivars on the root growth of Sinapis alba. (a) Secale cereale (SECCE) cv. Amilo versus cultivars of Avena sativa (AVESA; N = 8). (b) Triticum aestivum (TRZAX; N = 7) versus A. sativa cv. Jumbo. (c) S. cereale cv. Amilo, T. aestivum cv. Rimpaus Bastard II, T. durum cv. Extradur, and T. spelta cv. Ostar. (d) Correlation between allelopathic ability of cultivars of A. sativa (N = 9) and the exudation of scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one; 30 plants/100 ml/2 hr; 12-d-old plants). ED₅₀ = plant density causing 50% inhibition in root growth; B = slope; asymptotic 95% confidence interval in parentheses; nonlinear R² = 1 – residual SS / corrected SS; linear R² = coefficient of determination; r = Pearson correlation coefficient (significant at P = 0.01).

FIGURE 11

Comparison between the effects of root exudates of different plant species and cultivars on the root growth of Sinapis alba. (a) Secale cereale (SECCE) cv. Amilo versus cultivars of Avena sativa (AVESA; N = 8). (b) Triticum aestivum (TRZAX; N = 7) versus A. sativa cv. Jumbo. (c) S. cereale cv. Amilo, T. aestivum cv. Rimpaus Bastard II, T. durum cv. Extradur, and T. spelta cv. Ostar. (d) Correlation between allelopathic ability of cultivars of A. sativa (N = 9) and the exudation of scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one; 30 plants/100 ml/2 hr; 12-d-old plants). ED₅₀ = plant density causing 50% inhibition in root growth; B = slope; asymptotic 95% confidence interval in parentheses; nonlinear R² = 1 – residual SS / corrected SS; linear R² = coefficient of determination; r = Pearson correlation coefficient (significant at P = 0.01).

FIGURE 12

Differences in slope B and upper asymptote D depending on time of bioassay (April 2000; September 2000) exemplified by dose-response relations of 15 cultivars of Triticum aestivum (a) and lack of response levels near the lower limit for the dose-response relationship of T. aestivum (TRZAX) cv. Tamaro compared to cultivars of T. aestivum (TRZAX; N = 4), T. spelta (TRZSP; N = 3), and Secale cereale (SECCE; N = 1) (b). Asymptotic 95% confidence interval in parentheses; R² = 1 –residual SS / corrected SS.