Interference Potential of the Perennial Grasses Eragrostis Curvula, Panicum Maximum and Digitaria Eriantha with Parthenium Hysterophorus
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Tropical Grasslands (2008) Volume 42, 88–95 88 Interference potential of the perennial grasses Eragrostis curvula, Panicum maximum and Digitaria eriantha with Parthenium hysterophorus M. VA N DER LAAN1, C.F. REINHARDT1, In a laboratory study, seeds of the 3 grass R.G. BELZ2, W.F. TRUTER1, L.C. FOXCROFT3 species were exposed to pure parthenin in a AND K. HURLE2 germination bioassay. Based upon germination 1 University of Pretoria, Department of and early radicle development, E. curvula was Plant Production and Soil Science, Pretoria, the least sensitive and P. maximum the most sen- South Africa sitive to parthenin. Therefore, if P. maximum was 2 University of Hohenheim, Institute of to be used in a control or rehabilitation program, Phytomedicine, Department of Weed Science, it might be diffi cult to establish P. maximum Stuttgart, Germany stands from seed in areas already infested with 3 Scientifi c Services, Kruger National Park, parthenium. While transplanting seedlings of the Skukuza, South Africa grass would avoid the seed germination problem and give the grass a head-start, the practicality of this method on a large scale is open to question. Abstract The successful invasiveness of Parthenium Introduction hysterophorus (parthenium) is attributed to its competitive ability and high allelopathic The invader plant Parthenium hysterophorus potential. The compound, parthenin, has been (common name: parthenium) is globally rec- implicated as a major allelochemical in the plant. ognised for its threat to agricultural activities, A fi eld trial was established in Kruger National biodiversity and human health. The success of Park (South Africa) to investigate the inter ference parthenium as an invasive species is attributed to between parthenium and 3 indigenous grass its broad ecological adaptation (Hedge and Patil species, namely: Eragrostis curvula, Panicum 1982), strong competitive ability (Parsons and maximum and Digitaria eriantha. Grass seedlings Cuthbertson 1992), high fecundity and absence were transplanted from a glasshouse into fi eld of natural predators and diseases. Par thenium plots after failure to establish the seed in situ. Par- readily invades disturbed areas, such as along thenium seedlings were introduced at densities roadsides, and degraded rangeland (Haseler of 5 and 7.5 plants/m2 from areas adjacent to 1976; Pandey and Dubey 1989). It predominantly the fi eld trial. invades pastures in Australia, being vigorous in both developing and established pastures (Navie P. maximum displayed best overall growth per- et al. 1996). Nath (1988) reported that the weed formance and was able to completely suppress reduced forage production in grasslands by parthenium growth with time. The other 2 grass up to 90%. Parthenium is currently invading species performed less favourably, in terms of several prominent game reserves in South Africa, both growth rate and ability to suppress parthe- including Hluhluwe-iMfolozi and the Kruger nium. The ability of P. maximum to interfere National Park (Strathie et al. 2006). effectively with parthenium growth indicates that It is hypothesised that parthenium, in addition to this species has good potential for use as an antag- its competitive ability, is able to impede the recruit- onistic or rehabilitative species in containing the ment and/or growth of naturally occurring plant spread of the weed. species through the release of allelo chemicals. Phenolics and sesquiterpene lactones are impli- Correspondence: C.F. Reinhardt, Department of Plant Production and Soil Science, University of Pretoria, Pretoria cated as the 2 major chemical groups in parthenium 0002, South Africa. E-mail: [email protected] allelopathy. In particular, the sesquiterpene Interference between parthenium and grasses 89 lactone, parthenin, has been observed to exhibit then only garden refuse has been dumped, stop- dose-dependent toxicity effects on a range of test ping with the commencement of the trial. The plants, including aquatic species (Patil and Hedge trial site was cleared of vegetation and debris and 1988; Belz et al. 2007). Reinhardt et al. (2004; 36 plots, each measuring 4 m2, were demarcated 2006) observed that parthenin is sequestered at in a completely randomised design. Following high volumes in the capitate-sessile trichomes on failure to establish the grasses in situ from seed the surfaces of the leaves, and that a single parthe- in December 2003, E. curvula, P. maximum and nium plant can potentially introduce around 270 D. eriantha seedlings were raised in seedling trays mg of parthenin into the environment in a single in the University of Pretoria glasshouses. Seeds growing season. obtained commercially were sown into each tray Few studies have investigated the mecha- cell to form a tuft consisting of 5–8 seedlings. nisms of interference of P. hysterophorus with Once the seedlings attained a height of 4–7 cm, other plant species. Interactions between plants each species was transplanted into fi eld plots at are often the result of complex combinations 16 tufts/m2. Tufts were planted 25 cm apart in of specifi c mechanisms (Welden and Slauson rows parallel to 4 dripper lines (2 plant rows, each 1986; Callaway et al. 1991; Chapin et al. 10 cm off the dripper line) that spanned the plots 1994). Although the fundamentals of competi- at 50 cm intervals. P. hysterophorus seedlings, tion and allelopathy are generally understood as growing in the immediate vicinity of the fi eld isolated mechanisms, little is known about the trial, were transplanted into the plots at densities relative contributions of these 2 mechanisms to of 5 and 7.5 plants/m2. For the 5 plants/m2 den- overall interference between plants (Ridenour sity, parthenium plants were established between and Callaway 2001). Ecologists have identifi ed pairs of grass tufts along the dripper lines, and the importance of defi ning the individual effects additional plants were planted between the grass more precisely (Ridenour and Callaway 2001), rows that ran along the dripper lines for the but diffi culties in separating the effects experi- 7.5 plants/m2 density. Plots without parthenium mentally have hampered better understanding served as controls for the respective grass spe- (Fuerst and Putnam 1983). cies. A gravitational drip-irrigation system was The objective of this study was to investigate installed to supplement the region’s unreliable the growth of P. hysterophorus and 3 indigenous rainfall and as a safeguard against failure of the perennial grass species, Eragrostis curvula, trial. In the second growth season, parthenium Panicum maximum and Digitaria eriantha, at control plots at the 5 and 7.5 plants/m2 densities differing parthenium densities. These grasses were included. A wire fence enclosed the trial to were selected because they were inherently dif- prevent interference from animals. ferent in terms of growth form, vigour and Eight representative grass tufts and 6 repre- adaptability. In addition to the fi eld trial con- sentative parthenium plants were harvested from ducted in the Kruger National Park, a laboratory each plot after 18- and 14-weeks growth for germination bioassay was conducted to assess the 2003–2004 and 2004–2005 growth seasons, the sensitivity of these grass species to the allelo- respectively, and dry matter yields determined. chemical parthenin, as this may have implications Relative grass dry matter, as a percentage of when trying to establish an antagonistic or reha- the control, was logarithmically transformed bilitative species in a parthenium stand. to achieve normal distribution of the data and analysed using SAS®. P. hysterophorus dry Materials and methods matter accumulation data were analysed without transformation. A general linear model (GLM) Field trial of ANOVA was used, and means were separated In the 2003–2004 summer growing season, a by the multiple comparison Tukey test at P<0.05 2-year fi eld trial was established on an abandoned (unless otherwise stated). dumpsite (24°98′S, 31°60′E; elevation 263 m asl), which had been invaded by P. hysterophorus at Laboratory bioassay Skukuza in Kruger National Park, South Africa. The soil on the experimental site is classifi ed as Grass seed used for the germination bioassay was a solonetz form. Dumping of general refuse at from the same seed lot as for the fi eld trial. Pure the site ceased around 12 years earlier, and since parthenin was supplied by our research partner 90 M. van der Laan, C.F. Reinhardt, R.G. Belz, W.F. Truter, L.C. Foxcroft and K. Hurle based at the University of Hohenheim, Stuttgart, E. curvula, 8 days for D. eriantha and 10 days Germany. Extraction and purifi cation of parthenin for P. maximum, because of differences in rates were done from parthenium plants growing in the of germination between the species. Non-linear University glasshouses as described by Belz et al. regression analysis was conducted to fi t logistic (2007). A dose-response bioassay was conducted dose-response curves according to Streibig (1988) using a series of 12 parthenin concentrations using SPSS® regression models. ED50 (dose ranging from 0 to 500 µg/ml. Each concentra- causing 50% of the total response) values for tion in the series, including the control, contained radicle length and germination were calculated 1% acetone as solvent for parthenin. Owing to from equations for best-fi t regression lines, and differences in pre-determined seed germination compared using the F test for lack-of-fi t based on between the grass species, 10, 25 and 30 seeds of analysis of variance (P<0.05). E. curvula, P. maximum and D. eriantha, respec- tively, were placed into 9 cm diameter Petri dishes Results containing one layer of fi lter paper. The same volume of treatment solution (5 ml) was added to Field trial the Petri dishes, and 5 replications were used for each concentration. Petri dishes were sealed with Grass performance. Based on dry matter accumu- parafi lm, and placed in a growth chamber and lation, P.