Other studies also indicate that P. juliflora has substantial allelopathic potential [26,45,46]. Allelochemicals could directly affect plant growth or also impact soil microbes [47]. Our unpublished studies suggest that soils amended with P. juliflora litter have higher microbial activity compared to soil amended with P. cineraria litter. P. juliflora allelochemicals might affect plant growth directly and also by influencing microbial activity or microbial communities. Higher concentrations of allelochemicals in P. juliflora compared to P. cineraria could be due to higher ploidy levels in P. juliflora. Trenchard et al. [48] studied ploidy of 10 Prosopis species including P. juliflora and found all 9 species to be diploid (2n = 2x = 28) except P. juliflora which was tetraploid (2n = 4x = 56). Invasive species are commonly polyploids [49–51] and polyploids may have an advantage over their diploid progenitors in having novel phenotypic variation resulting from increased variation in expression of dosage-regulated genes, altered regulatory interac- tions, rapid genetic and epigenetic changes [52]. The observed higher levels of phenolics or L-tryptophan from the leaf leachate of P. juliflora than that of P. cineraria could be a novel phenotype resulting from any of these novel gene expression mechanisms following polyploidization. Although many chemicals or combinations of chemicals could cause the apparent allelopathic effects we measured, we focused on the release of L-tryptophan from litter and in soil because of reports of this chemical in leachates of P. juliflora foliage by Nakano et al. [27]. We detected L-tryptophan in leaf leachates of both P. juliflora and P. cineraria, but the amounts of L-tryptophan in leaf leachate of P. juliflora were 73% higher than that of leachate of P. cineraria. The presence of higher concentrations of L-tryptophan in soil amended with P. juliflora leachate and its rapid disappearance from soil amended with P. cineraria leachate supports the potential for L-tryptophan to affect the phytotoxic nature of P. juliflora litter. In India the annual leaf litterfall of P. juliflora has been reported at 8.1 Mg/ha, and litterfall is highest between February and May [53] when native species (such as Butea monosperma, Holoptelea integrifolia, Carissa spinarum, Capparis sepiaria, C. decidua, Cenchrus ciliaris or Azadirachta indica) germinate and recruit. Also, the evergreen habit of P. juliflora ensures the consistent presence of L- tryptophan in soils. Thus P. juliflora litter possesses large amounts of a chemical which is present at far lower amounts in the leaf litter of the native congener P. cineraria. There are likely to be other allelopathic or antibiotic chemicals in the litter, leachate or root exudates of P. juliflora that we did not measure, but such higher concentrations of a particularly biologically active chemical produced by an invasive species is only partially consistent with the Novel Weapons Hypothesis. The Novel Weapons Hypothesis References 1. Callaway RM, Maron JL (2006) What have exotic plant invasions taught us over the past 20 years? Trends Ecol Evol 21: 369–374. 2. Callaway RM, Schaffner U, Thelen GC, Khamraev A, Juginisov T, et al. (2012) Impact of Acroptilon repens on co-occurring native plants is greater in the invader’s non-native range. Biol Invas 14: 1143–1155. 3. Pasiecznik NM, Felker P, Harris PJC, Harsh LN, Cruz G, et al. (2001) The Prosopis juliflora - Prosopis pallida Complex: A Monograph. Coventry, UK: HDRA. 4. Burkart A (1976) A monograph of the genus Prosopis (Leguminosae subfam. Mimosoideae). (Part 1 and 2). Catalogue of the recognized species of Prosopis. J Arnold Arborat 57: 219–249; 450–525. 5. Palacios RA, Burghardt AD, Frr´as-Herna´ndez JT, Olalde-Portugal V, Grados N, et al. (2012) Comparative study (AFLP and morphology) of three species of Prosopis of the Section Algarobia: P. juliflora, P. pallida, and P. limensis. Evidence for resolution of the ‘‘P. pallida–P. juliflora complex’’. Plant Systemat Evol 298: 165–171. 6. Larrea-Alca´zar DM, Soriano PJ (2008) Columnar cacti-shrub relationships in an Andean semiarid valley in Western Venezuela. Plant Ecol 196: 153–161. Invasion of Prosopis juliflora poses that some successful invaders may possess allelopathic [54], antibiotic [41] or herbivore defense chemicals [55] that are unique in the non-native range, giving the invasive species an advantage against evolutionary naive native species [44]. As reported here, small amounts of L-tryptophan also occur in the litter of the Indian native P. cineraria. However, the striking differences in the inhibitory effects of litter between the two Prosopis species suggest that L-tryptophan is unlikely to be the only active biochemical in the leaves of the invader. Our results add to a growing body of literature indicating that there is substantial species specificity in the effects of plant-released secondary metabolites [56]. Also, these species-specific interactions suggest that assemblages of plants, or communities, may be less individualistic than often thought (see [57]). Finally, our results suggest that regional evolutionary trajectories exist and that novel competitive mechanisms have the potential to disrupt coevolved interactions among long-associated native species. Supporting Information Table S1 Summary of statistical analysis of pH, electrical conductivity (EC), organic carbon (OC), phosphate-P and total organic N (TON) of soil treated with no leachate (control, C), P. cineraria (PC) or P. juliflora (PJ) leaf leachate (one-way ANOVA and posthoc Tukey’s test at p,0.05). (DOCX) Table S2 Summary of statistical analysis of total phenolic content of soil treated with no litter (control, C), P. cineraria (PC) or P. juliflora (PJ) leaf litter at rate of 12 mg/g soil, and incubated for 0, 1, 2, 3, 4, 6, 8, 10 and 14 days. (DOCX) Acknowledgments We thank Professors C.R. Babu and S. R. Yadav for their help in the identification of plant species. Field work at Hawaii, USA was carried out during 9-month appointment of Professor Inderjit as G.P. Wilder Chair Professor at the University of Hawaii Manoa´. Author Contributions Conceived and designed the experiments: RK RMC Inderjit. Performed the experiments: RK WG LDL PJS MR TG Inderjit. Analyzed the data: RK RMC Inderjit. Contributed reagents/materials/analysis tools: RMC Inderjit. Wrote the paper: RK RMC Inderjit. Contribution to write site description of Peru: WG. Contribution to write site description of Venezuela: PJS LDL. Contribution to write site description of Hawaii: TG. 7. Tiedemann AR, Klemmedson JO (1973) Nutrient availability in desert grassland soils under Mesquite (Prosopis juliflora) trees and adjacent open areas. Soil Sci Soc Am J 37: 107–111. 8. Tiedemann AR, Klemmedson JO (1977) Effect of mesquite trees on vegetation and soils in the desert grassland. J Range Manage 30: 361–367. 9. Tiedemann AR, Klemmedson JO (1986) Long-term effects of mesquite removal on soil characteristics: I. Nutrients and bulk density. Soil Sci Soc Am J 50: 472– 475. 10. Virginia RA, Jarrell WM (1983) Soil properties in a mesquite-dominated Sonoran desert ecosystem. Soil Sci Soc Am J 47: 138–144. 11. 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