Abstract
In this study, aqueous extracts of Calliandra haematocephala Hassk. leaves and inflorescences were tested on seeds of quinoa (Chenopodium album L.) and rice (Oryza sativa L.), and on some of the most noxious-associated weeds, Chenopodium album L. and Holcus lanatus L. in quinoa, and Echinochloa crus-galli (L.) P. Beauv., Echinochloa colona L., Eclipta prostrata L. and Rottboellia cochinchinensis (Lou.) W.D. Clayton in rice. The objectives were to identify extract concentrations at which 50 and 90% of germination (GR_{[50,90]}) and radicle elongation (RR_{[50,90]}) were inhibited, to fractionate inflorescence extracts for facilitating identifying the chemical group causing allelopathic effects, and to evaluate the fraction showing the stronger weed suppression effects and the least crop damage. Increasing extract concentration rates (0, 6.25, 12.5, 25, 50 and 100% crude extract) were applied to seeds of target crops and weeds. Flower extracts at rates < 0.30 produced GR_{[50]} and RR_{[50]} on H. lanatus, and GR_{[90]} and RR_{[90]} in C. album, while quinoa seeds were not affected. Rice and its target weeds were minimally affected by flower extracts, whereas radicle elongation of all species was significantly reduced. A concentration rate > 0.52 caused the RR_{[50]} on rice and all its target weeds. Fractions were quantitatively and qualitatively analysed to detect phytochemical groups, using specific chemical reagents and thin-layer chromatography (TLC). The fraction F3 from aqueous flower extract showed a high content of flavonoids, assumed as the potential allelochemical substance. Total flavonoid content in F3 was quantified as 2.7 mg of quercetin per g F3, i.e., 12.8 mg of quercetin per g of inflorescence material. Additionally, field equivalent extract rates obtained from the harvested fresh inflorescence biomass could be determined. These rates ranged between 90 and 143 mL l−1 of F3 aqueous fraction, while for ethanol F3 were 131 mL l−1. Our results are encouraging for finding sustainable and ecologically friendly alternatives for weed management in crops of high nutritional value, contributing also to counteract the growing problem of herbicide resistance.
Highlights
The advent of synthetic pesticides in the 1930s and 1940s shaped agriculture as it is known today (Hall et al 2000; Rattner 2009)
These effects prevailed for bioassays on quinoa during 2015 and 2016, without any significant difference between years ( P > 0.1 ), results are presented averaged over years
Seed germination was minimally affected by C. haematocephala leaf extract, while radicle elongation was reduced somewhat more (Fig. 1a, c)
Summary
The advent of synthetic pesticides in the 1930s and 1940s shaped agriculture as it is known today (Hall et al 2000; Rattner 2009). Crop yields have increased remarkably with the use of synthetic fertilizers and pesticides. Aspects referred to water and soil pollution and their persistence in the environment due to the misuse of pesticides are well documented Vonberg et al (2014), based on monitoring data of a shallow aquifer exposed to the effects of intensive agriculture in western Germany, reported the presence of atrazine (in concentrations above 0.1 μg l−1) after 20 years of being banned. The need of alternative crop management strategies, such as the use of natural products, is evident. Natural pesticides may show advantages, assuming that their persistence in the environment is considerably shorter than those synthetically produced
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