Persistence of Root Exudates of Sorghum bicolor and Solidago canadensis: Impacts on Invasive and Native Species

Root exudates of the invasive Solidago canadensis and the cereal crop Sorghum bicolor (L.) Moench cv. 'Hybridsorgo' were tested for allelopathic interactions against native and invasive plant species in a controlled environment. After the surface was sterilized, the seeds of two invasive species (Bromus sterilis and Veronica persica) and two native species (Youngia japonica and Rumex acetosa) were germinated and transplanted into the soil (1:1 mixture of coco peat and sand) that had been conditioned for one month by the cultivation of Solidago canadensis and Sorghum bicolor, both in combination or as unplanted controls. After an additional eight weeks of growth, morphometric measurements of the shoot and root, including foliar characteristics and above- and below-ground biomass accumulation, were performed. The results revealed significant inhibitory effects of root exudates released by Sorghum bicolor and Solidago canadensis on native species' productivity and physiology. The invasive species exhibited variable growth responses, with Veronica persica showing reduced shoot and root expansion, but Bromus sterilis revealed increased shoot and root biomass allocation and nutrition under the exudate treatments. Exudates from Solidago canadensis and Sorghum bicolor together showed synergistic negative effects on native species, while they promoted growth and nutrition in Veronica persica. Taken together, the differential species responses indicate that the tested native species were more sensitive to the allelopathic compounds than the invasive species, which is in line with the theory of novel weapons. The legacy effects of root exudates of both Sorghum bicolor and Solidago canadensis could promote invasive establishment through imposing allelochemical interference competition against native plant species. Understanding the specific allelopathic mechanisms may help with the development of integrated strategies for managing invasive species.

Caffeine: The Allelochemical Responsible for the Plant Growth Inhibitory Activity of Vietnamese Tea (Camellia sinensis L. Kuntze)

The present study aimed to examine the phytotoxic potential of seven Vietnamese tea samples based on the specific and total activity of caffeine and tea extracts on test plants. The sandwich method results indicated that the inhibitory effect of tea samples on the radicle and hypocotyl growth of lettuce seedlings was dependent on the concentration and type of tea samples, and also the presence of agar soluble allelochemicals. Among the seven tea samples, the leachates from Vinatea-green tea showed the highest inhibition on the radicle growth of lettuce seedlings with 50% suppression at 0.12 mg dry leaves/mL of agar. Caffeine concentration in tea samples analyzed by high-performance liquid chromatography (HPLC) varied from 20.7 to 38.2 µg/mL of dry leaves. The specific activity (EC50 value) of pure caffeine was 75 µg/mL, and the highest total activity of caffeine estimated in Vinatea-green tea was 0.51 [no unit]. Caffeine from green and oolong tea may be considered as one of the contributors to the inhibitory activity of the crude extract. Moreover, the phytotoxicity of pure caffeine and aqueous tea extracts was highly selective on the growth of different plant species. The concentration of caffeine detected from tea farm soil ranged from 0.137 to 0.145 µg/g soil. The results indicated that caffeine might be considered as a promising allelochemical from Vietnamese tea and can be a good candidate for weed management.

Herbicidal activity of formulated sorgoleone, a natural product of sorghum root exudate

BACKGROUND

The allelochemical sorgoleone, a major component of the hydrophobic root exudates of Sorghum bicolor, was formulated as a wettable powder [4.6 WP] and evaluated as a natural herbicide on several weed and crop species under different growth conditions.

RESULTS

Formulated sorgoleone [4.6 WP] suppressed germination and shoot growth of weeds, with broadleaf species showing greater susceptibility than grass weed species. Germination and growth of broadleaf weed species were completely suppressed (100%) at 0.2 g a.i. L(-1) sorgoleone in a growth chamber study. Post-emergence applications of the wettable formulation of sorgoleone [4.6 WP] inhibited 20-25% higher growth of weeds than pre-emergence applications under greenhouse conditions. Broadleaf weeds were more susceptible than grass species to both methods of application. In all studies, growth was suppressed in more than 90% of the broadleaf weeds and two species, in particular, Rumex japonicus and Plantago asiatica, were completely suppressed at 0.4 kg a.i. ha(-1) sorgoleone. The crop species, on the other hand, were much more tolerant to sorgoleone, with 30% inhibition, at most, at the highest rate of 0.4 kg a.i. ha(-1) sorgoleone.

CONCLUSION

The results of this study reveal that sorgoleone, after formulation as a WP, is more effective in inhibiting weed growth, and crop species are tolerant to it. The strong weed suppressive ability of formulated sorgoleone therefore offers interesting possibilities as an effective natural environment-friendly approach for weed management.