Host influence on germination and reproduction of the facultative hemi‐parasitic weed Rhamphicarpa fistulosa

Abstract

Rice Vampireweed, Rhamphicarpa fistulosa, was a minor parasitic weed until recently when rice cultivation in sub-Saharan Africa was expanded into marginal wetlands, that are the parasite's natural habitat. Unlike most of the parasitic weeds, R. fistulosa is facultative, meaning that the parasite is able to complete its life cycle without a host. However, when not connected to a host plant, its biomass and seed production is lower. Because very little is known regarding the germination ecology of the parasite, the main objective of our study was to identify the cues that favour germination. We hypothesised that, first, being a wetland species, germination of R. fistulosa is stimulated by light and high soil moisture. Second, we hypothesised that if host plant presence increases its reproductive output then a germination stimulatory effect from host presence is likely to have developed. A Petri-dish and pot experiment showed that light and completely saturated soils were a requirement for germination, demonstrating that germination requirements of R. fistulosa are typical of species that grow in environments with fluctuating water levels. A pot experiment in which five infestation levels of R. fistulosa were installed in the absence and presence of a rice plant, showed that host plant presence resulted in a 3.7 times higher seed production rate and a 15% larger average seed size. Despite this reproductive advantage, a pot experiment with three rice cultivars, selected because of their difference in strigolactone production, showed that host plant presence, regardless of the development stage, did not influence the emergence rate of R. fistulosa. In a follow-up study, the germination stimulation effect of root exudates collected from the same three rice cultivars and a treatment consisting of an artificial germination stimulant (GR24) was compared with a treatment consisting of plain water. In these treatments, seeds of R. fistulosa were compared with seeds of the obligate parasite Striga hermonthica. Germination of S. hermonthica was strongly advanced by the presence of root exudates and GR24 but was completely absent in water, whereas germination of R. fistulosa in all treatments was similar to that in plain water. The absence of a host recognition mechanism at the germination stage suggests that the regulation of germination through light and soil moisture is near optimal. Our finding might also indicate that for this facultative parasitic plant species, a more opportunistic germination strategy is superior. Implications of the findings for management of R. fistulosa in rice cultivation are discussed.

Inefficiency of manual weeding in rainfed rice systems affected by parasitic weeds

“…In the case of parasitic weeds, an early intervention may be even more important as it will reduce the period of parasitism. In the case of Rhamphicarpa fistulosa , a facultative parasitic weed that starts as an ordinary weed and only starts parasitizing its host once it has developed into a seedling, early weeding may even completely avert parasitism (Kabiri et al., ).…”

Developing parasitic weed control strategies for rainfed rice production environments

“…Despite its wide application, it was only in Songea that hand weeding was selected as the preferred strategy. In this district, R. fistulosa is thriving and removing the weed at early stages of growth, before it connects to its host plant, is indeed assumed to be effective in reducing the damage (Kabiri et al, 2016). In Kyela, the other district with R.…”

“…The obvious reason for this is that very few crops can sustain the hydromorphic conditions typical for the lowlands (Kirchhof et al, 2000) where this species is found. Kabiri et al (2016) further suggested that crop diversification would not be effective because of the facultative parasitic nature of this species, rendering it relatively independent from a suitable host.…”

“…These results suggest that under lowland conditions early sowing is an effective way of escaping the detrimental effect of R. fistulosa infection. Previously, it was established that germination of R. fistulosa, being a facultative parasite, is independent of the presence of a host plant (Kabiri et al, 2016). Germination and establishment are stimulated by saturated soil conditions and occur more or less independent of the sowing time of the crop.…”

“…Rhamphicarpa fistulosa is a facultative root parasitic weed, which entails that it can also complete its life cycle without a host plant. Attachment to a host plant however provides R. fistulosa with a benefit, as attached parasites produce considerably more seeds than independently growing plants (Kabiri et al, 2016). Rhamphicarpa fistulosa is found in areas characterized by temporary and uncontrolled floods (Houngbedji et al, 2014;Rodenburg et al, 2011b;Hansen, 1975).…”