Ecology and biology of Rhamphicarpa fistulosa, a new parasitic weed of Rain-fed Rice (Oryza sativa) in sub-Saharan Africa

Abstract

Rice is an important staple food crop in Africa. The increasing scarcity of agricultural land has driven rice growers to expand into marginal areas that have natural infestations of Rhamphicarpa fitulosa. In return, R. fistulosa has increasingly become a serious problem to rice production in sub-Saharan Africa. To date, the understanding of the ecology and biology of the species and its dependence and effects on a host, is rather limited. The discrepancy between the emergence of this weed problem and the virtual absence of knowledge on the weed species motivated the study presented in this thesis. In a field survey in Tanzania, Striga asiatica was observed in higher lying and drier fields, while R. fistulosa was observed in the lower lying wetter fields. Experiments confirmed that S. asiatica is favoured by free-draining soils and R. fistulosa by water-logged soils. These results imply that changes in climate, specifically moisture regimes, will be crucial for the future prevalence of both parasitic weed species. In a second investigation, I found that daylight and completely saturated soil conditions were prerequisites for germination, demonstrating that R. fistulosa is a typical species of environments with fluctuating water levels. Neither root exudates collected from rice host plants, nor the synthetic germination stimulant GR24, triggered germination of R. fistulosa seeds. Host plant presence resulted in a 3.7 times higher seed production rate and a 15% larger average seed size. The absence of a host recognition mechanism at the germination stage suggests that either the regulation of germination through light and soil moisture is near optimal, or that for this parasitic plant species an opportunistic germination strategy is superior. In a third study, I observed that infection by R. fistulosa led to significant reductions in leaf photosynthetic rate, stomatal conductance, the quantum efficiency of PSII (ΦPSII) and chlorophyll content of rice. In addition, there was a 19-32% negative deviation of the linear relationship between quantum yield of CO2 assimilation (ΦCO2) and quantum efficiency of PSII (ΦPSII) of infected plants in comparison to un-infected plants. This indicated a parasite induced influence on the photochemical process of the host. Furthermore, there was a considerable time lag between the parasite’s gains in growth and the reduction of host photosynthesis. The reduction in host growth, coincided with suppression of host photosynthesis. This indicates that R. fistulosa affects host growth by first extracting assimilates and making considerable gains in growth, before it affects the host photosynthesis. In the final investigation, I examined how the interaction between host plant and parasite influenced growth and (re)production of R. fistulosa and rice. Infection by R. fistulosa increased root:shoot ratio and decreased plant height, leaf area and tiller number of rice. Reductions in light interception of the host were followed by reductions in light use efficiency, causing 22-71% losses in host plant biomass and 78-100% losses in host kernel production. Parasitism eventually caused a complete standstill of host plant growth, while the parasite managed to gradually increase its share of the total host plant-parasite biomass up to 50-82%. This implies that ultimately the host plant was producing solely for the sake of the parasite. In a final chapter, I discuss the implications of my findings for the future expansion of this parasitic weed, specifically in light of climate change. I also discuss how the divergent ecology and biology of R. fistulosa is likely to influence the effectiveness of measures that are currently applied to manage Striga spp. I argue that more than the current attention needs to be paid to R. fistulosa, specifically for the problems it causes to the rice sector in sub-Saharan Africa.