Heterotrophic carbon gain by the root hemiparasites, Rhinanthus minor and Euphrasia rostkoviana (Orobanchaceae)

Abstract

Hemiparasitic plants gain virtually all mineral nutrients and water from their host plant whilst organic carbon is provided, at least in part, by their own photosynthetic activity, although their rates of assimilation are substantially lower than that found in non-parasitic plants. Hence, hemiparasites must gain at least some of their organic carbon heterotrophically from the host plant. Despite this, heterotrophic carbon gain by root hemiparasites has been investigated only for a few genera. We investigated heterotrophic carbon gain by two root hemiparasites, Rhinanthus minor L. and Euphrasia rostkoviana Hayne (Orobanchaceae), using natural abundance stable isotope (delta(13)C) profiles of both parasites attached to C(3) (wheat) and C(4) (maize) hosts coupled to a linear two-source isotope-mixing model to estimate the percentage of carbon in the parasite that was derived from the host. Both R. minor and E. rostkoviana attached to maize hosts were significantly more enriched in (13)C than those attached to wheat hosts with R. minor becoming more enriched in (13)C than E. rostkoviana. The natural abundance (13)C profiles of both parasites were not significantly different from their wheat hosts, but were less enriched in (13)C than maize hosts. Using a linear two-source isotope-mixing model, we estimated that R. minor and E. rostkoviana adult plants derive c. 50 and 25% of their carbon from their hosts, respectively. In light of these results, we hypothesise that repeatedly observed negative effect of competition for light on hemiparasites acts predominantly in early ontogenetic stages when parasites grow unattached or the abstraction of host nutrients is less effective.

Interactions Between Rhinanthus minor and Its Hosts: A Review of Water, Mineral Nutrient and Hormone Flows and Exchanges in the Hemiparasitic Association

“…There is thus much speculation as to 'how parasitic' Rhinanthus is for C; moreover, we do not know how much C Rhinanthus actually extracts from its host. This question motivated Těšitel et al (2010) to use natural abundance isotope profiling coupled to a linear two-source isotope mixing model to estimate the degree of heterotrophic C gain by R. minor. Těšitel et al (2010) grew R. minor on maize and wheat hosts and investigated the isotopic enrichment of the parasite in 13 C on each host plant.…”

“…This question motivated Těšitel et al (2010) to use natural abundance isotope profiling coupled to a linear two-source isotope mixing model to estimate the degree of heterotrophic C gain by R. minor. Těšitel et al (2010) grew R. minor on maize and wheat hosts and investigated the isotopic enrichment of the parasite in 13 C on each host plant. Because maize undertakes C 4 photosynthesis and wheat undertakes C 3 photosynthesis, maize is significantly more enriched in 13 C than wheat, thus by measuring the extent to which R. minor (a C 3 plant) takes on the 13 C isotope signature of a C 4 plant when attached to maize, Těšitel et al (2010) were able to estimate that an average of 50% of the total non-structural C in mature R. minor leaves was host-derived.…”

Interactions between hemiparasitic plants and their hosts

“…22 A similar, approach was then used to estimate carbon budget of numerous hemiparasitic species demonstrating that ca 20% -80% of hemiparasite biomass is derived from the host assimilates differing across species and developmental stages. [23][24][25] Early developmental stages of obligatory hemiparasites (Striga spp.) were demonstrated to be highly dependent on host-derived carbon as expected from their belowground growth-habit (Fig.…”

“…It is not likely that a moderate level of light deficiency would have detrimental effects on obligate hemiparasites or adult root hemiparasites given the high efficiency of heterotrophic carbon gain. 22,23,25 In contrast, seedlings of facultative hemiparasites germinate without a host induction, start growing unattached and hence, they must be extremely sensitive to decrease of irradiance at this stage of development especially considering their low rates of photosynthesis. 36,37 Experiments evaluating effect of shading on different life stages of hemiparasites are missing from the literature although e.g., Striga spp.…”

“…in some cases, only isotopic composition was reported in the original studies. Proportion of heterotrophic carbon was then calculated using an isotope mixing model as in Těšitel et al 25 African mistletoes* ,1 = Septulina glauca, Tapinanthus oleifolius, African mistletoes* ,2 = Odonthella welwitchii, Plicosepalus undulatus, Septulina glauca, Tapinanthus oleifolius, Viscum rotundifolium; no separate data for individual species were given in the original study. 28 …”