Interactions between hemiparasitic plants and their hosts: The importance of organic carbon transfer

Hemiparasitic plants display a unique strategy of resource acquisition combining parasitism of other species and own photosynthetic activity. Despite the active photoassimilation and green habit, they acquire substantial amount of carbon from their hosts. The organic carbon transfer has a crucial influence on the nature of the interaction between hemiparasites and their hosts which can oscillate between parasitism and competition for light. In this minireview, we summarize methodical approaches and results of various studies dealing with carbon budget of hemiparasites and the ecological implications of carbon heterotrophy in hemiparasites.Key words: haustorium, heterotrophy, parasitic plant, mistletoe, Rhinanthus, Striga, δ¹³CHemiparasitic plants withdraw resources from the vascular system of their hosts through a specialized transfer organ called haustorium.¹ Hemiparasites attack the host''s xylem, in contrast to the holoparasites that infect both phloem and xylem, and as a consequence, hemiparasitic plants have access to water and mineral nutrients but little carbon.¹ Due to their reduced or non-existing root networks, hemiparasitic plants acquire virtually all mineral nutrients and water from the host while organic carbon is provided, at least in part, by their own photosynthetic activity.^2,3 This is in contrast to holoparasitic plants which rely on the host for the supply of both organic and inorganic nutrients. The location of the attachment to the host and the degree of host dependency represent the most important characters defining the three basic functional types within hemiparasitic plants. Root hemiparasites attack host roots but their above-ground appearance is usually not substantially different from that of a non-parasitic plant. This group can be further divided in two—facultative and obligate hemiparasites consisting of plants that are able (at least sometimes) or unable to complete their life cycle without an attachment to the host respectively. Stem hemiparasites are attached to the host stem (usually trunk or branches) and are all obligate parasites, unable to survive without a host.Hemiparasitic plants have an ambiguous relationship with their hosts which, on the one hand, represent exclusive sources of inorganic nutrients but on the other hand, the co-occurrence of these host plants in the hemiparasite vicinity imposes competition for light. The nature and intensity of this competitive relationship varies across different groups and species of hemiparasites. The ability of hemiparasites to acquire organic carbon (largely in the form of xylem-mobile organic and amino acids) is certainly the key factor affecting this interaction since hemiparasites that are capable of efficient organic carbon abstraction should be minimally affected by shading from their host. The fact that hemiparasites can exhibit substantial carbon heterotrophy is now supported by a large number of studies, although a traditional point of view on hemiparasites that highlights the importance of inorganic resources (mainly nitrogen) acquisition is still prevailing. Therefore, we decided to summarize available information on hemiparasite heterotrophy, outline techniques for assessing the proportion of heterotrophy and estimating the overall carbon budget, and discuss possible implications of this phenomenon on hemiparasite ecology.