Parasitic Flowering Plants Chapter 6. Host ranges and various ecological aspects of parasitism The present chapter is about the relationships potential to be facultative, competition from other between the parasite and other organisms, pri- species in the plant community will sooner or later marily the host. Some of these relations are eliminate the parasite. Therefore, it does not make physiological in nature and could as well have much sense to distinguish between facultative been discussed in the previous chapter, while and obligate parasites until facultative parasites the pollination and dispersal biology discussed have been demonstrated to occur in nature. Un- in Chapter 2-4 are equally relevant under the der laboratory conditions it is, none the less, pos- present heading. The most important questions sible to grow some hemiparasitic Orobanchaceae to be discussed in the following concern host throughout the reproductive phase without a host. range (host specificity), host reactions to attacks In the near future it may also be possible under by parasites, and the role of parasites in plant laboratory conditions to grow, e.g., a loranth and communities and ecosystems. Cuscuta veatchii perhaps even a holoparasite without a host, but (Fig. 372) grows on a Bursera species, but will clearly, it will not make sense to call such culti- it accept other hosts? How does the host react? vated plants facultative parasites. Can the host defend itself against a parasite at- tack? Do other species depend on the parasite? Has the parasite any influence on the plant com- Host specificity munity or ecosystem? These questions are rel- evant to any parasite. Some parasites use only one species as host while others use more species, and the majority will ac- Facultative and obligate parasites cept a very wide spectrum of host species. The Some authors find it useful to distinguish be- last group of parasites has a low degree of host tween facultative and obligate parasites. The last specificity. Practically any seed plant may be ones cannot survive without a host. In the first used as host for such a parasitic flowering plant. group, productivity is better when water and/or Naturally, stem parasites, apart from Cuscuta and nutrients (organic and inorganic solutes) are sup- Cassytha, cannot use soft herbaceous plants as plied from one or more hosts, but the parasite hosts but need some kind of woody plant or suc- may survive for a shorter or longer period with- culents such as cacti or euphorbia. Root parasites, out a host. Naturally, this group can only have on the contrary, can in principle use any other members amongst hemiparasitic root parasites. plant producing roots and even epiphytic orchids However, there do not seem to be any records or other epiphytes such as ferns as shown in the from nature where a parasitic flowering plant has case of Pedicularis dendrothauma (Fig. 24). completed its life cycle without haustorial connec- However, dicotyledons are more common as tions to host plants. Even if a hemiparasite has the hosts than monocotyledons, although grasses are 307 Parasitic Flowering Plants Table 6. Growth experiment with Viscum minimum on potential hosts from different genera. Number of hosts Min. number Total Number of seeds Number of Number of species in genus of seeds sown number germinated after hosts with exophytes per host species of seeds 2 months holdfast after after ½ sown 6 months years 63 Euphorbia 4-6 >300 219 = c73% 48 = 76% 28 = 44% 8 Monadenium 4-6 40 36 = 90% 8 = 100% 0 = 0% 7 Jatropa 3-5 25 8 = 72% 4 = 57% 0 = 0% 2 Pedilanthus 5 0 0 = 00% 2 = 00% 0 = 0% 2 Synadenium 5-6 11 10 = 91% 2 = 100% 0 = 0% Plumeria 5 5 5 = 00% = 00% 0 = 0% Peniocereus 3 3 0 = 0% 0 = 0% 0 = 0% quite common hosts. Some monocotyledons are ceptable hosts. Most of the problems are related to rarely seen hosting parasites. In fact there are only interpretation of negative observations. Conoph- a few rather casual reports on parasitic plants es- olis americana (Fig. 249) is in nature only known tablished on orchids, although germination on or- from the oak Quercus borealis, but in the Copen- chids may occur more regularly. In a Costa Rican hagen Botanical Garden it uses at least one of the botanical garden an Oryctanthus (Loranthaceae) species Q. garryana and Q. cerris. If a certain was found fully established on a leaf of the orchid parasite does not occur on a certain host species Trichocentrum pfavii, and in Ecuador at 3,000 m it is not necessarily because the parasite cannot elevation, Antidaphne andina (Eremolepidaceae) grow on that particular host species. There may was found on a leaf of Lepanthes lingulata. At very well be ecological reasons. Viscum mini- first one could assume these two parasites would mum from the Eastern Cape Province in South disappear when the orchid drops the leaf but they Africa is an example. In nature it has never been actually have a good chance to survive. This found on other hosts than Euphorbia horrida is because they both produce epicortical roots (Fig. 373A) and E. polygona. However, growth (Fig. 0B) and these roots will grow towards the experiments with Viscum minimum have shown branches supporting the epiphytic orchid. When that many other species are acceptable hosts. secondary haustoria are established on perennial The tiny mistletoe was sown on 84 different spe- branches the life of the parasite is secured. cies, all except one cactus species (Peniocereus sp.) belonging to six genera in Euphorbiaceae. Determining the host range As shown in Table 6, only species of Euphorbia When trying to determine host range (specificity) were accepted as hosts and of these are all succu- of a certain parasite, one encounters a number of lent species. It is also worth noting that all hosts problems which often result in listing too few ac- are closely related, belonging to 3 of 19 tribes in 308.