Biology of Hemiparasitic Rhinanthus Species in the Context of Grassland Biodiversity.

The aim of the present review is to compile and analyze information on biology of Rhinanthus species in the context of grassland biodiversity. Root hemiparasites have been relatively less studied in comparison to economically important holoparasitic weed species. Rhinanthus species appear to be genetically polymorphic, but also possess high phenotypic plasticity, and ecological factors are important determinants in evolution of specialization to most appropriate hosts. Rhinanthus individuals have a relatively short life span, and flowering is a photoperiod- or host plant-independent phenomenon. Both insect pollination and self-pollination can occur. Seeds do not form a persistent soil seed bank and have physiological dormancy broken by stratification. In general, Rhinanthus species have low host specificity, but there clearly are 'preferred' or 'avoided' hosts in natural conditions. In controlled conditions, interaction with most grass species result in more prominent parasite growth stimulation in comparison to that of legumes, and, especially, forbs, but there are significant gradations and exceptions. Ecological requirements of Rhinanthus species have been rarely studied, but it can be expected that significant tolerance against mineral nutrient heterogeneity and water shortage can be found. It seems that host plant characteristics are important determinants of the environmental resilience of Rhinanthus. Parasites not only obtain resources (water and minerals) from host plants but also negatively affect their physiological functions. The most intriguing and practically unexplored question is the exchange of chemical signals between the Rhinanthus parasite and the host plant. Extending this idea, it can be predicted that signals will also be exchanged between multiple host plants whose roots are connected through the parasite. It is highly possible that the exchange of small RNAs between plants could influence their environmental tolerance. Host selectivity forms the functional basis of changes in species diversity in grasslands, but the outcome seems to be highly dependent on other conditions, especially, soil edaphic factors. Development of new model systems is necessary to further expand our knowledge about the complex effects of parasitic plants on ecosystems. [ABSTRACT FROM AUTHOR]