The reproductive biology of the introduced root holoparasite Orobanche ramosa subsp. mutelii (Orobanchaceae) in South Australia

An understanding of the timing and constraints on seed production is necessary for the successful control of many invasive plant species. In the present study, we investigated the reproductive traits of the annual root holoparasite, Orobanche ramosa L. subsp. mutelii (F.W.Shultz) Cout. (branched broomrape), which occurs in the western Murray mallee area of South Australia. Flowering stems emerge above the ground from early September. Each flower spike has from 2 to 24 flowers and the first flower opens on the base 8 days after emergence and senesces 6 days later. An additional flower opens acropetally every 1–2 days. The species is autogamous and self-pollination can occur before anthesis. Mature seeds were observed from 9–10 days after flower senescence and capsules dehisced 12 days later. Although each plant is in flower only for an average of 20 days, in large populations or multi-branched plants, the flowering period extends over several weeks, with a peak in mid-October. Seed production in sampled populations was highly variable, ranging from 1000 to over 200000 per plant. Large plants with several branching stems produced the most seeds, although capsules produced per stem and seeds per capsule were also variable. There is a brief window of opportunity for control of O. ramosa subsp. mutelii after it emerges above the soil surface and before it sets seed. Weedy root parasites such as Orobanche ramosa can have devastating impacts on host crops, but are difficult to control because most growth occurs underground. We identified key characters that contribute to the success of this introduced plant, including self-pollination, rapid maturation of the reproductive stages and high seed output. Vigilance is required so that plants can be controlled before they set seed.