Growth, Resource Allocation and Haustorial Biology of the Root Hemiparasite Olax phyllanthi (Olacaceae)

Olax phyllanthi was found to parasitize a wide range of taxa in the native habitat in coastal heath, South-West Australia. All major life and growth forms were regularly exploited, including most woody dicotyledons (except members of the Myrtaceae), a few monocotyledons and cohabiting root hemiparasites. Initiation of haustoria occurred mostly in autumn (southern hemisphere), with some senescing the following summer, and others surviving until replaced by a new generation of haustoria the following autumn. Seedlings increased in dry matter and contents of N, P, K, Mg and Ca during preparasitic development, but did not survive beyond 6 months if failing to establish haustoria on a suitable host. Plant dry matter, mineral content and haustorial number increased exponentially during subsequent parasitic development. Mean shoot:root d. wt ratios of 1st to 3rd-year plants lay within the range 1.2–1.4. Root systems were laterally extensive and restricted to the top 40 cm of rooting substrate. Mean total root lengths of 2nd- and 3rd-year plants were 7.1m (n = 5) and 60.9 m (n = 5), respectively. Haustoria comprised 0.7–3.5% of plant d. wt, with a mean of 7.9 haustoria (n = 10, 2nd- and 3rd-year plants) per metre of root length. Comparisons of mineral concentrations in dry matter of O. phyllanthi and of a range of commonly parasitized hosts showed the parasite to be much richer on average than its hosts in K, P, and to a lesser extent in N, but not noticeably different in Mg and Ca.