Nutritional studies of Hemiparasitic Orthocarpus (Scrophulariaceae)

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Abstract
Excised shoot tips from Orthocarpus attenautus and O. purpurascens were cultured in vitro to ascertain whether the stem tips of these hemiparasites require complex organic substances for their development, and to determine if the capacity for haustoria formation is retained by the resulting plantlets. Explants consisted of the apical meristem plus the four smallest leaf primordia, having a volume of less than 0.5 mm3. A variety of mineral media, sucrose concentrations, root extract, soil extract, yeast extract, and malt extract were tested for effects on growth. Both species completed development in sterile culture on simple media. Orthocarpus attenuatus grew best on Knop‘s minerals with Ball’s microelements + 0.1 g ferric citrate 1–1+ 1% (w/v) sucrose, while MurashigeSkoog‘s minerals+ 2% sucrose provided the best growth of the media tested for O. purpurascens. Haustoria formed on the roots of all plantlets chemically induced by cotton string. The mean number of haustoria per plantlet was about half that of control plants raised from seed. Growth of intact O. Purpurascens seedlings was also compared on mineral agar, mineral agar supplemented with yeast extract, and in soil cultures supplemented with yeast extract and a host. While yeast extract has variable effects on the growth of shoot tip explants and intact plants raised under axenic conditions, it is highly stimulatory to the autotrophic growth of intact plants in soil culture.