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  • Author or Editor: Michael R. Sweatt x
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Castilleja indivisa grows hemiparasitically attached to the roots of various nearby plants. Studies were done using several host plants to determine the effects of the parasitic relationship on the growth of C. indivisa and the host plants. Transpiration rates, and leaf water potentials of C. indivisa, and various hosts, were also measured at various soil moisture levels. Carbon transfer between C. indivisa and each host was examined using a 14CO2 tracing technique.

The various hosts used in this experiment enhanced the growth of C. indivisa by 200-700% compared to non-parasitic controls. Transpiration rates of non-parasitic controls remained relatively low at all soil moisture levels while transpiration rates of parasitic C. indivisa increased rapidly as soil moisture increased, and generally exceeded that of its host at low to medium soil moisture levels. Leaf water potentials of non-parasitic controls were generally more negative than other treatments. Carbon exchange between C. indivisa and its hosts was insignificant and appears not to be a major nutritional factor.

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Castilleja indivisa (Indian Paintbrush normally grows hemiparasitically in association with various hosts. Research at Texas A&M University has shown that depending on which host plant is selected, growth rates are increased two to five times over Castilleja grown without a host.

Haustorial connections form between the roots of Castilleja and the roots of its host plant. Published papers show that water and nutrients are transferred from the host to the hemiparasite, although this has not been characterized on a quantative level.

Water relations of Castilleja and various hosts are being examined by psychrometry to determine the importance of water transfer in the relationship. Carbon transfer and partitioning is being examined using steady state 14CO2 labeling. Water relations, carbon transfer, and host specific growth enhancement will be discussed.

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To determine the role of endomycorrhizae on water relations and plant growth of geraniums under nonrate-limiting soil P conditions, hybrid seedlings (Pelargonium X hortorum Bailey cv. Cherry Glow) were planted in moderately high P (40 ppm) media, grown under high (−0.4 MPa) or low (−1 MPa) soil water potential (ψs), and either inoculated with VA mycorrhizal fungi [Glomus mosseae (Nicol. & Gerd.) and G. fasciculatus (Thaxt. sensu Gerd.) Gerd. & Trappe] (VAM) or left as noninoculated controls. Geraniums grown under high moisture had greater shoot growth, more advanced floral development, and higher P uptake than the low moisture plants. Mycorrhizal plants under high moisture conditions had higher P levels than noninoculated plants, while mycorrhizal plants under low moisture regimes had greater shoot growth, more advanced floral formation, and greater N uptake than noninoculated plants. Xylem water potential in leaves (ψL) was lower under −1 MPa than −0.4 MPa ψS moisture regimes and lowest in mycorrhizal geraniums grown under −1 MPa ψS. One hundred minutes after plants had recovered from water stress, the greatest change in ψL was recorded for mycorrhizal geraniums acclimatized to low moisture regimes. Geraniums under low moisture regimes are more mycorrhizal-dependent. Total estimated root length and root water conductivity were lower with mycorrhizal geraniums under high water and P regime. Under water stress, the larger mycorrhizal geraniums have greater total water demands than controls. Consequently, mycorrhizal geraniums stressed more rapidly yet more efficiently recovered from water deficits. Data suggests that VAM plants acclimate more efficiently to water stress because of more frequent or extreme drought.

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