Obligate shade plants possess adaptations that enable them to photosynthesise in the low light environment of the forest floor. Adaptations that facilitate light scavenging may compromise capacity for high rates of photosynthesis. This study compares the responses of obligate shade and facultative shade plant species upon exposure to elevated light. The obligate shade plants were two commercially grown medicinal herb species of ginseng, Panax ginseng C.A. Meyer and Panax quinquefolius L.; and goldenseal - Hydrastis canadensis L. Comparison was made to Arabidopsis thaliana and Pisum sativum L. as facultative shade species. Panax ginseng (Korean ginseng) and Panax quinquefolius (American ginseng) are obligate shade plants found in broadleaf forests of Eastern Asia and North America, respectively. Studies on these plants have shown optimal growth at light intensities between 200-300 [mu]mol photons. m⁻�. s⁻�, or 10-15% of full sunlight, and at intensities greater than 500 [mu]mol photons. m⁻�. s⁻� characteristic photoinbibitory symptoms develop. An atypical response to methyl viologen in photosynthetic electron transport assays was observed in ginseng in both isolated thylakoid membranes and whole leaves. No correlation was found between detectable superoxide dismutase activity and altered methyl viologen reactions. In a mutagenesis study using the model cyanobacterium Synechocystis sp. PCC 6803, a unique amino acid residue in the terminal electron acceptor PsaC, found only in ginseng, was changed and found to have no effect on methyl viologen reactions. Electron transfer to methyl viologen was examined in both isolated thylakoid membranes and whole leaves using chlorophyll a fluorescence and the apparent ability for methyl viologen to act as an electron acceptor was observed to differ between ginseng species. Obligate shade species were observed to possess alternate pools of photosystem II centres that potentially provide a mechanism to maximise photosynthetic gain under low light and during short periods of increased illumination. In experiments designed to identify physiological processes that contribute to increased susceptibility to photoinhibition in obligate shade plants, responses were observed and characterised following a moderate increase in illumination (140 to 400 [mu]mol photons. m⁻� . s⁻�) using chlorophyll a fluorescence induction curve analysis. The obligate shade species exhibited varied responses to elevated light and showed increased susceptibility, to photoinhibition. Photoprotective non-photochemical dissipative capacity was quantified and found to be comparable between all species studied.
| Identifer | oai:union.ndltd.org:ADTP/234759 |
| Date | January 2009 |
| Creators | Woods, Matthew Alan, n/a |
| Publisher | University of Otago. Department of Biochemistry |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Matthew Alan Woods |
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