Liverworts were the first plants to successfully make the transition from water to land and thus had to develop morphological and physiological features to overcome novel environmental constraints. The relative simple structure of liverworts in comparison to vascular plants implies that they have to combat environmental stress at a cellular level. Liverworts are an important part of the New Zealand flora. Leafy liverworts such as Jamesoniella colorata, Isotachis lyallii and Lepidolaena taylorii display a spectacular range in colouration within a single population. The leaves can vary from pale green in shaded habitats to bright red in more exposed places. A thalloid species, Monoclea forsteri, lacks such pigmentation but it might possess some unique cellular features to deal with stressful conditions. This thesis aims to study the cellular characteristics that allow liverworts to cope with extreme environmental conditions, with emphasis on (red) auxiliary pigments. Genetic analysis of liverwort populations revealed a highly clonal structure, which suggests that red colouration of liverwort leaves is a phenotypic response, and not a result of genotypic differences. Indeed, the red pigment could be induced in I. lyallii at relatively modest irradiance levels. The red leaves absorbed about 10% more photosynthetically active radiation than green leaves from the same species. Although the chemical characteristics of the red pigment remained elusive, it has light attenuation characteristics similar to anthocyanins in vascular plants. Its cellular location inside the cell wall is ideal for intercepting potentially damaging light quanta and can thereby protect the underlying chloroplasts from high light fluxes. The light energy that is absorbed by the red pigment did not translate into an enhanced protection from chilling-stress through leaf warming. The photosynthetic apparatus of liverworts has acclimated to the environmental conditions that these plants are most likely to experience in their habitat. Gametophytes from shady environments had lower chlorophyll a to b ratios than gametophytes found in sunnier places. This illustrates the need for a more efficient light-harvesting system in the former to collect all of the available light. In addition, plants from sunny environments had lower chlorophyll to carotenoid ratios than plants found in shady environments, which suggests that the photoprotective properties of carotenoids are better developed in the former. This was confirmed by measurements of chlorophyll a fluorescence which clearly showed a greater potential for nonphotochemical quenching in gametophytes from exposed habitats than gametophytes found in sheltered places, both during high light treatment and desiccation. A better developed photo-protective system (i.e. non-photochemical processes and lightscreening pigments) allowed the red-leaved gametophytes to better deal with high light stress as well as desiccation than their green-leaved counterparts. Measurements of photosynthetic responses revealed that red gametophytes were photoinhibited less and their recovery was more complete from a high light treatment of 1800 μmol/m2/s than the greens. Similarly, measurements of photosynthetic responses and oxidative damage indicated that the red morph was better protected from high light fluxes that occur during drought-stress than the green morph. Determination of cell water relations showed that the red pigment in J. colorata did not influence the water balance during drought. In conclusion, this study has presented evidence that liverworts from exposed habitats are better equipped to deal with abiotic stressors such as high light and dehydration than liverworts from more sheltered environments. Changes in pigment composition, concentration and location are likely to play an important role in liverwort protection from environmental stress – most noticeably a red pigment with photo-protective attributes.
| Identifer | oai:union.ndltd.org:ADTP/276035 |
| Date | January 2006 |
| Creators | Hooijmaijers, Cornelis Antonius Maria. |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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