AbstractThis thesis examines the effect of high light and water stress and solar UV-B radiation on photoinhibition of photosynthesis and the xanthophyll cycle in Acacia auriculiformis, Acacia mangium, Acacia crassicarpa, Acacia aulacocarpa, Eucalyptus camaldulensis and Eucalyptus pellita in both shade-house and field conditions. Photosynthesis and the xanthophyll pool size increased 2- and 4-fold respectively with growth irradiance in shade-house grown plants. In contrast, chlorophyll content and the epoxidation state (EPS) decreased with growth irradiance. Photosynthetic activities of these plants were comparable to field-grown acacias which formed larger xanthophyll pools (especially zeaxanthin) apparently in response to the higher light and higher water deficit conditions prevailing in the field.
The role of leaf angle in high light environment was examined in a number of species including A. crassicarpa, A. mangium, A. auriculiformis and E. pellita in Chapter 4. The near vertical leaf angle of A. crassicarpa reduced light absorption directly and lowered leaf temperature for optimal photosynthesis. Such a strategy seemed cost effective and eventually led to increased productivity (Table 4.4). In contrast, E. pellita with moderate photosynthetic capacity had enormous xanthophyll cycle activity to help dissipate excessive energy and reduce photoinhibition (Chapter 4). Furthermore, A. auriculiformis with small leaf size and leaf angle exhibited high photosynthetic activity and substantial xanthophyll cycling activity, useful for the dissipation of excessive energy (Chapter 5).
The leaf angle of A. crassicarpa was altered artificially (90°, 67.5º, 45°, 22.5° and 0º) in an experiment. Reducing the leaf angle below 90° increased incident irradiance, leaf temperature and xanthophyll cycle activities but photosynthetic activity and the total CO, fixed in a day were decreased significantly. The results indicate that reducing incident irradiance by altering leaf angle can be an effective photoprotective mechanism against photoinhibition by high light.
The effect of water stress was examined in clones of A. auriculiformis grown in irrigated and unirrigated sites during the wet and dry seasons (Chapter 5). Unirrigated plants during the dry season had very low leaf water potential and thick leaf cuticles compared to irrigated plants. The plants were photoinhibited under these conditions and they showed greatly reduced Fv/Fm ratio and photosynthetic activity. Chlorophyll and leaf soluble protein contents decreased but carotenoid and xanthophyll, especially zeaxanthin, contents increased greatly compared to plants growing under irrigation. Unirrigated plants showed large diurnal changes in photosynthesis and zeaxanthin and violaxanthin content. Photosynthetic activity was 3-7 fold less and zeaxanthin level 20-50 fold higher than the values determined in the wet season or under irrigated conditions. The high level of xanthophyll cycling observed in the unirrigated or dry season plants was presumably associated with the dissipation of excess light energy and serves to protect the photosynthetic apparatus from photoinhibition.
Phyllode pinitol levels also increased 1.4-1.7 fold in plants under water stress conditions (Chapter 6). The pinitol content constituted 50% of the phyllode total sugars content compared to 17% for fructose, 20% for glucose and less than 10% for sucrose. Drought stress increased the accumulation of pinitol and total soluble sugars apparently to help regulate the osmotic condition in the cell and to reduce damage by water stress. Soluble sugar levels especially pinitol were higher in the dry season and/or in unirrigated plants. Interestingly, no diurnal variations in sugar level were observed in either irrigated or unirrigated plants. This was probably because, unlike experiments with pot plants, water stress in the field can not be induced rapidly.
Furthermore, my results show that field-grown A. auriculiformis accumulated more pinitol but less sucrose compared to seedlings grown in the shade house. Under water stress condition, phyllode total sugars decreased 15% in seedlings but increased 30% in trees in the field. These differences probably reflect the different developmental stages between seedlings and trees. Differences in sugar accumulation were also observed between species. Acacias had high pinitol and sucrose but low fructose contents whereas leaves of E. pellita had high fructose but low pinitol and sucrose levels. These differences are likely to be due to differences in metabolism between acacias and eucalyptus. Thus different sugars may be involved in osmotic adjustment during water stress in acacia and eucalyptus.
The effects of natural solar UV-B radiation on the growth, photosynthesis and the xanthophyll cycle of four acacia and two eucalyptus species were examined in Chapter 7. Solar UV-B radiation seemed to delay plant growth in all species examined even though it did not affect photosynthetic activity significantly. Under solar UV-B radiation, a reduced SLA, and an increased leaf thickness and size of palisade cell and epidermis were observed in plants. Chlorophyll content decreased but leaf soluble protein content increased in plants under solar UV-B radiation. Solar UV-B radiation apparently had a strong effect on chlorophyll degradation rather than the size of xanthophyll cycle pooi. It seemed that xanthophyll formation was more strongly affected by PPFD than by solar UV-B radiation. Overall, the effect of solar UV-B on plant growth and form was much less detrimental than reported for indoor experiments where high UV-B dosage (relative to growth irradiance) were used.
Photoinhibition of photosynthesis was observed in acacia and eucalyptus species (and probably in other plant species as well) during the dry season in the NT. My studies show that different species may use different methods to dissipate excess energy to protect against photoinl-iibition. This includes vertical leaf angle orientation to reduce incident irradiance in A. crassicarpa; high photosynthetic capacity and electron transport activity in A. auriculformis; and high xanthophyll cycling in E. pellita. This is complemented by high soluble sugar (pinitol) content to protect against water stress during the dry season where conditions of high light and high water deficit prevail for a considerable period.
|Date of Award||Aug 1998|