Ecophysiological attributes could be causes for rarity in plants. We tested the hypothesis that a species’ ability to regulate photosynthesis and growth in response to environmental factors is indicative of its environmental resilience and that this is linked to its conservation status. In this study, the ecophysiology of Graptophyllum reticulatum, an Australian endangered endemic species, was compared with that of its three closely related and more common congeners G. ilicifolium, G. excelsum and G. spinigerum. Ecophysiological attributes were measured on the four species in their natural habitats and under artificially imposed environmental stresses, including changed soil conditions, excess light and low water availability, in a glasshouse experiment. Photosynthesis was determined at the photosystem II and leaf level using chlorophyll a fluorescence and gas exchange techniques. Applied to the chlorophyll fluorescence transient of leaves, the JIP test provides a Performance Index which quantifies the main steps in PSII photochemistry including light energy absorption, excitation energy trapping, and conversion of excitation energy into electron flow. At the leaf level, gas exchange measurements allow determination of maximum CO2 assimilation rates, intercellular CO2 concentrations, stomatal conductance for water vapour and instantaneous water use efficiency. Growth analysis was performed to assess relative growth rates and physiological and morphological responses. Analysis of physiological differences and responses indicated that, compared to its more common relatives, the endangered G. reticulatum was an intrinsically slow growing species, exhibited the lowest fitness when growing in favorable environments and was most sensitive to excess light stress. Photoinhibition is therefore likely to restrict the endangered species to shade habitats. Compared with the endangered G. reticulatum, the vulnerable G. ilicifolium and common G. spinigerum species were better adapted to high light and changed nutrient levels, but were more susceptible to water stress. The rare G. excelsum had the fastest growth rate and the highest fitness in favorable environments. Based on the ecophysiological attributes examined here, it is proposed that excess light is likely to be the most critical abiotic factor restricting distribution of the endangered species in a fragmented landscape. The survival of the species may be most dependent on the intactness of the habitat over-storey. In contrast, the vulnerable G. ilicifolium showed strong susceptibility to water limitation, and survival might be threatened if climate change alters habitat water relations to cause, for example, more pronounced dry periods. The rare G. excelsum which had highest carbon gain and growth in the experiments carried out in this study, may become the most successful adaptation out of the rainforest environment due to its tolerance to higher light and limited water availability. To examine the generality of the link between rarity and ecophysiology with Graptophyllum species, two dipterocarp species, narrowly endemic Dipterocarpus condorensis and local common Shorea roxburghii that are actually co-located in South-eastern Vietnam were studied. Findings in this case study confirmed the usefulness of the comparative approach based on physiological measurements, either in situ or ex situ, to explain plant rarity. The results of this study indicate ecophysiological research is a tool for examining causes of rarity and possible abiotic threats. The information gained allows assessment of environmental resilience of species and contributes essential knowledge for management and conservation of threatened plants. Such knowledge is also useful for ex situ conservation including propagation, translocation and re-introduction in restoration programs.

Plant responses to damage, including defoliation, are complex. Plants have been shown to have a marked ability to compensate for damage, including by producing new or larger fruiting structures and by regrowing vegetative components following damage. While compensation for the loss of reproductive components has been widely investigated, compensation for the loss of vegetative components has been much less thoroughly studied. Similarly, biomass has been used as the index for studies of compensation in plants, while spatial aspects of compensation have been ignored. The spatial aspects of compensation include the topological position of lost and compensating structures, which is an indicator of resource allocation strategies in the plant following damage. Due to the complexity of interactions between structural and non-structural aspects of plant growth, particularly in response to damage, a computational approach to the problem is likely to provide advantages over more traditional approaches. In computational plant science, a simulation step may be added to the established scientific paradigm of hypothesis-experiment, so that it becomes hypothesis-simulation-experiment. The simulation step formalises what is currently known and allows for the generation of potentially more relevant and more testable hypotheses than ad hoc methods. Cotton (Gossypium hirsutum L.) is an important crop in Australia, and a significant part of the agricultural economy. Cotton’s growth is constrained by a number of environmental factors, including the activities of pests. Cotton’s morphological responses to damage are particularly complex. Accordingly, cotton was chosen as a suitable model crop for investigating morphological responses to damage with computational tools. L-system models of cotton morphogenesis were produced by linking hypotheses about physiology, initially from the literature and later from data gathered during the research, with the structural outputs of physiology. The resulting simulations were used to generate morphological hypotheses that could be tested with real plant experiments. During the experiments, morphological data were gathered with a sonic digitiser, allowing comparison of the architectures of cotton plants at the level of each individual component. Because the measuring technique was non-destructive, plants could be measured many times, creating a set of data representing dynamic plant growth. Relationships from the data for several experiments on cotton's responses to defoliation of the main stem and branches of seedlings and older plants were used to adjust the initial model, rejecting some physiological hypotheses and confirming others. The result is a model that represents aspects of the interaction between cotton's carbon allocation, topology, and signalling behaviour following defoliation. Initial experiments found that apical development rates are not limited by photosynthate supply, at least at the levels of defoliation applied here. Unperturbed, obligate apical development was shown by subsequent versions of the model to be a significant contributor to cotton's ability to compensate for early season defoliation. Experiments on branch defoliation illustrate that while the vegetative branches respond in ways that are similar to those of the main stem, the sympodial branches behave in qualitatively different ways. Sympodial branch development was found to be significantly perturbed by the loss of the subtending main stem leaf, somewhat perturbed by the loss of significant leaf area within the branch, and very seriously perturbed by the removal of both the main stem leaf and branch leaves. The ability of a branch to produce mature bolls was likewise severely affected by defoliation, either of the branch leaves or of the subtending main stem leaf. However, no effect was noted where defoliation was applied to nearby branches or main stem leaves. This supports the theory that cotton branches are independent physiological units with regards to photosynthate. Branch independence interacts with cotton's propensity to produce many more fruiting structures than it can mature as a compensation mechanism for defoliation and subsequent fruit loss. This research demonstrates the utility of the hypothesis-simulation-experiment approach to biological investigation. This approach provides a way to use qualitative and quantitative physiological knowledge to produce readily tested morphological hypotheses for use in real plant experiments. The results of such experiments are demonstrated to be not only empirical evidence of a response, but evidence in support of or contradicting proposed physiological mechanisms for the response. Thus, the hypothesis-simulation-experiment approach as used in this thesis represents a method for generating and testing more meaningful hypotheses than are generally produced using ad hoc hypothesis generation techniques.

Ecophysiological attributes could be causes for rarity in plants. We tested the hypothesis that a species’ ability to regulate photosynthesis and growth in response to environmental factors is indicative of its environmental resilience and that this is linked to its conservation status. In this study, the ecophysiology of Graptophyllum reticulatum, an Australian endangered endemic species, was compared with that of its three closely related and more common congeners G. ilicifolium, G. excelsum and G. spinigerum. Ecophysiological attributes were measured on the four species in their natural habitats and under artificially imposed environmental stresses, including changed soil conditions, excess light and low water availability, in a glasshouse experiment. Photosynthesis was determined at the photosystem II and leaf level using chlorophyll a fluorescence and gas exchange techniques. Applied to the chlorophyll fluorescence transient of leaves, the JIP test provides a Performance Index which quantifies the main steps in PSII photochemistry including light energy absorption, excitation energy trapping, and conversion of excitation energy into electron flow. At the leaf level, gas exchange measurements allow determination of maximum CO2 assimilation rates, intercellular CO2 concentrations, stomatal conductance for water vapour and instantaneous water use efficiency. Growth analysis was performed to assess relative growth rates and physiological and morphological responses. Analysis of physiological differences and responses indicated that, compared to its more common relatives, the endangered G. reticulatum was an intrinsically slow growing species, exhibited the lowest fitness when growing in favorable environments and was most sensitive to excess light stress. Photoinhibition is therefore likely to restrict the endangered species to shade habitats. Compared with the endangered G. reticulatum, the vulnerable G. ilicifolium and common G. spinigerum species were better adapted to high light and changed nutrient levels, but were more susceptible to water stress. The rare G. excelsum had the fastest growth rate and the highest fitness in favorable environments. Based on the ecophysiological attributes examined here, it is proposed that excess light is likely to be the most critical abiotic factor restricting distribution of the endangered species in a fragmented landscape. The survival of the species may be most dependent on the intactness of the habitat over-storey. In contrast, the vulnerable G. ilicifolium showed strong susceptibility to water limitation, and survival might be threatened if climate change alters habitat water relations to cause, for example, more pronounced dry periods. The rare G. excelsum which had highest carbon gain and growth in the experiments carried out in this study, may become the most successful adaptation out of the rainforest environment due to its tolerance to higher light and limited water availability. To examine the generality of the link between rarity and ecophysiology with Graptophyllum species, two dipterocarp species, narrowly endemic Dipterocarpus condorensis and local common Shorea roxburghii that are actually co-located in South-eastern Vietnam were studied. Findings in this case study confirmed the usefulness of the comparative approach based on physiological measurements, either in situ or ex situ, to explain plant rarity. The results of this study indicate ecophysiological research is a tool for examining causes of rarity and possible abiotic threats. The information gained allows assessment of environmental resilience of species and contributes essential knowledge for management and conservation of threatened plants. Such knowledge is also useful for ex situ conservation including propagation, translocation and re-introduction in restoration programs.

Ecophysiological attributes could be causes for rarity in plants. We tested the hypothesis that a species’ ability to regulate photosynthesis and growth in response to environmental factors is indicative of its environmental resilience and that this is linked to its conservation status. In this study, the ecophysiology of Graptophyllum reticulatum, an Australian endangered endemic species, was compared with that of its three closely related and more common congeners G. ilicifolium, G. excelsum and G. spinigerum. Ecophysiological attributes were measured on the four species in their natural habitats and under artificially imposed environmental stresses, including changed soil conditions, excess light and low water availability, in a glasshouse experiment. Photosynthesis was determined at the photosystem II and leaf level using chlorophyll a fluorescence and gas exchange techniques. Applied to the chlorophyll fluorescence transient of leaves, the JIP test provides a Performance Index which quantifies the main steps in PSII photochemistry including light energy absorption, excitation energy trapping, and conversion of excitation energy into electron flow. At the leaf level, gas exchange measurements allow determination of maximum CO2 assimilation rates, intercellular CO2 concentrations, stomatal conductance for water vapour and instantaneous water use efficiency. Growth analysis was performed to assess relative growth rates and physiological and morphological responses. Analysis of physiological differences and responses indicated that, compared to its more common relatives, the endangered G. reticulatum was an intrinsically slow growing species, exhibited the lowest fitness when growing in favorable environments and was most sensitive to excess light stress. Photoinhibition is therefore likely to restrict the endangered species to shade habitats. Compared with the endangered G. reticulatum, the vulnerable G. ilicifolium and common G. spinigerum species were better adapted to high light and changed nutrient levels, but were more susceptible to water stress. The rare G. excelsum had the fastest growth rate and the highest fitness in favorable environments. Based on the ecophysiological attributes examined here, it is proposed that excess light is likely to be the most critical abiotic factor restricting distribution of the endangered species in a fragmented landscape. The survival of the species may be most dependent on the intactness of the habitat over-storey. In contrast, the vulnerable G. ilicifolium showed strong susceptibility to water limitation, and survival might be threatened if climate change alters habitat water relations to cause, for example, more pronounced dry periods. The rare G. excelsum which had highest carbon gain and growth in the experiments carried out in this study, may become the most successful adaptation out of the rainforest environment due to its tolerance to higher light and limited water availability. To examine the generality of the link between rarity and ecophysiology with Graptophyllum species, two dipterocarp species, narrowly endemic Dipterocarpus condorensis and local common Shorea roxburghii that are actually co-located in South-eastern Vietnam were studied. Findings in this case study confirmed the usefulness of the comparative approach based on physiological measurements, either in situ or ex situ, to explain plant rarity. The results of this study indicate ecophysiological research is a tool for examining causes of rarity and possible abiotic threats. The information gained allows assessment of environmental resilience of species and contributes essential knowledge for management and conservation of threatened plants. Such knowledge is also useful for ex situ conservation including propagation, translocation and re-introduction in restoration programs.

Summary: 1. A Study was made of some aspects of the utilization by Spirodela oligorrhiza of ammonium, nitrate, nitride, and organis nitrogenous compounds as sole sources of nitrogen. 2. S. oligorrhiza was grown in axenci culture under carefully defined conditions, and the only factor to be consciously varied were those relating to the nitrogen source. 3. Ammonium, or some product of assimilation, inhibited the utilization of nitrate by inhibiting, at least partially, the uptake of nitrate, and by inhibiting almost completely the reduction of nitrate to nitrite. 4. Nitrite also inhibited the utilization of nitrate. 5. Ammonium and nitrite were taken up and assimilated simultaneously when they were supplied together in the medium. 6. Nitrate reductase and nitrite reductase were found to be adaptive enzymes, being present in S. ologorrhiza only when it was supplied with nitrate or nitrite. There was good correlation between the concentration of nitrate in plants and the level of nitrate reduction that they contained...

The mangrove seedlings Bruguiera and Aviceania develop in a swamp environment where oxygen may be deficient and carbon dioxide in high concentration. Both of these states may be inimical to respiratory processes in plants and especially to the early stages of germination. While vivipary allows the development of the seedlings to a stage where they may better withstand the unfavourable conditions of the environment, nevertheless mangrove seedlings are still liberated when low oxygen concentrations can have severe effects. The developing Avicennia seedling, while on the parent tree, is surrounded by a tough pericarp which restricts gaseous diffusion and induces a partially anaerobic environment around the cotyledon tissues. The detection of high lactic acid levels in Avicennia seedlings may be linked with the carbon dioxide-rich atmospheres which build up around the tissues. It is possible that high levels of carbon dioxide induce the production of lactic acid, a non-toxic product of fermentation, rather than of the more toxic ethanol. Lactic acid is also formed during the initial stages of anaerobiosis in Bruguiera seedlings.

Summary: 1. A Study was made of some aspects of the utilization by Spirodela oligorrhiza of ammonium, nitrate, nitride, and organis nitrogenous compounds as sole sources of nitrogen. 2. S. oligorrhiza was grown in axenci culture under carefully defined conditions, and the only factor to be consciously varied were those relating to the nitrogen source. 3. Ammonium, or some product of assimilation, inhibited the utilization of nitrate by inhibiting, at least partially, the uptake of nitrate, and by inhibiting almost completely the reduction of nitrate to nitrite. 4. Nitrite also inhibited the utilization of nitrate. 5. Ammonium and nitrite were taken up and assimilated simultaneously when they were supplied together in the medium. 6. Nitrate reductase and nitrite reductase were found to be adaptive enzymes, being present in S. ologorrhiza only when it was supplied with nitrate or nitrite. There was good correlation between the concentration of nitrate in plants and the level of nitrate reduction that they contained...

The mangrove seedlings Bruguiera and Aviceania develop in a swamp environment where oxygen may be deficient and carbon dioxide in high concentration. Both of these states may be inimical to respiratory processes in plants and especially to the early stages of germination. While vivipary allows the development of the seedlings to a stage where they may better withstand the unfavourable conditions of the environment, nevertheless mangrove seedlings are still liberated when low oxygen concentrations can have severe effects. The developing Avicennia seedling, while on the parent tree, is surrounded by a tough pericarp which restricts gaseous diffusion and induces a partially anaerobic environment around the cotyledon tissues. The detection of high lactic acid levels in Avicennia seedlings may be linked with the carbon dioxide-rich atmospheres which build up around the tissues. It is possible that high levels of carbon dioxide induce the production of lactic acid, a non-toxic product of fermentation, rather than of the more toxic ethanol. Lactic acid is also formed during the initial stages of anaerobiosis in Bruguiera seedlings.

Summary: 1. A Study was made of some aspects of the utilization by Spirodela oligorrhiza of ammonium, nitrate, nitride, and organis nitrogenous compounds as sole sources of nitrogen. 2. S. oligorrhiza was grown in axenci culture under carefully defined conditions, and the only factor to be consciously varied were those relating to the nitrogen source. 3. Ammonium, or some product of assimilation, inhibited the utilization of nitrate by inhibiting, at least partially, the uptake of nitrate, and by inhibiting almost completely the reduction of nitrate to nitrite. 4. Nitrite also inhibited the utilization of nitrate. 5. Ammonium and nitrite were taken up and assimilated simultaneously when they were supplied together in the medium. 6. Nitrate reductase and nitrite reductase were found to be adaptive enzymes, being present in S. ologorrhiza only when it was supplied with nitrate or nitrite. There was good correlation between the concentration of nitrate in plants and the level of nitrate reduction that they contained...

The mangrove seedlings Bruguiera and Aviceania develop in a swamp environment where oxygen may be deficient and carbon dioxide in high concentration. Both of these states may be inimical to respiratory processes in plants and especially to the early stages of germination. While vivipary allows the development of the seedlings to a stage where they may better withstand the unfavourable conditions of the environment, nevertheless mangrove seedlings are still liberated when low oxygen concentrations can have severe effects. The developing Avicennia seedling, while on the parent tree, is surrounded by a tough pericarp which restricts gaseous diffusion and induces a partially anaerobic environment around the cotyledon tissues. The detection of high lactic acid levels in Avicennia seedlings may be linked with the carbon dioxide-rich atmospheres which build up around the tissues. It is possible that high levels of carbon dioxide induce the production of lactic acid, a non-toxic product of fermentation, rather than of the more toxic ethanol. Lactic acid is also formed during the initial stages of anaerobiosis in Bruguiera seedlings.