Comparative ecophysiology of Graptophyllum species in Australia

Abstract

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