Physiological responses of Australian native and agricultural plant species to smoke from bushfires and prescribed burns.

Abstract

The impact of smoke from bushfires or prescribed burning has in the last decade emerged as a major risk for agricultural industries in Australia. This is especially true for the wine industry, as many wineries have recently experienced financial losses due to smoke taint in wine. A considerable amount of research has been done on the role of smoke in seed germination and on the effects of smoke on human health but research about the effects of smoke on the physiology of agricultural and native plant species is lacking. The aim of my research is to investigate the physiological responses of native and agricultural plants to smoke from bushfires or prescribed burns. The first component of my project involved exposing a broad range of species (n = 14) to smoke generated from combustion of leaf litter of Eucalyptus saligna. Physiological measures such as photosynthesis, stomatal conductance and transpiration were measured using leaves before and after exposure to smoke. Leaf anatomical features were described to determine possible morphological adaptations for comparing native plant species from fire-prone environments with agricultural species. Characteristics such as leaf surface area and thickness, moisture content, presence or absence of wax layers or hairs and tissue distribution within leaves were measured for comparisons. The outcome of these two studies showed that smoke had a substantial negative impact on leaf gas exchange of native and agriculture species. Immediate responses were recorded for rates of photosynthesis and similar, but slower, responses for stomatal conductance and transpiration. There was a clear indication that native species from fire-prone environments were able to withstand smoke exposure better than those from non-fire-prone environments. However, it was shown that leaf characteristics, such as a thick lamina and encrypted stomata that are common in native Australian species, can provide a means of protection against smoke from bushfires and prescribed burns. However, leaf traits such as trichomes and rough leaf surfaces promote the adherence of particulate matter and thus may have a negative effect on plant physiology. As smoke composition is variable and largely depends on the fuel burnt, the second part of my project was designed to investigate the effects of smoke from different types of fuel on leaf gas exchange. For this study five different fuel types were used: leaf litter from Eucalyptus saligna, litter from a temperate Eucalyptus forest, Pinus radiata needles, a mixture of native and exotic grasses and straw. Four native and exotic plant species (three woody and one herbaceous species) were used in this study. The impact of smoke on the leaf gas exchange varied according to the sources of smoke and among plant species. For example, smoke had a relatively small impact on Orange var. Valencia most likely due to the rapid response of stomatal closure. Since all leaves died after exposure to smoke from burning straw and the smoke from this fuel type had the highest emission factor for volatile organic compounds, it is possible that this component of smoke may have an important role in affecting plant physiology. The next component of my project focussed on the change in physiological responses of two herbaceous species after exposure to smoke in a controlled environment. Using a laboratory-based system, Strawberry and Sunflower plants were exposed to smoke for a fixed period of time (5, 10 or 15 min), and as the CO2 concentration of smoke could be measured it could be categorised as ‘high’ or ‘low’ concentration. This study showed that both ‘exposure time’ and ‘smoke concentration’ had a negative impact on leaf gas exchange of both species, but the effect of increased smoke concentration outweighed the effect of exposure time. In the final study, the parameters of a biochemical C3 photosynthesis model for plants exposed to different smoke exposure times and concentrations were estimated. Gas exchange measurements were done by combined fluorescence and gas exchange using a LI-6400 fluorescence chamber open system. Modelling the biochemical C3 photosynthesis parameters allows for a better understanding of what potential mechanism, a gas exchange or a biochemical limitation, may have caused the decline in photosynthesis after plants were exposed to smoke. The result of this study was that smoke causes impairment of the biochemical capacity of leaves, including the maximum rate of Rubisco activity-limited carboxylation (Vcmax) and the maximum e- transport under saturated light (Jmax). As rates of photosynthesis were back to normal 24 h after smoke exposure it was hypothesised that particulate matter deposited on leaf surfaces had no impact on the photosynthetic apparatus. This research will contribute to a better understanding of the consequences of exposure to smoke on agricultural and native plant physiology. This knowledge can be included in management plans for a range of stakeholders. For example, land managers may work with land holders to adjust the timing of prescribed burning to suit nearby agricultural properties and in this way may reduce the financial losses for wineries and other agricultural industries.