Seedlings of two Acacia species from contrasting habitats show different photoprotective and antioxidative responses to drought and heatwaves

Abstract

Two Acacia species adapted to contrasting habitats showed different response of photoprotective and antioxidative defence systems to imposed drought and heatwave. Predicted increases in drought frequency and intense heatwaves are expected to lead to dieback of sensitive tree species. Stomatal closure restricts CO2 input into the leaf, resulting in imbalances between light energy-driven electron transport rate and electron consumption in the Calvin cycle. Reactive oxygen species formed under these circumstances have to be kept under control by photoprotective and antioxidative defence systems. We hypothesised that these defence systems behave differently in tree species from contrasting habitats. Acacia aneura (adapted to arid habitats) and Acacia melanoxylon (adapted to humid habitats) were exposed to two water treatments for 50 days including two short heatwave periods. Responses were assessed by gas exchange, chlorophyll fluorescence and concentrations of antioxidants (phyllodes, roots). Photosynthesis and quantum yield of photochemistry decreased significantly in both Acacia species, especially after water was withheld in combination with the second heatwave episode. In phyllodes, the concentration of antioxidants remained unchanged until exposure to severe drought and heatwave conditions (except for A. melanoxylon where changes in glutathione concentration were observed prior to exposure to severe stress), but after water was withheld and the second heatwave occurred, oxidised forms of glutathione increased. After exposure to the second heatwave, well-watered seedlings of A. melanoxylon but not A. aneura increased ascorbic acid concentration in phyllodes. Under well-watered conditions, Acacia species also showed increased concentration of antioxidants in roots following heatwaves. Both Acacia species showed photodamage to photosystem II (PSII) after water was withheld and the second heatwave imposed, but with more gradual response in A. aneura. Total concentration of investigated antioxidants increased in response to the first (A. melanoxylon) and second (A. aneura) heatwaves rather than drought stress alone.