Abstract
Two of the ways in which plants cope with water deficits are stomatal closure and “osmotic adjustment”. We sought to assess the contributions of these processes to maintenance of leaf hydration in field-grown, 7-year-old Eucalyptus marginata. Plants were exposed to their normal summer drought (controls) or supplied with additional water (irrigated). Irrigation increased photosynthesis by 30% in E. marginata. These increases in photosynthesis were related to an 80% increase in g s. However, there was no difference in substomatal CO2 concentrations between treatments, or in chloroplast CO2 concentrations, as indicated by carbon isotope composition of leaf soluble sugars. This suggests that impaired mesophyll metabolism may partially explain slower rates of photosynthesis in plants exposed to their normal summer drought. There was no difference in concentrations of solutes or osmotic potential between non-irrigated and irrigated individuals, perhaps because relative water content was the same in non-irrigated and irrigated plants due to stomatal sensitivity to water deficits. Irrespective of the absence of osmotic adjustment, analysis of leaf solutes gave a clear indication of the major groups of compounds responsible for maintaining cell osmotic potential. Soluble sugars were three times as abundant as amino acids. Proline, a putatively osmotically active amino acid, contributed less than 1% of total solutes. These patterns of solutes in E. marginata are consistent with a growing body of literature arguing a greater role for carbohydrates and cyclitols and lesser role for amino acids in maintaining osmotic potential. Our data suggest the primary mechanism by which E. marginata coped with drought was partial stomatal closure; however, we cannot discount the possibility of osmotic adjustment under more severe water deficits.
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Acknowledgments
We gratefully acknowledge the financial assistance of The Australian Research Council. We thank Stephen Vlahos and Belinda Bastow from Worsley Alumina Pty. Ltd., for their help selecting the field site and for generous in-kind support. Alcoa World Alumina Australia provided further in-kind support and financial assistance. TMB was funded by an APA(I) scholarship while undertaking PhD research as part of this project. We thank Stefan Arndt, Stephen Easton and Alasdair Grigg for their help with field work, and we acknowledge general support provided by the School of Plant Biology and the Ecosystems Research Group at UWA, and Rob Jackson at Duke University.
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Communicated by Robert Pearcy.
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Warren, C.R., Bleby, T. & Adams, M.A. Changes in gas exchange versus leaf solutes as a means to cope with summer drought in Eucalyptus marginata . Oecologia 154, 1–10 (2007). https://doi.org/10.1007/s00442-007-0803-2
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DOI: https://doi.org/10.1007/s00442-007-0803-2