Long-term water stress leads to acclimation of drought sensitivity of photosynthetic capacity in xeric but not riparian Eucalyptus species.
OBJECTIVE
Experimental drought is well documented to induce a decline in photosynthetic capacity. However, if given time to acclimate to low water availability, the photosynthetic responses of plants to low soil moisture content may differ from those found in short-term experiments. This study aims to test whether plants acclimate to long-term water stress by modifying the functional relationships between photosynthetic traits and water stress, and whether species of contrasting habitat differ in their degree of acclimation.


METHODS
Three Eucalyptus taxa from xeric and riparian habitats were compared with regard to their gas exchange responses under short- and long-term drought. Photosynthetic parameters were measured after 2 and 4 months of watering treatments, namely field capacity or partial drought. At 4 months, all plants were watered to field capacity, then watering was stopped. Further measurements were made during the subsequent 'drying-down', continuing until stomata were closed.


RESULTS
Two months of partial drought consistently reduced assimilation rate, stomatal sensitivity parameters (g1), apparent maximum Rubisco activity (V'(cmax)) and maximum electron transport rate (J'(max)). Eucalyptus occidentalis from the xeric habitat showed the smallest decline in V'(cmax) and J'(max); however, after 4 months, V'(cmax) and J'(max) had recovered. Species differed in their degree of V'(cmax) acclimation. Eucalyptus occidentalis showed significant acclimation of the pre-dawn leaf water potential at which the V'(cmax) and 'true' V(cmax) (accounting for mesophyll conductance) declined most steeply during drying-down.


CONCLUSIONS
The findings indicate carbon loss under prolonged drought could be over-estimated without accounting for acclimation. In particular, (1) species from contrasting habitats differed in the magnitude of V'(cmax) reduction in short-term drought; (2) long-term drought allowed the possibility of acclimation, such that V'(cmax) reduction was mitigated; (3) xeric species showed a greater degree of V'(cmax) acclimation; and (4) photosynthetic acclimation involves hydraulic adjustments to reduce water loss while maintaining photosynthesis.
Department of Biological Sciences, Macquarie University, NSW 2109, Australia, shuangxi.zhou2014@gmail.com.
Shuang-Xi
SX
Zhou
Department of Biological Sciences, Macquarie University, NSW 2109, Australia, Hawkesbury Institute for the Environment, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia and.
Belinda E
BE
Medlyn
Department of Biological Sciences, Macquarie University, NSW 2109, Australia, AXA Chair of Biosphere and Climate Impacts, Grand Challenges in Ecosystems and the Environment and Grantham Institute - Climate Change and the Environment, Department of Life Sciences, Imperial College London, Silwood Park, Buckhurst Road, Ascot SL5 7PY, UK.
Iain Colin
IC
Prentice
D000064:Acclimatization / Q000502:physiology* / Q000528:radiation effects; D002245:Carbon Dioxide / Q000378:metabolism; D002247:Carbon Isotopes; D003681:Dehydration; D055864:Droughts; D017753:Ecosystem; D005052:Eucalyptus / Q000033:anatomy & histology / Q000502:physiology* / Q000528:radiation effects; D008027:Light; D010788:Photosynthesis / Q000502:physiology / Q000528:radiation effects; D018515:Plant Leaves / Q000033:anatomy & histology / Q000502:physiology / Q000528:radiation effects; D025341:Principal Component Analysis; D013045:Species Specificity; D014867:Water / Q000378:metabolism
D002247:Carbon Isotopes; D014867:Water; D002245:Carbon Dioxide
Drought acclimation; Huber value; Jmax; Vcmax; hydraulic adjustment; mesophyll conductance; photosynthesis; riparian Eucalyptus camaldulensis; stomatal conductance; water stress; water use efficiency; xeric Eucalyptus occidentalis