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JXB Advance Access published online on April 18, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm023
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© The Author [2007]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

Whole-tree level water balance and its implications on stomatal oscillations in orange trees [Citrus sinensis (L.) Osbeck] under natural climatic conditions

S Dzikiti1,*, K Steppe2, R Lemeur2 and JR Milford1

1Agricultural Meteorology Group, Department of Physics, University of Zimbabwe, PO Box MP167, Mt Pleasant, Harare, Zimbabwe
2Faculty of Bioscience Engineering, Laboratory of Plant Ecology, Ghent University, Coupure links 653, B-9000 Ghent, Belgium

* To whom correspondence should be addressed. E-mail: sdzikiti{at}science.uz.ac.zw

Sustained cyclic oscillations in stomatal conductance, leaf water potential, and sap flow were observed in young orange trees growing under natural conditions. The oscillations had an average period of approximately 70 min. Water uptake by the roots and loss by the leaves was characterized by large time lags which led to imbalances between water supply and demand in the leaves. The bulk of the lag in response between stomatal movements and the upstream water balance resided downstream of the branch, with branch level sap flow lagging behind the stomatal conductance by approximately 20 min while the stem sap flow had a much shorter time lag of only 5 min behind the branch sap flow. This imbalance between water uptake and loss caused transient changes in internal water deficits which were closely correlated to the dynamics of the leaf water potential. The hydraulic resistance of the whole tree fluctuated throughout the day, suggesting transient changes in the efficiency of water supply to the leaves. A simple whole-tree water balance model was applied to describe the dynamics of water transport in the young orange trees, and typical values of the hydraulic parameters of the transpiration stream were estimated. In addition to the hydro-passive stomatal movements, whole-tree water balance appears to be an important factor in the generation of stomatal oscillations.

Key words: Citrus sinensis, model, oscillations, sap flow, stomata, stem diameter variation, time lag

Received 29 September 2006; Revised 20 January 2007 Accepted 23 January 2007


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