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Tree Health :: Mechanical Harvesting, Drought Stress
and ABA in Citrus Trees


Determine interactions with mechanical harvesting, drought stress and ABA in citrus trees on different rootstocks. Determine the short-term physiological responses to drought (ABA-mediated stomatal closure) of sweet orange grafted on rootstocks with varying drought tolerance.

Key Findings (Updated on 09/10)

  • We determine the short-term physiological changes in sweet orange trees under drought stress in a climate controlled growth chamber.
  • ‘Valencia’ sweet orange trees on Swingle citrumelo (drought sensitive) or Volk (drought tolerant) rootstocks were compared to examine drought stress responses during a 72 hr period. We observed the highest peak of leaf ABA after only 12 hrs of water stress as levels of leaf ABA after 30 hr of water stress similar to those of leaf ABA after 12 hr of rehydration recovery.
  • High leaf ABA after 12-hr watering suggested that ABA compounds were translocated via the xylem to the shoots due to increased or restored xylem movement. Root ABA started accumulating after 30 hr of drought (perhaps by concentration in xylem fluid), peaked at 36 hr and drastically decreased after 42 hr of drought. The root ABA apparently was not translocated through xylem to the shoots during drought. The ABA reductions after 42 hr of drought stress may be due to higher rates of ABA degradation than local synthesis.

Activities Planned for 2010-2011

A second ABA field study is not completed yet. Leaf ABA and antioxidant enzymes change in response to drought stress and exogenous ABA but all data are not analyzed yet. Only one exogenous ABA concentration was compared to drought stress in the field and we would like to run an experiment with different concentrations of ABA.



Melgar, J.C., A.W. Schumann and J.P. Syvertsen. 2010. Irrigation and nitrogen application frequency affects growth and improves water and N use efficiencies of Swingle citrumelo citrus rootstock seedlings. HortScience (in press)

Melgar, J.C , J. Dunlop and J.P. Syvertsen. 2009. Tree Water Status and Exogenous ABA Affect Freeze Tolerance in ‘Valencia’ Trees. Proc FSHS 122: 78-80.

Syvertsen, J.P. and J.M. Dunlop. 2009. Imidacloprid Has Little Effect On Growth or Drought Tolerance of Citrus Rootstock Seedlings Without Pests. Proc FSHS 122: 81-84.

For more information:

Jim Syvertsen, Plant Physiologist

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