Leaf gas exchange and water potential responses to
experimental drought
Following the cessation of
soil moisture additions on Day 0, large impacts on leaf water use and
CO2 metabolism could already be observed by Day 1 of the
drought (Figure 1 ). For example, mean stomatal conductance
(gs) values of drought treated plants declined from 1.1
mol m-2 s-1 on day 0 to 0.026 mol
m-2 s-1, representing a 97%
decrease. These low conductance values were maintained throughout the
drought treatment on day 4 and 7. As expected from a strong
drought-induced decrease in gs, leaf gas exchange of
CO2 and H2O in the light showed a large
suppression in drought treated plants. Under standard environmental
conditions, average net photosynthesis (A) decreased from 13.4 µmol
m-2 s-1 on Day 0 to -0.5 µmol
m-2 s-1 on Day 1, representing a
104% decrease and loss of net carbon assimilation. These near zero and
often negative net CO2 assimilation values continued in
the drought plants through days 4 and 7. Likewise, leaf transpiration
(E) decreased by 94% on Day 1 as a result of the experimental drought
treatment with average values declining from 5.7 mmol
m-2 s-1 on Day 0 to 0.33 mmol
m-2 s-1 on Day 1. These low leaf
transpiration values continued through Days 4 and 7. The strong
reduction in gs, A, and E observed during on Days 1, 4,
and 7 in drought treated individuals was associated with a decrease in
leaf water potential (LWP ), indicating dehydration of the leaves.
Average LWP declined from -0.56 MPa in drought treated leaves on
Day 0 to -1.0 on Days 1, 4, and 7, representing a 79% decline. After
the drought treatment, despite daily soil moisture additions resuming
for the droughted trees, all the trees lost their leaves.