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.