1. Supporting Information Figures S1-S6

2. SNP - + cPTIO - +

3. Supporting Information Figure S1. Representative images showing that pre-treatment of epidermal peels with SNP (an NO donor) dramatically increases subsequent guard cell DAF-FM fluorescence while co-treatment with both SNP and cPTIO (an NO scavenger) dramatically reduces the subsequent DAF-FM signal. See Materials and Methods for further details.

5. Supporting Information Figure S2. Fresh weight loss over time of detached leaves incubated at 150 PPFR (a); 500 PPFR (b); or 900 PPFR (c). The leaves were from well-watered plants grown at 150 PPFR. WT, open circles; RI9, closed triangles; RI29, closed squares. The fresh weight is expressed as a % of the fresh weight at the beginning of the incubation period (time 0). Each independent experiment included three leaves for each plant line and irradiance, and the data are the mean +/- S.E. of three independent experiments (n=3).

7. Supporting Information Figure S3. Stomatal density (a); epidermal cell density (b); and stomatal index (c) of WT tobacco and two knockdown lines (RI9, RI29) with reduced AOX. Plants were growing at 150 PPFR and are either well-watered plants (Day 1) or plants subjected to drought stress (Day 4). WT, open bars; RI9, shaded bars; RI29, black bars. Data are the mean +/- S.E. of 3 independent experiments (n=3), with further details described in the Materials and Methods. Data were analyzed by 2-way ANOVA followed by a Bonferroni post-test to compare, within each day, the WT to each transgenic line. Number of asterisks above the bar indicates the level of significant difference: * P<0.05; ** P<0.01; *** P<0.001. Bars without an asterisk are not significantly different from the WT.

9. Supporting Information Figure S4. Relative changes in NO (a); H 2 O 2 (b); and NO + H 2 O 2 (c) in response to short-term changes in irradiance. For (c), the values in (a) and (b) have been summed and divided by 2. This figure re-plots data in Fig. 1 but with the NO and H 2 O 2 amounts at 150 PPFR normalized across plant lines to a value of 1. This data presentation is meant to facilitate examination of the relative changes in NO and H 2 O 2 amount in each plant line after transfer from 150 to 500 PPFR for 30 min, followed by transfer from 500 to 900 PPFR for another 30 min. WT, open bars; RI9, shaded bars; RI29, black bars. See Fig. 1 for complete details.

11. Supporting Information Figure S5. A n rate (a); and g s (b) of WT tobacco and two knockdown lines (RI9, RI29) with reduced AOX. Plants were grown at 150 PPFR and measurements were taken over the range of 0 to 2000 PPFR, as outlined in Materials and Methods. WT, open circles; RI9, closed triangles; RI29, closed squares. Data are the mean +/- S.E. of three independent experiments (n=3).

13. Supporting Information Figure S6. Leaf A n rate as a function of C i for WT tobacco and two knockdown lines (RI9, RI29) with reduced AOX. These were well-watered plants grown at 150 PPFR. A n was measured at saturating PPFR as described in Materials and Methods. WT, open circles; RI9, closed triangles; RI29, closed squares. Data are the mean +/- S.E. from three independent experiments (n=3).