*2.2. Faster 2-PG Degradation is Beneficial for Photosynthetic CO2 Assimilation under Water-Limiting Conditions*

Next, we tested if the photosynthetic and stomatal features of *PGLP* overexpressors showed differences to wild-type plants under water-limiting conditions. To this end, all plants were conjointly grown under environmental controlled conditions in one pot (28 cm diameter, one plant per genotype) for 6 weeks with regular watering. Values from gas-exchange measurements at this time point served as control. Thereafter, watering was stopped, and gas-exchange measurements were repeated after 13 days under water-limiting conditions. This time point was chosen since previous studies showed upregulation of the photorespiratory pathway on the protein level 13 days after onset of water shortage [25]. As shown in Figure 3A, and in agreement with previous results [10], *PGLP* overexpressors displayed no significant change in *A*, the CO2 compensation points (Γ), *gs* and the ratio between the internal versus the external CO2 concentration (*Ci*/*Ca*) under control conditions. Only minor changes were observed in the transpiration rate (*E*) and the water use efficiency (*AN*/*E*) in line O9. After 13 days of withholding water, wild-type plants compromised photosynthetic performance, whereas both overexpressor lines displayed significantly higher *A*, *gs* and *E* levels, while Γ was decreased (significant in line O1). The *Ci*/*Ca* and *AN*/*E* parameters remained almost unchanged (Figure 3B). In summary, overexpression of *PGLP* permits a longer maintenance of the photosynthetic capacity under water-limiting conditions.

**Figure 2.** Light and temperature dependent gas exchange parameters of wild-type Arabidopsis and *PGLP* overexpressors. Plants were grown under standard conditions to growth stage 5.1 [33] and subsequently used for gas exchange measurements at (**A**) 20 ◦C and (**B**) 30 ◦C block temperature. Shown are (**A1**,**B1**) net CO2 uptake rates (*A*), (**A2**,**B2**) stomatal conductance (*gs*), (**A3**,**B3**) transpiration (*E*) and (**A4**,**B4**) relative electron transport rates (rETR). Values presented are mean values ± SD from at least four biological replicates (Col.0—green, solid line with circles, O9—black, dotted lines with squares and O1—black, dashed line with triangles). Asterisks indicate values statistically different from the wild type as determined by Student's *t*-test (*p* < 0.05).

**Figure 3.** Photosynthetic parameters of wild-type Arabidopsis and *PGLP* overexpressors under water-limiting conditions. Wild-type and *PGLP* overexpressor (O9 and O1) plants were grown in one pot (28 cm diameter, 1 individual per genotype) for 6 weeks with regular watering (twice a week) under standard conditions. Following control measurements (**A**), watering was stopped, and plants measured after 13 days of withholding water (**B**). Shown are mean values ± SD from at least 5 individuals per genotype, grown as 5 technical replicates. Asterisks indicate values statistically different from the wild type as determined by Student's *t*-test (*p* < 0.05; n. s.—not significant). Abbreviations: *A*—net CO2 uptake rates, Γ—CO2 compensation points, *gs*—stomatal conductance, *E*—transpiration, *AN*/*E*—water use efficiency and *Ci*/*Ca*—ratio of internal versus external CO2 concentration.
