Photorespiration Is Associated with Nitrogen Metabolism and Nitrogen Productivity in Kentucky Bluegrass

Photorespiration links carbon metabolism with nitrogen assimilation in C₃ plants, but its relationship with nitrogen productivity in perennial turfgrass remains unclear. Ten Kentucky bluegrass (Poa pratensis L.) cultivars were screened for photorespiration rate and intensity. 'Heidi' and 'My Holiday' exhibited the highest and lowest photorespiration intensities (31.17 ± 2.75% and 6.05 ± 1.15%, respectively), and their absolute photorespiration rates were 2.36 ± 0.22 and 0.49 ± 0.09 μmol CO₂·m^-2·s^-1, representing a 4.8-fold difference. Under baseline conditions, 'Heidi' showed higher NDVI, shoot N concentration, and N accumulation, as well as component-specific differences in selected epidermal traits and biochemical measurements, whereas the measured gas-exchange variables were unchanged. Nitrogen treatments further showed that cultivar responses depended on nitrogen form and application rate. Under low nitrate supply, 'My Holiday' had a higher photorespiration rate than 'Heidi', whereas their partial factor productivity of applied nitrogen based on shoot dry matter (PFP_N) did not differ significantly. Comparative transcriptomic analysis identified serine hydroxymethyltransferase 4 (SHM4, hereafter PpSHMT) as a photorespiration-related candidate transcript, with a log₂ fold change of 3.37 in 'Heidi' relative to 'My Holiday'. qRT-PCR confirmed higher PpSHMT transcript abundance in 'Heidi', but bulk-leaf SHMT relative protein abundance did not differ significantly between the cultivars. Transient delivery of pBI121-PpSHMT into Nicotiana tabacum L. cv. K326 leaves was associated with changes in selected biochemical measurements. Overall, the results demonstrate a treatment-dependent association between photorespiration and nitrogen productivity and identify PpSHMT as a candidate requiring further functional investigation.