Differential temperature responses of diurnal and nocturnal leaf respiration in four alpine herbaceous species

Current estimates of diel respiratory carbon release depend on accurate predictions of the temperature sensitivity (Q₁₀) of leaf respiration during the day and night. Such predictions typically rely on measurements of the Q₁₀ of respiration in the light (R_L) and dark (R_D) made during the day, and assuming that the Q₁₀ of nocturnal respiration (R_N) equals that of R_D. Using R_D as a surrogate for R_N, however, creates errors in estimates of diel respiration whenever the Q₁₀ of R_D and R_N differ. Using measurements made on field-grown, high-altitude alpine plants, our study investigated whether the Q₁₀ of leaf respiration differs between the day and night.

We characterised diurnal R_L and R_D from 15 to 35 °C, and R_N from 10 to 25 °C at night, in four common herbaceous species widely distributed in alpine meadows on the Qinghai-Tibetan Plateau. We measured leaf temperature every second for 24 h over a period of 18 days. By combining leaf temperature with respiratory physiological measurements, we calculated leaf carbon loss to assess the consequences of differences in temperature response of leaf respiration between day and night.

R_N exhibited a higher Q₁₀ than R_L and R_D by about one third. Although there were no significant differences in Q₁₀ between R_L and R_D, light inhibition of leaf respiration (i.e. 100 % - R_L / R_D) was at its lowest at a moderate leaf temperature (22−25 °C). G. straminea and S. pulchra showed lower levels of inhibition than L. sagitta and L. virgaurea. Respiratory carbon loss (C_{loss_day}) based on R_N exceeded C_{loss_day} based on R_L by up to 47 %, which varied considerably between the species.

These results suggest that the temperature sensitivity (Q₁₀) of leaf respiration differs significantly between day and night, a finding that needs to be taken into account when modelling the diel rates of respiratory carbon loss in plants, especially at high altitudes and some high latitudes with a large diurnal variation and low mean temperature. Therefore, considering that neither R_N nor R_D can accurately represent R_L, we strongly recommend that the observations of R_L should be prioritized when estimating daytime leaf carbon loss.