As plants in ombrotrophic bogs are well adapted to nutrient-poor conditions, we hypothesized that reactive-nitrogen (N) addition would affect their biochemistry and physiology. For the past 12 years, a section of Mer Bleue bog (Ontario, Canada) has been fertilized with different concentrations of N (as NO3 and NH4 combined) with or without phosphorus (P) and potassium (K) to investigate the impacts of nutrient addition on CO2 exchange at both ecosystem and leaf-level scales. Previous results from this experiment found that while fertilization increased foliar N contents, it did not increase photosynthetic capacity of three dominant ericaceous shrubs. They instead exhibited some indications of stress associated with nutrient imbalance. Our study examined more closely the stress responses of leatherleaf (Chamaedaphne calyculata Moench), the dominant evergreen shrub, in terms of light harvesting capacity and carbon assimilation. Maximum CO2 assimilation rates (Amax), Rubisco carboxylation rate (Vcmax) and chlorophyll content were measured in current-year and older top-canopy leaves at each triplicate treatment plot at control, 5 and 20 times ambient wet N deposition. Dark-adapted chlorophyll fluorescence (Fv/Fm), a widely used stress indicator assessing maximum leaf efficiency in absorbing light for photosynthesis, was also measured. An Fv/Fm ratio between 0.75 and 0.83 signifies healthy leaves.
Results/Conclusions
Fv/Fm ratios were significantly higher in old leaves (0.80 ± 0.034) than current-year ones (0.77 ± 0.043). Overall, leaves were not under stress, as the Fv/Fm ratio for leaves in all treatments ranged between 0.66 and 0.86. Fertilization seemed to benefit light-harvesting capacity in old leaves, with an increase in Fv/Fm in 5N (0.82 ± 0.007) and 20NPK (0.81 ± 0.009) treatments compared to control (0.79 ± 0.010), but have no effect on new ones. However, net assimilation rates (Amax) did not improve at high nutrient levels. Vcmax and chlorophyll content, on the other hand, both increased in the highest nutrient treatments (by 38% and 21% in 20N and 17% and 20% in 20NPK, respectively), indicating investment of newly added N to both light and dark phases of photosynthesis. There was also a strong positive correlation between Vcmax and chlorophyll content, yet no relationship between chlorophyll content and Amax. These responses indicate that the added nutrients are being used to improve both light harvesting and carbon assimilation capacity. The fact that there is no change in Amax warrants more investigation into changes in foliar respiration or photorespiration in response to nutrient addition.