Sequential Effects of Acidic Precipitation and Drought on Photosynthesis and Chlorophyll Fluorescence Parameters of Pinus radiata D. Don Seedlings

We studied the sequential effect of simulated acid rain (SAR) and water stress on net photosynthesis (A), internal CO2 concentration (Ci), stomatal conductance (g), and instantaneous water use efficiency (A/E), and the photochemical damage caused by these stressors by analysing chlorophyll fluorescence parameters such as Fv/Fm, Fv/Fo and Fm/Fo. SAR caused inhibition of photosynthesis. The depletion of CO2 assimilation seems not to be solely a stomatal effect as effects on chloroplasts contribute to this photosynthetic inhibition. No noticeable damage seems to occur to the photochemical apparatus since well-watered plants treated with acid rain did not show any change in the chlorophyll fluorescence. Water stress produced a large effect on both gas exchange and chlorophyll fluorescence parameters, indicating structural and functional damage. Interactions among SAR and drought had a larger impact on all parameters studied. However, after rewatering, significant recovery in both gas exchange and chlorophyll fluorescence parameters occurred within 6 days, although the values measured prior to the imposition of treatments were not recovered. We conclude that Pinus radiata exposed to acid rain were affected in a wide range of components in the photosynthetic apparatus, thereby modifying its response to sequential drought stress.