The potential for photoinhibition of Pinus sylvestris L. seedlings exposed to high light and low soil temperature

The effect of high light and root chilling on gas exchange, chlorophyll fluorescence, and bulk shoot water potential (φ_shoot) was examined for Pinus sylvestris seedlings. Transferring plants from low light (200 μmol m^-2s^-1, PAR) and a soil temperature of 15 °C to high light (850 μmol m^-2 s^-1) and 1 °C caused >90% decrease in net photosynthesis and leaf conductance measured at 350 mm³ dm^-3 CO₂, and a decrease in the ratio of variable to maximum fluorescence (F_v/F_m) from 0.83 to 0.63. The decrease in Fv/Fm was, however, only marginally greater than when seedlings were transferred from low to high light but kept at a soil temperature of 15 °C. Thus, photoinhibition was a minor component of the substantial decrease observed for net photosynthesis at 1 °C soil temperature. The decrease in net photosynthesis and φ_shoot at 1 °C was associated with an increase in calculated intracellular CO₂ concentration, suggesting that non-stomatal factors related to water stress were involved in inhibiting carbon assimilation. Measurements at saturating external CO₂ concentration, however, indicate that stomatal closure was the dominant factor limiting net photosynthesis at low soil temperature. This interpretation was confirmed with additional experiments using Pinus taeda and Picea engelmannii seedlings. Decreases in gas-exchange variables at 5 °C soil temperature were not associated with changes in φ_shoot Thus, hormonal factors, localized decreases in φ_needles or changes in xylem flux may mediate the response to moderate root chilling.