Acclimation of loblolly pine (Pinus taeda) seedlings to high temperatures

To determine the extent to which loblolly pine seedlings (Pinus taeda L.) acclimate to high temperatures, seedlings from three provenances-southeastern Texas (mean annual temperature 20.3 degrees C), southwestern Arkansas (mean annual temperature 16.2 degrees C) and Chesapeake, Maryland (mean annual temperature 12.8 degrees C)-were grown at constant temperatures of 25, 30, 35 or 40 degrees C in growth chambers. After two months, only 14% of the seedlings in the 40 degrees C treatment survived, so the treatment was dropped from the experiment. Provenance and family differences were not significant for most measured variables. Total biomass was similar in the 25 and 30 degrees C treatments, and less in the 35 degrees C treatment. Foliage biomass was higher, and root biomass lower, in the 30 degrees C treatment compared with the 25 degrees C treatment. Net photosynthesis and dark respiration of all seedlings were measured at 25, 30 and 35 degrees C. Both net photosynthesis and dark respiration exhibited acclimation to the temperature at which the seedlings were grown. For each temperature treatment, the highest rate of net photosynthesis was measured at the growth temperature. Dark respiration rates increased with increasing measurement temperature, but the basal rate of respiration, measured at 25 degrees C, decreased from 0.617 &mgr;mol m(-2) s(-1) in the 25 degrees C treatment to 0.348 &mgr;mol m(-2) s(-1) in the 35 degrees C treatment, resulting in less carbon loss in the higher temperature treatments than if the seedlings had not acclimated to the growth conditions. Temperature acclimation, particularly of dark respiration, may explain why total biomass of seedlings grown at 30 degrees C was similar to that of seedlings grown at 25 degrees C.