LOW NIGHT TEMPERATURES CAUSE REDUCED TRACHEID EXPANSION IN PODOCARPUS LATIFOLIUS

The influence of low night temperatures on the kinetics of tracheid expansion of two-year-old Podocarpus latifolius (Thunb.) R.Br. ex Mirb. was studied in a growth chamber experiment. In experiment 1 the plants were exposed to an almost constant air temperature of 18 to 20 degrees C, while in experiment 2 the air temperature was reduced from 20 degrees C during the day to 6 degrees C during the night. The formation of the cambium derivative cells and the kinetics of tracheid expansion were analysed by high resolution laser increment measurements in combination with microscopic methods (accuracy: +/- 2 mu m, spatial resolution of 9.7 to 13.1 mu m, temporal resolution: 60 s). Low night temperatures had no significant influence on the rate of cambial cell divisions and the proportion of tracheids and parenchyma cells in the xylem. However, they irreversibly interrupted the expansion of differentiating tracheids causing a high proportion of flattened tracheids in the xylem of the plants grown under the conditions of experiment 2. The results indicate that large differences between day and night temperatures can be an important trigger for the variation of the wood structure in subtropical and tropical gymnosperms.