Temperature and irradiance and the total daily photosynthetic production of the crown of a Pinus radiata tree

Summary Carbon dioxide exchange rates were recorded for different ages and positions of foliage and parts of the main stem of a 7-m tall Pinus radiata D. Don tree growing in a large, artificially lit, controlled-environment room. Irradiance levels were varied from dark to approximately full sunlight, and air temperatures from 10° to 35°C in 5°C steps. Leaf temperatures within the cuvettes used for CO₂ exchange measurements, however, were up to 5°C higher than the room air temperature set but this varied with position in the tree crown, the shaded lower crown being at approximately room temperature. A balance sheet was prepared to show the photosynthetic gains and respiratory losses of different parts of the crown over 24 h at each air temperature and at irradiances of 400, 270, and 135 W m^-2 during the 8-h photosynthetic period. The greatest daily photosynthetic gain was at 10° C, although this temperature is considered sub-optimal for growth. At temperatures greater than 25° C, even at the greatest irradiance level for 8 h, total respiration was greater than photosynthesis.