Photosynthesis and Transpiration of Pinus Radiata D. Don Under Plantation Conditions in Southern Australia. Ii. First-Year Seedlings and 5-Year-Old Trees on Aeolian Sands at Rennick (South-Western Victoria).

Abstract

Measurements of net CO2 assimilation and transpiration rates were made over 4 days in the field in February 1980, on first-year seedlings and 5-year-old trees of Pinus radiata growing in plantation conditions in SW. Victoria. Gas exchange, enclosure, methods were used. Very high needle temperatures (to 45°C) and vapour pressure differences (to 70 mbar) were encountered. Watering treatments allowed estimates to be made of the effect of existing soil water deficits. The maximum rates of assimilation observed were 3.78 µmol CO2 m-² s-¹ in the watered seedlings and 3.15 µmol m-² s-¹ in the unwatered tree. These values are 40% less than has been recently reported for P. radiata in New Zealand but agree with other data for P. radiata and other conifers. The light saturation of net photosynthesis occurred at c. 350 W m-². Increase in needle-air vapour pressure difference reduced needle conductance. Decrease of needle temperature and of vapour pressure difference from very high values, due to a rapid drop in air temperature, was accompanied by an increase of assimilation rate and of needle conductance, confirming that needle temperatures above about 30°C are supraoptimal for P. radiata. Watering increased assimilation rate, needle water potential and needle conductance. This was most apparent in the seedlings; the responses of the older trees were much less marked.