Interactive effect of CO2 enrichment and temperature on the photosynthesisof field-grown hinoki cypress (Chamaecyparis obtusa) branches

Two branches of a field-grown Chamaecy-paris obtusa tree were enclosed in c hambers of an open gas exchange system for continuous CO2 exchange measurem ents. One branch was subjected to ambient air (CO2, 370 mu mol mol(-1)) and the other was subjected to CO2-enriched air (800 mu mol mol(-1)). The CO2 exchange rate of the branches, air temperature and photosynthetic photon fl ux density were recorded every 4 min by a computer during the two experimen tal periods of July 1994 to June 1995 (experiment 1) and April 1996 to Augu st 1997 (experiment 2). The response of CO2 gas exchange rate to light chan ged with the seasonal temperature. The highest saturated rate of net photos ynthesis on a leaf area basis was observed in May and October in both CO2 t reatments when the mean daytime temperature was-about 18-19 degrees C. This temperature was almost equal to the yearly mean daytime temperature. Above and below this temperature, the saturated net photosynthesis rate decrease d. The net photosynthesis rate was usually higher in the elevated CO2 treat ment. The ratio of monthly net photosynthesis rate in elevated CO2 to that in ambient CO2 was linearly related to the monthly mean daytime temperature . This ratio increased by 3.3%: for each 1 degrees C increase in the monthl y mean daytime temperature; the highest ratio of 1.8 occurred in August. Wh en the ratio was 1.0, the temperature was about 5-6 degrees C, which was cl ose to the mean daytime temperature of the coldest month. Elevated CO2 incr eased per unit area net photosynthesis by 38.5% and 43.7% in experiments 1 and 2, respectively.