Barley and its environment. 3. Carbon budget of the stand.
The 2 sources of CO2 for canopy photosynthesis are the soil and the atmosphere. For a barley crop, calculations of root respiration rate based on measured changes in root dry weight showed that approximately half of the CO2 released from the soil could be attributed to root respiration which is recycled within the crop system. The CO2 exchange between the crop and the atmosphere was calculated using either the Bowen ratio or aerodynamic method. These measurements enabled the net photosynthesis and dark respiration of the canopy to be calculated and hence the respiration in the light and 'gross' photosynthesis to be estimated. Weekly C fixation was estimated for 11 consecutive weeks during the growing season and these estimates agreed very well with the measured increase in C from growth analysis sampling. A relation between canopy photosynthesis rate and irradiance was demonstrated. Maximum photosynthesis rates decreased as a result of leaf senescence and when severe water stress occurred. Weekly amounts of 'gross' photosynthesis, total respiration and net CO2 fixation were established and measurements of respiration agreed well with estimates from McCree's 1970 formula [see HbA 41, 3115(11)] throughout the growing season. Initially net CO2 fixation was controlled by the expansion of the leaf surface and the amount of radiation intercepted, but after the emergence of the ear, respiration increased until net CO2 fixation was only 13% of 'gross' photosynthesis. The mean photosynthetic efficiencies of the canopy in terms of 'gross' and net photosynthesis were initially 5.5 and 2.8%, respectively. After ear emergence there was a brief increase in efficiency associated with the additional green area, but the efficiencies then declined to 4.3 and 0.5%, respectively, 3 weeks before harvest.