Measuring the Canopy Net Photosynthesis of Glasshouse Crops

A null balance method is described for measuring net photosynthesis of mature canopies of cucumber and other protected crops over periods of 10 min in a single-span glasshouse (c. 9m × 18m in area). Accuracy of control of the CO2 concentration in the greenhouse atmosphere is within ±10 vpm of the normal ambient level (c. 350 vpm). The amounts of CO2 used in canopy net photosynthesis are measured with linear mass flowmeters accurate to within ±0.80g. The total errors incurred in measuring canopy net photosynthesis at an ambient CO2 level are estimated to be of the order of ± 1·2% in bright light (350 W m−2, PAR)and ±3·6% in dull light (100 W m−2, PAR). Measurements of the rates of net photosynthesis of a mature canopy of a cucumber crop were made at near-ambient CO2 concentrations over a range (0–350 W m−2) of natural light flux densities. A model of light absorption and photosynthesis applicable to row crops was used to obtain a net photosynthesis versus light response curve for the cucumber crop. At a light flux density of 350 W m−2 the fitted value of canopy net photosynthesis was 2.65 mg CO2 m−2s−1 (equivalent to over 95 kg CO2 ha−1h−1). The results are discussed in relation to the need for CO2 supplements to avoid depletion in both ventilated and unventilated glasshouses during late spring and summer.