AN EXPERIMENTAL SET-UP TO STUDY CARBON, WATER AND NITRATE UPTAKE RATES BY HYDROPONICALLY GROWN PLANTS

The experimental system described allows concomitant hourly measuremen ts of CO2, H2O, and NO3 uptake rates by plants grown hydroponically in a greenhouse. Plants are enclosed in an airtight chamber through whic h air flows at a controlled speed. Carbon dioxide exchange and transpi ration rates are determined from respective differences of concentrati ons of CO2 and water vapor of the air at the system inlet and outlet. This set-up is based on the ''open-system'' principle with improvement s made on existing systems. For instance, propeller anemometers are us ed to monitor air flow rates in the chamber. From their signal it is p ossible to continuously adjust air speed to changing environmental con ditions and plant activity. The air temperature inside the system ther efore never rises above that outside. Water and NO3 uptake rates are c alculated at time intervals from changes in the volume and the NO3 con centration of the nutrient solution in contact with the roots. The pre cise measurement of the volume of solution is achieved using a balance which has a higher precision than any liquid level sensors. Nitrate c oncentration is determined in the laboratory from aliquots of solution sampled at time intervals. A number of test runs are reported which v alidate the measurements and confirm undisturbed conditions within the system. Results of typical diurnal changes in CO2, H2O, and NO3 uptak e rates by fruiting tomato plants are also presented.