Sweet potato (cv. Beniazuma) plants were cultured for 123 days
from June to October in 2010 in a plastic house at Osaka Prefecture
University. A sweet potato cutting was planted in each of 15 cylindrical
vinyl chloride pots (8.3 cm diameter and 60 cm deep) filled with sand (Fig. 2) and was grown for 2 weeks. After 2 weeks, vessels
containing either sodium hydrogen carbonate (to absorb CO2)
or calcium hydroxide (to produce CO2) were placed 7 cm from the
bottom of each pot to regulate at CO2 concentrations of 1.8%, 2.5%
and 4.9% in the root zone. The bottom of each pot was sealed with
a water layer. The number of pots in each CO2 concentration treatment
was five. Nutrient solution was applied periodically to the
soil surface, as outlined for Experiment 2 above. We supplied CO2
to the sand soil from the bottom of the cylindrical container in order
to increase the CO2 concentration in the root zone. There were no
significant increase in the atmospheric CO2 and no effects on photosynthesis
and transpiration, because the CO2 release rate from
the soil surface was negligibly small and the air ventilation rate
was sufficiently large for rapid dispersion inside the plastic house
with fully opened side windows.