Re: What is the effect of a plant on CO2 level in a closed system?

Normally plants should absorb CO2 and release O2 during photosynthesis, 
however,there are some factors that influence photosynthesis, such as 
humudity(low humidity will limit photosynthesis), temperature(high 
temperature promotes photorespiration, which is competitive with 
photosynthesis for resources, releases CO2 and ammonium) , light intensity. 
Plants also do respiration throughout its lifetime, which absorbs O2 and 
release CO2. Humidity and tempetature also will influence respiration rate.

In your system, somehow the plant did more respiration than photosynthesis, 
which means this system is stressful for this plant. Reason for the stress 
is hard to speculate, but plants need some time to adapt to the new 
environment, so the CO2 level may increase during the first day and may 
decrease after several days.

Reference:
1. http://unisci.com/stories/20013/0831012.htm
2. Salisbury and Ross (1997), Plant Physiology, 4th edition; 
Taiz & Zeiger (1998), Plant Physiology 2nd edition