It's difficult to generalize about plant CO2 uptake because it ranges
widely over time and for different crops and biomes. Plant productivity
is often expressed as grams of dry matter produced per square meter of
ground area per day or kilograms/square meter/year. One of the highest
short term figures I've seen is 50 g/square meter/day for a corn crop
during grain filling. Search google.com for net primary productivity
(NPP) to find more data.

Here's a neat, color global map showing how NPP changes over two years:

http://earthobservatory.nasa.g ov/Newsroom/NPP/npp.html

Salisbury and Ross's 1985 Plant Physiology text had Table 11-1 with NPP
data for natural ecosystems and cultivated areas. Their annual average
for tropical rain forest ranged from 2.7 to 9.6 g/square meter/day and
0.27 to 9.6 g/square meter/day for cultivated land.

If you want an idea of the percentage of the carbon fixed during the
day that is respired at night, search for the term maintenance
respiration.

The following webpage gives a generalization that plant respiration
consumes "25% of gross photosynthesis plus 1.5% of existing plant dry
weight per day."

http://www.life.uiuc.edu/plantbio/wimovac/respirat.htm


David R. Hershey