I've often read that plants grow better when exposed to higher concentrations of carbon dioxide.

Yet, when the Associated Press mentions the subject, what it says is: Global warming boosts poison ivy.

The AP report, as published May 29 by the Boston Globe, begins:

Another reason to worry about global warming: more and itchier poison ivy. The noxious vine grows faster and bigger as carbon dioxide levels in the atmosphere rise, researchers report Monday.

And a CO2-driven vine also produces more of its rash-causing chemical, urushiol, conclude experiments conducted in a forest at Duke University where scientists increased carbon-dioxide levels to those expected in 2050.

Carbon dioxide is a greenhouse gas -- a chemical that traps heat similar to the way a greenhouse does -- that's considered a major contributor to global warming...

The Associated Press may be being a bit selective in the studies it chooses to cover.

Where, I wonder, is the news coverage of a 2006 study of the impact of increased atmospheric carbon dioxide on tobacco plants? The study that shows that tobacco plants grown in higher-CO2 conditions contain less nicotine in their plant leaves?

As the Center for the Study of Carbon Dioxide reports it (subscription required):

The authors [of the CO2-tobacco plant study] report that "plants grown at elevated CO2 and 5 mM NH4NO3 showed a marked and significant decrease in content of nicotine in leaves as well as in roots," while at 8 mM NH4NO3 the same was found to be true of upper leaves but not of lower leaves and roots..."

What it means

Keeping the story simple, Matros et al. concluded, in their words, that "tobacco plants grown under elevated CO2 show a slight decrease of nicotine contents." ...these findings would likely be considered by most people to be beneficial, as... nicotine is nearly universally acknowledged to have significant negative impacts on human health (Topliss et al., 2002).

I checked Lexis-Nexis for combinations of the term "tobacco" and "carbon dioxide" and found no news service reports about the tobacco/C02 study.