[remotemass (OP)]

How much O2 and CO2 is released and absorbed by average lettuce until it reaches its maximum size and is ready to harvest?

[alancalverd]

There is no "average lettuce". Some varieties weigh less than 200g at market size, others up to 800g. But I like iceberg, which is usually sold at around 400g in the UK. The root system would add another 150  g or so.

Of this, about 95% is water and the remainder is organic material which you can estimate as being half CO2 and half water in origin - our correspondent BC will almost certainly have a better estimate. So 2.5% of 550g gives us 13.75 gram of CO2 incorporated as sugars and suchlike in a lettuce, of which you can eat about 10 gram and ....exhale about half as CO2!.

[Bored chemist]

There is no "average lettuce"

You don't eat service station salad, do you?
I'm told it's a very average salad.

You could get a more precise estimate of the CO2 from the dietary information, but Alan's figure is in the right ballpark.

On a slightly longer time scale, the net CO2 consumed by the plant is zero unless it's turned to coal.

[evan_au]

If you want a lifecycle estimate of lettuce CO₂, you have to include:
- Any artificial fertilizer (a very CO₂-intensive process)
- Any transport involved in planting, harvesting and retail
- Any cooking (not much if it's used in a salad...)

[alancalverd]

Although I travel a lot, I rarely eat service station salads. I grow lettuce from seed without artificial fertiliser - not sure how to estimate the carbon footprint of chicken poo and rotted nettles.  But home-grown lettuce uses a lot of water which has been processed and pumped to the property, so a supermarket iceberg (that variety doesn't grow too well in my garden) may actually be more efficient.