When to use CO2

Experiments with tomato have shown that morning and afternoon CO2 enrichment give similar yield benefits.

Consequently, it is best to spread the use of the available CO2 over the whole course of the day, and so avoid canopy depletion. However, the optimal CO2 concentration will change over the course of the day. The benefits of CO2 enrichment increase at higher light levels, but the efficiency of enrichment decreases with higher ventilation rates. Simulations carried out as part of PC 110a suggest that CO2 levels should ideally be highest around noon in spring and autumn, but that in the summer they should be highest in the morning when ventilation rates tend to be lower.         

a close up of a plant

Excess CO2 damage in tomato 

MAFF funded work HH1303SPC  (1994) with four tomato cultivars grown in a multifactorial experimental glasshouse enriched with 425 ppm CO2 in either the morning or the afternoon or all day in comparison to no enrichment were compared. Yield was found to be related to the daily average CO2 concentration, there was no significant difference of adding CO2 in the morning or the afternoon.
Plants move from respiration to photosynthesis at first light (after 4:00 am) and fix carbon strongly until a maximum is reached around midday closure. At midday closure the leaves have accumulated starch (end product inhibition) and are also transpiring rapidly to cool the leaf surfaces. At some point in the diurnal cycle plants will close stomata to rehydrate (midday closure) and also metabolise the starch which has accumulated in the chloroplasts. Levels of assimilation will be more variable in the afternoon.

Preliminary results from commercial unreplicated monitoring in PE 021 suggest that there was a reduced photosynthetic response in the afternoon of a large vine (cv. Roterno) and cocktail cherry (Piccolo) tomato. The data suggest there may be merit in tailoring CO2 enrichment to the morning and up to and including the brightest parts of the day. This will shift from up to 15:00 h in the early season and move back to 12-13:00 h in September/October. Piccolo was shown to be less efficient at scavenging available CO2 compared with Roterno and as such is potentially more sensitive to lower CO2 enrichment. It should be stressed that these are preliminary results and that a variety of environmental variables will have an influence on these responses.