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The reasoning behind such a conclusion could to do with the differences in leaf anatomical features, stomatal conductance, and/or metabolic rates between the C3 and C4 crops.

To limit infection, plants close stomata on recognizing such an attack, in a defence response called stomatal immunity2.

And in contrast to paleosols, the stomatal technique is insensitive to high CO2. 

Management of stomatal apertures feeds in to management of the plant's hydration status and photosynthetic metabolism.

Conversely, when atmospheric carbon dioxide is low, stomatal density tends to increase — it’s like getting more mouths to breathe with.