The stimulatory effect of e[CO2] on plant growth is dependent on adequate nutrient supply. For example,
N concentrations in plant tissues generally decrease under e[CO2], which in leaves is commonly
related to a decrease in Rubisco concentration and activity, and therefore linked to photosynthetic downward
acclimation. This effect is also of direct concern for food production where decreased N and protein
content can have negative effects on product quality (e.g. grain protein). Plant nutrient metabolism
appears to adjust to a new physiological equilibrium under e[CO2] which limits the extent to which
nutrient application can ameliorate the situation. What the control points are for an adjustment of plant
N metabolism is unclear. Rubisco metabolism in leaves, N assimilation, N translocation or N uptake are
all potential key steps that may be inhibited or down regulated under e[CO2]. To achieve the best possible
growth response whilst maintaining product quality, it is important to understand plant nutrient
metabolism under e[CO2].
Comparatively little is known about mechanisms of potential changes in plant stress tolerance under
e[CO2]. Defence metabolites such as antioxidants are, in part, directly linked to primary carbohydrate
mechanism and so potentially impacted by e[CO2]. It is unknown whether photoprotective and antioxidative
defence systems, key to plant stress tolerance, will be affected, and if so, whether the response
will be strengthened or weakened by e[CO2]. Better understanding of underlying principles is particularly
important because it is virtually impossible to test all possible stress factor combinations with e[CO2] in
realistic field settings.