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Rising atmospheric carbon dioxide (CO2) levels can impact plant photosynthesis and productivity and threaten food security, especially when combined with additional environmental stressors. This study addresses the effects of elevated CO2 in combination with low nutrient supply on Lemna minor (common duckweed). We quantified plant growth rate and nutritional quality (protein content) and evaluated whether any adverse effects of elevated CO2, low nutrients, or the combination of the two could be mitigated by plant-microbe interaction. Plants were grown under controlled conditions and were either uninoculated or inoculated with microorganisms from a local pond that supported L. minor populations. Under low nutrients in combination with high CO2, growth (plant area expansion rate) decreased and biomass accumulation increased, albeit with lower nutritional quality (lower percentage of protein per plant biomass). Inoculation with plant-associated microorganisms restored area expansion rate and further stimulated biomass accumulation while supporting a high protein-to-biomass ratio and, thus, a high nutritional quality. These findings indicate that plant-microbe interaction can support a higher nutritional quality of plant biomass under elevated atmospheric CO2 levels, an important finding for both human and non-human consumers during a time of rapid environmental change. 

Climate change impacts environmental conditions that affect photosynthesis. This review examines the effect of combinations of elevated atmospheric CO2, long photoperiods, and/or unfavorable nitrogen supply. Under moderate stress, perturbed plant source–sink ratio and redox state can be rebalanced but may result in reduced foliar protein content in C3 plants and a higher carbon-to-nitrogen ratio of plant biomass. More severe environmental conditions can trigger pronounced photosynthetic downregulation and impair growth. We comprehensively evaluate available evidence that microbial partners may be able to support plant productivity under challenging environmental conditions by providing (1) nutrients, (2) an additional carbohydrate sink, and (3) regulators of plant metabolism, especially plant redox state. In evaluating the latter mechanism, we note parallels to metabolic control in photosymbioses and microbial regulation of human redox biology. 

This review focuses on recently characterized traits of the aquatic floating plant Lemna with an emphasis on its capacity to combine rapid growth with the accumulation of high levels of the essential human micronutrient zeaxanthin due to an unusual pigment composition not seen in other fast-growing plants. In addition, Lemna’s response to elevated CO2 was evaluated in the context of the source–sink balance between plant sugar production and consumption. These and other traits of Lemnaceae are compared with those of other floating aquatic plants as well as terrestrial plants adapted to different environments. It was concluded that the unique features of aquatic plants reflect adaptations to the freshwater environment, including rapid growth, high productivity, and exceptionally strong accumulation of high-quality vegetative storage protein and human antioxidant micronutrients. It was further concluded that the insensitivity of growth rate to environmental conditions and plant source–sink imbalance may allow duckweeds to take advantage of elevated atmospheric CO2 levels via particularly strong stimulation of biomass production and only minor declines in the growth of new tissue. It is proposed that declines in nutritional quality under elevated CO2 (due to regulatory adjustments in photosynthetic metabolism) may be mitigated by plant–microbe interaction, for which duckweeds have a high propensity.