How Plants May Maintain Protein Homeostasis Under Rising Atmospheric CO ₂

ABSTRACT Vascular plants may employ several physiological mechanisms to stabilize their protein contents as atmospheric CO 2 concentrations change over a day, year, decade, or century. One mechanism is that plants may rely more on soil ammonium as their nitrogen source when CO 2 increases. Another is that plants may convert more nitrate into amino acids in their roots. A third is that plants may increase the ratio of manganese over magnesium in their chloroplasts and accelerate a previously unrecognized biochemical cycle that generates organic acids such as malate instead of carbohydrates. This proposed cycle coordinates photorespiration with the metabolism of nitrogen and sulfur, the malate valve, the pentose phosphate shunt, the C 1 pathway, the C 2 glycolate pathway, and the C 3 carbon fixation pathway. The three mechanisms tend to improve the energy efficiency of most vascular plants.