Atmospheric CO2 enrichment influences the synthesis and mobilization of putative vacuolar storage proteins in sour orange tree leaves

Concentrations of three soluble proteins with molecular masses of 33, 31 and 21 kDa were measured weekly for a period of 1 year in leaves of sour orange (Citrus aurantium L.) trees that had been grown for 6 years at atmospheric CO2 concentrations of 400 and 700 ppm. Abundances of the proteins were generally lower in CO2-enriched leaves than in ambient-treatment leaves during the central portion of the year. Over the early and latter parts of the year, however, they typically were much greater in leaves of the CO2-enriched trees. The decrease from their high wintertime levels in the CO2-enriched trees possibly provided a source of nitrogen required for the enhanced new branch growth observed in the spring on the trees growing in CO2-enriched air. The hypothesis that they are vegetative storage proteins (VSPs) is also supported by the N-terminal amino acid sequence obtained for the 21-kDa protein, which has homology with sporamin B, an implicated storage protein in sweet potato tubers. In addition, immunoelectron microscopy demonstrated the presence of these proteins within amorphous material in the vacuoles of mesophyll cells, where VSPs are commonly located. The fact that elevated CO2 had little impact on the amount of leaf rubisco suggests that enhanced branch and fruit growth observed in the CO2-enriched trees was not correlated with an increased rate of breakdown of this major protein, another potential source of the nitrogen. The 33-, 31- and 21-kDa proteins appear to be specific to citrus species, as immunologically related proteins were detected in a variety of orange, grapefruit, lemon, tangelo and kumquat trees, but were not found in a large number of herbaceous plants and unrelated woody species.