Ashwani Kumar1 and Sven Schubert2 Abstract Phosphoenolpyruvate carboxylase (PEPcase) catalyzes the first step in the fixation of atmospheric CO2 during C4 photosynthesis. Under saline conditions, Hatzig et al. (2010) found an increase in PEPcase activity in young shoots of maize, whereas activity in old shoot parts and roots was identical for control and salt-stress conditions. Also, no difference in PEPcase activity under control and salt-stress conditions was found for maize. Although the effect of salt stress on PEPcase activity was found for the C4 and not the C3 species, the C4 pathway apparently was not the reason for enhanced PEPcase activity because sugar concentrations were increased. Instead, it was concluded that PEPcase activity may play an important role in organic-acid metabolism (Hatzig et al. 2010). Organic-acid synthesis may be important for cytoplasmic pH regulation (Davies 1973) or the supply of carbon skeletons for amino acid synthesis. Our recent study with the newly developed salt-resistant maize hybrid SR 12 (Schubert et al. 2009) in comparison with the less-resistant hybrid Pioneer 3906 found an increase in PEPcase (EC activity in young shoots of SR 12 but not in Pioneer 3906) under salt stress. This may indicate that PEPcase plays a role in the salt-resistance of maize. References: Davies D. D. (1973): Control of and by pH. Symp. Soc. Exp. Biol. 27: 513-529. Hatzig, S., Kumar A., Neubert A., Schubert S. (2010): J. Agron. Crop Sci.: 196: 185-192. Schubert S., Neubert A., Schierholt A., Sümer A., Zörb C. (2009): Plant Sci. 177: 196-202. Acknowledgment: Alexander von Humboldt Foundation grant of fellowship to Ashwani Kumar is gratefully acknowledged.