Utilization of urea by leaves of bromeliad Vriesea gigantea under water deficit: much more than a nitrogen source

Abstract

Vriesea gigantea Gaudichaud is an epiphytic bromeliad with a high capacity to take up urea. In plants, urea is hydrolyzed by urease into ammonium and CO2, providing nitrogen to the plant. Most studies of urea nutrition have focused only on nitrogen metabolism, whereas scarce attention has been given to CO2 assimilation. Therefore, this study attempted to investigate whether urea could play an important role as a carbon source, which could be of a significant importance under water deficit conditions because of the limitation in atmospheric CO2 influx into the leaves due to stomatal closure. In this study, detached leaves of V. gigantea were exposed to water deficit and supplied with urea. The most photosynthetic parts of the leaf (mainly the apical leaf portion) showed higher urease activities and CO2 buildup near chloroplasts, particularly during the nighttime under water deficit conditions when compared to urea application without the water deficit. Moreover, part of the CO2 generated from urea hydrolysis was fixed into malate, probably via phosphoenolpyruvate carboxylase. Therefore, urea may contribute to the carbon balance of plants under water deficit conditions. Our data suggest that, besides being a source of nitrogen, urea might also be an important carbon source during CO2-limited conditions in leaves of epiphytic bromeliads.