Further knowledge of plant response to environmental modifications is required to improve our understanding and potentially predict the broader effects of global climate change over terrestrial ecosystems. Molecular techniques, particularly gene expression analysis, can help investigate the response of non-model plant species to abiotic stress such as drought. To gain a mechanistic understanding of a grassland ecosystem's response to global climate change from the level of gene expression to the whole ecosystem function, we designed an experimental setup that allowed precise control of precipitation regimes and collection of plant samples (leaf and roots) at specific soil moisture values. A. barbata plants (the dominant species in many California grasslands) were grown in monoculture or in mixed communities including additional grass and forbs species. The plants, grown on natural soil, were submitted to three precipitation regimes (low, ambient and high) dispensed in several cycles of watering and drought periods. Analysis of leaf samples collected at peak physiology and during the drought period indicated that ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) mRNA levels tended to be higher in plants grown under ambient conditions compared to those exposed to low or high precipitation treatments. However, the changes in Rubisco transcript abundance did not always relate to changes in leaf level photosynthesis. The mRNA levels for genes involved in nitrogen metabolism such as glutamine synthetase and nitrate reductase were correlated to some measures of soil water and nitrogen availability. These results suggest that analysis of transcript abundance in the leaf may be a good indicator that the plants are under environmental stress. However, establishing relationships between gene expression and leaf physiology may be more challenging partly because of post-transcriptional and post-translational regulation.