Drought limits light harvesting,resulting in lower plant growth and reproduction.One trait important for plant drought response is water-use efficiency (WUE).We investigated (1) how the joint genetic architecture of WUE,reproductive characters,and vegetative traits changed across drought and well-watered conditions,(2) whether traits with distinct developmental bases (e.g.leaf gas exchange versus reproduction) differed in the environmental sensitivity of their genetic architecture,and (3) whether quantitative variation in circadian period was related to drought response in Brassica rapa.Overall,WUE increased in drought,primarily because stomatal conductance,and thus water loss,declined more than carbon fixation.Genotypes with the highest WUE in drought expressed the lowest WUE in well-watered conditions,and had the largest vegetative and floral organs in both treatments.Thus,large changes in WUE enabled some genotypes to approach vegetative and reproductive trait optima across environments.The genetic architecture differed for gas-exchange and vegetative traits across drought and well-watered conditions,but not for floral traits.Correlations between circadian and leaf gas-exchange traits were significant but did not vary across treatments,indicating that circadian period affects physiological function regardless of water availability.These results suggest that WUE is important for drought tolerance in Brassica rapa and that artificial selection for increased WUE in drought will not result in maladaptive expression of other traits that are correlated with WUE.