Background/Question/Methods Phenotypic plasticity is widespread in plants, underlying acclimation to variable environmental conditions, and when adaptive, enhancing fitness. There has been extensive research to characterize phenotypic plasticity, however few studies explore the role of plasticity across developmental stages, i.e. ontogenetic stages. There is extensive evidence that functional traits vary across ontogeny, reflecting adaptation to changing environmental conditions and intrinsic constraints. How plasticity and genetically regulated ontogenetic trait change together drive phenotypic variation as plants develop remains difficult to disentangle. To address this, we investigated phenotypic plasticity to water and light availability throughout ontogeny in the endemic Hawaiian prickly poppy, Argemone glauca (pua kala in Hawaiian; Papaveraceae). We manipulated light and water availability in a full factorial experiment, characterizing traits within treatment groups at the seedling, juvenile, and mature ontogenetic stages. We targeted functional traits likely to drive fitness effects under light and water limitation, including leaf morphological (LMA, leaf area and dissection, prickle density), as well as physiological (photosynthesis, chlorophyll content, latex exudation, and water use efficiency) traits. Trait variation across stages reflects ontogenetic patterns, while shifts in the slope or magnitude of these patterns among treatment groups indicate phenotypic plasticity in ontogenetic trait trajectories. Results/Conclusions We detected trait plasticity in response to both light and water availability. The patterns and magnitude of plasticity varied among traits, with leaf morphology demonstrating more plasticity in response to light availability. Trait expression was highly variable across ontogeny, and plasticity for some traits shifted with ontogeny. These results reveal the role of ontogeny in driving plant response to stress and provides insight into the drivers of phenotypic plasticity as plants face a changing environment across ontogeny. Plans to expand this research include greenhouse experiments on congeneric species from Mexico (A. mexicana, A. platyceras), and a field experiment to test the fitness of different ontogenetic trajectories in response to abiotic stress.