Molecular and physiological studies in walnut (Juglans regia) are combined to establish the putative role of leaf plasma membrane aquaporins in the response of leaf hydraulic conductance (K-leaf) to irradiance. The effects of light and temperature on K-leaf are described. Under dark conditions, K-leaf was low, but increased by 400% upon exposure to light. In contrast to dark conditions, K-leaf values of light-exposed leaves responded to temperature and 0.1 mM cycloheximide treatments. Furthermore, K-leaf was not related to stomatal aperture. Data of real-time reverse transcription-polymerase chain reaction showed that K-leaf dynamics were tightly correlated with the transcript abundance of two walnut aquaporins (JrPIP2,1 and JrPIP2,2). Low K-leaf in the dark was associated with down-regulation, whereas high K-leaf in the light was associated with up-regulation of JrPIP2. Light responses of K-leaf and aquaporin transcripts were reversible and inhibited by cycloheximide, indicating the importance of de novo protein biosynthesis inthisprocess. Our results indicate that walnut leaves can rapidly change their hydraulic conductance and suggest that these changes can be explained by regulation of plasma membrane aquaporins. Model simulation suggests that variable leaf hydraulic conductance in walnut might enhance leaf gas exchanges while buffering leaf water status in response to ambient light fluctuations.