Putative Role of Aquaporins in Variable Hydraulic Conductance of Leaves in Response to Light

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 (Kleaf) to irradiance. The effects of light and temperature on Kleaf are described. Under dark conditions, Kleaf was low, but increased by 400% upon exposure to light. In contrast to dark conditions, Kleaf values of light-exposed leaves responded to temperature and 0.1 mM cycloheximide treatments. Furthermore, Kleaf was not related to stomatal aperture. Data of real-time reverse transcription-polymerase chain reaction showed that Kleaf dynamics were tightly correlated with the transcript abundance of two walnut aquaporins (JrPIP2,1 and JrPIP2,2). Low Kleaf in the dark was associated with down-regulation, whereas high Kleaf in the light was associated with up-regulation of JrPIP2. Light responses of Kleaf and aquaporin transcripts were reversible and inhibited by cycloheximide, indicating the importance of de novo protein biosynthesis in this process. 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.