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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 …