Reduced blue light irradiance is known to enhance leaf elongation rate (LER) in grasses but the mechanisms involved have not yet been elucidated. We investigated if leaf elongation response to reduced blue light could be mediated by stomatal induced variations of plant transpiration.Two experiments were carried out on tall fescue in order to monitor LER and transpiration under reduced blue light irradiance. Additionally, LER dynamics were compared to those observed in the response to VPD-induced variations of transpiration. Finally, we developed a model of water flow within a tiller to simulate the observed short-time response of LER to various transpiration regimes.LER dramatically increased in response to blue light reduction and then reached new steady states, which remained higher than the control. Reduced blue light triggered a simultaneous stomatal closure which induced an immediate decrease of leaf transpiration. The hydraulic model of leaf elongation accurately predicted the LER response to blue light and VPD, resulting from an increase in the growth-induced water potential gradient in the leaf growth zone.Our results suggest that the blue light signal is sensed by stomata of expanded leaves and transduced to the leaf growth zone through the hydraulic architecture of the tiller.