The Earth's rotation and its orbit around the Sun leads to continual changes in the environment. Many organisms, including plants and animals, have evolved circadian clocks that anticipate these changes in light, temperature, and seasons in order to optimize growth and physiology. Circadian timing is thought to derive from a molecular oscillator that is present in every plant cell. A central aspect of the circadian oscillator is the presence of transcription translation loops (TTLs) that provide negative feedback to generate circadian rhythms. This review examines the evidence that the 24 h circadian clocks of plants regulate the fluxes of solutes and how changes in solute concentrations can also provide feedback to modulate the behaviour of the molecular oscillator. It highlights recent advances that demonstrate interactions between components of TTLs and regulation of solute concentration and transport. How rhythmic control of water fluxes, ions such as K(+), metabolic solutes such as sucrose, micronutrients, and signalling molecules, including Ca(2+), might contribute to optimizing the physiology of the plant is discussed.