Physiological implications of ultradian oscillations in plant roots

Oscillatory processes are ubiquitous in the Plant Kingdom. Surprisingly, many plant physiologists ignored these as physiologically unimportant unwanted 'noise'. Based on the application of the non-invasive ion-selective flux measuring (the MIFE) technique, this paper provides experimental evidence that ultradian oscillations in roots are a widespread phenomenon and reviews some physiological implications of ultradian rhythms in root nutrient acquisition. It is shown that the rhythmical character of root nutrient uptake is a characteristic feature for all measured species (both monocots and dicots; C₃ and C₄ type of photosynthesis). These oscillations were present in all major functional root zones, including root meristem, elongation and mature zone, and root hair region. For the first time, ultradian ion flux oscillations have been reported from the developing root hairs and from vertically grown roots exhibiting circumnutations. Several types of ultradian oscillations were distinguished, including those associated with extension growth of root tissues, more slow oscillations associated with either root circumnutation or nutrient acquisition in the mature zone, and rhythmical fluctuation in nutrient acquisition, associated with root adaptive responses to environmental stresses. Some underlying ionic mechanisms are discussed. Overall, these results show a crucial role of the rhythmical membrane-transport processes in plant-soil environmental interaction.