H⁺ flux kinetics around plant roots after short-term exposure to low temperature: Identifying critical temperatures for plant chilling tolerance

H⁺ flux kinetics were measured in solution around the roots of chilling-tolerant pea (Pisum sativum) and bean (Vicia faba), chilling-sensitive cucumber (Cucumis sativus) and pumpkin (Cucurbita pepo), and intermediate corn (Zea mays) species using a microelectrode technique to measure net flux. As a root warmed to room temperature after 90 min at 4 °C, at which temperature the H⁺ flux was near zero, the flux rose (influx) and then fell. These changes occurred at two apparent critical temperatures, which were higher for the more chilling-sensitive species. The first, lower, apparent critical temperature may represent the start of passive inward H⁺ transport. The higher critical temperature may represent the start of active H⁺ extrusion. From these apparent critical temperatures we have calculated the real critical temperature and the time delay of the chilling signal transduction process. Passive and active H⁺ transporters appear to have the same real critical temperature of chilling sensitivity, about 9 °C, but have, respectively, 4 min and 11 min time delays. Measurement of these apparent critical temperatures may provide quick and reliable screening for chilling sensitivity in plant breeding programmes. Future ion flux studies may show the cellular location of chilling stress perception and the signal transduction pathways.