DIURNAL AND ULTRADIAN RHYTHMS IN K-REPENS UNDER NATURAL LIGHT PATTERNS - EVIDENCE FOR SEGMENTATION AT DIFFERENT ROOT TEMPERATURES( UPTAKE BY TRIFOLIUM)

The patterns and variability in the diurnal and ultradian fluctuations in net uptake of K+ by 85-day-old plants of white clover (Trifolium r epens L. cv. Huia) were analysed during 21 days of vegetative growth u nder natural light conditions, at constant root temperatures of 13 and 25 degrees C. Plants were established in flowing solution culture, an d hourly rates of K+ uptake from solutions containing 2.5-6.5 mu M Kwere automatically measured without physical disturbance. Total daily solar radiation ranged between 1.3 and 12.5 MJ m(-2) day(-1) over the 21 days of measurements. Substantial diurnal changes in rates of K+ up take, both absolutely and proportionately, were recorded on all but th e day of lowest radiation (day 19), during which there was a period of net efflux of K+ from the root system into the flowing nutrient solut ions at both temperatures. Typically, uptake rates increased severalfo ld during the first half of the light period, followed by several hour s of relative stability during the afternoon; then a rapid decline thr oughout the evening and first half of the night, and finally gradual d ecline during the second half of the night. The data from a sequence o f typical days were normalised and indicated that the pattern could be resolved into 4 distinct and linear phases, successively: (1) rapid a cceleration, (2) stable uptake, (3) rapid deceleration, and (4) slow d eceleration. The full data set (days 1-21) of hourly K+ uptake rates w as Fourier-transformed and subjected to time-series analysis to determ ine whether there were significant ultradian rhythms in K+ uptake. Per iodograms indicated a strong 24 h periodicity for uptake at both root temperatures. In addition, there were strong ultradian periodicities o f 7 h for uptake at 13 degrees C and 4 h for uptake at 25 degrees C. T he results are interpreted in terms of the possible mechanisms respons ible for diurnal fluctuations in nutrient uptake.