DIURNAL-VARIATIONS OF AMINO-ACIDS AND ORGANIC-ACIDS IN XYLEM FLUID FROM LAGERSTROEMIA-INDICA - AN ENDOGENOUS CIRCADIAN-RHYTHM

Diurnal variations in the concentrations of major organic compounds oc curred in xylem fluid extracted from Lagerstroemia indica L. The conce ntration of amino acids and the NIC ratio was at a maximum and that of organic acids was at a minimum between 1230 and 2030 h. Since the con centrations of total organic nitrogen, total amino acids and most indi vidual amino acids (but not organic acids or sugars) were also proport ional to xylem tension two experiments were performed to discern wheth er variations in chemistry were a consequence of diurnal changes in mo isture stress. In the first experiment, L. indica, exposed to variable levels of moisture stress during midday, manifested an increase in or ganic acids and a reduction in the NIC ratio. In the second experiment , chemical profiles of xylem fluid were collected and compared for pla nts exposed to a natural photoperiod, constant darkness or continuous light at noon and midnight. After day amino acids increased in concent ration during midday for all treatments; the variation was greatest (1 0-fold) for plants in constant darkness where xylem tension varied fro m 0.20 to 0.25 MPa. Only plants exposed to continuous light lost a diu rnal rhythm after 3 days. Thus, the circadian rhythm was endogenous, t erminated in continuous light and was not mediated by changes in moist ure stress. Glutamine accounted for most of the diurnal variation in t otal amino acids, organic nitrogen and the NIC ratio in xylem fluid.