Pc. Andersen et al., DIURNAL-VARIATIONS OF AMINO-ACIDS AND ORGANIC-ACIDS IN XYLEM FLUID FROM LAGERSTROEMIA-INDICA - AN ENDOGENOUS CIRCADIAN-RHYTHM, Physiologia Plantarum, 89(4), 1993, pp. 783-790
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.