strategy of carbon utilization in uniculm barley. II. The effect of continuous light and continuous dark treatments

After a photoperiod of 8.25 h during which the youngest fully expanded leaf of uniculm barley plants was allowed to assimilate 14CO2 for 30 min, groups of plants were transferred either to continuous light or to continuous dark. Plants were harvested over a 72 h period to examine the effect of the treatments (compared with control plants growing in normal light/dark cycles) on the transport of 14C from the exposed leaf, the distribution of 14C assimilates to the rest of the plant, and the chemical fate of assimilated 14C. In continuous light a substantial quantity (22% at 72 h) of the 14C assimilated by the leaf remained in that leaf in the form of starch and neutral sugars compared with only 4% in the control fed leaf. Also the total amount of 14C respired from plants maintained in continuous light was significantly less (c. 18% of the total originally fixed by 24 h) than that respired from control plants (c. 36%). The result was that approximately equal amounts of 14C were accumulated in the growing leaves and roots of plants given continuous light or normal light/dark cycles. In continuous dark the fate of 14C was similar to that of control plants. This is probably because the two treatments shared a common light/dark environment for the first 22 h, during which time almost complete distribution and utilization of 14C occurred.