[Changes in labelling patterns after feeding Bryophyllum tubiflorum with (14)CO 2 at different moments during the light/dark period : II. Relations between malate content of the tissue and the labelling patterns after (14)CO 2 light fixation].

The distribution of radioactivity between the products of (14)CO2 light fixation in phyllodia of Bryophyllum tubiflorum could be influenced experimentally by manipulating the malic acid content of the cells. Accelerating the deacidification of the tissue during the light period by application of higher light intensities accelerated the increase of malate labelling and the decrease of the sucrose labelling after (14)CO2 light fixation under our standard conditions (10 min preillumination, 15 min (14)CO2 light fixation, 8000 lux).In other experiments different malate contents of the tissues were induced by treating the phyllodia with different temperatures during the night period. In the morning, phyllodia with low malate content transferred most of the label into malate, and phyllodia with high malate content incorporated most of the (14)C radioactivity into sugars. However, this was true only after preillumination of 1 hour. When the phyllodia fixed (14)CO2 without preillumination, no differences in the labelling patterns between acidified and non-acidified phyllodia could be observed.In experiments using leaf tissue slices of Bryophyllum daigremontianum we could again observe that malate was labelled more heavily in the deacidified tissue than in the acidified controls, with less radioactivity being transferred into phosphate esters and sugars. The rates of (14)CO2 light fixation were identical in tissue slices with high and low malate content. However, the rates of CO2 dark fixation in the acidified samples were clearly lower than those in the deacidified ones. The low rate of CO2 dark fixation in acidified samples could not be inhibited by an inhibitor of PEP-carboxylase as the high CO2 dark fixation rate of the deacidified tissue could be inhibited.The results are discussed in relation to the feed back inhibition of PEP-carboxylase in vivo by malate. Compartmentation also seemed to be involved in controlling the flow of carbon during CO2 light fixation in succulent tissue.