EFFECTS OF PHOTOPERIOD ON GROWTH OF AND DENITRIFICATION BY PSEUDOMONAS-CHLORORAPHIS IN THE ROOT-ZONE OF GLYCERIA-MAXIMA, STUDIED IN A GNOTOBIOTIC MICROCOSM

The emergent macrophyte Glyceria maxima was subjected to different pho toperiods and grown with ammonium or nitrate as nitrogen source in pre sterilized microcosms with spatially separated root and non-root compa rtments. The microcosms were inoculated with the denitrifying bacteriu m Pseudomonas chlororaphis. The effect of the plant and the photoperio d on growth and denitrification by P. chlororaphis was assessed. The p lant had a strong positive effect on the growth of the bacteria. The b acterial numbers in the root compartment of the planted microcosms wer e 19-32 times higher than found in the non-root sediment of the unplan ted systems. Lengthening the photoperiod resulted in elevated bacteria l numbers due to the higher carbon exudation of the plant. This effect was greater still with the nitrate-fed plants, where additional P. ch lororaphis growth could proceed via denitrification, indicating oxygen -limiting conditions in the microcosms. Higher porewater N2O concentra tions in the root compartments as compared to the non-root compartment s, which were highest for the long photoperiod, were also indicative o f a plant-induced stimulation of denitrification. An effect of a diurn al oxygen release pattern of G. maxima on denitrification could not be detected. The gnotobiotic microcosm used in this study represents st potential system for the study of the behaviour and interactions of im portant bacterial groups, such as nitrifying and denitrifying bacteria where plant roots drive bacterial activity.