The Physiology of Flowering Plants - KHAM PHA MOI

NITROGEN ASSIMILATION, FIXATION AND CYCLING 127have been identified. The two partners of the symbiosis controlgenetic activity in each other, inducing the appropriate genes bychemical signals. For instance, plant mutants are found in whichnon-functional nodules are formed, the bacteria failing to synthesizenitrogenase: this indicates that a plant signal is needed for thesynthesis of the bacterial enzyme.While the legume–Rhizobium symbioses are the most intenselystudied, they are not the only N 2 -fixing symbioses known for floweringplants. The filamentous actinobacterium Frankia sp. formsN 2 -fixing nodules in the roots of numerous woody dicotyledons,e.g. alders (Alnus spp.) and the bog myrtle (Myrica gale).4.4.3 The nitrogen cycleThe cycling of N in the biosphere is shown in Fig. 4.8. For floweringplants it is the supply of available N in the soil that is critical.Fig: 4:8 The nitrogen cycle. Themain N source for flowering plantsis NO – 3 . The soil NO – 3 isdepleted by plant uptake, bydenitrification and by runoff. It isreplenished by regenerationfrom decaying organic matter viaammonium, by N fixation(by symbiotic and free-livingmicroorganisms) and by the actionof lightning on atmospheric N 2 .Processes omitted from thediagram are (1) addition of oxides ofN to the atmosphere by volcanicactivity and by industrial pollution,and (2) addition to the soil ofnitrogen fertilizer derived fromindustrial N fixation fromatmospheric N 2 .