| Abstract: | Ryle, G. J. A., Powell, C. E. and Gordon, A. J. 1988. Responsesof N2 fixation-linked respiration to host-plant energy statusin white clover acclimated to a controlled environment.—J.exp. Bot. 39: 879–887. Single plants of white clover, acclimated to a controlled environmentand dependent for nitrogen on N2 fixation in their root nodules,were darkened, defoliated or exposed to enhanced CO2 levelsto establish the quantitative relationships between the photosynthesisof the host plant and the N2 fixation metabolism of root nodules. The nodule respiration associated with N2 fixation (FLR) declinedrapidly to 10–15% of its normal rate following plant darkeningearly in the photoperiod. Darkening at progressively later intervalsduring the photoperiod demonstrated a positive, apparently linearrelationship between duration of illumination and total FLRduring the photoperiod and the following night period. Completeor partial defoliation reduced FLR according to the leaf arearemoved: again, there was a strong positive correlation betweencurrent rate of photosynthesis, whether of defoliated or undefoliatedplants and the FLR of root nodules. Doubling the current rateof photosynthesis, by enhancing CO2 levels around the shoots,promoted FLR within 1–2 h when plants were stressed bylack of light. However, enhanced CO2 levels increased FLR onlyslowly over a period of several hours in plants entrained tothe normal growing conditions. It is concluded that, in these plants acclimated to a uniformand favourable controlled environment, the supply and utilizationof photosynthetic assimilate in N2 fixation was finely balancedand quantitatively linked during a single diurnal period andthat nodule functioning was not depressed by lack of energysubstrate. Key words: White clover, N2 fixation, photosynthesis. |
