The Carbohydrate Requirements for Symbiotic Nitrogen Fixation: A "Whole-Plant" Growth Analysis Approach

Abstract

Anexamination was made of the carbohydrate requirements for the establishment and function of the legume-root nodule symbiosis in Trifolium subterraneum L. The method adopted involved the use of relative growth rates in order to minimize the effect of time and initial differences in plant size. Essential factors in this approach were that the nodulated plants and ammonium nitrate-supplied control plants assimilated nitrogen at the same relative rate, and that both sets of plants has similar initial photosynthetic capacity. During the period of active nitrogen fixation, the carbohydrate requirements (C.R.) for fixation and nodule maintenance ranged from 3.2 mg carbohydrate per 1 mg nitrogen fixed (i.e. C.R. = 3.2 : 1) down to [long dash] 3.0 : 1, such values being relative to the carbohydrate requirement for the assimilation of combined nitrogen by control plants. Statistically, it was not possible to show that the values differed from a zero requirement. Using 3 host varieties and 2 strains of nodule bacteria, the overall mean was 0.7 : 1. Previously used methods for determining the carbohydrate requirement for symbiotic nitrogen fixation were discussed. Differences in the carbohydrate requirements were found for different bacterial strains nodulating the same host variety; the relative positions of these strains changed with different host varieties. While thd plants were first forming nodules, and during the early stages of nitrogen fixation, the carbohydrate consumption by the nodulated plants was greater than that observed in the control plants. This affected the shoots more than the roots, and contributed to absolute differences in dry weight between control and nodulated plants in the later stages of growth. The carbohydrate requirement for the assimilation of combined nitrogen was also considered.