![Sample our Environment & Agriculture journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5934/TOC-Subject-Sample-2021-Environment-Agriculture.jpg"})
Summary
On the basis of Schanderl's findings (1939), according to wich leguminous plants usually harbour Rhizobium not only in the roots, but also in the stems and leaves, the Authors asked themselves why these organs behave in such different ways and notably for what reason the roots proliferate forming root nodules while the stems, which undergo the same stimulus, show no morfological reaction.
Schanderl's results have been confirmed, and more definetively substantiated inasmuch the Authors progressed experimentally beyond the mere observation of the presence of Rhizobium in the roots, stems and leaves by means of histological examination and of isolation of rhizobia from several organs making pure cultures thereof. The Authors have gone still further infecting the roots of cultivated bean plants under sterile conditions with cultures of bacteria obtained from stems, leaves and seeds of other bean plants, and induced the formation of root nodules in the former.
It was assumed by the Authors, as a working hypothesis, that the different consequences caused by the rhizobia in the roots and in the shoot system should be attributed to the ascorbic acid (AA) contents in the two different organs. In other words, the AA, wich is always present in considerable quantities in stems and leaves, would annul the activity of the growth-promoting substances produced in these organs by the existing bacteria, so that their presence would not cause any particular growth reactions. On the other hand in the roots, wich are particularly poor in AA, the growth-promoting substances of bacterial origin could exert freely their action causing those growth reactions wich lead to the formation of nodules.
In support of their explanation, and after a preliminary report of a series of researches showing that AA has a tendency to move within the plant in a polar acro-petous direction, what helps to explain the scarcity of AA in the roots, the Authors describe the results of their experiments.
The first group of tests had the aim of proving that roots infected by Rhizobium cause those growth reactions which end forming nodules, only because of the very poor content in AA. As a matter of fact bean plants grown in pots and experimentally enriched with AA by watering the roots daily with a 8/1000 solution (so that the AA attained a concentration which was comparable to the normal concentration found in the stems) did not form root nodules, which were on the contrary regularly produced by the controls. It was also found that there is a relationship between the AA content experimentally induced in the roots and the frequency and size of the nodules.
The second series of experiments was designed to prove that the lack of growth reactions by the epigeal apparatus, also infected with Rhizobium, was due to its high AA contents. Bean plants have been cultivated in the dark so as to prevent AA synthesis, and — for the same reason — carbonated substances were supplied in form of glycerine instead of sugar. Eight days after sowing, their hypocotyls were inoculated with Rhizobium and seven days later it was noted that, while the control plants infected in the identical manner, but kept in the light, showed no reaction, those plants kept in the dark showed the formation of a callous tissue in the hypocotyl area previously inoculated. No reaction, on the other hand, has been observed in plants infected in the same manner, kept in the dark and artificially supplied with AA.