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Index >> Actinorhizal Plants (Frankia Induced Nodulation) >>Leaf Nodules

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Actinorhizal Plants Frankia Induced Nodulation Root Nodules Caused by Frankia Actinorhizal Symbiosis Cross Inoculation Experiments Isolation and Characteristics of Frankia Cultural Characteristics of Frankia Taxonomy of Frankia Entry of Frankia to the Host Plant Structure of Actinorhizal Nodule

Leaf Nodules

	Leaf Nodules The occurrence of leaf nodules is confined to the families of Rubiaceae and Myrsinaceae. Among the several genera of the family Rubiaceae reported to form leaf nodules (Pavetta, Chomelia, Psychotria), the genus Psychotria has received considerable attention from many workers. Whether or not the bacterial endophyte in the leaf nodule of this genus fixes nitrogen in association with the plant is a debatable point. Earlier reports claiming nitrogen fixation have later been disproved by experiments with nodulated leaves from plants grown in media free of combined nitrogen. Detached Psychotria leaves bearing leaf nodules were tested for nitrogenase activity by the acetylene reduction technique and also by exposing to an atmosphere containing ¹⁵N labelled nitrogen gas to determine the extent of nitrogen fixation.
The results showed no evidence of nitrogen-fixation. Experiments with nodulated leaves of several species of Pavetta, either by the use of acetylene reduction technique or by the use of ¹⁵N have also not shown any clear-cut indications of nitrogen-fixing ability. The identity of the endophyte of Psychotria nodules remains still in doubt in spite of repeated investigations. Isolates of bacteia from leaf nodules have been variously named Mycobacterium rubiacearum, Mycoplana rubra, Flavobacterium sp., Bacterium rubiacearum, Phyllobacterium rubiacearum and Klebsiella rubiacearum.
If fixation of nitrogen is not attributable to leaf nodules of Psychotria, the function of symbiosis may lie in the ability of the endophyte to produce phytohormones, a suggestion which has been put forth by some investigators. Some improvements in the growth of endophyte-free plants have been observed following the application of indole acetic acid and gibberellic acid. Evidence exists which suggests that cytokinins may be produced in leaf nodules. Retention of chlorophyll by leaves is a characteristic feature commonly used in the bio-assay for cytokinins. In keeping with this attribute for cytokinin activity, it is noteworthy that naturally occurring senescent and yellow leaves of Psychotria retain chlorophyll around leaf nodules indicating the presence of cytokinins.
Moreover, under experimental conditions when leaf-nodule discs from Psychotria were placed on oat leaf segments, the discs caused the retention of chlorophyll in oat leaves below them even after 6-8 days of incubation indicating cytokinin activity. The symbiotic association between bacteria and species of Ardisia of the family Myrsinaceae in leaf nodules has been investigated since the beginning of this century by several investigators. Using definitive techniques (acetylene reduction and ¹⁵N methods), it has been clearly shown that Ardisia symbiosis has no capacity to fix molecular nitrogen.
	Alternate explanations for the significance of symbiosis based on growth factor (cytokinin-like substances) production have been put forward by recent workers in the field. Dioscorea macroura, a member of the family Dioscoreaceae possesses leaf glands at the acuminate apices of leaves which are inhabited by bacteria whose identity is not known. The nitrogen content of leaf tissue containing glands is higher than the remainder of the leaf which indicates the ability of the symbiotic association to fix molecular nitrogen although tests with ¹⁵N have not been carried out. Mycorrhizal associations in members of Ericaceae and Orchidaceae, more particularly in Calluna vulgaris and Neottia nidusavis have been implicated in nitrogen fixation but not substantiated by ¹⁵N tests.

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Nitrogen Fixation and Assimilation Field Experiments on the Benefit of Frankia Inoculation Mycorrhizae and Actinorhizal Plants Proteoid Roots Genetics of Frankia Rhizobium Induced Root Nodulation of Parasponia Leaf Nodules Gymnosperm Nodules

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