Tracing the Path of Oxygen into Birdsfoot Trefoil and Alfalfa Nodules using Iodine Vapor

Abstract

Although physiological control of nodule 02 permeability is an active area of research, the gas diffusion pathway between the atmosphere and the infected zone has not been firmly established. Previous studies have used infiltration of ink or dyes to identify points of entry, but such water‐soluble tracers could give a misleading picture of gas diffusion pathways. We therefore used iodine vapor (and its reaction with starch) to trace gas‐phase pathways into the infected zone of determinate birdsfoot trefoil (Lotus corniculatus) and indeterminate alfalfa (Medicago sativa) nodules. We also used histochemical methods to identify suberized or lignified layers that could act as barriers to gas diffusion. Birdsfoot trefoil nodules were surrounded by a suberized periderm, but nonsuberized cells and intercellular spaces were observed in the periderm between lenticels and their associated vascular bundles. Iodine entered birdsfoot trefoil nodules only through lenticels. The periderm appears to provide a significant barrier to gas diffusion. Although airspaces were rare in the nodule parenchyma (also referred to as the “inner cortex”), we found some evidence that a few air‐filled pathways cross this secondary barrier, also in the vicinity of vascular bundles. Alfalfa nodules were cylindrically surrounded by a suberized endodermis which ended near the meristematic tip; iodine entered principally at the end of the endodermis near the meristem. Future research on physiological control of nodule O2 permeability should concentrate on strategic “choke points”, associated with lenticels in determinate nodules, or in the zone proximal to the meristem in indeterminate nodules.

Chemical composition and ultrastructure of broad bean ( Vicia faba L.) nodule endodermis in comparison to the root endodermis

“…Microelectrode measurements indicated that a gas diffusion barrier is located in the nodule cortex (Witty et al 1987). This has been confirmed by tracing the diffusion of iodine vapor into alfalfa nodules (Jacobsen et al 1998). Symbiotic N 2 fixation can be regulated according to the plant's nitrogen demands.…”

“…Infiltration of indeterminate alfalfa nodules with iodine vapor demonstrated that only the suberin-and lignin-free submeristematic regions of the nodule cortex could be permeated by the gas (Jacobsen et al 1998). If O 2 behaves in the same way as I 2 , the nodule endodermis may be (part of) a gas diffusion barrier located in the nodule cortex.…”

Development of N2‐fixing nodules on the wetland legume Lotus uliginosus exposed to conditions of flooding

“…K. James, unpublished). Flavolan (condensed tannin)-containing cells (Pankhurst et al, 1979 ;Jacobsen et al, 1998) were also seen in the developing cortical tissue (Fig. 4c).…”

“…Moreover, in common with previous studies of legumes that are tolerant to flooding (Arber, 1920 ;Walker et al, 1983 ;James et al, 1992a,b ;Loureiro et al, 1994Loureiro et al, , 1995, as well as of terrestrial legumes that were subjected to flooding and\or artificially lowered pO # (Pankhurst & Sprent, 1975 ;Parsons & Day, 1990 ;Dakora & Atkins, 1990 ;Shiferaw et al, 1992 ;Arrese-Igor et al, 1993 ;Pugh et al, 1995), stems and roots of L. uliginosus became highly aerenchymatous after flooding. In addition, lenticels, which in Lotus nodules have recently been identified as strategic choke points for gaseous diffusion (Jacobsen et al, 1998), were very prominent in the flooded L. uliginosus nodules (Fig. 5a, and James & Crawford, 1998).…”

Adenylate-Coupled Ion Movement. A Mechanism for the Control of Nodule Permeability to O2 Diffusion

“…Since the spaces between the cells of the inner cortex are smaller than the spaces in the central, infected zone of the nodules (Parsons and Day, 1990;Jacobsen et al, 1998), capillary action would draw the water (and the K 1 it contains) into these spaces. As the water replaces the gas within these spaces, the permeability of the nodule to O 2 diffusion would decline sharply since diffusivity of O 2 in water is about 350,000 times lower than that in air (Thumfort et al, 2000).…”