TRACING THE PATH OF OXYGEN INTO BIRDSFOOT-TREFOIL AND ALFALFA NODULESUSING IODINE VAPOR

Although physiological control of nodule O-2 permeability is an active area of research, the gas diffusion pathway between the atmosphere an d 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 dif fusion pathways. We therefore used iodine Vapor (and its reaction with starch) to trace gas-phase pathways into the infected zone of determi nate birdsfoot trefoil (Lotus corniculatus) and indeterminate alfalfa (Medicago sativa) nodules. We also used histochemical methods to ident ify suberized or lignified layers that could act as barriers to gas di ffusion. Birdsfoot trefoil nodules were surrounded by a suberized peri derm, 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. Al though 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 vascul ar bundles. Alfalfa nodules were cylindrically surrounded by a suberiz ed endodermis which ended near the meristematic tip; iodine entered pr incipally at the end of the endodermis near the meristem. Future resea rch on physiological control of nodule O-2 permeability should concent rate on strategic ''choke points'', associated with lenticels in deter minate nodules, or in the zone proximal to the meristem in indetermina te nodules.