The barrier to radial oxygen loss impedes the apoplastic entry of Fe into the roots of Urochloa humidicola

Lack of O2 and high concentrations of Fe and Mn commonly occur in waterlogged soils. The development of a barrier to impede radial O2 loss (ROL) is a key trait improving internal O2 transport and waterlogging tolerance in plants. We evaluated the ability of the barrier to ROL to impede the entry of excess Fe into the roots of the waterlogging tolerant grass Urochloa humidicola. Plants were grown in aerated or stagnant deoxygenated nutrient solution with 5 or 900 µM Fe. Quantitative X-ray-microanalysis was used to determine cell-specific Fe concentrations at two positions behind the root apex in relation to ROL and the formation of apoplastic barriers. At a mature zone of the root, Fe was ‘excluded’ at the exodermis where a suberized lamella was evident, a feature also associated with a strong barrier to ROL. By contrast, the K concentration was similar in all root cells, indicating that K uptake was not affected by apoplastic barriers. The hypothesis that the formation of a tight barrier to ROL impedes the apoplastic entry of toxic concentrations of Fe into the mature zones of roots was supported by the significantly higher accumulation of Fe on the outer-side of the exodermis.

Occurrence of the Suberized Lamella in Leaves of Grasses of Different Photosynthetic Type. II. In Herbarium Material

Dead air-dried leaves have been conventionally prepared for transmission electron microscopy to ascertain if the occurrence of a suberized lamella in cell walls can be detected. Our species sample includes representatives of all known photosynthetic types within the Poaceae (viz. C3, C4 NAD-malic enzyme type, C4 NADP-malic enzyme type and PEP carboxykinase type). Each photosynthetic type exhibits a characteristic pattern of suberized lamella occurrence in mestome sheath and/or 'photosynthetic carbon reduction' (PCR or 'Kranz') cell walls, consistent with that in fixed living material. Plasmodesmatal structure, and even on occasion chloroplast structure, is remarkably well preserved. Leaves from herbarium specimens, therefore, could be used to assign C4 species to their C4 acid decarboxylation type. This has potential application in large-scale systematic surveys for which living material may be difficult to obtain.

δ13 Values of C4 Types in Grasses

δ13 values were determined for leaves of C4 grasses (Poaceae) of different photosynthetic types. For plants grown in the same environment at the same time, mean δ13 values for the three C4 types were: NADP-ME-type, - 11.350 � 0.13 s.e. (11 spp.); PCK-type, - 11.950 � 0.19 s.e. (11 spp.); and NAD-ME-type, -12.70 � 0.21 s.e. (9 spp.). Although there is some overlap between the values for individual species of the three groups, the difference between any two means is highly significant [P(t) < 0.01] and is not due to taxonomic sampling bias at the subfamily level. The differences in means may suggest that C4 types differ in rates of leakage of CO2 and HCO3- from PCR tissue ('photosynthetic carbon reduction' tissue: equivalent to 'Kranz' tissue), and/or, using Farquhar's (Appendix) expression for plant δ values, that C4 types differ in their average intercellular CO2 concentrations (c1). It is also possible that differences between C4 types exist in some other, unknown, leaf fractionation process. Apoplastic CO2 leakage from PCR tissue in NAD-ME-type C4 grasses, which do not possess a PCR 'suberized lamella' as found in NADP-ME- type and PCK-type C4 grasses, may give these species the most negative δ13 values. Expressions for C4 plant δ13 values, and a model for the δ13 values of CO2 and HCO3- in various pools and fluxes in C4 plant leaves, are given.

Development of the Suberized Lamella in the Mestome Sheath of Wheat Leaves

The suberized lamella in the cell walls of the mestome sheath of wheat leaves developed asynchronously. The lamella formed first in the cells which were adjacent to the protophloem sieve tubes and formed last in the cells that abutted on the tracheary elements. In the latter case, the suberized lamella formed first in the outer tangential and radial walls and last in the inner tangential wall adjacent to the tracheary element. Eventually, the suberization was completed opposite the tracheary elements and the cell walls developed tertiary thickenings in all mestome sheath cells. Cytoplasmic structures that were clearly involved in suberin synthesis and the development of tertiary thickenings could not be identified.