Aerenchyma formation and radial O2 loss along adventitious roots of wheat with only the apical root portion exposed to O2 deficiency

This study investigated aerenchyma formation and function in adventitious roots of wheat (Triticum aestivum L.) when only a part of the root system was exposed to O₂ deficiency. Two experimental systems were used: (1) plants in soil waterlogged at 200 mm below the surface; or (2) a nutrient solution system with only the apical region of a single root exposed to deoxygenated stagnant agar solution with the remainder of the root system in aerated nutrient solution. Porosity increased two‐ to three‐fold along the entire length of the adventitious roots that grew into the water‐saturated zone 200 mm below the soil surface, and also increased in roots that grew in the aerobic soil above the water‐saturated zone. Likewise, adventitious roots with only the tips growing into deoxygenated stagnant agar solution developed aerenchyma along the entire main axis. Measurements of radial O₂ loss (ROL), taken using root‐sleeving O₂ electrodes, showed this aerenchyma was functional in conducting O_2. The ROL measured near tips of intact roots in deoxygenated stagnant agar solution, while the basal part of the root remained in aerated solution, was sustained when the atmosphere around the shoot was replaced by N₂. This illustrates the importance of O₂ diffusion into the basal regions of roots within an aerobic zone, and the subsequent longitudinal movement of O₂ within the aerenchyma, to supply O₂ to the tip growing in an O₂ deficient zone.     

Ammonium Tolerance and Carbohydrate Status in Maize Cultivars

Four maize (Zea mays L.) hybrids were grown hydroponically for4 weeks with 20 mM ammonium or nitrate as the sole nitrogensource. Dry matter production was strongly depressed by ammoniumnutrition in the hybrid Helga relative to plants grown on nitrate,and moderately decreased in the hybrid Melina. Ammonium hadno inhibitory effect on total yield in the other two hybrids(Ramses and DK 261). The relative growth rate (RGR) of rootsand shoots of the sensitive hybrid Helga decreased significantlyunder ammonium nutrition during the first 2 weeks of the experiment,while at the end of the experiment nitrogen form had no effecton the RGR in any of the four hybrids. The strong reductionin RGR of Helga in the early seedling stage was correlated withthe accumulation of twice the concentration of free ammoniumin the shoot tissue relative to the other hybrids. Helga wastherefore unable to sufficiently detoxify ammonia in the roots.Root concentrations of water soluble carbohydrates (WSC) inHelga and Melina in the early seedling stage did not differunder ammonium and nitrate nutrition. In contrast, Ramses andDK 261 both had elevated WSC concentrations in ammonium-fedroots. It is hypothesized that a sufficient supply of carbonskeletons for ammonium assimilation in the roots is requiredfor maximum growth under high ammonium concentrations, and thatthere is genotypic variability in this physiological trait. Ammonium; carbohydrates; growth rate; maize; nitrate; roots; Zea mays L     

Soil microbial responses to increased concentrations of atmospheric CO2

Terrestrial ecosystems respond to an increased concentration of atmospheric CO₂. While elevated atmospheric CO₂ has been shown to alter plant growth and productivity, it also affects ecosystem structure and function by changing below-ground processes. Knowledge of how soil microbiota respond to elevated atmospheric CO₂ is of paramount importance for understanding global carbon and nutrient cycling and for predicting changes at the ecosystem-level. An increase in the atmospheric CO₂ concentration not only alters the weight, length, and architecture of plant roots, but also affects the biotic and abiotic environment of the root system. Since the concentration of CO₂ in soil is already 10–50 times higher than that in the atmosphere, it is unlikely that increasing atmospheric CO₂ will directly influence the rhizosphere. Rather, it is more likely that elevated atmospheric CO₂ will affect the microbe–soil–plant root system indirectly by increasing root growth and rhizodeposition rates, and decreasing soil water deficit. Consequently, the increased amounts and altered composition of rhizosphere-released materials will have the potential to alter both population and community structure, and activity of soil- and rhizosphere-associated microorganisms. This occurrence could in turn affect plant health and productivity and plant community structure. This review covers current knowledge about the response of soil microbes to elevated concentrations of atmospheric CO₂.