Root O 2 consumption, CO 2 production and tissue concentration profiles in chickpea, as influenced by environmental hypoxia.

Roots in flooded soils experience hypoxia, with the least O-2 in the vascular cylinder. Gradients in CO2 across roots had not previously been measured. The respiratory quotient (RQ; CO2 produced : O-2 consumed) is expected to increase as O-2 availability declines. A new CO2 microsensor and an O-2 microsensor were used to measure profiles across roots of chickpea seedlings in aerated or hypoxic conditions. Simultaneous, nondestructive flux measurements of O-2 consumption, CO2 production, and thus RQ, were taken for roots with declining O-2. Radial profiling revealed severe hypoxia and c. 0.8 kPa CO2 within the root vascular cylinder. The distance penetrated by O-2 into the roots was shorter at lower O-2. The gradient in CO2 was in the opposite direction to that of O-2, across the roots and diffusive boundary layer. RQ increased as external O-2 was lowered. For chickpea roots in solution at air equilibrium, O-2 was very low and CO2 was elevated within the vascular cylinder; the extent of the severely hypoxic core increased as external O-2 was reduced. The increased RQ in roots in response to declining external O-2 highlighted the shift from respiration to ethanolic fermentation as the severely hypoxic/anoxic core became a progressively greater proportion of the root tissues.