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

Summary Roots in flooded soils experience hypoxia, with the least O 2 in the vascular cylinder. Gradients in CO 2 across roots had not previously been measured. The respiratory quotient (RQ; CO 2 produced : O 2 consumed) is expected to increase as O 2 availability declines. A new CO 2 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, CO 2 production, and thus RQ, were taken for roots with declining O 2 . Radial profiling revealed severe hypoxia and c.  0.8 kPa CO 2 within the root vascular cylinder. The distance penetrated by O 2 into the roots was shorter at lower O 2 . The gradient in CO 2 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 CO 2 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.