Search

Acclimation by plants to hypoxia and anoxia is of importance in various ecological systems, and especially for roots in waterlogged soil. We present evidence for acclimation by roots via 'anoxic' cores rather than being triggered by O 2 sensors. The evidence for 'anoxic' cores comes from radial O 2 profiles across maize roots and associated metabolic changes such as increases in the 'anaerobic enzymes' ADH and PDC in the 'anoxic' core, and inhibition of Cl− transport to the xylem. These cores are predicted to develop within 15–20 min after sudden transfer of a root to hypoxia, so that the cores are 'anoxically-shocked'. We suggest that 'anoxic' cores could emanate a signal(s), such as ACC the precursor of ethylene and/or propagation of a 'Ca2+ wave', to other tissue zones. There, the signalling would result in acclimation of the tissues to energy crisis metabolism. An O 2 diffusion model for tissues with an 'anoxic' core, indicates that the phytoglobin-nitric oxide (Pgb-NO) cycle would only be engaged in a thin 'shell' (annulus) of tissue surrounding the 'anoxic' core, and so would only contribute small amounts of ATP on a whole organ basis (e.g. whole roots). A key feature within this annulus of tissue, where O 2 is likely to be limiting, is that the ratio (ATP formed) / (O 2 consumed) is 5–6, both when the NAD(P)H of glycolysis is converted to NAD(P)+ by the Pgb-NO cycle or by the TCA cycle linked to the electron transport chain. The main function of the Pgb-NO cycle may be the modulating of NO levels and O 2 scavenging, thus preventing oxidative damage. We speculate that an 'anoxic' core in hypoxic plant organs may have a particularly high tolerance to anoxia because cells might receive a prolonged supply of carbohydrates and/or ATP from the regions still receiving sufficient O 2 for oxidative phosphorylation. Severely hypoxic or 'anoxic' cores are well documented, but much research on responses of roots to hypoxia is still based on bulk tissue analyses. More research is needed on the interaction between 'anoxic' cores and tissues still receiving sufficient O 2 for oxidative phosphorylation, both during a hypoxic exposure and during subsequent anoxia of the tissue/organ as a whole. [ABSTRACT FROM AUTHOR]

• Some recent data on O2 scavenging by root segments showed a two-phase reduction in respiration rate starting at ⁄ above 21 kPa O2 in the respirometer medium. The initial decline was attributed to a down-regulation of respiration, involving enzymes other than cytochrome oxidase, and interpreted as a means of conserving O2. As this appeared to contradict earlier findings, we sought to clarify the position by mathematical modelling of the respirometer system. • The Fortran-based model accommodated the multicylindrical diffusive and respiratory characteristics of roots and the kinetics of the scavenging process. Output included moving images and data files of respiratory activity and [O2] from root centre to respirometer medium. • With respiration at any locus following a mitochondrial cytochrome oxidase O2 dependence curve (the Michaelis-Menten constant Km = 0.0108 kPa; critical O2 pressure, 1–2 kPa), the declining rate of O2 consumption proved to be biphasic: an initial, long semi-linear part, reflecting the spread of severe hypoxia within the stele, followed by a short curvilinear fall, reflecting its extension through the pericycle and cortex. • We conclude that the initial respiratory decline in root respiration recently noted in respirometry studies is attributable to the spread of severe hypoxia from the rootcentre, rather than a conservation of O2 by controlled down-regulation of respiration based on O2 sensors. [ABSTRACT FROM AUTHOR]

Background and Aims: Respiratory critical oxygen pressures (COPR) determined from O2-depletion rates in media bathing intact or excised roots are unreliable indicators of respiratory O2-dependency in O2-free media and wetlands. A mathematical model was used to help illustrate this, and more relevant polarographic methods for determining COPR in roots of intact plants are discussed. [ABSTRACT FROM PUBLISHER]