Oxygen release and consumption have traditionally been assessed by measuring changes in bulk water
DBL - diffusive boundary layer KPa - kilo Pascals
Rubisco - Ribulose-1,5-bisphosphate carboxylase-oxygenase.
oxygen concentrations in incubation chambers. This technique is feasible for measurements ofphotosyn-thesis and respiration of isolated leaves, although potential problems with lacunar oxygen accumulation, and especially with poor simulation of natural boundary layer conditions around leaves, may interfere significantly with rate measurements. Measurements of respiration in isolated roots and rhizomes using chamber techniques can be more problematic. It has been argued that measuring respiration of isolated below-ground tissues under aerobic conditions may overestimate the respiration that would occur in anoxic sediments (e.g. Smith et al., 1988; Touchette and Burkholder, 2000). However, such a procedure may also underestimate respiration, because the la-cunar oxygen supply from leaves to roots and rhizomes is disrupted when the tissues are separated from the leaves. Hence, respiration has to be fueled by oxygen diffusing from the bulk water through boundary layers and through the more or less permeable root and rhizome tissues, and this diffusion may be too slow to sustain an adequate internal oxygen supply and mimic natural conditions of intact plant gas phase transport (Saglio et al., 1984).
Chamber techniques provide reliable estimates of whole plant metabolism if intact plants with leaves, roots and rhizomes are incubated for longer time intervals allowing equilibration of oxygen between lacunae and bulk water (Kemp et al., 1986). In addition, split chambers with leaf compartments separated from root compartments by water- and gas-tight seals have been used to estimate oxygen transport from leaves to roots and subsequent oxygen release to the sediments (e.g. Sand-Jensen et al., 1982; Kemp and Murray, 1986). Results based on this technique have, however, to be interpreted with caution. Transport from leaves to roots is driven by gradients between sources and sinks, and the steepness of these gradients depend greatly on the experimental conditions (Sorrell and Armstrong, 1994). The oxygen gradient from the root to the sediment is especially important, because it determines the rate of oxygen loss to the sediment and because it can vary by an order of magnitude depending on the oxygen consumption within the root medium (Sorrell and Armstrong, 1994). To mimic natural sediments as proper sinks the rooting media must not only be anoxic but also reducing and oxygen consuming to generate the sufficiently steep gradients between root and sediment forcing the release of oxygen. Such conditions can be established by adding titanium citrate buffer to the root medium thereby increasing measured rates of oxygen release from the roots (Sorrell and Armstrong, 1994). However, if simulating natural conditions properly the split chamber techniques provide the most reliable estimates of whole plant oxygen transport.
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