The oxygen uptake and energy charge under hypoxia and anoxia of excised plant roots from flood-tolerant and flood-sensitive species

Abstract

A study is presented of two aspects of plant adaptation to soil waterlogging. The first part of this study is an investigation into the relation between the uptake of oxygen by plant root tips and the external concentration of oxygen. A range of plant species was collected and grown in the glasshouse in both flooded and unflooded sand cultures. Experiments were carried out on these plants after at least one month's growth in the glasshouse, and also on plants collected from the wild without any pre-treatment in the glasshouse. Oxygen uptake measurements were carried out using excised root apices in a Rank Memebrane Oxygen Electrode assembly. It was found in nearly all experiments that the rate of oxygen uptake was highly dependent on the external oxygen concentration in both flood-tolerant and flood-sensitive species. Samples which had been grown in a flooded sand culture, however, were less dependent on the supply of external oxygen than those which had been grown in an unflooded sand culture These results were interpreted with the aid of a simple mathematical model of oxygen uptake in a plant root apex. The model was designed to predict the relation between oxygen uptake rate and the external concentration of oxygen for roots with different porosities, different rates of uptake and possessing respiratory systems with different affinities for oxygen. Using this model, it was found that a normal plant root apex, in which the respiratory systems have a high affinity for oxygen and in which the presence of intercellular air spaces allows a rapid radial movement of oxygen, should exhibit a rate of oxygen uptake which is largely independent of the external concentration of oxygen. Prom this prediction and from theoretical calculations on the movement of oxygen within individual root cells, it was concluded that the high dependence of oxygen uptake on external oxygen concentration which was found in the experiments was largely an artefact resulting from flooding of the intercellular air spaces under the in vitro conditions of the experiments. Affinity for oxygen and the ability to maintain high rates of oxygen uptake under conditions of low oxygen availability are not therefore considered to be criteria by which flood-tolerant and flood-intolerant species can be distinguished. The second part of this study was an investigation into the energy relations of excised plant root systems, by the measurement of ATP/ADP ratios under anoxia. Root systems were harvested from plants which had previously been grown under a flooded or an unflooded sand culture regime in the glasshouse, and were incubated under nitrogen gas in deoxygenated phosphate buffer for up' to 4 hours. Control samples were incubated in air-saturated buffer under an atmosphere of air. It was found that ATP/ADP ratios were lower when the excised root systems were incubated under nitrogen than when they were incubated in an aerobic environment. However, the values obtained suggest a higher energy charge in the root tissues under anoxia than has generally been reported by other workers. No difference was found between flood-tolerant and flood-sensitive species, but it is pointed out that since the experiments did not measure the total size of the adenylate pool, such differences may have been overlooked. It is suggested that energy charge measurements may be more useful as a general indication of the metabolic activity within the root tissue rather than as a criterion for distinguishing flood tolerant and flood intolerant plant species.