A correlation has been firmly established, in a wide range of plants, between environmental stress, the onset of senescence, loss of viability in seeds and the development and accumulation of a stable organic free radical. On the basis of the EPR response obtained at 95 GHz (W-band) and ENDOR spectra, and comparisons with quinone radical anions, we present evidence from contrasted plant species, plant tissues and sub-cellular fractions that this stable radical originates from one or more quinones possibly, though perhaps not exclusively, associated with stressed or age-impaired photosynthetic and respiratory electron transport chains. The radical appears to be ubiquitously associated with sub-lethal stress-induced damage and with senescence and arises during the sub-cellular structural and biochemical processes associated with the final phases of metabolism prior to death. As the free radical persists for some considerable time after death, it may have value in long-term studies of seed viability and in broader areas of plant pathology and stress physiology.

The electrolytic reduction of oxygen which occurs at a wetted ‘unattackable’ cathodic electrode of platinum or gold when polarised in conjunction with a Ag/AgCl anode, forms the basis of most polarographic oxygen measurements in plant biological work.

Various types of polarographic electrode and their uses are reviewed. These include cylindrical sleeving electrodes for quantifying localised oxygen fluxes from intact roots, ‘bare’, membrane-coated, and Clark-type microelectrodes suitable for measuring concentrations and profiles both inside roots and in the rhizosphere, and macro-Clark electrodes most frequently used in respiratory studies.

Some details concerning equipment and electrode construction are given and some pitfalls in electrode application are discussed.

Nuclepore membranes of known pore size and porosity, attached to small glass chambers were used to investigate the mechanism of humidity-induced convection found in Phragmites and other species. The convective flow of gases and the development of static pressures were studied in relation to a variety of conditions imposed on opposite sides of the membrane. Data were obtained in relation to pore size (0.015 to 3 μm diameter) and porosity, temperature and humidity gradients, membrane area and resistance to venting.

A humidity gradient across the Nuclepore membranes was far more effective than a temperature gradient per se for inducing flow; also pores of the order 0.2 μm diameter were the most effective. With a steep enough humidity gradient, however, significant flows could also be induced across pores as large as 2 μm diameter. The results accord with previous experiments and observations on Phragmites and with the mathematical treatment of the mechanism. It is concluded that humidity-induced convection should play a greater role than thermal transpiration (thermo-osmosis) in effecting convections in Phragmites and other non-floating-leaved species.

A twig chamber system was developed for the exposure of mature trees to ozone (O3) under field conditions. The fumigation system allowed the exact control of O3 concentrations in the chambers, the measurement of O3 uptake as well as gas exchange measurements under ambient and controlled conditions during and after O3 fumigation. Because of differences in individual twigs the system should provide the exposure of replicates to different O3 treatments. Tests showed that temperature, humidity and O3 concentrations inside the chambers were comparable with diurnal courses observed in the field. Comparative gas exchange measurements indicated that there were no differences in net photosynthesis and conductance of twigs outside the chambers and twigs which remained within the chambers for 23 weeks receiving ambient air.