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SUMMARYFlushing of nitrogen gas through culture solutions, for periods between 4 and 15 h, increased activities of alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC) in roots of intact maize plants (activities expressed as //mol mg protein"^ min~^). The activity of ADH was six to nine times greater than the activity of PDC.Under root anaerobiosis of intact maize plants, activities of both ADH and PDC were similar in all root segments taken between o and 37 mm behind the apex, but lower activities were found in the older root segments. These differences in enzyme levels between younger and older root tissues were affected neither by the light regime of the shoots nor by imposing anaerobiosis of the shoots as well as the roots; i.e. the differences in ADH and PDC activities found in root segments of different ages were not due to a superior oxygen supply from the shoots to the tissues near the shoot-root junction.Roots of intact maize plants were also exposed to a range of oxygen concentrations, by flushing the nutrient solution with gases of different composition. ADH activities in the roots were highest with gas mixtures containing 8-13% oxygen, intermediate with pure nitrogen, and lowest when the solution was flushed with gas containing 20% oxygen.It is concluded that ADH activity cannot be used to diagnose the degree of oxygen deficiency to which maize roots have been exposed.

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SUMMARYFlooding usually reduced shoot and root growth of barley. In addition, root growth was inhibited much more in flooding-sensitive than flooding-tolerant cultivars. Prolonged flooding resulted in larger increases in activity of alcohol dehydrogenase (ADH) in roots of floodingtolerant than flooding-sensitive cultivars.Barley and rice were also grown in solutions flushed with gases of difl^erent oxygen concentration. In rice roots, high activities of ADH and pyruvate decarboxylase (PDC) were found only when the solutions were flushed with pure Nj gas. In barley roots, activities of ADH and PDC were highest using gases between 3 and 13% O2, lower with pure N2, and very low when using 20% O2. These results indicate that the level of ADH occurring in plant roots will be very dependent on experimental conditions, as for example root density. Species difl^erences in activity of ADH might be mainly related to different oxygen tensions inside the roots, due in turn to difl^erences in volume of aerenchyma and morphology of roots.

A comparative study was made of the effects of high concentrations of NaCl, KCl and MgCl2 on two electron transport reactions of thylakoids isolated from a mesophyte, Pisum sativum and a halophyte, Aster tripolium. The rate of photosystem I mediated electron transport from reduced N, N, N′, N′‐tetramethyl‐p‐phenylenediamine (TMPD) to methyl viologen was determined polarographically, and photosystem II mediated electron flow from water to 2,6‐dichlorophenolindophenol (DCPIP) was monitored spectrophotometrically. The response of photosystem II to increasing in vitro salt concentrations was similar for thylakoids isolated from both A. tripolium and P. sativum, but differences in the response of photosystem I to salinity changes were observed for the two species. Increasing NaCl, KCl and MgCl2 concentrations produced similar patterns of response of photosystem I activity in P. sativum thylakoids, whilst for A. tripolium KCl induced a completely different response pattern compared to NaCl and MgCl2. The salinity of the culture medium in which A. tripolium was grown also had an effect on both the absolute in vitro activities of photosystems I and II and their response to changes in salt concentration of the reaction media.