Extracts of healthy and rusted leaves of Little Club and Ichapli wheats were made under anaerobic conditions. Oxidation-reduction potentials in the extracts were rneasured a t two different levels both in the presence and in the absence of both methylene blue and riboflavin as redox mediators. At the lower level (i.e. in the absence of mediators) a reversible rise in potential was noted during the course of rust infection on Little Club; with Ichapli the rise was similar but was not reversed. The response to rust infection of resistant and susceptible plants was also reflected in characteristic changes in the average potentials a t higher levels ( i t . in the presence of mediators). After infection the oxidationreduction balance of susceptible tiss~ie was reorganized by the introduction of new systems which mere found in the red~lced state only. After infection the oxidation-reduction potential of resistant tissue was only poorly poised as indicated by a rise in the average potentials and as demonstrated by oxidationreduction titrations.The r e s~~l t s are discussed and the possibility is considered that a rust-induced catalyst may "short circuit" the chain of hydrogen transport, thus initiating all the observed redox changes. Results(a) Alterations in Potentials at Lower Levels (Eh ca. -100 mv) During the Progress of Infection As described earlier (lo), polarized platinum electrodes were used in malting E h measurements a t relatively low E h values, even though the poteiltials so measured were not stable for more than about 10 minutes. The E h values 'Manuscript

A simple rllethod of subjecting wheat seecllings to progressive water stress and of measuring the respiration rates is described. Slight water tleficits increased CO? output by about 20y0, and more severe water stress decreased it by about SOYo. Water stress caused signilicnrit changes in the coilcentratioils of malate and citrate and of an unknown organic acid. Water stress stril;iilgly decreased the level of malate in the roots and increased it in the shoots. It is suggested that the clecrcase in respiratioil caused by water stress cannot be attributed to an increased rate of dark iixatiou of carbon dioxide.

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