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Inoculation of winter barley plants in a glasshouse with Pyrenophora teres at three different growth stages (GS 11, 13 or 30) greatly decreased root and shoot dry matter production and the size of healthy leaves produced subsequently, but there was no significant yield loss with a single inoculation at GS 11 or 30. Inoculation at GS 13, GS 31 or GS 39 decreased grain yield by 12,17 and 20% respectively, and also straw yield. Larger yield losses resulted from repeated inoculations on five successive occasions (GS 11, 13, 30, 31 and 39) which caused much disease on all leaves throughout the life of the plant. All components of yield measured were decreased by the cumulative effect of successive inoculations; ear number by 15%), grain sites per ear by 20%, grain yield by 48% and straw yield by 32%.

Growth and nitrate reductase activity were measured in Paul's Scarlet rose cell suspensions, cultured in media purified from molybdenum and containing nitrate or urea as sole nitrogen source with or without added Mo. Urea could replace nitrate to yield 80% of the fresh weight in nitrate medium. Nitrate reductase activities were compared by in vivo and in vitro assays. The latter varied due to inactivation during extraction. Compared with activities in cells in complete NO3 (-) medium, activity in NO3 (-)-Mo cells was reduced to 30% and, in urea-grown cells, to trace amounts. Increases in nitrate reductase activity were found when NO3 (-) alone was added to NO3 (-) or urea+Mo cultures. In NO3 (-)-Mo cultures, Mo alone or with NO3 (-) caused a similar increase in activity, whereas urea-Mo cultures required both NO3 (-) and Mo for enzyme induction.