Changes in the activities of three enzymes (nitrate reductase, L-phenylalanine ammonia-lyase, and a dehydronicotinamide adenine dinucleotide-oxidase complex) were measured during development of water stress in young maize (Zea mays) plants.L-Phenylalanine ammonia-lyase and nitrate reductase activities decreased markedly with water deficits of 10 to 20%. 'Abbreviation: PAL, L-phenylalanine ammonia-lyase. MATERIALS AND METHODSThe dominant tall form of the d1 dwarf maize (Zea mays) was grown in vermiculite in the greenhouse. Nutrient requirements were provided with a modified Hoagland's solution 1 (9). Moisture stress was initiated by withholding water for varying periods beginning approximately 10 days after germination. In early experiments, the plants remained in the greenhouse until sampled for assay, but in later experiments they were transferred to a growth chamber when water stress was initiated. The chamber environment was 27 C, 30 to 50% relative humidity, and had a light period of 12 hr/day of 1000 ft-c. The plants were sampled at the middle of the light period. In the rehydration experiments, the vermiculite base was saturated with water 24 hr prior to sampling.Water deficit was estimated by the method of Weatherly (21). Duplicate or triplicate 5-cm sections were taken from the midportion of the third leaf of two or three plants. The original weight was determined immediately after harvest, and the tissue was floated in deionized water for 4 hr. At that time, the saturated weight was taken and the tissue was dried at 95 C for 24 hr for dry weight determination. The water deficit was expressed as the ratio of water taken up divided by the final water content, multiplied by 100.The midportion of the leaf blades from four to six seedlings was used as the source of tissue for nitrate reductase and NADH oxidase. PAL was prepared from the leaf sheaths of the same or comparable seedlings. Nitrate reductase activity was determined by the method of Hageman and Flesher (6), except that the tissue was ground in a chilled mortar with pestle and sand. PAL was prepared and assayed as described by Reid and Marsh (15). NADH oxidase activity was determined in the same extract used for nitrate reductase. The reaction mixture for the oxidase assay contained 200 ymoles of tris buffer, pH 8.2; 20 ,tmoles of (NH4)2SO4; 0.66 umole of NADH; plus enzyme in a final volume of 3 ml. The reaction was run at 30 C and was initiated by the addition of NADH. Changes in absorbancy at 340 nm were followed spectrophotometrically for 5 min.One unit of activity for all three enzymes was defined as the amount of enzyme which catalyzed the formation of 1 ,tmole of product in 1 min under standard conditions. Protein was determined by standard biuret or Lowry methods (12) with bovine serum albumin as a standard. RESULTSThe results of water deficit measurements (Fig. 1)