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Effects of Mild Water Stress on Enzymes of Nitrate Assimilation and of the Carboxylative Phase of Photosynthesis in Barley¹

The effects of water stress were determined on enzymes of nitrate assimilation, of the carboxylative phase of photosynthesis, and of dark CO₂ fixation in the first leaves of Hordeum vulgare L. After water removal, decreased activity of nitrate reductase was detected at the same time that water potential decreased in the first leaves and growth of the second leaves ceased. Up to 58% of the nitrate reductase activity was lost during the 4-day stress period. Nitrite reductase activity showed a tendency to decrease. Phosphoenolpyruvate carboxylase activity decreased 1 day later in the stress period and by only half as much as did nitrate reductase activity. Twenty-four hours after rewatering, the activities of nitrate reductase and phosphoenolpyruvate carboxylase had recovered completely. This recovery occurred during the same time that water potential increased in the first leaves and elongation of the second leaf began.

Activities of phosphoribulokinase and ribulose-l,5-diP carboxylase were affected little by water stress. The reported rapid decrease in photosynthetic fixation of CO₂ at the onset of water stress may be due less to changes in these enzyme levels than to restrictions in gaseous exchange by reduction of stomatal aperture. Concentrations of chlorophyll, soluble protein, and nitrate were slightly lower in the first leaves of water-stressed plants.

The reduction in the activities of phosphoenolpyruvate carboxylase and, especially, nitrate reductase could be part of a biochemical adaptation to stress conditions, causing a decrease in synthetic capabilities so as to reduce overall energy requirements.

Key Words: Hordeum vulgare L. cv. ‘Blanco Mariout’ • Water stress • Nitrate and nitrite reductases • Phosphoenolpyruvate carboxylase • Photosynthetic carboxylative phase enzymes

² Associate Professor of Agronomy, Research Assistant, Laboratory Technician, and Laboratory Technician, respectively.

Received for publication September 13, 1969.