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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.

Turnover of ribulose 1, 5-diphosphate carboxylase in barley leaves (Hordeum vulgare L.) was followed over time in light and dark. The enzyme was degraded in prolonged darkness and was resynthesized after the plants were returned to light. Labeling with 14C showed that simultaneous synthesis and degradation (turnover) did not occur in light. In contrast, the remaining soluble protein was turned over rapidly in light. Although ribulose 1, 5-diP carboxylase can be both degraded and synthesized, these processes seem not to occur simultaneously, but can be induced independently by changing environmental conditions. Much information is available on the increase of RuDPI carboxylase activity (6, 3, 9, 12, 13. 16), RuDP carboxylase protein (9), and fraction I protein (9, 10), in response to leaf development and light, and relatively little on the fate of the enzyme after its synthesis. Fraction I protein and RuDP carboxylase are very probably identical (9,14,15,18,19). RuDP carboxylase activity and fraction I protein decrease during senescence of tobacco (1,8) and Perilla (7) leaves, but it is not known whether the enzyme is controlled by a turnover system or is degraded without simultaneous synthesis.Since RuDP carboxylase protein makes up a large percentage of the total soluble protein of many plant leaves, it can be considered to be a major component of storage proteins (9, 14) as well as a major catalyst of CO2 fixation. A study was therefore done to determine whether RuDP carboxylase is controlled by a turnover system in barley leaves. MATERIALS AND METHODSPlant Materials. Hordeumn vulgar-e L. var. Numar was grown in vermiculite in 24-cm plastic pots or 28-X 33-cm plastic pans. Moisture was supplied by cotton wicks connecting the vermiculite to a full strength nutrient supply (9). The seedlings were grown for 6 days in either complete darkness or in continuous light (21,000 lux) at 27 C and 55% relative humidity. Plant Treatments. Loss of RuDP carboxylase from barley seedlings was followed in both darkness and continuous light. Six-day-old light-grown seedlings were placed in darkness, and the level of the enzyme was assayed every 6 hr for 72 hr. The seedlings were then returned to light, where the enzymatic assays were continued.'Abbreviation: RuDP: ribulose 1.5-diphosphate. This report defines turnover as simultaneous synthesis and degradation of protein. To determine whether RuDP carboxylase was being turned over while plants were in continuous light, RuDP carboxylase protein was labeled with "C by introducing CO2 when the leaves were rapidly synthesizing the enzyme (6, 9, 13). Six-day-old dark-grown barley seedlings were given 12 hr of light and then placed in a gas-tight chamber at 27 C under light (16,000 lux) and treated with 2 mc of "CO2. Continuous monitoring showed that at the end of 6 hr, about 80% of the "CO2 had been removed from the atmosphere of the chamber by the seedlings. At that point the plants were removed from the chamber and placed in continuous light (21,000 lux) at 27 C and 55% rel...

Nitrogen fertilizer applications, for maximum fertilizer efficiencies and crop yields, should be based on the N required by the crop during its various growth stages. The objectives of this paper were to identify the N requirements of the potato plant (Solanum tuberosumL.) during growth and to evaluate selected soil and plant tissue tests as indicators of the plant's N status. Growth analysis data and soil and petiole NO3‐N concentrations were obtained at predetermined time intervals from N fertilization treatments in replicated field studies on a coarse‐silty mixed, mesic Durixerollic Calciorthrid soil. Maximum early tuber growth occurred when leaf area index was between 2.5 and 3.2 and the tops contained between 79 and 100 kg N ha−1at the start of linear tuber growth. A preplant N fertilizer application between 67 and 134 kg ha−1gave these characteristics under the experimental conditions. The maximum dry matter production rate per day (approx. 250 kg ha−1) occurred when there was between 80 and 140 kg N ha−1in the plant tops and roots. An average tuber growth rate of 0.75 Mg ha−1day−1required a N uptake rate of 3.7 kg ha−1day−1to prevent the loss of N and dry matter from the tops and roots. Sufficient N was available for this rate when the soil NO3‐N concentration was > 7.5 mg kg−1(0.46‐m soil depth), corresponding to 15 000 mg kg−1NO3‐N in the fourth petiole. Soil and petiole NO3‐N concentrations may be used to adjust the N fertilization rates during the growing season. This practice has the potential of increasing the overall N fertilizer use efficiency and final tuber yields within the climatic, disease, and variety limitations.