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.
The rate and duration of tuber growth largely determines the final potato (Solanum tuberosum L.) tuber yields. Cultivars that continue leaf development and nutrient uptake while maintaining maximum tuber growth rates may have higher final tuber yields, yet different N requirements. The objective of this study was to obtain information relating plant growth rates to N availability for selected potato cultivars. Total dry matter accumulation and N assimilation patterns of indeterminant cultivars, ‘Russet Burbank’, ‘Lemhi Russet’, ‘Centennial Russet’, and one advanced selection A66107‐51, were compared with that of two determinant cultivars, ‘Pioneer’ and ‘Norgold Russet’, at three N levels. Cultivars were grown in a field experiment on a Portneuf silt‐loam soil (Xerollic Calciorthids). High available soil N levels at planting delayed the linear potato tuber growth period 7 to 10 days but had minor effects on the time of tuber initiation for the indeterminant varieties. Maximum tuber growth rates (tuber bulking) were 900 to 1,300 kg/ha/day. A66107‐51 was superior in N‐use efficiency to the other cultivars. Between 70 and 100% of the total available N was utilized by this cultivar in producing high yields. This information may be used to select lines and cultvars that will optimize production. A knowledge of plant growth and N uptake rates can improve the fertilizer recommendations for each cultivar.
Variability in specific gravity of Russet Burbank potatoes was documented in a single 32 hectare field. Maximum variation was 40 units among individual tubers within hills (one unit equals one part in 1000th of specific gravity measurement) and 15 units among hills. Field site variability of 10-15 units was common between sampled grid lines in the field; but bulked samples (truckloads) reduced the sampled variability to 8-10 units. When the samples were taken from bulked lots as opposed to single hill samples, the variability decreased. Grower lots, which were pooled samples from several truckloads, showed specific gravity differences of 2-7 units even though all lots were from the same field. These variations among specific gravity samples should be taken into account when considering total solids content in any lot of potatoes.Degree of russetting of the skin and hollow heart also influenced specific gravity measurements. Measured differences between peeled and unpeeled lots of 10 units in specific gravity corresponded to 2% difference in total solids content. Statistically, the variance of the peeled lot was one half that of the unpeeled lot, therefore, to minimize the measured differences due to skin type, peeled potatoes could be used for the specific gravity measurements. CompendioSe probe) la variabilidad en la gravedad especifica de las papas Russet Burbank en un solo campo de 32 hectareas. La variaciOn maxima entre tuberculos individuales dentro de los puntos de siembra fue de 40 unidades (una unidad es igual a una parte por milesimo de la gravedad especifica medida) y 15 unidades entre puntos de siembra. Fue comtin encontrar una variabilidad de 10-15 unidades entre las diferentes muestras tomadas en el terreno convenientemente dividido; pero muestras al granel (camionadas) redujeron la variabilidad de las muestras a 8-10 unidades. Cuando las muestras fueron tomadas de lotes al granel en oposiciOn a muestras de un ' solo golpe o punto de siembra, la variabilidad disminuy6. Los lotes de los productores, que fueron una mezcla de muestras tomadas de varias camionadas, mostraron diferencias de 2-7 unidades en la gravedad especifica aim en el caso que todos los lotes fueran del mismo campo. Estas variaciones en las muestras, en cuanto a gravedad especifica, deben tomarse en cuenta cuando se considere el contenido total de sOlidos en cualquier lote de papas.El grado de reticulado de la piel y el de corazOn vacio influencian tambièn las mediciones de la gravedad especifica. Las diferencias en las mediciones de la gravedad especifica entre lotes pelados y sin pelar, que fueron de 10 unidades, corresponden a un 2% de diferencia en el contenido de sOlidos totales. Estadisticamente, la variancia de los lotes pelados fue la mitad que la de los lotes sin pelar, por lo canto, para minimizar las diferencias en las mediciones, debidas a las condiciones de la piel, se podrian usar papas peladas para las medici6n de la gravedad especifica.
Beans (Phase()las valgtais L.) often respond to N fertilization; however, N fertilization is not practiced for maximum seed production in southern Idaho. This suggests that the symbiotic relationship and/or soil N sources can provide most of the N needed by this legume. Our objective was to evaluate the relative contribution of the symbiotic-nonsymbiotic N sources by studying the effects of N fertilization on the symbiotic N. fixation and seed yields under field conditions. Experiments were conducted on silt loam soils belonging to the Portneuf series (Xerollic Calciortnids). An acetylene reduction (AR) method was used to determine the effect of N fertilization treatments on the relative seasonal N, (AR) fixation. The symbiotic N, fixation was also estimated by the equation, N. = N"" -(N, + N", N.) -aN., where N", is the accumulated N uptake measured near physiological maturity, N, and N,, are the amounts of soil NO,-N in the root zone before planting and near physiological maturity, N" is the N mineralized from soil organic N sources, and a is the recovery of the N fertilizer (N,) applied. Estimates of the N fertilizer recoveries were obtained from two experiments using 15N-depleted (NH4), SO,.The symbiotic N, relationship contributed up to 90 kg N/ha, which was 40 to 50% of the total N found in bean plants near physiological maturity. The amount of symbiotic N. fixed decreased as the available soil N or fertilizer N increased, and increased as the N required by the individual cultivars increased. The response to N fertilization depended upon the cultivar, as well as on the N_available from soil sources. Measured fertilizer N recoveries ranged from 7 to 33%. An average of 52% of the total N uptake near physiological maturity was taken up after the maximum symbiotic NAAR) rate occurred; while the seed contained an average of 60% of the total N uptake. A low N fertilization rate (< 50 kg Nilia) when the soil N, was low (<50 kg N/ha) ensured an early vi us plant growth but did not always increase seed yields. Higher N fertilization rates may be required on soils with lower amounts of mineralizable N.
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