Stochastic modeling of cargo transport by teams of molecular motors

Many different types of cellular cargos are transported bidirectionally along microtubules by teams of molecular motors. The motion of this cargo-motors system has been experimentally characterized in vivo as processive with rather persistent directionality. Different theoretical approaches have been suggested in order to explore the origin of this kind of motion. An effective theoretical approach, introduced by M\"uller et al., describes the cargo dynamics as a tug-of-war between different kinds of motors. An alternative approach has been suggested recently by Kunwar et al., who considered the coupling between motor and cargo in more detail. Based on this framework we introduce a model considering single motor positions which we propagate in continuous time. Furthermore, we analyze the possible influence of the discrete time update schemes used in previous publications on the system's dynamic.Comment: Cenference proceedings - Traffic and Granular Flow 1

Nitrogen and Phosphorus Fertilization of Sugarbeets

Nitrogen and phosphorus fertilization for sugarbeet production has been practiced in the United States for the past 30 to 40 years. During this period numerous studies have been conducted and summarized (5). Since nitrogen plays a dominant role in the production of high quality roots and maximum sucrose yields, its supply must be accurately controlled. Recent methods developed for predicting N fertilizer needs for sugarbeets in Washington and Colorado (4, 7) are based on the amount of NO?-N in the root zone. However, mineralizable soil N can be a major source of N for plant growth and varies widely in Idaho from one area to another (2, 3). It must be considered if a general procedure for estimating N fertilizer needs is used over a wide area with many soil types and management conditions. Phosphorus is also important in the nutrition of the sugarbeet. Low P levels depress root yields, whereas high levels generally maintain maximum root yields without lowering root quality. Methods have been developed for estimating P fertilizer needs based on the NaHCO?-extractable soil P level (6). Soil test data from England and many U. S. areas suggest that the available soil P levels in many soils are sufficient for maximum root and sucrose production without additional P fertilization. Soil test correlation data establishing P fertilization guidelines for sugarbeets has been limited in Idaho until recently. We conducted 30 field experiments in 1971 and 1972 dealing with N, and two field experiments in 1972 and 1973 dealing with the P fertilization needs of sugarbeets. Since much of this information is published elsewhere, this report summarizes only the soil test results as related to root and sucrose yields

Effect of Irrigation Method and Late Season Nitrate-Nitrogen Concentration on Sucrose Production by Sugarbeets

For maximum sucrose production by sugarbeets (Beta vulgaris L.) in southern Idaho, the available soil nitrogen (N) level should be highest during early July, when the plant N uptake (Nup) rate is highest, and lowest by the latter part of August. Inadequate N during the early growth stages limits root and sucrose yield, whereas excess N stimulates top growth, increases root impurities, and decreases sucrose percentage and extractable sucrose. Inadequate irrigation limits root and sucrose yields, whereas overirrigation leaches N and may affect the sugarbeet response to N application

Phosphorus Fertilization of Sugarbeets

Sugarbeets (Beta vulgaris L.) have been fertilized with phosphorus (P) in the U.S.A. for the past 30 to 40 years. During this period, numerous studies on P fertilization rates and application methods have been conducted. Much of this information has been summarized (12, 14, 15, 4). Recent soil test data from England and the U.S.A. suggest that the available P levels in many soils are sufficient for maximum root and sucrose production without P fertilization. Detrimental effects of P fertilization at excessive soil P levels have been reported (6, 5) but not all data support these conclusions (12). Increasing fertilizer costs have also made it essential that growers have adequate guidelines upon which to base fertilizer applications. Information has been limited in Idaho for establishing an adequate soil test P level for optimum sugarbeet production. With this as background, we conducted two field experiments evaluating 1) the P fertilizer requirements of sugarbeets at different soil test P levels, and 2) the effects of P fertilization on soils already containing adequate available P levels

Root-zone mineral nitrogen changes as affected by crop sequence and tillage

Crop sequence and tillage affect soil mineral N (NH4 plus NO3) and NO3 leaching below the root zone following alfalfa (Medicago sativa L.). A 2-yr field experiment was conducted in south-central Idaho to determine the effect on soil NO3 levels of a corn (Zea mays L.)- wheat (Triticum aestivum L.) rotation compared with a bean (Phaseolus vulgaris L.)-bean rotation and to demonstrate improved N utilization with a corn-wheat rotation. Alfalfa, growing on an irrigated Portneuf silt loam (coarse-silty, mixed, mesic Durixerollic Calciorthid), was killed in October 1989 with herbicide. Treatments were: (i) BT-BT: conventional tilled bean grown in 1990 and 1991; (ii) CNT-WNT: no-till silage corn grown in 1990, and no-till winter wheat grown in 1990-1991; and (iii) CT-WT: same as CNT-WNT but under conventional tillage. Similar amounts of soil N were mineralized the first (275 kg N ha-1) and second (213 kg N ha-1) year after killing the alfalfa in all treatments. The BT-BT treatment had the highest growing-season soil mineral N (up to 251 kg ha-1, 0-0.45-m depth) because the N uptake by bean was lower (187 kg N ha-1) than corn (252 kg N ha-1, average of CT-WT and CNT-WNT treatments) in 1990 and later than winter wheat uptake in 1991. Most wheat N uptake had occurred by late June when bean uptake was just starting. A rotation that follows alfalfa with corn or a crop with a similar N uptake pattern, instead of bean, will save N fertilizer, lower soil NO3 levels, and reduce NO3 leaching potentia

Predicting Nitrogen Fertilizer Needs for Sugarbeets from Residual Nitrate and Mineralizable Nitrogen

Nitrogen (N) fertilizer management for sugarbeet (Beta vulgaris L.) production requires more precise information than for most crops. Inadequate N limits plant growth and root yield, but excess N may reduce both sucrose percentage and recoverable sucrose (7). Also, excess N may stimulate more leaf growth than necessary. The rate and timing of N fertilizer applications are not only important in supplying crop N needs, but can influence the amount of N lost by leaching and denitrification. Soil and plant tissue tests can provide essential data For decision-making for efficient and economical use of N fertilizer. Recent studies have shown that the NO?-N level in the soil before planting is closely related to sucrose production when N is limiting (8, 12). Inclusion of the N mineralization capacity of the soils would be expected to improve the relationship. Stanford and Smith (14) showed that the mineralization capacity varies with soil type and location. Therefore, a soil test for N that would have general applicability should include the mineralization capacity of the soil, and the interpretation of these tests should include some knowledge of expected irrigation practices. A soil test for NO?-N may suffice as an index of N fertilizer needs for a given soil and irrigation level. Recently, Carter et al. (5) showed that sucrose production was closely related to available soil N, as indicated by a soil test that included both mineralizable N and NO?-N. The objective of our study was to evaluate the soil test-yield relationship, developed from experimental data at one location in south central Idaho, for predicting N fertilizer needs throughout southern Idaho under various irrigation management practices

Residual Nitrate and Mineralizable Soil Nitrogen in Relation to Nitrogen Uptake by Irrigated Sugarbeets

Previously reported studies on N fertilization of sugarbeets (Beta vulgaris L.) in southern Idaho revealed considerable variation among sites in amounts of residual soil NO? and N mineralized during short-term laboratory incubations. Consequently, the amount of N fertilizer needed to achieve near-maximum yields of sucrose differed markedly. The purpose of this study was to determine the feasibility of estimating amounts of N mineralized in the root zone during the season, taking into account site variations in temperature and soil water regimes. Residual soil NO?--N and mineralizable N to approximate rooting depth were estimated for 21 field sites in 1971 and six sites in 1972. The relative contributions of these two N sources to total N uptake by the crop, in the absence of applied fertilizer N, were then assessed. Estimates of N mineralized in the upper 45- cm soil layer for each successive month, ?N, over a 6- month period were derived using the expression, ?N/ ?t kWN (k = fraction of N mineralized during each month, ?t, adjusted for average air temperature; and W the estimated soil water content expressed as a fraction of the available water storage capacity). Resulting estimates of the fraction of potentially mineralizable N converted to (NO?- + NH?+)-N between 1 April and 30 September ranged from 0.15 to 0.22 (mean ± S.D. = 0.18 ± 0.02) in 1971 and 1972. On the average, mature sugarbeets recovered about 73% of the estimated N mineralized (6 months) plus residual NO?--N. The relative contributions of these two sources of soil derived N, respectively, were approximately 66 and 75%, as estimated from multiple regression analyses

Residual Nitrate and Mineralizable Soil Nitrogen in Relation to Nitrogen Uptake by Irrigated Sugarbeets

Previously reported studies on N fertilization of sugarbeets (Beta vulgaris L.) in southern Idaho revealed considerable variation among sites in amounts of residual soil NO? and N mineralized during short-term laboratory incubations. Consequently, the amount of N fertilizer needed to achieve near-maximum yields of sucrose differed markedly. The purpose of this study was to determine the feasibility of estimating amounts of N mineralized in the root zone during the season, taking into account site variations in temperature and soil water regimes. Residual soil NO?--N and mineralizable N to approximate rooting depth were estimated for 21 field sites in 1971 and six sites in 1972. The relative contributions of these two N sources to total N uptake by the crop, in the absence of applied fertilizer N, were then assessed. Estimates of N mineralized in the upper 45- cm soil layer for each successive month, ?N, over a 6- month period were derived using the expression, ?N/ ?t kWN (k = fraction of N mineralized during each month, ?t, adjusted for average air temperature; and W the estimated soil water content expressed as a fraction of the available water storage capacity). Resulting estimates of the fraction of potentially mineralizable N converted to (NO?- + NH?+)-N between 1 April and 30 September ranged from 0.15 to 0.22 (mean ± S.D. = 0.18 ± 0.02) in 1971 and 1972. On the average, mature sugarbeets recovered about 73% of the estimated N mineralized (6 months) plus residual NO?--N. The relative contributions of these two sources of soil derived N, respectively, were approximately 66 and 75%, as estimated from multiple regression analyses

Gluon self-energy in a two-flavor color superconductor

The energy and momentum dependence of the gluon self-energy is investigated in a color superconductor with two flavors of massless quarks. The presence of a color-superconducting quark-quark condensate modifies the gluon self-energy for energies which are of the order of the gap parameter. For gluon energies much larger than the gap, the self-energy assumes the form given by the standard hard-dense loop approximation. It is shown that this modification of the gluon self-energy does not affect the magnitude of the gap to leading and subleading order in the weak-coupling limit.Comment: 21 pages, 6 figures, RevTeX, aps and epsfig style files require

Drum vortons in high density QCD

Recently it was shown that high density QCD supports of number of topological defects. In particular, there are U(1)_Y strings that arise due to K^0 condensation that occurs when the strange quark mass is relatively large. The unique feature of these strings is that they possess a nonzero K^+ condensate that is trapped on the core. In the following we will show that these strings (with nontrivial core structure) can form closed loops with conserved charge and currents trapped on the string worldsheet. The presence of conserved charges allows these topological defects, called vortons, to carry angular momentum, which makes them classically stable objects. We also give arguments demonstrating that vortons carry angular momentum very efficiently (in terms of energy per unit angular momentum) such that they might be the important degrees of freedom in the cores of neutron stars.Comment: 11 pages, accepted for publication in Physical Review