Simulated Soil Organic Carbon Response to Tillage, Yield, and Climate Change in the Southeastern Coastal Plains.

Intensive tillage, low-residue crops, and a warm, humid climate have contributed to soil organic carbon (SOC) loss in the southeastern Coastal Plains region. Conservation (CnT) tillage and winter cover cropping are current management practices to rebuild SOC; however, there is sparse long-term field data showing how these management practices perform under variable climate conditions. The objectives of this study were to use CQESTR, a process-based C model, to simulate SOC in the top 15 cm of a loamy sand soil (fine-loamy, kaolinitic, thermic Typic Kandiudult) under conventional (CvT) or CnT tillage to elucidate the impact of projected climate change and crop yields on SOC relative to management and recommend the best agriculture management to increase SOC. Conservation tillage was predicted to increase SOC by 0.10 to 0.64 Mg C ha for six of eight crop rotations compared with CvT by 2033. The addition of a winter crop [rye ( L.) or winter wheat ( L.)] to a corn ( L.)-cotton ( L.) or corn-soybean [ (L.) Merr.] rotation increased SOC by 1.47 to 2.55 Mg C ha. A continued increase in crop yields following historical trends could increase SOC by 0.28 Mg C ha, whereas climate change is unlikely to have a significant impact on SOC except in the corn-cotton or corn-soybean rotations where SOC decreased up to 0.15 Mg C ha by 2033. The adoption of CnT and cover crop management with high-residue-producing corn will likely increase SOC accretion in loamy sand soils. Simulation results indicate that soil C saturation may be reached in high-residue rotations, and increasing SOC deeper in the soil profile will be required for long-term SOC accretion beyond 2030.

Reexposure breeds recall: effects of experience on 9-month-olds' ordered recall.

On the basis of three experiences, 9-month-olds recall specific events after one month. We tested whether multiple experiences are necessary, or merely facilitative, of 9-month-olds' long-term recall. In two experiments, using deferred imitation, infants were exposed to multi-step sequences either one time, two times, or three times prior to a one-month delay. In Experiment 1, regardless of the number of experiences, infants did not demonstrate recall. In Experiment 2, infants were tested on and reexposed to the events after one week; recall was tested again one month later. Performance after 1 week in Experiment 2 was greater than performance after one month in Experiment 1. Moreover, presumably as a function of reexposure after one week, infants recalled the individual actions of the events one month later. Infants in the three-experience condition also evidenced ordered recall after one month. Implications for the developmental status of the neural substrate supporting long-term recall are discussed.

Episodic recall of specifics and generalisation coexist in 25-month-old children.

It is clear that children generalise their knowledge of events from one instantiation to another. The means by which generalisation is accomplished are unclear. In three experiments, we used elicited imitation of multi-step sequences to test whether 25-month-olds' generalisation occurs as a function of forgetting of the features of the original event. Experiment 1 was an initial test of generalisation from one version of an event to another version involving perceptually different yet functionally analogous props. After a 1-week delay, children showed evidence of generalisation by enacting the event using the analogous props. Experiment 2 was a within-subject test of generalisation and memory for the original version of an event. After a 1-week delay, when paired with unrelated distractor props, analogous props served as effective retrieval cues; when paired with the original props, analogous props were treated as functionally equivalent to unrelated distractors. This within-subject reversal in the functional role of analogous props is compelling evidence that children's event representations include specific features and, at the same time, are generalisable. In Experiment 3, children showed evidence of generalisation immediately after exposure to an event, thereby making clear that generalisation occurs even in the face of robust memory for the specific features of the original event.

The local Ca concentration profile in the vicinity of a Ca channel.

Intracellular Ca signaling is governed by diffusion and buffering of Ca ions. Mobile endogenous buffers increase the redistribution of Ca ions and tend to restrict the regions of elevated Ca concentrations to the close vicinity of the channel. These Ca microdomains dominate fast Ca signaling, as observed, e.g., in synaptic transmission. The steady-state solution of the linearized differential equations of buffered Ca diffusion, as developed by Neher and Naraghi and Neher, will be reviewed and generalized to the case of more than two buffers. Immobile buffers do not enter the steady-state equations, but instead slow down Ca diffusion and prolong the time to reach the steady state. Based on this phenomenon, a quite different putative mechanism to localize Ca will be suggested that is likely to be realized in photoreceptors where Ca source, Ca sink, and Ca sensor form a complex, as was recently reported.

The dawning of a past: the emergence of long-term explicit memory in infancy.

The ability to recall information about the past is thought to emerge in the 2nd half of the 1st year of life. Although there is evidence from both cognitive neuroscience and behavioral psychology to support this hypothesis, there is little longitudinal evidence with which the question can be addressed. Infants' memory abilities were tested between the ages of 9 and 16 months using elicited and deferred imitation. Infants' memory for events was tested after delays ranging from 1 to 6 months. The results suggest that at 9 months of age, infants are able to store and retrieve representations over delays of as many as 4 weeks but not over long delays. In contrast, 10-month-olds have at their disposal a system that allows encoding and retrieval of event representations over delays of up to 6 months. These results support the idea that the system that underlies long-term ordered recall emerges near the end of the 1st year of life.

Deaf and hearing adults' recollections of childhood and beyond.

Whereas the average age of earliest reportable personal memory among adults is 3 to 3 1/2, there is considerable individual and group variability in the age of earliest autobiographical memory. Some of the variability is thought to be attributable to differential narrative socialisation. In the present research we tested the hypothesis that by virtue of later exposure to language, individuals born deaf to hearing parents will have earliest memories from later in life, relative to hearing individuals. The average age of single earliest identifiable memory for adults who are deaf and adults who are hearing did not differ. Nevertheless, adults who are deaf were found to have less dense representations of early autobiographical memories and to include in their narrative reports fewer categories of information, including visual-spatial information, relative to hearing adults. Participants' ratings of their memories on a number of dimensions were found to have low utility in predicting the content of autobiographical reports from both early and later in life (i.e., after age 10 years).

Explicit memory in low-risk infants aged 19 months born between 27 and 42 weeks of gestation.

The aim of this study was to determine whether there are primary effects of prematurity on the development of explicit memory. Elicited imitation of action sequences was used to compare immediate and 15-minute delayed memory in term and preterm infants (19 months corrected age; n=48) who were at low risk: none had experienced the medical or social risk factors often associated with preterm birth. Relative to infants born at term (38 to 40 weeks' gestation), children who had been born at 27 to 34 weeks' gestation showed lower levels of ordered recall; performance of healthy infants born at 35 to 37 weeks' gestation was intermediate and did not differ significantly from that of the other groups. These results suggest that specific cognitive deficits can occur as a function of preterm birth even in low-risk infants.

Binding of the cGMP-gated channel to the Na/Ca-K exchanger in rod photoreceptors.

The intracellular Ca(2+) concentration in rod outer segments of vertebrate photoreceptors is controlled by Ca(2+) influx through cGMP-gated channels and by Ca(2+) efflux driven by Na/Ca-K exchangers. Previously, we suggested that channel and exchanger are associated (Bauer, P. J., and Drechsler, M. (1992) J. Physiol. (Lond. ) 451, 109-131). This suggestion has been thoroughly examined using a variety of biochemical approaches. First, we took advantage of the fact that cGMP-gated channels bind calmodulin (CaM). Using CaM affinity chromatographic purification of the channel in 10 mm CHAPS, a significant fraction of exchanger was co-eluted with the channel indicating a binding affinity between channel and exchanger. Binding of channel and exchanger was examined more directly by cross-linking of proteins in the rod outer segment membranes. Activation of the channel with cyclic 8-bromo-GMP lead to exposure of a cysteine, which allowed cross-linking of the channel to the exchanger with the thiol-specific reagent dl-1,4-bismaleimido-2,3-butanediol. Cleavage of the cross-links and electrophoretic analysis indicated that a cross-link between the alpha-subunit of the channel and the exchanger formed. Furthermore, a cross-link between two adjacent alpha-subunits of the channel was found, suggesting that the alpha-subunits of the native channel are dimerized. Further support for an interaction between alpha-subunit and exchanger was obtained by in vitro experiments. Specific binding of the exchanger to the alpha-subunit but not to the beta-subunit of the channel was observed in Western blots of purified channel incubated with purified exchanger. This study suggests that two exchanger molecules bind to one cGMP-gated channel and, more specifically, that binding of exchanger molecules occurs at the alpha-subunits, which in the native channel are dimerized. The implications of these findings regarding the possibility of local Ca(2+) signaling in vertebrate photoreceptors will be discussed.

Parameters of remembering and forgetting in the transition from infancy to early childhood.

The ability to recall is something that most intact adults take for granted. For much of the last century, this feature of mental life was not considered to extend to very young children. There now is evidence that 1- to 2-year-olds are able to recall specific events after delays of several months. Over the short term, 1- to 2-year-olds' recall is affected by the same factors that affect older children's recall; it is not clear whether similar effects are apparent over the long term. Moreover, although age-related increases in long-term recall are assumed, there have been few empirical tests of the question. We examined recall by 14- to 32-month-olds for events experienced at 13 to 20 months. Using elicited imitation of novel multistep event sequences we examined effects of (a) delay length, (b) age at the time of experience, (c) temporal structure of events, (d) mode of experience of events, and (e) availability of verbal reminders, on long-term recall. Participants were 360 children enrolled at 13 (n = 90), 16 (n = 180), and 20 (n = 90) months. All of the 13-month-olds and half of the 16-month-olds were tested on 3-step event sequences; all of the 20-month-olds and half of the 16-month-olds were tested on 4-step event sequences. Within each age and step-length group, equal numbers of children were tested after intervals of 1, 3, 6, 9, and 12 months (n = 18 per cell). Children were tested on a variety of sequence types. For half of the events, imitation was permitted prior to the delay; for the other half, children were not permitted imitation. At delayed testing, children experienced a recall period during which they were cued by the event-related props alone, followed by a period in which recall was cued both by the event-related props and by verbal labels for the event sequences. Within step-length groups, the length of time for which older and younger children showed evidence of memory did not differ. Nevertheless, when the children were prompted by the event-related props alone, there were age-related differences in the robustness of children's memories (as indexed by higher levels of recall for older children relative to younger children). When the children were prompted by the props and by verbal labels for the event sequences, at the longer retention intervals, there were age-related differences in the robustness of children's memories and in the reliability with which recall was evidenced (as indexed by the larger numbers of older children evincing recall). Age-related effects were particularly apparent on children's ordered recall. Across the entire age range, the children were similarly affected by the variables of sequence type, opportunity for imitation, and verbal reminding.

Planning ahead: goal-directed problem solving by 2-year-olds.

On tasks that require overcoming an obstacle along an existing path to a physically present goal, infants and very young children evince planning. In each of 3 experiments, the authors tested 21- and 27-month-olds' ability to construct a path to a mentally re-presented goal. Across experiments, the authors varied the number and type of cues to the solution provided. After exposure to the goal-state configuration of problems, both age groups showed evidence of planning (Experiment 1). Demonstration of the initial step in the solution path in Experiment 2 was not as effective as exposure to the goal state in Experiment 1. Even with specification of a greater proportion of the goal path, goal-state configuration information was particularly effective in facilitating performance (Experiment 3). The results suggest productive generation of solutions to novel problems by young children; planning is facilitated by goal-state configuration information.

Direct evidence of Na+/Ca2+ exchange in squid rhabdomeric membranes.

Na+/Ca2+ exchange has been investigated in squid (Loligo pealei) rhabdomeric membranes. Ca2+-containing vesicles have been prepared from purified rhabdomeric membranes by extrusion through polycarbonate filters of 1-micrometer pore size. After removal of external Ca2+, up to 90% of the entrapped Ca2+ could be specifically released by the addition of Na+; this finding indicates that most of the vesicles contained Na+/Ca2+ exchanger. The Na+-induced Ca2+ efflux had a half-maximum value (K1/2) of approximately 44 mM and a Hill coefficient of approximately 1.7. The maximal Na+-induced Ca2+ efflux was approximately 0.6 nmol Ca2+. s-1. mg protein-1. Similar Na+-induced Ca2+ effluxes were measured if K+ was replaced with Li+ or Cs+. Vesicles loaded with Ca2+ by Na+/Ca2+ exchange also released this Ca2+ by Na+/Ca2+ exchange, suggesting that Na+/Ca2+ exchange operated in both forward and reverse modes. Limited proteolysis by trypsin resulted in a rate of Ca2+ efflux enhanced by approximately fivefold when efflux was activated with 95 mM NaCl. For vesicles subjected to limited proteolysis by trypsin, Na+/Ca2+ exchange was characterized by a K1/2 of approximately 25 mM and a Hill coefficient of 1.6. For these vesicles, the maximal Na+-induced Ca2+ efflux was about twice as great as in control vesicles. We conclude that Na+/Ca2+ exchange proteins localized in rhabdomeric membranes mediate Ca2+ extrusion in squid photoreceptors.

Assumptions of infantile amnesia: are there differences between early and later memories?

Most adults are able to provide few, if any, reports of autobiographical memories from the first years of life. Early memories that do exist have been characterised as highly emotional, containing an abundance of perceptual as opposed to propositional information, and more often in the third than the first person perspective. These qualities figure prominently in theories of the source of the phenomenon of infantile amnesia. However, early and later memories have not been directly compared with regard to these features. In the present research, we compared early and later memories in the same individuals, with the same methodology. Results indicated few objective differences between the early and later memories of women (Experiment 1) and men (Experiment 2). The findings are discussed in light of their implications for theories as to the source of infantile amnesia.

When the event is more than the sum of its parts: 9-month-olds' long-term ordered recall.

Research suggests that 9-month-old infants are able to recall single object-specific actions over delays of 24 hours. In the present research we investigated whether 9-month-olds are able to recall over more extended delays, and to recall the temporal order of events, as well as the individual actions in them. In addition, we investigated whether recall can be enhanced by pre- and/or re-exposure to target events. Using elicited imitation of novel, multi-step event sequences, we demonstrated that, as a group, 9-month-olds are able to recall target actions after delays of five weeks. However, after this long delay, only 45% of the infants recalled the temporal order of the events. Re-exposure to events during the delay interval proved necessary for boys, but not for girls; pre-exposure to events did not affect later recall. The implications of individual differences in infants' recall ability for the understanding of the development of the neural correlates of declarative memory are discussed.

When even arbitrary order becomes important: developments in reliable temporal sequencing of arbitrarily ordered events.

By late in the first year of life, children show temporally ordered recall of event sequences, the orders of which are constrained by enabling relations; they do not reliably recall arbitrarily ordered events. Using elicited imitation, in two experiments, we examined age- and experience-related changes in young children's recall of events, the orders of which are arbitrary. The changes were found to have implications for the efficacy of verbal reminding and to be related to developments in language. Specifically, on the basis of a single experience, 16-month-olds did not accurately recall arbitrarily ordered event sequences either immediately or after a two-week delay (Experiment 1); 22-month-olds recalled the events immediately, but not after the delay; by 28 months, children recalled the events even after the delay (Experiment 2A). This development was accompanied by changes in the ability to benefit from verbal reminders: 28-month-olds' recall was facilitated by provision of verbal reminders, whereas that of the younger children was not. Moreover, age-related changes in accurate reproduction of lengthy arbitrarily ordered event sequences were found to be related to developments in language (Experiment 2B). Critically, the limitations on 1-year-olds' performance that are overcome with age are not absolute: After three experiences, 16-month-olds accurately recalled the events after a two-week delay; their recall was facilitated by verbal reminders (Experiment 1). The implications of these findings are discussed.

If memory serves, will language? Later verbal accessibility of early memories.

Of major interest to those concerned with early mnemonic process and function is the question of whether early memories likely encoded without the benefit of language later are accessible to verbal report. In the context of a controlled laboratory study, we examined this question in children who were 16 and 20 months at the time of exposure to specific target events and who subsequently were tested for their memories of the events after a delay of either 6 or 12 months (at 22-32 months) and then again at 3 years. At the first delayed-recall test, children evidenced memory both nonverbally and verbally. Nonverbal mnemonic expression was related to age at the time of test; verbal mnemonic expression was related to verbal fluency at the time of test. At the second delayed-recall test, children evidenced continued accessibility of their early memories. Verbal mnemonic expression was related to previous mnemonic expression, both nonverbal and verbal, each of which contributed unique variance. The relevance of these findings on memory for controlled laboratory events for issues of memory for traumatic experiences is discussed.

The Na/Ca-K exchanger of rod photoreceptor exists as dimer in the plasma membrane.

The oligomeric state of the Na/Ca-K exchanger in the plasma membrane of bovine photoreceptors was investigated using chemical cross-linking techniques. In the natural membrane, virtually all Na/Ca-K exchanger could be cross-linked mainly to a complex having an apparent molecular mass of 490 kDa by cupric phenanthroline catalyzed disulfide bonding as evidenced by Western blotting. Stable cross-links of the exchanger were also obtained with the thiol-specific reagent N,N'-p-phenylidenedimaleimide. Neuraminidase treatment reduced the apparent molecular mass of the highly glycosylated Na/Ca-K exchanger and of the 490 kDa cross-link product by 50 and 85 kDa, respectively. DL-1,4-Bismaleimido-2,3-butanediol (BMBD), a novel cleavable dimaleimide, was synthesized in order to produce cross-links that were stable to reductive conditions. Purification of the BMBD cross-linked exchanger followed by two-dimensional SDS polyacrylamide electrophoresis identified the cross-linked homodimers of the exchanger. There was no indication of higher oligomers, suggesting that the exchanger exists as a dimer in the plasma membrane. Hydrodynamic properties of the detergent-solubilized exchanger were determined by velocity sedimentation and gel filtration chromatography. The Triton X-100-solubilized exchanger ran as a single species having a Stokes radius of 10.0 nm, a sedimentation coefficient of 5.4 S, and a partial specific volume of 0.74 mL/g in Triton X-100. Similar results were obtained for the CHAPS-solubilized exchanger. A molecular mass of 236 and 205 kDa was calculated for the exchanger-detergent complex and the detergent-free protein, respectively. Neuraminidase treatment further reduced the molecular mass of the exchanger indicating that glycosylation contributes significantly to the mass of the exchanger. Cross-links of the exchanger were not detected if cross-linking was attempted after solubilization in 10 mM CHAPS. However, after reconstitution of the purified exchanger into soybean phosphatidylcholine vesicles, chemical cross-linking yielded again dimers. On the basis of these cross-linking and hydrodynamic studies, we conclude that the exchanger exists as a homodimer in the rod outer segment plasma membrane but dissociates into a monomer when solubilized in detergent.

Clavibacter michiganensis subsp. Sepedonicus Elicits a Hypersensitive Response in Tobacco and Secretes Hypersensitive Response-Inducing Protein(s).

ABSTRACT Strains of Clavibacter michiganensis subsp. sepedonicus, causal agent of bacterial ring rot of potato, showed marked differences in virulence on host plants. When infiltrated into tobacco leaves, virulent strains caused a rapid localized necrotic response (within 24 to 48 h) characteristic of the hypersensitive response (HR), whereas nonpathogenic strains did not. Concentrated cell-free culture supernatants (CCS) from virulent strains caused a necrotic reaction on tobacco, whereas CCS from nonpathogenic strains did not. The necrosis-inducing activity was heat stable and protease sensitive. Inhibitors of eukaryotic metabolism suppressed the necrotic reaction of tobacco to CCS. No necrotic response was observed when host plants were infiltrated with either cells or CCS from virulent strains. HR-inducing protein(s) from a virulent strain separated from the majority of other proteins on DEAE cellulose at 250 to 300 mM NaCl. Ammonium sulfate-precipitated proteins from a virulent strain produced a necrotic reaction at a total protein concentration of 18 mug/ml, whereas those from a nonpathogenic strain did not, even at a concentration of 180 mug/ml. We conclude that virulent strains of C. michiganensis subsp. sepedonicus elicit a typical HR in tobacco and secrete proteinaceous elicitor(s) of the nonhost HR.

Cyclic GMP-gated channels of bovine rod photoreceptors: affinity, density and stoichiometry of Ca(2+)-calmodulin binding sites.

1. Ca(2+)-loaded vesicles of bovine rod outer segment (ROS) membranes were used to examine the influence of Ca(2+)-calmodulin (Ca(2+)-CaM) on the activity of cGMP-gated channels. 2. In vesicles prepared from ROS membranes which were washed at zero free Ca2+, Ca(2+)-CaM reduced the Ca2+ flux to maximally 40%. The dose-response curve for activation of the cGMP-gated channel had a half-maximal value of 36.8 +/- 2 microM in the CaM-free state, and of 55.6 +/- 5.2 microM in the Ca(2+)-CaM-bound state. In both cases the Hill coefficients were 2.2 +/- 0.2. 3. In vesicles prepared from ROS membranes which were washed at 100 microM Ca2+, the dose-response curve was identical to the Ca(2+)-CaM-bound state. 4. Titration of the Ca(2+)-CaM-dependent decrease of the channel activity upon addition of 40 microM cGMP yielded half-maximal Ca(2+)-CaM concentrations (EC50CaM) which were linearly correlated with the concentration of membrane vesicles. Extrapolation of EC50CaM to infinite dilution of vesicles yielded a Ca(2+)-CaM affinity constant for the cGMP-gated channel of 1.01 +/- 0.20 nM. Hill analysis of the Ca(2+)-CaM titrations resulted in a Hill coefficient of 1.36 +/- 0.15. 5. From the slope of the linear regression of EC50CaM plotted vs. the rhodopsin concentration, the molar ratio of rhodopsin to externally accessible Ca(2+)-CaM binding sites of fused ROS membranes was determined to be 1439 +/- 109. Therefore, there are about 720 molecules of rhodopsin per Ca(2+)-CaM binding site present in ROS. 6. Based on these data, a density of 560 Ca(2+)-CaM binding sites micron-2 is estimated for the plasma membrane of bovine ROS, suggesting that there are two Ca(2+)-CaM binding sites per channel. 7. The Ca(2+)-CaM effect did not become noticeable until the ROS membranes were hypotonically washed at free [Ca2+] below 100 nM, suggesting that an endogenous Ca(2+)-binding protein was washed off in the absence of Ca2+. 8. If the endogenous Ca(2+)-binding protein of bovine ROS membranes was washed off at zero Ca2+ and then Ca(2+)-CaM added, Ca(2+)-CaM could only be washed off again at free [Ca2+] below 100 nM. 9. These findings strongly suggest that the endogenous Ca(2+)-binding protein of the bovine cGMP-gated channel is CaM.

The dynamics of preschoolers' categorization choices.

The present research explored the effects of stimulus and task factors on preschoolers' (Experiments 1 and 3) and adults' (Experiment 2) tendency to categorize according to taxonomic relations, when those relations conflict with appearances. In Experiment 1, we examined the effects of and interactions among (a) available information, operationalized by using more- or less-informative stimulus types (objects vs. line drawings) and by the presence or absence of labeling, and (b) task constraints, operationalized by comparing sorting questions with inductive inferences questions. When provided with information that constrained the categorization decision, either through the availability of labels or a combination of enhanced physical informativeness of objects and an inference question, preschoolers reliably based their categorization decisions on taxonomic relations between physically dissimilar items. In Experiment 2, stimulus type (objects vs. line drawings) was shown to have a similar effect on adults. In Experiment 3, we examined the effects of stimulus type on preschoolers' inductive inferences and accuracy of naming. The effects in the two tasks were closely related, suggesting that the amount of available information affects different responses in similar ways. These data demonstrate the interactive effects of available information and task constraints on categorization decisions.

What do infants recall of their lives? Memory for specific events by one- to two-year-olds.

Until as recently as a decade ago, it was widely believed that infants and children younger than age three were unable to recall the events of their lives. Several developments, including findings of representational competence in infants in the first year of life and evidence of long-term recall abilities in children as young as three, have led to revision of this assumption. Through application of the technique of elicited imitation of action sequences, my colleagues and I have gathered evidence that children in the one- to two-year age range are able to recall specific events. Within this age period, age is not the major determinant of whether an event will be recalled: Even 13-month-olds recall events after long periods of time. Children demonstrate event memory both nonverbally and verbally. Rather than by age, recall primarily is determined by what the child is asked to remember, the number of exposures to the event, and the availability of cues or reminders of the event. These findings have led to rejection of the traditional assumption of a mnemonically incompetent one- to two-year-old and provided impetus for identification of the factors that maintain the accessibility of early memories over the transition from infancy to early childhood.