Soil P resources,plant growth and rooting characteristics in nutrient poor upland grasslands

A field study was undertaken to establish the demand for P by mixed herbage, manipulated by cutting regimes, and the extent to which orthophosphate alone in soil solution could meet this demand from three cambisols derived from different parent materials. Differences in soil types were sufficient to produce significantly different rooting patterns at each site. Yields for 7-and 10-cm treatments generally exceeded those for swards cut to 2-and 4-cm. The highest yields were from plots cut once at the end of the season, or when herbage was cut in June and October only. Yields fell in the second season by an average of 30%. Two cuts in the season resulted in almost twice the P uptake compared with other treatments, leading to the view that a silage cut stimulated root growth. Rooting was deepest in Tarves Association soil (Dystric cambisol), densest in Insch Association soil (Eutric cambisol) and intermediate in Foudland Association soil (Dystric cambisol) but herbage yield at each site was similar. Whole season mean P and N content in roots ranged from 1.0 to 3.4 and from 8.1 to 27.9 mg g^–1 dry weight, respectively. The lowest values were in once cut herbage and were half those in herbage cut in June and October only. Data for the total P resources of the soils, extractable P, and shoot and root P at each site are presented together with data for P in soil solution (principally organic) from an associated soil solution study. There was a disparity between daily uptake and orthophosphate in soil solution. These findings suggested that it was probable that soluble organic forms of P are important for P nutrition in these nutrient poor soils, and could account for the excess of observed P uptake (from soils low in P) over that predicted by mechanistic mathematical models.     

Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L.

From compartmental analysis of radioisotope elutin measurements, fluxes of Ca²⁺ were estimated for cortical cells in root segments of onion, Allium cepa L., relative to complete nutrient solutions containing a range of calcium concentrations ([Ca₀]) from 2 eq l^-1 to 20 meq l^-1, increasing in 10-fold steps for Ca²⁺. Except for the calcium counter-ion (usually NO ₃ ^- , sometimes Cl^- at the highest [Ca₀]), the composition of the nutrient solution was other-wise the same at all calcium concentrations. Compartmental analysis indicated that the cytoplasm had a high content of exchangeable Ca²⁺ but, in the light of evidence from animal studies, ionic activity of calcium in the cytoplasm was assumed to be no greater than 0.002 eq ml^-1. With the Ussing-Teorell flux equation as the criterion, it was concluded that at all values of [Ca₀] tested, Ca²⁺ entered the cytoplasm passively and was actively pumped back into the external solution. Entry of calcium to the vacuole from the cytoplasm was active in all cases. The conclusions regarding the character of ion transport across the plasmalemma were the same as when the whole calcium content of the cytoplasm was taken to contribute to the ionic activity. However, the electrochemical activity gradient was very much steeper than formerly estimated. Calcium was transported to the stele in proportion to the calcium content of the cytoplasm and moved in the xylem almost exclusively in the basipetal direction.     

Uptake and transport of phosphorus by Agrostis capillaris seedlings from rapidly hydrolysed organic sources extracted from32 P-labelled bacterial cultures

Cultures of the soil bacterium Serratia liquifaciens grimesii were grown with³² P labelled phosphate, to produce a uniformly³² P labelled source of microbial P. Extracts of the bacteria were prepared by sonication, dialysis and filtration to provide a clear sterile solution which was characterised in terms of dissolved organic and condensed P (DOP and DCP) and molecular weight range. The extract was used as a source of P to Agrostis capillaris L. seedlings in nutrient solution from which orthophosphate was omitted. In a time course experiment, root surface phosphatase activity increased as soon as extract was added to the root medium, DOP was rapidly hydrolysed and orthophosphate concentration increased rapidly. These processes were complete within about 8 h, after which phosphatase activity fell to its original level, and the plants absorbed molybdate reactive P from the nutrient solution so that it reached its original concentration over 48 h. DCP concentrations did not change significantly throughout the experiment. This work clearly demonstrated that DOP but not DCP, as a component of a bacterial extract produced by a relatively straightforward method, was quickly hydrolysed and the P made available for plant uptake.     

Active and passive transport of potassium in cells of excised pea epicotyls

The Ussing-Theorell equation, which provides a fundamental test for the independent passive movement of ions under conditions of nonequilibrium, has been used to assess the active and passive components of K⁺ uptake by segments of pea epicotyl (Pisum sativum L. cultivar Alaska), incubated for 24 hours in both 1-fold and 10-fold concentrations of a complete nutrient solution. Measurements of the rates at which ⁴²K diffused out of the segments provided data from which were estimated the K⁺ content of, and the fluxes to and from, the nonfree space compartments, interpreted as being cytoplasm and vacuole. For this analysis the serial model of MacRobbie and Dainty and Pitman for the spatial arrangement of cell compartments was used. On the basis of these values, and measurements of electrical potential across the cell membranes, the vacuolar K⁺ concentration was found to be fairly close to that expected as a result of passive diffusion between the cytoplasm and vacuole provided that no potential exists across the tonoplast. Cytoplasmic K⁺ concentration, however, was much too high in both treatments to be accounted for in passive terms. It was concluded, therefore, that, on the basis of the model, the high ratio of influx to efflux was maintained in the cells by an active K⁺ pump located at the plasmalemma. There is some reason to question the applicability of this model for flux analysis to the conditions of high net influx as encountered here; nonetheless, it provides a first approach to an over-all flux analysis in pea stem tissue.     

Natural Selection of Human Embryos: Decidualizing Endometrial Stromal Cells Serve as Sensors of Embryo Quality upon Implantation

Background

Pregnancy is widely viewed as dependent upon an intimate dialogue, mediated by locally secreted factors between a developmentally competent embryo and a receptive endometrium. Reproductive success in humans is however limited, largely because of the high prevalence of chromosomally abnormal preimplantation embryos. Moreover, the transient period of endometrial receptivity in humans uniquely coincides with differentiation of endometrial stromal cells (ESCs) into highly specialized decidual cells, which in the absence of pregnancy invariably triggers menstruation. The role of cyclic decidualization of the endometrium in the implantation process and the nature of the decidual cytokines and growth factors that mediate the crosstalk with the embryo are unknown.

Methodology/Principal Findings

We employed a human co-culture model, consisting of decidualizing ESCs and single hatched blastocysts, to identify the soluble factors involved in implantation. Over the 3-day co-culture period, approximately 75% of embryos arrested whereas the remainder showed normal development. The levels of 14 implantation factors secreted by the stromal cells were determined by multiplex immunoassay. Surprisingly, the presence of a developing embryo had no significant effect on decidual secretions, apart from a modest reduction in IL-5 levels. In contrast, arresting embryos triggered a strong response, characterized by selective inhibition of IL-1β, -6, -10, -17, -18, eotaxin, and HB-EGF secretion. Co-cultures were repeated with undifferentiated ESCs but none of the secreted cytokines were affected by the presence of a developing or arresting embryo.

Conclusions

Human ESCs become biosensors of embryo quality upon differentiation into decidual cells. In view of the high incidence of gross chromosomal errors in human preimplantation embryos, cyclic decidualization followed by menstrual shedding may represent a mechanism of natural embryo selection that limits maternal investment in developmentally impaired pregnancies.     

Natural Selection of Human Embryos: Impaired Decidualization of Endometrium Disables Embryo-Maternal Interactions and Causes Recurrent Pregnancy Loss

Background

Recurrent pregnancy loss (RPL), defined as 3 or more consecutive miscarriages, is widely attributed either to repeated chromosomal instability in the conceptus or to uterine factors that are poorly defined. We tested the hypothesis that abnormal cyclic differentiation of endometrial stromal cells (ESCs) into specialized decidual cells predisposes to RPL, based on the observation that this process may not only be indispensable for placenta formation in pregnancy but also for embryo recognition and selection at time of implantation.

Methodology/Principal Findings

Analysis of mid-secretory endometrial biopsies demonstrated that RPL is associated with decreased expression of the decidual marker prolactin (PRL) but increased levels of prokineticin-1 (PROK1), a cytokine that promotes implantation. These in vivo findings were entirely recapitulated when ESCs were purified from patients with and without a history of RPL and decidualized in culture. In addition to attenuated PRL production and prolonged and enhanced PROK1 expression, RPL was further associated with a complete dysregulation of both markers upon treatment of ESC cultures with human chorionic gonadotropin, a glycoprotein hormone abundantly expressed by the implanting embryo. We postulated that impaired embryo recognition and selection would clinically be associated with increased fecundity, defined by short time-to-pregnancy (TTP) intervals. Woman-based analysis of the mean and mode TTP in a cohort of 560 RPL patients showed that 40% can be considered “superfertile”, defined by a mean TTP of 3 months or less.

Conclusions

Impaired cyclic decidualization of the endometrium facilitates implantation yet predisposes to subsequent pregnancy failure by disabling natural embryo selection and by disrupting the maternal responses to embryonic signals. These findings suggest a novel pathological pathway that unifies maternal and embryonic causes of RPL.     

Phosphate fluxes, compartmentation and vacuolar speciation in root cortex cells of intact Agrostis capillaris seedlings: effect of non-toxic levels of aluminium

From compartmental analysis of ³²P elution measurements, concentrationsand fluxes of orthophosphate were estimated for root corticalcells of intact seedlings of the indigenous grass Agrostis capillarisL. cv. Highland when in complete nutrient solution containing10 mmol m^–3 or 100 mmol m^–3 phosphate with or without3.7 and 37 mmol m^–3 Al, respectively, during loading andelution. When plotted as counts min^–1 remaining in thetissue as a function of time, the data failed to meet the criteriafor first order kinetics. Transformation of the data to meetthe kinetic criteria gave corrected values for compartmentalconcentrations and fluxes of phosphate, and estimates of theotherwise unresolved slowly exchanging compartment within thevacuole considered to be the cause of the discrepancy in fluxanalysis. In the control, the discrepancy was considered dueto sequestration of phosphate with Ca in the vacuole and a small,but not significant, increase in sequestered P occurred in thepresence of Al, the presence of which was confirmed by X-raymicroanalysis. A chemical speciation model was used to demonstrate,for various values of pH and carboxylic acid concentration,the possibility that phosphate was precipitated in root cellvacuoles as Ca and Al phosphates (hydroxy-apatite and variscite).The ecological significance of the ability of A. capillarisplants to make efficient use of scarce P resources by minimizingthe sequestration of P by Al in vacuoles, compared with Loliumperenne, was recognized. Key words: Phosphate sequestration, phosphate use efficiency, X-ray microanalysis, super-saturation     

Aspects of ovarian follicle development throughout life

The pool of primordial follicles present in the female ovary reaches its maximum number around 20 weeks of gestational age and then decreases in a logarithmic fashion throughout life until complete depletion occurs around the age of the menopause. Reproductive life is initiated when less than 10% (0.5 million) of primordial follicles are left. The entire growth trajectory of the follicle takes at least 3 months. Follicle growth up to the antral stage occurs during fetal life and infancy. While the role of gonadotropins in early follicular development remains controversial, the last 2 weeks of development are FSH dependent. The intercycle rise in FSH and decreasing levels thereafter are crucial for recruitment of a cohort of healthy, early antral follicles and subsequent single dominant selection. Following puberty, anovulation may persist for years and this may presage the development of adult anovulatory infertility. The menopause is preceded by a period of reduced fertility. The development of reliable and sensitive markers for ovarian ageing will be the challenge of the near future.     

Light-enhanced Chloride Uptake by Wheat Laminae: A Comparison of Chopped and Vacuum-infiltrated Tissue

The time course of (36)Cl(-) uptake from 5 mm KCl by 1.5-mm leaf segments of Triticum aestivum L. seedlings has been determined over 24 hours both in the light and in the dark. A light-enhanced uptake of Cl(-) develops after a few hours. Using whole laminae which have been water-injected by vacuum infiltration, a light-enhanced uptake is apparent from zero time. Uptake values achieved in the light by the two types of tissue are similar but in the dark there is a restricted uptake by the whole infiltrated laminae. It is considered that the slower uptake by whole laminae relative to chopped tissue in the dark is due to cuticular resistance to solute penetration, whereas in the light the impediment is overcome by stomatal opening. A light-enhanced uptake unrelated to stomatal opening is discernible in both tissues. Its energetic basis has not been defined but may be related to substrate exhaustion. The absorption mechanism is not impaired by vacuum infiltration.