Photoreceptors and photopigments in a subterranean rodent,the pocket gopher (<Emphasis Type="Italic">Thomomys bottae</Emphasis>)

Pocket gophers (Thomomys bottae) are rodents that spend much of their lives in near-lightless subterranean burrows. The visual adaptations associated with this extreme environment were investigated by making anatomical observations of retinal organization and by recording retinal responses to photic stimulation. The size of the eye is within the normal range for rodents, the lens transmits light well down into the ultraviolet, and the retina conforms to the normal mammalian plan. Electroretinogram recording revealed the presence of three types of photopigments, a rod pigment with a spectral peak of about 495 nm and two types of cone pigment with respective peak values of about 367 nm (UV) and 505 nm (medium-wavelength sensitive). Both in terms of responsivity to lights varying in temporal frequency and in response recovery following intense light adaptation, the cone responses of the pocket gopher are similar to those of other rodents. Labeling experiments indicate an abundance of cones that reach densities in excess of 30,000 mm^–2. Cones containing UV opsin are found throughout the retina, but those containing medium-wavelength sensitive opsin are mostly restricted to the dorsal retina where coexpression of the two photopigments is apparently the rule. Rod densities are lower than those typical for nocturnal mammals.     

Survivorship of penstemon grandiflorus in an oak woodland: combined effects of fire and pocket gophers

Summary The effects of fire and pocket gophers, Geomys bursarius, on the survivorship of Penstemon grandiflorus growing in an oak woodland in Minnesota were studied from 1986 to 1990. Plants growing in sparse vegetation experienced mortality rates twice that of plants growing in dense vegetation. This difference was due partly to pocket gophers whose earth moving activities reduce the density of vegetation and bury and kill individual Penstemon plants. Laboratory feeding trials showed that gophers readily eat Penstemon, particularly the fleshy roots. An experiment involving the removal of 25–75% of the root tissue in 90 plants showed that root loss significantly reduced survivorship, suggesting that gopher herbivory might also kill plants. When gophers were experimentally excluded, plants growing in sparse vegetation exhibited significantly lower mortality rates than those growing in dense vegetation. Plants in the smallest size class exhibited reduced survivorship following a late spring burn; however, overall patterns of survivorship of plants in burned areas did not differ markedly from those in the unburned areas. A longitudenal analysis of plants with different reproductive histories revealed no survivorship cost to reproduction. Mortality rates decreased with increasing plant size. Small plants were more likely to be killed by fire and by being buried under gopher mounds. Differences in underground energy reserves of small and large plants can account for most of the survivorship patterns observed in this study. The study shows that within openings of the oak woodland, fire and gophers reduce the survival of individual Penstemon plants. Nevertheless, since both gophers and fire also serve to perpetuate suitable habitat in the woodland, Penstemon is ultimately dependent on both for its long term persistence in the landscape.     

Cophylogeny and disparate rates of evolution in sympatric lineages of chewing lice on pocket gophers

Although molecular-based phylogenetic studies of hosts and parasites are increasingly common in the literature, no study to date has examined two congeneric lineages of parasites that live in sympatry on the same lineage of hosts. This study examines phylogenetic relationships among chewing lice (Phthiraptera: Trichodectidae) of the Geomydoecus coronadoi and Geomydoecus mexicanus species complexes and compares these to phylogenetic patterns in their hosts (pocket gophers of the rodent family Geomyidae). Sympatry of congeneric lice provides a natural experiment to test the hypothesis that closely related lineages of parasites will respond similarly to the same host. Sequence data from the mitochondrial COI and the nuclear EF-1alpha genes confirm that the two louse complexes are reciprocally monophyletic and that individual clades within each species complex parasitize a different species of pocket gopher. Phylogenetic comparisons reveal that both louse complexes show a significant pattern of cophylogeny with their hosts. Comparisons of rates of nucleotide substitution at 4-fold degenerate sites in the COI gene indicate that both groups of lice have significantly higher basal mutation rates than their hosts. The two groups of lice have similar basal rates of mutation, but lice of the G. coronadoi complex show significantly elevated rates of nucleotide substitution at all sites. These rate differences are hypothesized to result from population-level phenomena, such as effective population size, founder effects, and drift, that influence rates of nucleotide substitution.     

Soil calcium and plant disease in serpentine ecosystems: a test of the pathogen refuge hypothesis

Ecologists have long sought mechanistic explanations for the patterns of plant distribution and endemism associated with serpentine soils. We conducted the first empirical test of the serpentine pathogen refuge hypothesis, which posits that the low levels of calcium found in serpentine soils provide associated plants with a refuge from attack by pathogens. We measured the range of soil calcium concentrations experienced by 16 wild population of California dwarf flax (Hesperolinon californicum) and experimentally recreated part of this range in the greenhouse by soaking serpentine soils in calcium chloride solutions of varying molarity. When flax plants grown in these soils were inoculated with spores of the rust fungus Melampsora lini we found a significant negative relationship between infection rates and soil calcium concentrations. This result refutes the pathogen refuge hypothesis and suggests that serpentine plants, by virtue of their association with low calcium soils, may be highly vulnerable to attack by pathogens. This interaction between plant nutrition and disease may in part explain demographic patterns associated with serpentine plant populations and suggests scenarios for the evolution of life history traits and the distribution of genetic resistance to infection in serpentine plant communities.     

Soil and plant water relations in a crested wheatgrass pasture: response to spring grazing by cattle

Summary Few field studies have attempted to relate effects of actual livestock grazing on soil and plant water status. The present study was initiated to determine the effects of periodic defoliations by cattle during spring on soil moisture and plant water status in a crested wheatgrass (Agropyron cristatum (L.) Gaertn. and A. desertorum (Fisch. ex Link) Schult.) pasture in central Utah. Soil moisture in the top 130 cm of the soil profile was depleted more rapidly in ungrazed plots than in grazed plots during spring and early summer. Soil moisture depletion was more rapid in grazed plots in one paddock after 1 July due to differential regrowth, but there was no difference in soil water depletion between plots in another paddock during the same period. This difference in soil water depletion between paddocks was related to a difference in date of grazing. Although more water had been extracted from the 60 cm to 130 cm depths in ungrazed plots by late September, cumulative soil moisture depletion over the entire 193 cm profile was similar in grazed and ungrazed plots. Prior to 1 July, grazing had no effect on predawn leaf water potentials as estimated by a pressure chamber technique; however, after 1 July, predawn leaf water potentials were lower for ungrazed plants. Midday leaf water potentials were lower for grazed plants before 1 July, but did not differ between grazed and ungrazed plants after 1 July. A 4- to 8-day difference in date of defoliation did not affect either predawn or midday leaf water potentials. The observed differences in water use patterns during spring and early-summer may be important in influencing growth and competitive interactions in crested wheatgrass communities that are subject to grazing by domestic livestock.     

Boron translocation in coffee trees

Boron deficiency in coffee trees (Coffea arabica) is widespread, however, responses to B fertilizer have been erratic, depending on the year, method, and time of application. A better understanding of B uptake, distribution, and remobilization within the plant is important in developing a rational fertilization program. Field and greenhouse experiments were conducted to study B distribution and remobilization in coffee trees. Boron was provided either in the nutrient solution or sprayed on the leaves of trees grown under adequate or transient B deficiency. There was clear evidence for B translocation via symplast (remobilization) to coffee grains, even in well-nourished plants. When ¹⁰B was present in the nutrient solution during most part of fruit filling, from 33 to 40% of the B found in coffee fruits was absorbed during this period, depending on the timing and duration of the B deficiency treatment. In the field, when B was sprayed once on the leaves, around 4% of the fruit B was derived from the foliar fertilizer. Boron remobilization within coffee trees is limited in well nourished plants, but it can be significant during periods of temporary B deficiency in plants otherwise well nourished with B. The implications of these findings for B fertilization practice, are discussed.     

Soil nutrient status after bamboo flowering and death in a seasonal tropical forest in western Thailand

We have examined the surface (0–10 cm) soil characteristics of sites after bamboo (Cephalostachyum pergracile) mass flowering and death (DB sites) in comparison with sites with living bamboo (Bambusa tulda) (LB sites) in a seasonal tropical forest in Thailand. One year after bamboo flowering the DB sites were acidic with lower concentrations of exchangeable Ca and Mg and soil nitrogen than the LB sites. Therefore, although leaf and root litter of the dead bamboo was deposited in the DB sites after bamboo flowering, soil nutrient status decreased.     

Degradation of Fumonisin B1 by a Bacterial Strain Isolated from Soil

A mixed microbial culture degrading fumonisin B l was obtained from soil samples using an enrichment culture procedure. A bacterial isolate from the enrichment culture (strain NCB 1492) degraded fumonisin B1 after incubation for 3 h, as indicated by TLC and HPLC analysis. On the basis of the sequence analysis of 16S rDNA, strain NCB 1492 was related to the Delftia/Comamonas group. Thin-layer chromatographic analysis indicated the presence of metabolites in the NCB 1492 culture filtrates after degradation of fumonisin B1 supplied as sole carbon and nitrogen source in phosphate buffer. Four metabolites were identified by mass spectrometry analysis.     

连作改变土壤性状对甜叶菊产量和品质的影响

为了解连作障碍的产生机理,对甜叶菊(Stevia rebaudiana)连作后的土壤性状变化进行了研究,并探讨土壤性状与叶片干质量和甜菊糖苷之间的相关性。结果表明,连作2 a和3 a的土壤pH值、有机质、速效磷、脲酶、过氧化氢酶、蔗糖酶、磷酸酶和甜叶菊叶片干质量及甜菊糖苷组分含量均无显著差异。连作4 a后,土壤pH值、全氮和速效钾含量显著下降,分别比对照降低了10.07%、14.38%和24.79%,土壤电导率(EC)和速效磷含量显著增加,是对照的2.57和1.70倍;土壤脲酶、蔗糖酶、磷酸酶活性、微生物量碳和微生物量氮在连作4 a降到最低,比对照分别降低了63.68%、72.03%、47.43%、78.35%和41.07;多酚氧化酶则在连作4a达到最高,是对照的4.22倍;与对照相比,连作4a的叶片干质量和甜菊苷含量降低了29.51%和16.00%,莱鲍迪苷A含量则增加了22.19%。叶片干质量及甜菊糖苷含量与土壤性状间存在着相关性。因此,连作通过改变土壤性状影响甜叶菊产量和品质,生产中最大连作年限不宜超过3 a。     

Soil H2-uptake in relation to soil properties and rhizobial H2-production

Summary The biological nature of soil H₂-consumption has been investigated. Soil microorganisms were capable to remove H₂ present in the gas phase at concentrations in the range of 200 ppm at rates varying between 0.2 and 1.0 l.min^–1. 100 g^–1. Free soil enzymes did not contribute significantly at the H₂ concentrations tested. Oxygen seemed to be the predominant electron acceptor. The influence of microbiological and physical soil properties on the H₂-uptake activity was examined for 38 soils.A highly significant correlation between biomass-C and H₂-uptake rate of the soil was noted, suggesting that the latter parameter might be useful as an indirect estimation of soil microbial biomass. The correlation was however not applicable for soils recently grown with legumes. Indeed, soya plants nodulated with aRhizobium strain with a weak hydrogen uptake capability, strongly increased the hydrogen oxidizing capability of the surrounding soil.     

Soil acidity and nutrient deficiency in central Amazonian heath forest soils

Experiments were carried out to test the effects of liming and nutrient additions on plant growth and soil processes such as C and N mineralisation in three contrasting forest types in central Amazonia: the stunted facies of heath forest (SHF), the tall facies of heath forest (THF) and the surrounding lowland evergreen rain forest (LERF). Calcium-carbonate additions increased soil respiration in the field plots in the SHF; in laboratory incubations, soil respiration was higher in the SHF when soils were fertilised with N, and in THF and LERF after S additions. The addition of N alone or in different combinations generally induced a net immobilisation of soil N. Net nitrification increased during the incubation in SHF and THF soils fertilised with N+P, and in LERF soils fertilised with either N, or P, or CaCO₃. In a field experiment using ingrowth bags, a higher fine root production was observed in all forest types when bags were fertilised with CaCl₂ or CaCO₃, suggesting that Ca may be a limiting nutrient in these soils. Calcium-carbonate addition in a glasshouse bioassay experiment with rice showed an overall positive effect on the survival and growth of the seedlings. In other treatments where soil pH was not raised, the rice showed acute toxicity symptoms, poor root and shoot growth and high mortality. Similar results were yielded in a field experiment, using naturally established seedlings in the field plots in SHF, THF and LERF. It is concluded that the acute H⁺ ion toxicity is a major growth-limiting factor for non-adapted plants in heath forest soils in central Amazonia.     

Modification of the properties of a translocation in the German cockroach by selection

Summary A stock of Blattella germanica bearing the interchange T(3; 12)/3;12 was subjected to close inbreeding with selection for random disjunction at metaphase I. After 3–4 generations of selection, interchange quadrivalent chiasma frequency decreased, variability in free bivalent chiasma frequency increased sharply, and individuals with either random or directed disjunction were present in the stock. Random disjunction was modified from a ratio of 2112 (adj.-1; alt.-1; adj.-2; alt.-2) to a ratio of 1111. After 7–8 generations of selection, chiasma frequency appeared to stabilize at lower than normal levels and variability decreased for both quadrivalents and free bivalents. Directed disjunction was modified from a ratio of 2114 to 1112, and no individuals with the original high level of directed disjunction were detected. Chains-of-four tended to orient randomly, especially in individuals where the ring quadrivalents showed directed disjunction. Relaxation of inbreeding, but not selection, produced an increase in chiasma frequency and variability in both free bivalents and quadrivalents, but the modified ratios for both random and directed disjunction were retained. These results are discussed with respect to inbreeding effects and genetic control of chiasma frequency and metaphase I disjunction in interchange quadrivalents.     

Soil modification by invasive plants: effects on native and invasive species of mixed-grass prairies

Invasive plants are capable of modifying attributes of soil to facilitate further invasion by conspecifics and other invasive species. We assessed this capability in three important plant invaders of grasslands in the Great Plains region of North America: leafy spurge (Euphorbia esula), smooth brome (Bromus inermis) and crested wheatgrass (Agropyron cristatum). In a glasshouse, these three invasives or a group of native species were grown separately through three cycles of growth and soil conditioning in both steam-pasteurized and non-pasteurized soils, after which we assessed seedling growth in these soils. Two of the three invasive species, Bromus and Agropyron, exhibited significant self-facilitation via soil modification. Bromus and Agropyron also had significant facilitative effects on other invasives via soil modification, while Euphorbia had significant antagonistic effects on the other invasives. Both Agropyron and Euphorbia consistently suppressed growth of two of three native forbs, while three native grasses were generally less affected. Almost all intra- and interspecific effects of invasive soil conditioning were dependent upon presence of soil biota from field sites where these species were successful invaders. Overall, these results suggest that that invasive modification of soil microbiota can facilitate plant invasion directly or via ‘cross-facilitation’ of other invasive species, and moreover has potential to impede restoration of native communities after removal of an invasive species. However, certain native species that are relatively insensitive to altered soil biota (as we observed in the case of the forb Linum lewisii and the native grasses), may be valuable as ‘nurse’species in restoration efforts.     

Are boreal ecosystems susceptible to alien plant invasion? Evidence from protected areas

Although biological invasion by alien species is a major contributor to loss of indigenous biological diversity, few studies have examined the susceptibility of the boreal biome to invasion. Based on studies of other ecosystems, we hypothesized that alien plants will be restricted to disturbed areas near human activity and will not be found in natural areas of boreal ecosystems in Gros Morne National Park (Canada), a protected area experiencing a wide range of disturbance regimes. The distribution of alien plants in the region was evaluated using surveys, and study sites were established in naturally and anthropogenically disturbed habitats that had been invaded. Within study sites, randomization tests evaluated the importance of disturbance to alien plant invasion by examining changes in environmental conditions and species abundance within various disturbance regimes, while the importance of site characteristics limiting the distribution of alien plants were examined using Canonical Correspondence Analysis. Consistent with studies in a variety of biomes, areas of high disturbance and human activity had the greatest abundance of resources and the highest percentage of alien species. However, contrary to our hypothesis, natural areas of boreal ecosystems were found susceptible to alien plant invasion. Vegetation types vulnerable to invasion include forests, riparian areas, fens, and alpine meadows. Natural disturbance occurring in these vegetation types caused increases in bare ground and/or light availability facilitating alien plant invasion. Although high soil pH was associated with alien plants in these areas, disturbance was not found to cause changes in soil pH, suggesting susceptibility to invasion is pre-determined by bedrock geology or other factors influencing soil pH. Moose (Alces alces), a non-native herbivore, acts as the primary conduit for alien plant invasion in GMNP by dispersing propagules and creating or prolonging disturbance by trampling and browsing vegetation. The recurrent nature of disturbance within the boreal biome and its interaction with site conditions and herbivores enables alien plants to persist away from areas of high human activity. Managers of natural lands should monitor such interactions to decrease the invasion potential of alien plants.     

Soil solution phosphate,root uptake kinetics and nutrient acquisition: implications for a patchy soil environment

Summary The importance of increased root phosphate (P) uptake kinetics, root proliferation and local increases of soil solution P (P₁) for P acquisition from fertile soil microsites was explored with a simulation model and calculated uptake was compared with experimental data. Based on the partitioning of added P in microsites to P₁ and P adsorbed on soil particles and the results of a dual-isotope-labeling experiment (Caldwell et al. 1991a), acquisition of P from the fertile microsites was some 20 X that of uptake from an equal volume of soil which received only water. Simulations were in general agreement and also showed that elevation of root P uptake kinetics could contribute more to the increased acquisition than did root proliferation under these circumstances. Although increased physiological uptake capacity for P has generally been considered to be of little benefit because of diffusion limitation, in patchy soil environments selective elevation of P uptake kinetics in fertile microsites may be of considerable benefit. These tests were conducted in calcareous soil which releases much less P into the soil solution than do many other soils. In many noncalcareous soils the benefits of selective elevation of root uptake kinetics would likely be greater.     

Pocket gophers (Geomys bursarius), vegetation,and soil nitrogen along a successional sere in east central Minnesota

Summary Insect size tactics or developmental strategies are discussed in relation to decisions individuals make about when to mature. Such decisions carry with them costs and benefits in terms of that individual's reproductive success. Whenever size affects fitness, selection should act such that individuals evaluate the costs and benefits due to changes in size and should mature when the ratio of benefit to cost is maximized.Predictions about seasonal changes in adult sizes within a population are tested on two species of mole cricket, Scapteriscus acletus and vicinus. Specifically, individuals maturing in the fall should be larger than average because there is no cost associated with delayed reproduction since reproduction occurs only during spring months. Smaller than average individuals should remain in juvenile stages and get larger before reproducing. Also it is predicted that as the spring reproductive season progresses a greater proportion of smaller individuals should mature because the costs due to delaying reproduction increase. The changes in seasonal distribution of adult sizes of mole crickets support the predictions and suggest that individuals make decisions about when to mature based on costs and benefits associated with changes in size.     

5种相思树和尾巨桉人工林土壤养分和酶活性特征

为揭示固氮树种土壤养分转化的酶学机制,对固氮树种[厚荚相思(Acacia crassicarpa)、黑木相思(A. melanoxylon)、卷荚相思(A.cincinnata)、大叶相思(A.auriculiformis)和马占相思(A.mangium)]及非固氮树种尾巨桉(Eucalyptusurophylla×E.grandis)人工林的土壤养分含量、酶活性及其相关性进行研究。结果表明,相思林40～60cm土层的pH均高于尾巨桉林;5种相思林土壤各土层的TP、TK含量均低于尾巨桉林,而20～40 cm土层的TC、TN含量均高于尾巨桉林,黑木相思林和马占相思林各土层的有效养分均显著高于尾巨桉林(P0.05)。0～10 cm土层中,相思林的土壤酸性磷酸酶和纤维素酶活性均高于尾巨桉林,大叶相思林的土壤脲酶、蔗糖酶、纤维素酶和芳基硫酸酯酶活性显著高于尾巨桉林(P0.05),卷荚相思林的土壤脲酶、纤维素酶、几丁质酶和淀粉酶活性显著高于尾巨桉林(P0.05)。相关分析结果表明,土壤脲酶、蔗糖酶和几丁质酶活性与AP显著负相关(P0.05),蔗糖酶和纤维素酶活性与NH4+-N显著负相关(P 0.05),脲酶、纤维素酶、芳基硫酸酯酶与土壤TK显著负相关(P0.05),几丁质酶活性与TN含量呈显著正相关(P0.05),土壤淀粉酶活性与NH4+-N呈显著正相关(P 0.05),过氧化氢酶活性与土壤TK含量呈显著正相关。可见,与尾巨桉人工林相比,在我国南方退化山地引种相思树可提高土壤关键酶的活性,对土壤有效养分具有明显改良作用,有利于退化地土壤的生态修复及人工林长期生产力的维持。     

Local and regional differences in abundance of co-dominant grasses in the shortgrass steppe: a modeling analysis of potential causes

The shortgrass steppe is co-dominated by two C₄perennial grasses, Bouteloua gracilis andBuchloë dactyloides. At our site and throughouteastern Colorado Bouteloua gracilis has higher cover andbiomass than Buchloë dactyloides. We hypothesizedthatthe interaction between seedling recruitment differences and disturbance regimeand tolerance to drought conditions were the most likely causes of the observeddifferences in relative abundances. We used a simulation model to investigatethe relative importance of the three factors. We studied plant biomass of thesetwo species in 18 simulated treatments that resulted from a factorialcombination of seedling recruitment differences, disturbance regime, andtolerance to drought conditions. Analysis of the simulation outputs with ANOVAindicated that biomass of each species responded positively to increases inrecruitment probability. Increasing disturbance frequency favoredBuchloë dactyloides over Boutelouagracills, whereas the susceptibility Buchloëdactyloides to drought favored Boutelouagracilis. The results indicated that differences in droughttoleranceand seedling recruitment probabilities along with their interactions withdisturbance regimes exert a major control on the biomass of the species. In theabsence of or with intermediate disturbance, a higher recruitment probabilityand greater tolerance to drought of Bouteloua gracilisthanBuchloë dactyloides yielded patterns of relativebiomass similar to the patterns observed in the shortgrass steppe.     

Phloem translocation of gibberellins in three species of higher plants

Phloem sap was collected from white lupin (Lupinus albus L.), cowpea (Vigna unguiculata L.) and castor bean (Ricinus communis L.) and analysed for gibberellins (GAs) using gas chromatography-mass spectrometry (GC-MS). A large number of GAs were found in the phloem exudate of all three species, particularly where the sap was collected from pods (white lupin and cowpea) and in these legumes GAs representing both the early C-13-hydroxylation and non-hydroxylation pathways of biosynthesis were identified. In the sap collected from the vegetative tissues of castor bean the number of GAs identified was fewer than that in the other species, representing mainly the non-hydroxylation pathway. Data from sap collected from the pedicel and stylar ends of pods and by making feeds of radiolabelled GAs to seeds in situ in white lupin indicate that the GAs present in the phloem are derived mainly from the vegetative tissues of the plant. No evidence for metabolism of GAs in the phloem could be found.