Influence of compaction from wheel traffic and tillage on arbuscular mycorrhizae infection and nutrient uptake by Zea mays

Interactive effects of seven years of compaction due to wheel traffic and tillage on root density, formation of arbuscular mycorrhizae, above-ground biomass, nutrient uptake and yield of corn (Zea mays L.) were measured on a coastal plain soil in eastern Alabama, USA. Tillage and soil compaction treatments initiated in 1987 were: 1) soil compaction from tractor traffic with conventional tillage (C,CT), 2) no soil compaction from tractor traffic with conventional tillage (NC,CT), 3) soil compaction from tractor traffic with no-tillage (C,NT), and, 4) no soil compaction from tractor traffic with no-tillage (NC,NT). The study was arranged as a split plot design with compaction from wheel traffic as main plots and tillage as subplots. The experiment had four replications. In May (49 days after planting) and June, (79 days after planting), root biomass and root biomass infected with arbuscular mycorrhizae was higher in treatments that received the NC,NT treatment than the other three treatments. In June and July (109 days after planting), corn plants that received C,CT treatment had less above-ground biomass, root biomass and root biomass infected with mycorrhizae than the other three treatments. Within compacted treatments, plants that received no-tillage had greater root biomass and root biomass infected with mycorrhizae in May and June than plants that received conventional tillage. Corn plants in no-tillage treatments had higher root biomass and root biomass infected with mycorrhizae than those in conventional tillage. After 7 years of treatment on a sandy Southeastern soil, the interactive effects of tillage and compaction from wheel traffic reduced root biomass and root biomass infected with mycorrhizae but did not affect plant nutrient concentration and yield. ei]J H Graham     

A diazotrophic bacterial endophyte isolated from stems of Zea mays L. and Zea luxurians Iltis and Doebley

The apoplastic fluids of field-grown Zea mays and Zea luxurians plants were isolated from surface sterilized stem tissue by centrifugation and spread on agar plates containing a nitrogen-free, defined medium. The predominant bacterium isolated from these plates was characterized further. The ability of this bacterium to fix nitrogen was confirmed by its ability to grow on a semi-solid, nitrogen-free medium and reduce ¹⁵N₂ to ¹⁵NH₃ and acetylene to ethylene. Protions of the nifH and 16S rRNA genes from this organism were amplified by PCR and sequenced. The nifH gene, which codes for dinitrogenase reductase, from this organism is closely related to nifH from Klebsiella pneumoniae. Similarly, the 16S rRNA gene sequences and carbon utilization tests grouped it closely with K. pneumoniae. Based an these data, the isolates from Z. mays and Z. luxurians are tentatively classified as Klebsiella spp. (Zea). The ability of this bacterium to contribute to the nitrogen economy of the corn plant is unknown.     

Sperm competition and sex allocation in simultaneous hermaphrodites

Summary Sex allocation theory is developed for hermaphrodites having frequent copulations and long-term sperm storage. Provided the sperm displacement mechanisms are similar to those known in insects, the ESS allocation to sperm versus eggs satisfies a rather simple rule. There are no data to test this rule, as yet.     

Optimal flower lifetimes

Summary ESS floral lifetimes satisfy the product theorem from sex allocation theory. The dimensionless time investment per flower is a symmetric function of two dimensionless gain : cost ratios, one for each gender function.     

Use of low temperature sem to locate free water in frozen,hydrated seed tissues of Glycine max (Leguminosae)

Low temperature field emission electron microscopy was used to determine the location of free water in soybean seeds. Frozen, hydrated soybean seeds were fractured, the water etched from the fractured surface, and then part of the etched surface was refractured. The resulting surface, which contained a freeze-fractured face as well as a freeze-etched face was coated with platinum and viewed on the cryostage of a low temperature field emission electron microscope. Two surfaces could be viewed simultaneously to determine the location of water in the seed tissue. Viewing the fractured surface gave an indication of the extent of hydration of the tissue. Viewing the etched surface detailed the macro- and microanatomy of the tissue. Viewing the intersection between the fractured and etched surfaces allowed observation of the environment of partially etched cells and organelles. The technique avoids artifacts associated with chemical fixation, dehydration, and critical-point drying, procedures that affect the water content of the seed. The technique does not affect the degree of hydration of the seed and can be used to localize water in the inter- and intracellular environment of the seed. This technique could find wide application in studies of water relationships of seeds during development, maturation, and imbibition.     

Effects of light,moisture, temperature,and litter on the regeneration of five tree species in the tropical montane wet forest of Puerto Rico

Field experiments were conducted to investigate the effects of light, moisture, temperature, and litter on the regeneration of two early-, one mid-, and two late-successional tropical tree species. High light and litter seem to be universally good cues for regeneration, increasing seed/seedling survival for all species except for Cecropia (an early-successional species) whose small seeds may not be able to penetrate the litter layer. In addition, the high temperature environment in both artificially shaded and nonshaded areas of a natural gap exhibits less seed loss, an increase in the percent and rate of germination, and an increase in seedling survival for Dacryodes (a late-successional species), than the lower temperature environment under an intact canopy. Low soil water is also a good cue for Dacryodes germination as it is for Prestoea and Cecropia. Finally, the lower temperature environment found under the forest canopy (compared to the natural gap) leads to less seed loss and more germination for Guarea (a mid-successional species). Our results suggest that a good patch for regeneration of many species in this forest, early- as well as late-successional species, would have high light and a litter layer that moderates temperature and moisture extremes. The substantial variation in suitability among regeneration filters and species could: (1) contribute to low establishment success, i.e., most dispersed propagules do not become trees, (2) make it difficult to group species into germination strategies, and (3) make it hard to generalize about a net effect of any specific environmental variable on establishment. We suggest that tropical disturbances should be viewed in terms of their impact on a variety of environmental cues, which may signal germination and impact subsequent growth and survival.