The significance of small herbivores in structuring annual grassland

Question: Herbivores can play a fundamental role in regulating the composition and structure of terrestrial plant communities. Relatively inconspicuous but nevertheless ubiquitous gastropods and small mammals are usually considered to influence grassland communities through distinct modes. 1. Do terrestrial gastropods and small mammals, either alone or in combination, influence plant community composition of an intact annual grassland? 2. Do these herbivores influence the plant size structure of the dominant grass Avena? Location: Jasper Ridge Biological Preserve (37°24’ N, 122° 13’ W, elevation 150 m) in northern California. Methods: Animal exclosures were used to examine the single and combined influences of these herbivores on annual grassland production, community composition, and plant size structure during the growing season of an intact annual grassland. Results: The removal and exclusion of the herbivores increased the prevalence of grasses relative to legumes and non‐legume forbs; increased total production of above‐ground plant biomass; and increased mean plant size and exacerbated size hierarchies in populations of Avena. The effect of both gastropods and small mammals, alone and in combination, was characterized by temporal oscillations in the relative dominance of grasses in plots with vs. without herbivores. Conclusions: Both groups of herbivores are important controllers of California annual grassland that exert similar influences on production and composition. While other factors appear to determine the absolute number of individuals in this plant community, selective consumption of grasses by gastropods and small mammals partially offsets the competitive advantages associated with their early germination.     

Grass seed infection following inundation with Pyrenophora semeniperda

Seed infection by the seed-borne pathogen Pyrenophora semeniperda was demonstrated following inundative applications of the fungus to several annual grass weeds and wheat. The optimum time for applying inoculum was found to be around mid-anthesis in wheat. Applications of inoculum during and after anthesis in field experiments resulted in seed infection that manifested as stromatal development which affected germination or reduced seedling vigour. In an initial field experiment, >70% of Bromus diandrus seeds exhibited stromata of P. semeniperda. In a field comparison of inoculum types, conidial suspensions resulted in the greatest level of seed infection of B. diandrus compared with several types of inoculum containing mycelium fragments. Inundation of Avena fatua, Lolium rigidum, Hordeum leporinum, Vulpia bromoides and B. diandrus under field conditions with conidia of P. semeniperda also resulted in either the failure of infected seeds to germinate or a reduction of seedling vigour. It is concluded that the use of P. semeniperda as a seed-borne bioherbicide may be a biologically reasonable tactic, and the many logistical and technological constraints impeding its development are discussed.     

Site-directed monoclonal antibodies against the amino terminus of 124-kDa phytochrome from Avena sativa L.

Abstract Two monoclonal antibodies (AFRC MAC 184 and 185) have been raised in rats against a synthetic octadecapeptide corresponding to the N-terminus of Avena phytochrome. The peptide was conjugated to tuberculin purified protein derivative (PPD) for immunization and the cell lines screened by ELISA using the free peptide. Both antibodies bind to intact 124-kDa phytochrome on Western blots and in a double antibody sandwich ELISA. In the ELISA, they have an approximately four-fold higher affinity for Pr than Pfr. Conformational changes during photoconversion, therefore, involve the extreme N-terminus of the phytochrome molecule.     

Regulation of ethylene biosynthesis during senescence of oat leaf segments

The evolution of endogenous ethylene, the conversion of 1-aminocylopropane-1-car-boxylic acid (ACC) to ethylene and the amounts of ACC (free and conjugated) have been followed during the senescence of oat ( Avena sativa L. cv. Victory) leaf segments. During the first three days of incubation of leaf segments in darkness, endogenous ethylene evolution and ACC-dependent ethylene production displayed a close relationship, both showing an increase followed by a decrease to the basal rate. However, unlike ethylene production, the level of ACC increased during the five days of incubation in the dark without any decline. It is concluded that ACC synthesis does not limit ethylene production, at least in the last stages of leaf senescence when ethylene production markedly decreased. The level of conjugated ACC increased and reached a plateau already at the first day of incubation. Yet, at the progressive stages of senescence, when the level af ACC gradually increased, no further conjugation of ACC could be detected. Thus, conjugation of ACC cannot account for ethylene drop at the last stages of oat leaf senescence.     

The absorption and translocation of diclofop-methyl and amitrole in wheat and oat roots

The absorption and translocation of diclofop-methyl (methyl 2-[4(2',4'-dichlorophenoxy)phenoxy]propanoate) was examined by using a specially designed treatment apparatus that separated excised roots or roots of seedlings into four zones. [¹⁴C]-Diclofop-methyl was absorbed along the entire root length of both wheat ( Triticum aestivum L.) and oat ( Avena sativa L.). In both species, absorption was greatest in the apical region of the root. Absorption by the apical region of wheat roots was more than three times greater than the basal portions, and more than twice as great as the apical region of oat roots. Less than 5% of the absorbed diclofop-methyl was translocated in both wheat and oat roots. Diclofop-methyl and diclofop(2-[4(2',4'-dichlorophenoxy)phenoxy]propanoic acid) were the predominant translocated forms. The absorption and translocation of amitrole (3-amino-1,2,4-triazole) were also examined. Amitrole was absorbed along the entire length of wheat roots and translocated primatily in the basipetal direction. The usefulness of the specially designed apparatus for biochemical and physiological studies is discussed.     

Bulking generated considerable bias in detection of amplified fragment length polymorphism variations in oat,fringed brome and smooth bromegrass

Amplified fragment length polymorphism (AFLP) variations in bulk and plant‐by‐plant (PBP) samples of five oat (Avena sativa L.) accessions, one fringed brome (Bromus ciliatus L.) accession and two smooth bromegrass (B. inermis Leyss.) accessions were compared. The proportions of AFLP bands detected in PBP, but lost in bulk, samples of oat, fringed brome, and smooth bromegrass ranged from 19 to 31%, 40 to 44%, and 22 to 33% of the total bands scored, respectively. These lost bands had occurred at frequencies ranging from 0.1 to 1 in the PBP samples. These findings demonstrate bulking can generate substantial bias in detection of AFLP variations.     

The adaptive significance of drought escape in Avena barbata, an annual grass

Drought strongly influences plant productivity, suggesting that water limitation has shaped the evolution of many plant physiological traits. One functional strategy that plants employ to cope with decreasing water availability is drought escape. For drought-escaping species, high metabolic activity (gas exchange) and rapid growth are hypothesized to confer a fitness advantage, because this enables a plant to complete its life cycle before the most intense period of drought. By growing an annual grass species (Avena barbata) under well-watered or water-limited conditions in a greenhouse, we directly tested whether high photosynthesis, increased stomatal opening, and early flowering are adaptive under drought. We measured phenotypic selection on instantaneous gas exchange and flowering time as well as the underlying biochemical traits that regulate photosynthesis. We found strong selection for earlier flowering in the dry environment, but no evidence that increased photosynthesis was adaptive under drought. Photosynthetic rate (A) and stomatal conductance (gs) were both adaptively neutral in the dry environment. Increased photosynthetic capacity (Amax) was maladaptive in the dry environment, perhaps because of the respiratory cost associated with maintaining excess enzyme and substrate capacity. There was no correlational selection on the combination of physiology and flowering time in the dry environment, suggesting that accelerated development and high gas exchange may not need to be tightly linked to promote drought escape. In contrast, there was selection for both high photosynthetic function (Amax and A) and early flowering in the well-watered environment. These combinations of traits may have been favored because they maximize both energy and time available for reproduction. Our results suggest that the benefit of increased photosynthesis for plant fitness may be strongest in the absence of drought stress.     

Digestibility of whole-crop barley and oat silages in dairy heifers

This study evaluated the digestibility of whole-crop cereal silage (WCCS) made from oats and six-rowed barley harvested at the heading, early milk and early dough stages, and two-rowed barley harvested at the early milk and early dough stages of maturity. The eight WCCSs were fed to 32 Swedish Red heifers in a changeover design over three periods of 28 days each. The heifers were first fed ad libitum for 17 days and then at 0.95 of ad libitum for 11 days of each period. During the last 5 days all faeces and orts were collected to determine the digestibility of the silages. Only the maturity stage effect was significant for the WCCS organic matter (OM) digestibility and the average OM digestibility was higher at the heading stage (698 g/kg) than at early milk (647 g/kg) and early dough (652 g/kg) stages of maturity. For neutral detergent fibre (NDF) digestibility the crop × maturity stage effect was significant. The NDF digestibility decreased from the heading to the early milk stage for both six-rowed barley (746 to 607 g/kg) and oats (698 to 596 g/kg). There was no further significant decrease in NDF digestibility for six-rowed barley at the early dough stage (577 g/kg), but for two-rowed barley it decreased from the early milk (682 g/kg) to the early dough (573 g/kg) stage, and also for oats the NDF digestibility was lowest at the early dough stage (507 g/kg). The decrease in NDF digestibility during maturation was to a large extent compensated by an increase in starch concentration in the crops. The starch digestibility was lower for six-rowed barley at early dough stage (948 g/kg) than at early milk stage (977 g/kg), and was also lower compared with oats (979 g/kg) at early dough stage. The average crude protein (CP) digestibility was higher at the heading (646 g/kg) and the early milk (642 g/kg) stages than at the early dough stage (599 g/kg), and oats had higher average CP digestibility (650 g/kg) than six-rowed (613 g/kg) and two-rowed (624 g/kg) barley. Delaying the harvest of WCCS from the heading to the early milk and dough stage of maturity will decrease the OM digestibility; as a result there is a decreased NDF digestibility.