Prior hydration of Brassica tournefortii seeds reduces the stimulatory effect of karrikinolide on germination and increases seed sensitivity to abscisic acid

Background and Aims

The smoke-derived compound karrikinolide (KAR₁) shows significant potential as a trigger for the synchronous germination of seeds in a variety of plant-management contexts, from weed seeds in paddocks, to native seeds when restoring degraded lands. Understanding how KAR₁ interacts with seed physiology is a necessary precursor to the development of the compound as an efficient and effective management tool. This study tested the ability of KAR₁ to stimulate germination of seeds of the global agronomic weed Brassica tournefortii, at different hydration states, to gain insight into how the timing of KAR₁ applications in the field should be managed relative to rain events.

Methods

Seeds of B. tournefortii were brought to five different hydration states [equilibrated at 15 % relative humidity (RH), 47 % RH, 96 % RH, fully imbibed, or re-dried to 15 % RH following maximum imbibition] then exposed to 1 nm or 1 µm KAR₁ for one of five durations (3 min, 1 h, 24 h, 14 d or no exposure).

Key Results

Dry seeds with no history of imbibition were the most sensitive to KAR₁; sensitivity was lower in seeds that were fully imbibed or fully imbibed then re-dried. In addition, reduced sensitivity to KAR₁ was associated with an increased sensitivity to exogenously applied abscisic acid (ABA).

Conclusions

Seed water content and history of imbibition were found to significantly influence whether seeds germinate in response to KAR₁. To optimize the germination response of seeds, KAR₁ should be applied to dry seeds, when sensitivity to ABA is minimized.     

A scientific approach to agent selection

Abstract  The prioritisation of potential agents on the basis of likely efficacy is an important step in biological control because it can increase the probability of a successful biocontrol program, and reduce risks and costs. In this introductory paper we define success in biological control, review how agent selection has been approached historically, and outline the approach to agent selection that underpins the structure of this special issue on agent selection. Developing criteria by which to judge the success of a biocontrol agent (or program) provides the basis for agent selection decisions. Criteria will depend on the weed, on the ecological and management context in which that weed occurs, and on the negative impacts that biocontrol is seeking to redress. Predicting which potential agents are most likely to be successful poses enormous scientific challenges. 'Rules of thumb', 'scoring systems' and various conceptual and quantitative modelling approaches have been proposed to aid agent selection. However, most attempts have met with limited success due to the diversity and complexity of the systems in question. This special issue presents a series of papers that deconstruct the question of agent choice with the aim of progressively improving the success rate of biological control. Specifically they ask: (i) what potential agents are available and what should we know about them? (ii) what type, timing and degree of damage is required to achieve success? and (iii) which potential agent will reach the necessary density, at the right time, to exert the required damage in the target environment?     

IS MACROEVOLUTION MORE THAN SUCCESSIVE ROUNDS OF MICROEVOLUTION?

Abstract:  Whether macrovolution is reducible to microevolution is one of the persistent debates in evolutionary biology. Although the concept of emergence is important to answering this question, it has not been extensively discussed within palaeobiology. A taxonomy of emergence concepts is presented to clarify the ways in which emergence relates to this debate. Weak emergence is a particularly helpful way to understand the hierarchical nature of biology: it captures the ways in which higher-level traits depend on lower-level processes, while recognizing that emergent traits can nonetheless provide the basis for autonomous higher-level theories. A brief review of the biological literature suggests that geographical range size is weakly emergent. While some concepts of emergence do not block the attempt to reduce macroevolution (i.e. the attempt to explain all macroevolutionary phenomena in terms of microevolutionary processes), weak emergence does. Thus, if geographical range is weakly emergent, it provides a basis for arguing that macroevolutionary phenomena cannot be fully explained by microevolutionary processes.     

Spectroscopic and biochemical analysis of regions of the cell wall of the unicellular 'mannan weed', Acetabularia acetabulum

Although the Dasycladalean alga Acetabularia acetabulum has long been known to contain mannan-rich walls, it is not known to what extent wall composition varies as a function of the elaborate cellular differentiation of this cell, nor has it been determined what other polysaccharides accompany the mannans. Cell walls were prepared from rhizoids, stalks, hairs, hair scars, apical septa, gametophores and gametangia, subjected to nuclear magnetic resonance and Fourier transform infrared spectroscopy, and analyzed for monosaccharide composition and linkage, although material limitations prevented some cell regions from being analyzed by some of the methods. In diplophase, walls contain a para-crystalline mannan, with other polysaccharides accounting for 10-20% of the wall mass; in haplophase, gametangia have a cellulosic wall, with mannans and other polymers representing about a quarter of the mass. In the walls of the diplophase, the mannan appears less crystalline than typical of cellulose. The walls of both diploid and haploid phases contain little if any xyloglucan or pectic polysaccharides, but appear to contain small amounts of a homorhamnan, galactomannans and glucogalactomannans, and branched xylans. These ancillary polysaccharides are approximately as abundant in the cellulose-rich gametangia as in the mannan-rich diplophase. In the diplophase, different regions of the cell differ modestly but reproducibly in the composition of the cell wall. These results suggest unique cell wall architecture for the mannan-rich cell walls of the Dasycladales.     

Effects of forest management intensity on carbon and nitrogen content in different soil size fractions of a North Florida Spodosol

Pine plantations of the southeastern USA are regional carbon (C) sinks. In spite of large increases in woody biomass due to advanced growing systems, studies have shown little or even negative effects on the C content of the extremely sandy soils of this region. Hence, it is important to understand the mechanisms that determine the impact of intensive forest management on soil organic carbon (SOC) sequestration. This study was conducted to examine the C profile in a 4-year-old loblolly pine (Pinus taeda L.) plantation managed under two levels of management intensity (chemical understory control and fertilizer inputs). Soil organic C and nitrogen (N) pools were evaluated using two size fractionation methods, dry and wet sieving (2000–250 μm, 250–150 μm, 150–53 μm and <53 μm). Dry sieving was preferred over wet sieving for soil size fractionation, as it preserved more structure and water-soluble SOC components such as esters and amides and did not affect the N distribution. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were used to examine the chemical composition of the size fractions, which showed the presence of recently added organic matter in the largest sand fraction, as well as more decomposed organic matter in the <53 μm fraction. Intensive forest management reduced SOC in all three 2000–53 μm fractions, most likely due to reduced root input of understory plants that were controlled using herbicides. The 2000–250 μm fractions contained nearly half of the total SOC and showed a 23% decrease in C content due to the intensive management regime. Results from this study indicated the significance and responsiveness of sand size SOC fractions in Florida Spodosols. Results also showed that reductions in SOC due to intensive management occurred after four years and highlighted the need to understand the long-term impacts and the mechanisms responsible. Responsible Editor: Barbara Wick     

Distribution of flower morphs, ploidy level and sexual reproduction of the invasive weed Oxalis pes-caprae in the western area of the Mediterranean region

Background and Aims: Oxalis pes-caprae is a widespread invasive weed in regions witha Mediterranean climate. In its native habitat (southern Africa)this species has been reported as heterostylous with trimorphicflowers and a self- and morph-incompatible reproductive system.In most of the areas invaded, only a pentaploid short-styledmorphotype that reproduces mainly asexually by bulbils is reported,but this has only been confirmed empirically. This study aimsto analyse the floral morph proportions in a wide distributionarea, test the sexual female success, and explain the causesof low sexual reproduction of this species in the western areaof the Mediterranean Basin. Methods: Fifty-five populations of O. pes-caprae were sampled in theIberian Peninsula and Morocco to evaluate the floral morph ratioand individual fruit set. In plants from a dimorphic population,hand-pollination experiments were performed to evaluate theeffect of the pollen source on pollen tube growth through thestyle. The ploidy level and genome size of individuals of eachfloral morph were analysed using flow cytometry. Key Results: From the populations studied 89·1 % were monomorphic,with most of them containing the short-styled (SS) floral morph,and 10·9 % were dimorphic containing long-styled(LS) and SS morphs. In some of these, isoplethy was verifiedbut no fruit production was observed in any population. A sterileform was also recorded in several populations. Hand-pollinationexperiments revealed that pollen grains germinated over recipientstigmas. In intermorph crossings, pollen tubes were able todevelop and fruit initiation was observed in some cases, whilein intramorph pollinations, pollen tube development was sporadicand no fruit initiation was observed. All individuals withineach floral form presented the same DNA ploidy level: SS plantswere pentaploid and LS and the sterile form were tetraploid. Conclusions: The low or null sexual reproduction success of this speciesin the area of invasion studied seems related with the highfrequency of monomorphic populations, the unequal proportionof floral morphs in dimorphic populations and the presence ofdifferent ploidy levels between SS and LS morphs. The discoveryof the occurrence of an LS floral morph and a sterile form,whose invading capacity in these areas is as yet unknown, willbe valuable information for management programmes.     

施肥对苗期紫茎泽兰和黑麦草相对竞争力的影响

为种植黑麦草替代控制紫茎泽兰提供依据,运用取代实验法,研究了施肥(氮、磷)对苗期紫茎泽兰和黑麦草的相对竞争力以及生长表现的影响。结果表明:增施氮肥和磷肥均能够提高紫茎泽兰的相对竞争力,而仅磷肥对黑麦草的竞争力略有促进作用;但在各种施肥水平下,黑麦草的竞争力仍然明显强于紫茎泽兰。增施氮肥可以显著提高紫茎泽兰的株高、分枝数和干质量,而磷肥仅在较高时才显著提高其干质量;增施氮肥虽然可以提高黑麦草的分蘖数量,但对其干质量没有影响,而增施磷肥(2次)可以显著提高其干质量;在竞争中紫茎泽兰植株生长的能量分配(用根茎比表示)对磷肥不敏感。建议适当增施磷肥、不施或少施氮肥,以提高黑麦草的替代控制效果。