This review organizes ideas on the evolution of life histories. The key life-history traits are brood size, size of young, the age distribution of reproductive effort, the interaction of reproductive effort with adutl mortality, and the variation in these traits among an individual's progeny. The general theoretical problem is to predict which combinations of traits will evolve in organisms living in specified circumstances. First consider single traits. Theorists have made the following predictions: (1) Where adult exceeds juvenile mortality, the organism should reproduce only once in its lifetime. Where juvenile exceeds adult mortality, the organism should reproduce several times. (2) Brood size should maximize the number of young surviving to maturity, summed over the lifetime of the parent. But when optimum brood-size varies unpredictably in time, smaller broods should be favored because they decrease the chances of total failure on a given attempt. (3) In expanding populations, selection should minim...

Life-history tactics: a review of the ideas.

Maternal effects in plants were recognized as long ago as 1909 (32). Recent evidence, primarily over the last 15-20 years, shows that maternal effects can contribute substantially to the phenotype of an individual, and as we show, this has important consequences for the interpretation and' design of both ecofogical and genetic studies. Following a discussion of the consequences of maternal effects and an analysis of the different ways these effects can be estimated, we review the evidence for maternal effects from the fields of physiological ecology, crop science, and quantitative genetics. We do not review all of the literature because that would be a monumental task; rather, we focus on representative studies from each of these fields. It is our contention that despite evidence that maternal effects can have a large in­ fluence on offspring phenotype, few detailed studies have identified the specific causes of maternal effects, particularly in natural populations.

Maternal effects in plants

Summary
‘How are plants adapted to the low temperatures and other stresses of arctic and alpine environments ?’ At present it is not possible to answer this question completely. Much work remains to be done, particularly on low-temperature metabolism, frost resistance, and the environmental cues and requirements for flowering, dormancy, regrowth, and germination. However, in brief, we can say that plants are adapted to these severe environments by employing combinations of the following general characteristics:

1. Life form: perennial herb, prostrate shrub, or lichen. Perennial herbs have greatest part of biomass underground.

2. Seed dormancy: generally controlled by environment; seeds can remain dormant for long periods of time at low temperatures since they require temperatures well above freezing for germination.

3. Seedling establishment: rare and very slow; it is often several years before a seedling is safely established.

4. Chlorophyll content: in both alpine and arctic ecosystems not greatly different on a land-area basis from that in temperate herbaceous communities. Within a single species there is more chlorophyll in leaves of arctic populations than in those of alpine populations.

5. Photosynthesis and respiration:

(a) These are at high rates for only a few weeks when temperatures and light are favourable.

(b) Optimum photosynthesis rates are at lower temperatures than for ordinary plants; rates are both genetically and environmentally controlled with phenotypic plasticity very marked.

(c) Dark respiration is higher at all temperatures than for ordinary plants; rate is both genetically and environmentally controlled, with phenotypic plasticity very pronounced, i.e. low-temperature environment increases the rate at all temperatures.

(d) Alpine plants have higher light-saturation values in photosynthesis than do arctic or lowland plants; light saturation closely tied to temperature.

(e) There is some evidence that alpine plants can carry on photosynthesis at lower carbon dioxide concentrations than can other plants.

(f) Annual productivity is low, but daily productivity during growing season can be as high as that of most temperate herbaceous vegetation. Productivity can be increased by temperature, nutrients, or water.

6. Drought resistance: most drought stress in winter in exposed sites is due to frozen soils and dry winds. It is met by decreased water potentials, higher concentrations of soluble carbohydrates, and closed stomates. Little drought resistance in snowbank plants. Alpine plants adapted to summer drought stress can carry on photosynthesis at low water potentials; alpine or arctic plants of moist sites cannot do this.

7. Breaking of dormancy: controlled by mean temperatures near or above 0° C., and in some cases by photoperiod also.

8. Growth: very rapid even at low positive temperatures. Respiration greatly exceeds photosynthesis in early re-growth of perennials. Internal photosynthesis may occur in hollow stems of larger plants during early growth. Nitrogen and phosphorus often limiting in cold soil.

9. Food storage: characteristic of all alpine and arctic plants except annuals. Carbohydrates mostly stored underground in herbaceous perennials. Lipids in old leaves and stems of prostrate evergreen shrubs. Depleted in early growth, and usually restored after flowering.

10. Winter survival: survival and frost resistance are excellent after hardening. Cold resistance closely tied to content of soluble carbohydrates, particularly raffinose.

11. Flowering: flower buds are pre-formed the year before. Complete development and anthesis dependent upon temperature of the flowering year and also, in some cases, upon photoperiod.

12. Pollination: mostly insect-pollinated in alpine regions and even in Arctic, but to a lesser extent. Wind-pollination increasingly more important with increasing latitude. Diptera more important than bees in the Arctic and in the highest mountains.

13. Seed production: opportunistic, and dependent upon temperature during flowering period and latter half of growing season.

14. Vegetative reproduction: by rhizomes, bulbils, or layering. More common and important in Arctic than in alpine areas.

15. Onset of dormancy: triggered by photoperiod, low temperatures, and drought. Dormant plant extremely resistant to low temperatures.

The ecology of arctic and alpine plants

Rapid seed germination and stand establishment are critical factors to crop production under salt‐stress conditions. In many crop species, seed germination and early seedling growth are the most sensitive stages to salinity stress. Salinity may delay the onset, reduce the rate, and increase the dispersion of germination events, leading to reductions in plant growth and final crop yield. The adverse effects of salt‐stress can be alleviated by various measures, including seed priming (a.k.a. pre‐sowing seed treatment). The general purpose of seed priming is to partially hydrate the seed to a point where germination processes are begun but not completed. Most priming treatments involve imbibing seed with restricted amounts of water to allow sufficient hydration and advancement of metabolic processes but preventing germination or loss of desiccation tolerance. Treated seeds are usually redried before use, but they would exhibit rapid germination when re‐imbibed under normal or stress conditions. Various seed priming techniques have been developed, including hydropriming (soaking in water), halopriming (soaking in inorganic salt solutions), osmopriming (soaking in solutions of different organic osmotica), thermopriming (treatment of seed with low or high temperatures), solid matrix priming (treatment of seed with solid matrices), and biopriming (hydration using biological compounds). Each treatment has advantages and disadvantages and may have varying effects depending upon plant species, stage of plant development, concentration/dose of priming agent, and incubation period. In this article, we review, evaluate, and compare effects of various methods of seed priming in improving germination of different plant species under saline and non‐saline conditions. We also discuss the known metabolic and ultra‐structural changes that occur during seed priming and subsequent germination. To maximize the utility of various seed priming techniques, factors affecting their efficiency must be examined and potential benefits and drawbacks determined. For example, quality of the seed before treatment, concentration/dose of priming agent, time period for priming, and storage quality of the seed following priming treatment must be carefully determined. Furthermore, such assessments must be based on large‐scale experiments if seed priming is to be used for large‐scale field planting. A better understanding of the metabolic events that take place in the seed during priming and subsequent germination will improve the effective application of this technology. The incorporation of advanced molecular biology techniques in seed research is vital to the understanding and integration of multiple metabolic processes that can lead to enhanced seed germination, and consequently to improved stand establishment and crop yield under saline and non‐saline conditions.

Pre‐Sowing Seed Treatment—A Shotgun Approach to Improve Germination, Plant Growth, and Crop Yield Under Saline and Non‐Saline Conditions

There has been considerable recent interest in the assessment of the en­ ergy allocation of plants (38, 50, 52). It is likely that through a quantitative understanding of how different plants gain and allocate their resources it will be possible to make predictions as to their success in any given physical envi­ ronment in combination with any competitor and predator. We are still far from this reality. However, as a contribution toward this goal, this review seeks to gather the available. information on the various evolutionary strate­ gies which plants have evolved to gain energy and to account for the numer­ ous ways in which this energy is utilized to meet the demands of the environ­ ment, as well as to successfully produce progeny. To date, studies which have considered allocation have generally been at a rather gross level-e.g., reproductive versus nonreproductive (38), roots versus shoots (117), and photosynthetic versus nonphotosynthetic tissue (86). A study by Harper & Ogden (52) has attempted, however, to account in somewhat more detail for the various sources of energy allocation. To account in more detail for energy gain in allocation and for its even­ tual loss through a variety of routes, the approach that will be used here is the capture of carbon by plants and its SUbsequent diversion into various products which perform a multiplicity of functions. Carbon is the vehicle by which organisms store and transfer energy by chemical binding. This device enables the utilization of a large reservoir of physiological and ecological data into a single coherent body. However, it becomes rather clear that the total information required for any given plant simply does not exist. Thus, this review unfortunately can represent only a composite picture of the gen­ realized routes of carbon movement in plants. It does, however, call attention to the continual partitioning of resources and the multiplicity of carbon path­ ways. Emphasis is placed on the modes of capture of carbon. Hopefully, this analysis will stimulate studies which will provide, for a given series of bio­ types, the total information on carbon gain, use, and loss, so that quantitative models can be derived relating these to shortand long-term environmental influences.

The Carbon Balance of Plants

Provides a comprehensive overview of the physiology, biochemistry and ecology of the process of seed germination. This revised edition includes extended coverage of the influence of molecular biology on seed science and a new chapter on seed technology and propagation.

http://www.gbv.de/dms/bs/toc/025202073.pdf

Alexandra Poljakoff-Mayber

Papers

The germination of seeds