Relative Canopy Height Influences Wild Oat (Avena fatua) Seed Viability, Dormancy, and Germination

The environment in which a plant grows (maternal environment) can affect seed viability, germination, and dormancy. We assessed the effects of maternal environment on wild oat seed viability, germination, dormancy, and pathogen infection by collecting and analyzing wild oat seed from above and below a barley canopy at three field sites in Montana. The viability of wild oat seed collected below a crop canopy was consistently less than it was for seed from the overstory but varied among sites and years. Reductions in viability because of relative canopy position ranged from 10% to 30%. Effects of position relative to crop canopy on weed seed germination/dormancy rates varied by site and suggest that the direction and magnitude of the effects of maternal environment on dormancy depend on environmental conditions. These effects may be driven by crop competition or by changes in seed pathogen pressure or both. Seven species each of fungi and bacteria were isolated from wild oat seeds. The only fungi causing reductions in seed viability (15%) was isolated from understory seeds, and several bacteria from both overstory and understory sources reduced seed germination. Results suggest that, in addition to the known weed-suppressive effects of using taller or earlier emerging varieties of crops, such crops can reduce weed spread through effects on weed seed demography because weeds growing beneath the crop canopy produce a reduced amount of viable seed that is less likely to germinate in the following year.

On the use of fungicides in ecological seed burial studies

Evidence for effects of saprophytic fungi on buried seed demography is usually obtained from studies involving the simultaneous burial of fungicide-treated seeds and of untreated seeds. However, any potential influence of fungicide treatment on seed dormancy levels is generally ignored in these studies. Also, some studies assume that a combination of several fungicidal compounds provides better protection against a broader range of fungi, ignoring chemical interactions that may potentially occur between different compounds. To investigate these issues, we carried out a 6-month burial experiment using seeds ofAnthriscus sylvestris(L.) Hoffm.,Centaurea nigraL. andDaucus carotaL., and three substrates differing in organic matter content. Three fungicidal compounds, captan, iprodione and mancozeb, were applied alone and in combination, including an untreated control. All fungicidal compounds and combinations thereof provided protection against fungal-induced seed mortality and, except for a low efficacy of iprodione in protecting seeds ofAnthriscus, there were no pronounced differences in seed mortality between different fungicide treatments. Captan temporarily inhibited germination inCentaurea, whereas a similar inhibition inDaucusseeds caused by mancozeb was more long lasting, suggesting an induction of secondary dormancy. Organic matter content had only a negligible influence on these results. Our results suggest that the basic conclusions from most seed burial studies are robust with respect to their choice of fungicide. We conclude by discussing further implications of our findings for the design and interpretation of seed burial studies.

Seed demography

"Some seeds fell by the way side, … some fell upon stony places,... and some fell among thorns;.... but other fell into good ground" (The Bible, St. Matthew's gospel, Chapt. 13).Much of modern plant ecology is concerned with demography. One major topic is the study of mortality rates. A common belief, originating with Charles Darwin's Origin of Species is that the seedling stage of the plant's life cycle exhibits the highest mortality rates. Evidence is provided in this paper to show that the highest rates of mortality for many species occur at the seed stage. The major causes of mortality associated with five major functions served by seeds are discussed. These functions enable the population to (i) invade new areas, (ii) occupy all possible microsites within a habitat, (iii) survive unfavorable periods, (iv) avoid predators and pathogens, and (v) recruit new genotypes which will be more successful in growth, survival, and reproduction than their predecessors. Seed mortality, somewhat paradoxically, has an integral role in each of these functions. The discussion includes a consideration of mortality at all stages of the seed's life history from formation through to germination.