Improving measurements of the falling trajectory and terminal velocity of wind‐dispersed seeds

Seed dispersal by wind is one of the most important dispersal mechanisms in plants. The key seed trait affecting seed dispersal by wind is the effective terminal velocity (hereafter “terminal velocity”, V _t ), the maximum falling speed of a seed in still air. Accurate estimates of V _t are crucial for predicting intra‐ and interspecific variation in seed dispersal ability. However, existing methods produce biased estimates of V _t for slow‐ or fast‐falling seeds, fragile seeds, and seeds with complex falling trajectories. We present a new video‐based method that estimates the falling trajectory and V _t of wind‐dispersed seeds. The design involves a mirror that enables a camera to simultaneously record a falling seed from two perspectives. Automated image analysis then determines three‐dimensional seed trajectories at high temporal resolution. To these trajectories, we fit a physical model of free fall with air resistance to estimate V _t . We validated this method by comparing the estimated V _t of spheres of different diameters and materials to theoretical expectations and by comparing the estimated V _t of seeds to measurements in a vertical wind tunnel. V _t estimates closely match theoretical expectations for spheres and vertical wind tunnel measurements for seeds. However, our V _t estimates for fast‐falling seeds are markedly higher than those in an existing trait database. This discrepancy seems to arise because previous estimates inadequately accounted for seed acceleration. The presented method yields accurate, efficient, and affordable estimates of the three‐dimensional falling trajectory and terminal velocity for a wide range of seed types. The method should thus advance the understanding and prediction of wind‐driven seed dispersal.     

Effect of water deficits on seed development in soybean : I. Tissue water status

Water deficits during seed filling often decrease seed size in soybean (Glycine max L.). The physiological basis for this response is not known but may result from direct effects of low seed water potential (Ψ_w) on the seed filling process. To determine whether low Ψ_w occurred in reproductive tissues of soybean, we monitored the water status (Ψ_w, Ψ_s, and Ψ_p) of leaf, pericarp, and seed (embryo and testa) tissue of greenhouse-grown plants subjected to a brief water deficit during the linear period of seed growth. Water deficits were imposed by withholding water and monitored in the reproductive tissues by thermocouple psychrometry. When water was abundant, leaf, pericarp, and seed Ψ_w were −0.5 to −0.7 megapascal at midday. When water was withheld, leaf Ψ_w decreased to −2.3 megapascals within 6 days. Pericarp Ψ_w also decreased to −1.9 megapascal during this time. Pericarp Ψ_s followed the decline in Ψ_w, but osmotic adjustment was not evident as the pericarp lost turgor completely by day 6. However, seed Ψ_w, Ψ_s, and Ψ_p were not significantly different from the controls. These results indicate that the water status of the developing seeds of soybean is not altered by short-term water deficits severe enough to inhibit the metabolic activity of the maternal plant. Maintenance of a favorable water status may be important for the conservation of seed growth rate exhibited by soybean under dry conditions.     

Control of Seed Respiration and Growth in Vicia faba by Oxygen and Temperature: No Evidence for an Oxygen Diffusion Barrier

The rate of dry matter accumulation by seeds of Vicia faba L. cv. Minica increases with temperature in the range of 16 to 26°C. The duration of dry matter accumulation decreases with temperature, resulting in a decrease of final seed dry weight. In this study we test the hypothesis that a diffusion barrier for O₂, located in the seed coat, inhibits seed respiration and growth. The rate of O₂ uptake of intact seeds and of excised embryos and seed coats (separated seeds) was measured in air and buffer at 16, 20, and/or 26°C at various O₂ concentrations and developmental stages. Oxygen uptake rates of intact seeds in buffer were only 9 to 15% of those in air. In buffer, the respiration rate of intact seeds decreased at a pO₂ below air saturation (21 kilopascals), whereas separated seeds showed a decline of O₂ uptake only below 80% of air saturation. In air, embryo excision had no effect on the sensitivity of seed respiration to pO₂, at both 20 and 26°C. In air at 20°C, separated and intact seeds showed similar rates of O₂ uptake. Oxygen uptake by intact seeds, both halfway and beyond the linear growth phase, showed a temperature coefficient Q₁₀ of 2.3 and was insensitive to pO₂ in the range of 80 to 100% of ambient. These results indicate that V. faba seed respiration in air is not limited by the diffusion of O₂ into the seed.     

Identification of a single gene for seed coat impermeability in soybean PI 594619

Key message

Inheritance studies and molecular mapping identified a single dominant gene that conditions seed coat impermeability in soybean PI 594619.

Abstract

High temperatures during seed fill increase the occurrence of soybeans with impermeable seed coat, which is associated with non-uniform and delayed germination and emergence. This can be an issue in soybean production areas with excessively high-temperature environments. The objectives of the present study were to investigate the inheritance of impermeable seed coat under a high-temperature environment in the midsouthern United States and to map the gene(s) that affect this trait in a germplasm line with impermeable seed coat (PI 594619). Crosses were made between PI 594619 and an accession with permeable seed coat at Stoneville, MS in 2008. The parental lines and the segregating populations from reciprocal crosses were grown in Stoneville in 2009. Ninety-nine F_2:3 families and parents were also grown at Stoneville, MS in 2011. Seeds were assayed for percent impermeable seed coat using the standard germination test. Genetic analysis of the F₂ populations and F_2:3 families indicated that seed coat impermeability in PI 594619 is controlled by a single major gene, with impermeable seed coat being dominant to permeable seed coat. Molecular mapping positioned this gene on CHR 2 between markers Sat_202 and Satt459. The designation of Isc (impermeable seed coat) for this single gene has been approved by the Soybean Genetics Committee. Selection of the recessive form (isc) may be important in developing cultivars with permeable seed coat for high-heat production environments. The single-gene nature of impermeable seed coat may also have potential for being utilized in reducing seed damage caused by weathering and mold.     

Changes in seed water status as characterized by NMR in developing soybean seed grown under moisture stress conditions

Changes in water status of developing seeds of Soybean (Glycine max L. Merrill.) grown under different moisture stress conditions were characterized by proton nuclear magnetic resonance (NMR)- spin–spin relaxation time (T₂). A comparison of the seed development characteristics, composition and physical properties indicated that, characteristics like seed weight, seed number/ear, rate of seed filling increased with development stages but decreased with moisture stress conditions. The NMR- spin–spin relaxation (T₂) component like bound water increased with seed maturation (40–50%) but decreased with moisture stress conditions (30–40%). The changes in seed water status to increasing levels of moisture stress and seed maturity indicates that moisture stress resulted in more proportion of water to bound state and intermediate state and less proportion of water in free-state. These changes are further corroborated by significant changes in protein and starch contents in seeds under high moisture stress treatments. Thus seed water status during its development is not only affected by development processes but also by moisture stress conditions. This study strongly indicated a clear moisture stress and development stage dependence of seed tissue water status in developing soybean seeds.     

Soybean seed growth in response to long-term exposures to differing oxygen partial pressures

Short-term studies have indicated that alterations in the oxygen partial pressure (pO₂) around developing soybean (Glycine max [L.] Merr.) seeds may alter seed growth characteristics. A 2-year field study was undertaken to determine the effects on seed development of long-term exposures of individual pods to either sub-ambient or supra-ambient pO₂. Pod chambers were used through which fixed pO₂ were continuously flowed throughout seed development. No effects on maturity date were observed from exposures to either sub-ambient or supra-ambient pO₂. On the other hand, seed weight was reduced by 0.10 pO₂ in both years of the study implicating an O₂ limitation on seed growth rate at this fairly high pO₂. In 1 of the 2 years, supra-ambient pO₂ resulted in increased seed weight.     

Plant factors controlling seed set in maize : the influence of silk, pollen, and ear-leaf water status and tassel heat treatment at pollination

In maize (Zea mays L.) large decreases in kernel number result when low water potentials (Ψ_w) and high temperatures occur during pollination. To gain insight into the basis for the decreased seed set, silk, pollen, and ear-leaf Ψ_w, the capability for silk osmotic adjustment, and pollen appearance were measured to determine their relationship to seed set. A multiple-eared or prolific (high carbohydrate availability to the pistillate inflorescence) hybrid (B73 × FR25), a heat sensitive hybrid (WF9 × A632), and a commercial hybrid (B73 × Mo17) were studied. A cross-pollination experiment, with pollination limited by pollen amount, was conducted to determine the impact on seed set of water and heat stressing the tassel and water stressing the ear. At low Ψ_w, silk Ψ_w and seed set were decreased whereas pollen Ψ_w, appearance, and viability were unaffected. High temperature resulted in a 2 megapascal decrease in pollen Ψ_w, visually damaged pollen being shed, decreased pollen viability, and, in two of the hybrids, substantially decreased pollen shed. Prolificacy did not result in increased silk solute accumulation but did result in superior seed production by the pistillate inflorescence at low Ψ_w. The magnitude of the decrease in silk solute potential was small (0.2 megapascal) and similar for all genotypes. One hybrid maintained a relatively high silk turgor but this hybrid also decreased the most in seed production when the pistillate inflorescence was water deficient. These results indicated an adverse effect of high temperature on pollen development, a positive relationship between seed production and silk water status, and no advantage to high silk turgor after silk emergence in maintaining seed production. Additionally, there was no evidence of variation in silk solute regulation capability among hybrids which varied in prolificacy, a trait important in drought tolerance, but the seed production of the pistillate inflorescence of the prolific hybrid was least affected by water deficit.     

Genetic diversity of dispersed seeds is highly variable among leks of the long-wattled umbrellabird

Frugivorous animals frequently generate clumped distributions of seeds away from source trees, but genetic consequences of this phenomenon remain poorly resolved. Seed dispersal of the palm Oenocarpus bataua by long-wattled umbrellabirds Cephalopterus penduliger generates high seed densities in leks (i.e., multi-male display sites), providing a suitable venue to investigate how dispersal by this frugivore may influence seed source diversity and genetic structure at local and landscape levels. We found moderate levels of maternal seed source diversity in primary seed rain across five leks in northwest Ecuador (unweighted mean alpha diversity α = 9.52, weighted mean α_r = 3.52), with considerable variation among leks (α_r range: 1.81–24.55). Qualitatively similar findings were obtained for allelic diversity and heterozygosity. Higher densities of O. bataua adults around leks were associated with higher values of α_r and heterozygosity (non-significant trends) and allelic diversity (significant correlation). Seed source overlap between different leks was not common but did occur at low frequency, providing evidence for long-distance seed dispersal by umbrellabirds into leks. Our findings are consistent with the idea that seed pool diversity within leks may be shaped by the interaction between density of local trees, which can vary considerably between leks, and umbrellabird foraging ecology, particularly a lack of territorial defense of fruiting trees. Taken as a whole, this work adds to our growing appreciation of the ways resource distribution and associated frugivore foraging behaviors mechanistically shape seed dispersal outcomes and the distribution of plant genotypes across the landscape.     

Phosphorus-mediated changes in life history traits of the invasive New Zealand mudsnail (Potamopyrgus antipodarum)

The longer soil has to stay moist to allow germination the more likely that seedlings experience favourable moisture conditions. Since theory predicts that fitness variance-reducing traits will be negatively correlated, we tested the hypothesis that time to germination is negatively correlated with the ability of radicle growth to keep up with the drying front. We measured time to germination and root elongation rate (RER) in 14 Kalahari annuals. We controlled for habitat (canopy association and sand content), germinability, median base water potential for germination (ψ ₅₀), seed mass and seed shape as a persistence surrogate. For species and phylogenetically independent contrasts (PICs), we did not find a relationship between time to germination and RER. However, we found a negative relationship of time to germination with RER for PICs when controlling for sand content and ψ ₅₀. Seed shape increased with time to germination which can be explained by reduced opportunities for germination in slow-germinating species that select for persistence. We found a positive relationship between time to germination and ψ ₅₀, suggesting a continuum of risky to cautious germination. ψ ₅₀ was not correlated with RER suggesting that variation in ψ ₅₀ reflects different drought-adapted traits. Probably the relationship of time to germination with RER is not mediated by seed mass, which was not correlated with time to germination as found by others, though RER was positively correlated with seed mass. Instead of the seed size-seed number trade-off, a trade-off with resource capture may explain variation in RER: more root hairs or rootlets may increase resource capturing surface while reducing RER. For habitat, we found a (positive) relationship with time to germination only for canopy association. This may be explained by maximization of resource capture at the expense of RER being favoured by the higher nutrient and water availability under canopies. Future studies should clarify which trade-offs govern variation in time to germination, focussing on a possible resource capture-RER trade-off.     

Effects of smoke water and karrikin on seed germination of 13 species growing in China

Plant-derived smoke water (SW), derived from combusted plant material, has been shown to stimulate seed germination and improve seedling vigor of a number of plant species from fire-dependent Mediterranean-type climate areas. The effects of SW on seed germination of 13 plant species from southern tropical and subtropical monsoon climate regions of South China are reported for the first time in this study using laboratory and pot trials. Among the 13 species tested, only Aristolochia debilis showed a significant positive response to commercial SW when diluted 1:10. Seed germination of A. debilis was also stimulated by 1–100 nM 3-methyl-2H-furo [2, 3-c] pyran-2-one (karrikin 1 or KAR₁) and by 10–1000 µM gibberellic acid (GA₃). GA₃ stimulated seed germination of Santalum album and significantly elongated the radicles of A. debilis while SW could not. The functions and/or metabolic pathways of Kar₁ and GA₃ are likely to be separate and/or distinct.     

Oleosin isoforms of high and low molecular weights are present in the oil bodies of diverse seed species

Oleosins are unique and major proteins localized on the surface of oil bodies in diverse seed species. We purified five different oleosins (maize [Zea mays L.] KD 16 and KD 18, soybean [Glycine max L.] KD 18 and KD 24, and rapeseed [Brassica campestris L.] KD 20), and raised chicken antibodies against them. These antibodies were used to test for immunological cross-reactivity among oleosins from diverse seed species. Within the same seed species, antibodies raised against one oleosin isoform did not cross-react with the other oleosin isoform (i.e. between maize oleosins KD 16 and KD 18, and between soybean oleosins KD 18 and KD 24). However, the respective antibodies were able to recognize oleosins from other seed species. Where interspecies cross-reactivity occurred, the results suggest that there are at least two immunologically distinct isoforms of oleosins present in diverse seed species, one of lower M_r, and another one of higher M_r. This suggestion is also supported by the relative similarities between the amino acid sequence of a small portion of rapeseed oleosin KD 20 and those of maize oleosins KD 16 and KD 18. In maize kernel, there was a tissue-specific differential presentation of the three oleosins, KD 16, KD 18, and KD 19, in the oil-storing scutellum, embryonic axis, and aleurone layer. The phylogenetic relationship between the high and low M_r isoforms within the same, and among diverse, seed species is discussed.     

The <Emphasis Type="Italic">APX4</Emphasis> locus regulates seed vigor and seedling growth in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

The amino acid sequence of APX4 is similar to other ascorbate peroxidases (APXs), a group of proteins that protect plants from oxidative damage by transferring electrons from ascorbate to detoxify peroxides. In this study, we characterized two apx4 mutant alleles. Translational fusions with GFP indicated APX4 localizes to chloroplasts. Both apx4 mutant alleles formed chlorotic cotyledons with significantly reduced chlorophyll a, chlorophyll b and lutein. Given the homology of APX to ROS-scavenging proteins, this result is consistent with APX4 protecting seedling photosystems from oxidation. The growth of apx4 seedlings was stunted early in seedling development. In addition, APX4 altered seed quality by affecting seed coat formation. While apx4 seed development appeared normal, the seed coat was darker and more permeable than the wild type. In addition, accelerated aging tests showed that apx4 seeds were more sensitive to environmental stress than the wild-type seeds. If APX4 affects seed pigment biosynthesis or reduction, the seed coat color and permeability phenotypes are explained. apx4 mutants had cotyledon chlorosis, increased H₂O₂ accumulation, and reduced soluble APX activity in seedlings. These results indicate that APX4 is involved in the ROS-scavenging process in chloroplasts.     

Genetic Variability of Wild Cherry (Prunus avium L.) Seed Stands in Slovenia as Revealed by Nuclear Microsatellite Loci

Microsatellite markers were used to describe the genetic variability of four seed stands of wild cherry (Prunus avium L.). One hundred and thirty one individuals were genotyped at ten nuclear microsatellite loci. Total genetic diversity was high (H _E = 0.704), while differences between stands were small but significant (F _ST = 0.053, G′ _ST = 0.234). There was a significant amount of clonal reproduction in one stand, with only 11 genotypes identified among 36 trees. One stand showed a significant excess (F _IS = −0.044) of heterozygosity, and one showed a deficit (F _IS = 0.044). Our results demonstrate the importance of taking into account the biological and genetic characteristics of species in forest management, especially when determining a new seed stand. The small genetic differences found between seed stands indicate that a large number of stands are not required. However, they should be carefully selected and should possess adequate genetic variability to ensure low relatedness between seed trees.     

Hydrated mucilage reduces post-dispersal seed removal of a sand desert shrub by ants in a semiarid ecosystem

Post-dispersal seed removal by animals can lead to extensive seed loss and thus is an important factor in structuring plant communities. However, we know much less about post-dispersal seed predation than about other forms of herbivory. Mucilage plays many ecological roles in adaptation of plants to diverse environments; nevertheless, until now the role of mucilage in ant-mediated seed movement remains largely hypothetical. We studied the role of mucilage in seed removal of Artemisia sphaerocephala by ants in Mu Us Sandland in Inner Mongolia, China. Messor aciculatus was the most active seed predator of Artemisia sphaerocephala. Time to first ant collecting (T _1st) of wet intact seeds was longest and significantly different from that for dry intact seeds, wet demucilaged seeds, and dry demucilaged seeds; number of seeds removed to ant nests was lowest for wet intact seeds. After they were collected by ants, 5 % of wet intact seeds were dropped during transport. Our results indicate that seed mucilage of Artemisia sphaerocephala may play a significant role in post-dispersal seed removal by (1) making seeds less attractive to ants, thus resulting in a delay of collection time; (2) forming a strong bond to soil particles, making it difficult for ants to remove seeds; and (3) making seeds more likely to be dropped during transport, thereby allowing them to escape from predation even after collection by ants. This study demonstrates the importance of mucilage in reducing seed removal by ants and thus in anchoring seeds of desert plants in the vicinity of mother plants.     

Effect of variation in self-incompatibility on pollen limitation and inbreeding depression in Flourensia cernua (Asteraceae) scrubs of contrasting density

Background and Aims

Selection may favour a partial or complete loss of self-incompatibility (SI) if it increases the reproductive output of individuals in the presence of low mate availability. The reproductive output of individuals varying in their strength of SI may also be affected by population density via its affect on the spatial structuring and number of S-alleles in populations. Modifiers increasing levels of self-compatibility can be selected when self-compatible individuals receive reproductive compensation by, for example, increasing seed set and/or when they become associated with high fitness genotypes.

Methods

The effect of variation in the strength of SI and scrub density (low versus high) on seed set, seed germination and inbreeding depression in seed germination (δ_germ) was investigated in the partially self-incompatible species Flourensia cernua by analysing data from self-, cross- and open-pollinated florets.

Key Results

Examination of 100 plants in both high and low scrub densities revealed that 51% of plants were strongly self-incompatible and 49 % varied from being self-incompatible to self-compatible. Seed set after hand cross-pollination was higher than after open-pollination for self-incompatible, partially self-incompatible and self-compatible plants but was uniformly low for strongly self-incompatible plants. Strongly self-incompatible and self-incompatible plants exhibited lower seed set, seed germination and multiplicative female fitness (floral display × seed set × seed germination) in open-pollinated florets compared with partially self-incompatible and self-compatible plants. Scrub density also had an effect on seed set and inbreeding depression: in low-density scrubs seed set was higher after open-pollination and δ_germ was lower.

Conclusions

These data suggest that (a) plants suffered outcross pollen limitation, (b) female fitness in partially self-incompatible and self-compatible plants is enhanced by increased mate-compatibility and (c) plants in low-density scrubs received higher quality pollen via open-pollination than plants in high-density scrubs.Key words: Flourensia cernua, population density, seed set, seed germination, female fitness, partial self-incompatibility, Mapimí Biosphere Reserve     

Maternal tissue is involved in stimulant reception by seeds of the parasitic plant Orobanche

Background and Aims

A fundamental element in the evolution of obligate root-parasitic angiosperms is their ability to germinate only in response to chemical stimulation by roots, to ensure contact with a nearby nourishing host. The aim of this study was to explore inheritance of the unique germination control in this group of plants.

Methods

Analysis was made of the segregation of spontaneous (non-induced) germination that appeared in hybrid progenies derived from crosses between Orobanche cernua and O. cumana, which, like all other Orobanche species, are totally dependent on chemical stimulation for the onset of germination, and show negligible spontaneous germination in their natural seed populations.

Key Results and Conclusions

F₁ and F₂ seeds did not germinate in the absence of chemical stimulation, but significant spontaneous germination was found in some F₃ seed families. This indicates that the prevention of non-induced germination in Orobanche seeds, i.e. dependence on an external chemical stimulation for seed germination, is genetically controlled, that this genetic control is expressed in a seed tissue with maternal origin (presumably the perisperm that originates from the nucellus) and that genetic variation for this trait exists in Orobanche species. Similar segregation results were obtained in reciprocal crosses, suggesting that stimulated germination is controlled by nuclear genes.