Breaking of Seed Dormancy by Nitrate as a Gap Detection Mechanism

Taraxacum officinale G. H. Weber ex Wiggers (dandelion, pissenlit officinal) is a perennial weed occurring in parks, gardens, pastures, orchards, roadsides, vegetable gardens, agricultural crops an...

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The biology of Canadian weeds. 117. Taraxacum officinale G. H. Weber ex Wiggers

Summary
1Requirements for dormancy break and seed germination are specific for all species and depend chiefly on phylogeny, geographical distribution, habitat preference and life cycle. Studying germination requirements of closely related species with a similar geographic distribution allows one to attribute variation in germination requirements to differences in habitat preference between the species.
2We investigated requirements for dormancy break and the effect of environmental signals on induction of germination in seeds of four closely related Caryophyllaceae species growing in a variety of habitats (Moehringia trinervia, Stellaria holostea, S. nemorum and S. graminea). The species studied depend on disturbances in the vegetation for seed germination and subsequent seedling establishment.
3Seedlings of all four species emerged both in summer and spring. Stellaria nemorum and M. trinervia, both growing in temperate forests, emerged mainly in summer under a closed forest canopy. Seeds of S. graminea, occurring in grasslands, did germinate in summer at an open site, but could not germinate under a closed forest canopy. Seedlings of S. holostea were observed in late summer when buried at an open site or in early spring when sown in a forest patch.
4Seeds of S. holostea and M. trinervia were completely dormant at dispersal in early summer, while germination was low in fresh seeds of S. graminea and S. nemorum. Dormancy was broken, to a certain extent, during all three after-ripening treatments applied (dry storage, cold and warm stratification).
5The effect of three gap-detection signals (light, fluctuating temperatures, nitrates) on germination of fresh and dry stored seeds was tested. Seeds of S. holostea only germinated in response to daily fluctuating temperatures. Although light was the most important signal affecting germination of S. graminea and M. trinervia, we also observed a positive effect of fluctuating temperatures and nitrates on germination. The effect of fluctuating temperatures on germination of S. nemorum was small in both light and dark incubated seeds. Seed germination in this species generally occurred in response to addition of light and nitrates.
6This study on dormancy breaking and germination requirements of the four species enabled us to expose, sometimes subtle, differences in germination requirements. These contrasting germination patterns were related to differences in the species’ habitat preferences.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2008.01385.x

Environmental signals for seed germination reflect habitat adaptations in four temperate Caryophyllaceae

The temperate grass species, red fescue (Festuca rubra ssp. litoralis), perennial ryegrass (Lolium perenne) and Kentucky bluegrass (Poa pratensis), are often sown in mixtures for turfgrass. Differences in germination response to environmental conditions often result in different establishment success. The three species were compared in a study of imbibition at reduced water potential (Ψ) and in a study of the effects of Ψ and temperature on germination. In all three species, imbibition rate and extent were reduced with decreasing Ψ, causing a prolonged lag-phase before germination, particularly in Kentucky bluegrass. In perennial ryegrass and Kentucky bluegrass, water content at radicle emergence decreased non-linearly with decreasing Ψ, with a lower asymptote expressing the critical water content for germination. Seeds were germinated in factorial combinations of two temperatures (10 and 25°C) and nine Ψ levels (‐2 to 0 MPa). Kentucky bluegrass required a longer thermal time to germinate than the other species and exhibited conditional dormancy at 25°C, which affected the use of the thermal time model. The effects of Ψ were described by a hydrotime model. Kentucky bluegrass had higher base water potential (Ψ b ) and required a longer hydrotime for germination than the other species. However, Ψ b increased with temperature in Kentucky bluegrass and red fescue, but decreased in perennial ryegrass, and the base temperatures also changed with Ψ. The interacting effects of Ψ and temperature affected the ability of the hydrothermal time model to predict germination performance across all temperature and Ψ conditions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0960258504000042

Use of the hydrothermal time model to analyse interacting effects of water and temperature on germination of three grass species

Summary 1 We investigated the effect of light quality, nitrogen supply, fruit size and age on germination of the three types of fruit produced by Atriplex sagittata . 2 Regardless of fruit type, seed germination was significantly reduced in the dark. Light quality had no significant effect on germination of seeds from fruit types A and B, both of which are dormant, but seeds from fruit type C, which are non-dormant, germinated better in green light (low photon flux and red : far-red ratio, simulating conditions in the canopy) than in white light. 3 Optimum nitrate concentration differed between seed types but, for all, germination was reduced at the highest concentration used. 4 The effects of cold stratification and its interaction with light and nitrogen effects were also studied. Strong interactions between stratification and light were found in all seed types but other interactions were less frequent. 5 Three size classes of type C fruit showed similar final germination percentages but large seeds germinated more slowly. 6 The effect of seed age was highly significant: over a 3 year period, the germinability of dormant seeds (types A + B) increased while that of non-dormant seeds (type C) fell. 7 The salt concentration in bracteoles of freshly ripened fruits was too low to mediate an inhibitory effect on germination. Moreover, the first autumn rain would be sufficient to remove all the salt from bracteoles. 8 Differences between fruit types in depth of dormancy, and in response to the quality of light and to nitrate concentration in the soil, allows A . sagittata to respond to the spatial and temporal variation in environmental conditions encountered by this annual of disturbed habitats.

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The effects of light quality, nitrate concentration and presence of bracteoles on germination of different fruit types in the heterocarpous Atriplex sagittata

SUMMARY

The effects of factorial combinations of alternating (20/10 oC) and constant temperature (20 oC), of light (intermittent) and dark, and of distilled water and nitrate on the germination of a range of species of indigenous grassland were investigated in 1979, and in 1980 when the effects of pre-chilling were also studied



Species differed greatly in their response to the eight sets of environmental conditions in 1979 With the exception of Lolium perenne ssp perenne and Cynosurus cristatus, only a small percentage of seeds of most species was able to germinate in constant temperature in the dark However, when light was supplied there was moderate germination of Anthoxanthum odoratum, Cerastium fontanum ssp glabrescens, Festuca rubra, Holcus lanatus and Poa trivialis Alternating temperature greatly increased the germination of most species and nitrate further increased germination of Agrostis capillaris, Deschampsia caespitosa, H lanatus, Poa annua and P trivialis In alternating temperatures, light increased germination of these species even more than did nitrate A capillaris, D caespitosa and P annua required all three factors for maximum germination, and another 7% of seeds of A capillaris also required gibberellic acid Germination responses following sequential application of factors often differed from those resulting from simultaneous treatment: in particular, the germination of Ranunculus species was greatly enhanced



Although seeds of the species tested in 1980 were more germinable than those of the same species in 1979, they responded similarly to the different factors Light increased the germination of both Taraxacum officinale and Plantago lanceolata in constant but not in alternating temperature, while nitrate was much more stimulatory to the latter species About half the seeds of P lanceolata were dormant Pre-chilling at 4oC for 7 days increased subsequent germination of all species when followed by constant temperature, except of A capillaris in the dark and C cristatus (already maximal) in the light When followed by alternating temperature in the dark, pre-chilling greatly decreased germination of A capillaris In the light, where germination of most species was maximal, there was little effect of pre-chilling Longer durations (21 and 42 days) of chilling of older seed gave similar results to the 7 day pre-chilling

E. D. Williams

Papers

Effects of temperature, light, nitrate and pre chilling on seed germination of grassland plants