Light and Seed Germination in Diverse Taxa
Light-mediated germination is a feature of a broad range of species with small (<1 mg) seeds and is under the control of phytochrome. However, an unusual phenomenon of dark-mediated dormancy release has been observed in some seeds.
Seed germination triggered by light exposure (positive photoblastism) has been determined in quantitative studies for numerous plant families, including the cacti (Flores et al., 2011). In this family, across 136 species, we have observed that relative light germination (RLG) was lower for seeds from taller than shorter taxa and lower for taxa with heavier seeds than taxa with lighter seeds (Flores et al., 2011). These trends agree with the expectation for small-seeded species to have a light requirement to germinate more often than large-seeded species. For example, the large seeds (13 mg) of the endemic small tree of Central-Eastern Sardinia, Rhamnus persicifolia, are not light responsive (Mattana et al., 2009). However, there can be a seed size – seed number trade-off, as smaller seeds are produced in greater number but have a lower probability of establishment (Daws et al., 2007). We confirmed this expectation for nine neotropical gap-dependent tree species with seed dry mass < 1 mg, as the seeds were photoblastic and had a lower than expected root growth extension rate that contributed to reduced establishment success (Daws et al., 2007).
Phytochrome regulates seed germination of light-sensitive species, with red (R) light generally stimulating and far-red (FR) inhibiting germination. We investigated the dependency of seed germination in the light on seed mass for 27 herbaceous species from northern temperate deciduous forest in the context of the suitability of micro-sites for germination (Jankowska-Blaszczuk and Daws, 2007). We observed that light-dependent germination reduced with increasing seed mass. Moreover, for the light-dependent species, there was a significant negative relationship between the R : FR that resulted in 50% germination and seed mass. Higher R : FR signifies the absence of over-topping vegetation or leaf litter, conditions that could enable seedlings from small seeds to grow relatively rapidly.
Seed dormancy controls the quantity of light required to stimulate germination, thus changes in sensitivity to light can occur over time as a result of changes in dormancy status. However it is only recently that the light environment during dormancy release has been the primary subject of investigation. Following the discovery of a new dormancy release mechanism for Lolium rigidum seeds involving hydrated storage, which circumvents the need for months of dry after-ripening, the light environment has been found to play a pivotal role and a novel role for phytochrome in the inhibition of dormancy release has been described.
Along with seed water content, the light environment can regulate whether or not dormancy release will occur in dormant L. rigidum. Temperature controls the rate at which dormancy release takes place when light quality and water levels are permissive, being faster at warmer temperatures. Dormant seeds (seeds that are unable to germinate in light or darkness) lose their dormancy and become light-sensitive during incubation of hydrated seeds in darkness, thus becoming able to germinate in light (R or white) but not darkness. Incubation in FR light also results in dormancy release, but dormant seeds that are incubated in R or white light remain dormant. Thus, in addition to its role in stimulating germination, phytochrome now has a role in modulating dormancy release.
In summary, light quality appears to have a profound effect on both stimulation of germination and release from dormancy. The focus of this project is to further investigate the control of dormancy by light quality, by describing physiology in conjunction with biochemical/molecular changes in a taxonomic approach.
Project Partners and Collaborators
University of Buenos Aires
Instituto Nacional de Tecnología Agropecuaria
Banco Base de Germoplasma, CIRN-CNIA- INTA, Buenos Aires
Universidad Nacional de Salta
University of Western Australia
University of Queensland
Instituto de Investigaciones Agropecuarias
University of Cagliari, Sardinia
Instituto Potosino de Investigación Científica y Tecnológica, San Luis Potosí
Universidad Nacional Autónoma de México
Universidad Nacional Agraria La Molina, Lima
Pedagogical University of Kielce
University of Aberdeen
Australian Research Council (A$297 k, PI Univ. of Queensland)
Key papers published since 2006:
Goggin, D.E., Powles, S.B., Toorop, P.E. & Steadman, K.J. (2011) Dark-mediated dormancy release in stratified Lolium rigidum seeds is associated with higher activities of cell wall-modifying enzymes and an apparent increase in gibberellin sensitivity. Journal of Plant Physiology 168: 527–533 (IF 2.677).
Flores, J., Jurado, E., Chapa-Vargas, L., Ceroni-Stuva, A., Dávila-Aranda, P., Galíndez, G., Gurvich, D., León-Lobos, P., Ordóñez, C., Ortega-Baes, P., Ramírez-Bullón, N., Sandoval, A., Seal, C.E., Ullian, T. & Pritchard, H.W. (2011). Seed photoblastism and its relationship with some plant traits in 136 cacti taxa. Environmental and Experimental Botany 71: 79-88 (IF 2.699).
Mattana, E., Daws, M.I. & Bacchetta, G. (2009). Effects of temperature, light and pre-chilling on germination of Rhamnus persicifolia, an endemic tree species of Sardinia (Italy). Seed Science and Technology 37: 758-764 (IF 0.631).
Jankowska-Blaszczuk, M. & Daws, M.I. (2007) Impact of red : far red ratios on germination of temperate forest herbs in relation to shade tolerance, seed mass and persistence in the soil. Functional Ecology 21: 1055-1062 (IF 4.65).
Daws, M.I., Ballard, C., Mullins, C.E., Garwood, N.C., Murray, B., Pearson, T.R.H. & Burslem, D.F.R.P. (2007). Allometric relationships between seed mass and seedling characteristics reveal trade-offs for neotropical gap-dependent species. Oecologia 154: 445-454 (IF 3.517).