Why small seeds require light to germinate

Seed responses to light can control the timing of germination in the field, impacting seedling survival, as well as growth and fitness in subsequent life stages. Seeds that require light for germination are usually small. Milberg et al. (2000) suggested that a light response and seed mass coevolved as an adaptation to ensure germination of small-seeded species only when close to the soil surface. On the other hand, a phylogenetic component of light-promoted germination – regardless of seed size – has been suggested.

Temperature is a major factor modulating seed responses to light: a seed may require light to germinate at a given temperature but not at other temperatures. For some species, temperature fluctuations can fully or partially substitute for the light requirement. The amplitude of soil temperature fluctuations is highest on or close to the surface of bare soil and in vegetation gaps.

Phytochromes are well known to mediate light-promoted germination; they are also known to increase the amount of bioactive gibberellins in seeds. Exogenously applied gibberellins promote germination of photorequiring seeds in darkness. Conversely, nitrates which are naturally occurring in the soil, can also substitute for the light requirement in some cases.

A recent paper in Annals of Botany aims to determine the effect of light on germination for 131 taxa in the Campanulaceae. The authors tested germination in light and darkness at constant and alternating temperatures; associated the response to light with seed mass and alternating vs. constant temperatures; and examined whether gibberellic acid and nitrate can substitute for the light requirement. They find that the influence of light on germination was much stronger in smaller than in larger seeded species; thus germination is prevented when seeds are buried deep in the soil. Larger seeded species can germinate in deeper soil depths in the presence of fluctuating temperatures.

Koutsovoulou, K., Daws, M.I., & Thanos, C.A. Campanulaceae: a family with small seeds that require light for germination. (2014) Annals of Botany, 113(1), 135-143.
The Campanulaceae is a large cosmopolitan family, but is understudied in terms of germination, and seed biology in general. Small seed mass (usually in the range 10–200 µg) is a noteworthy trait of the family, and having small seeds is commonly associated with a light requirement. Thus, the purpose of this study was to investigate the effect of light on germination in 131 taxa of the Campanulaceae family, from all five continents of its distribution.
For all taxa, seed germination was tested in light (8 or 12 h photoperiod) and continuous darkness under constant and alternating temperatures. For four taxa, the effect of light on germination was examined over a wide range of temperatures on a thermogradient plate, and the possible substitution of the light requirement by gibberellic acid and nitrate was examined in ten taxa.
For all 131 taxa, seed germination was higher in light than in darkness for every temperature tested. Across species, the light requirement decreased significantly with increasing seed mass. For larger seeded species, germination in the dark reached higher levels under alternating than under constant temperatures. Gibberellic acid promoted germination in darkness whereas nitrates partially substituted for a light requirement only in species showing some dark germination.
A light requirement for germination, observed in virtually all taxa examined, constitutes a collective characteristic of the family. It is postulated that smaller seeded taxa might germinate only on the soil surface or at shallow depths, while larger seeded species might additionally germinate when buried in the soil if cued to do so by fluctuating temperatures.