Colder temperatures better for long-term maize seed storage

Seed banking of agricultural genetic resources such as crop landraces is crucial if agriculture is to adapt to changing climate conditions and human needs in the coming decades. As a globally important food crop, maize and its wild relatives have been heavily seed-banked to protect their genetic diversity. At CIMMYT (the International Maize and Wheat Improvement Center), which is the largest seedbank in the world, based in Mexico, more than 28,000 maize seed accessions are stored. Many of these are in long-term storage, but relatively little is known about variation among accessions in cold-storage longevity, a fundamental piece of information if all are to be successfully protected and preserved.

In a new article published in Annals of Botany, lead author Filippo Guzzon and colleagues undertook the largest ever study of maize seed longevity under seed bank cold storage conditions. Using both germination experiments and historical monitoring data, the researchers studied 987 seeds of five different maize types (dent, floury, flint, popcorn, and sweet corn), that had been in storage at CIMMYT for an average of 48 years, and up to 60 years in some cases. The seeds came from two different storage conditions: active storage at -3°C, meant for short-term storage and distribution, and base storage at -15°C, meant for long-term conservation.

The authors found that while seed accessions conserved at the lower base temperature had an average of 92.1% germination, those stored at the higher active temperature had only 81.4% germination, and a higher variation in viability. The different accessions varied in their response to cold storage, and those with shorter than average viability at one temperature were likely to behave similarly at the other temperature. While accessions of sweet and popcorn varieties were too few for full analysis, results showed that grain type was a major factor in longevity, with flint-type seeds outlasting both floury- and dent-type seeds.

These results suggest both that both base and active storage should be kept at the lower (-15°C) temperature, and that accessions would benefit from individual testing of their cold storage longevity, with this data being used to inform regeneration intervals for the seeds, which are currently around 15-20 years for all seed types in active storage.

“This [finding] raises doubts about the current strategy, employed by several international seedbanks, of conserving the same accessions in two chambers with different temperature conditions,” write the authors. “Our data indicate that base chamber conditions (cold storage between -15 and -20°C) should be preferred for the conservation of long-term collections of maize seeds, [and] also for “active” seed lots (intended for regeneration, distribution and characterization) to decrease their loss of viability and therefore the frequency of their regeneration.”