Testing the Relative Humidity of Seed Accessions Prior to Freezing

Authors: Stacy Anderson, Tobin Weatherson, Joe Davitt – San Diego Zoo Native Plant Seed Bank

We are all taught in school that water expands when frozen. That’s why ice floats, sealed bottles explode in the freezer, and frost kills living tissues. All living cells are comprised largely of water and can rupture and die when the water they contain expands under freezing conditions. The cells of seeds are no different.  Research has shown there is a “goldilocks zone” of relative humidity that our seeds must reach before freezing. This perfect range of relative humidy is greater than 25% and less than 35%. The final and most crucial step before freezing our seeds, is testing their relative humidy to ensure it lies within this range. At SDZGs native plant seed bank we use small air tight desiccation chambers and color changing silica gel to obtain the relative humidity needed in our seed accessions. It’s also common to use salt solutions to desiccate. We have found that a very quick and convenient way to read the relative humidy is to place a BlueMaestro Tempo Disc inside our sealed chambers. Given amble time to stabalize, we can infer the seeds reach an equalibrium with their ambient environment. These discs can quickly tell us the relative humidy inside the desiccation chambers without opening them and exposing the contents to humid air. There is a simple app on our phones that gives us temperature and relative humidity levels, and makes it easy to track these data over time. It’s important to note that the size of the desiccation chamber, the amount of silica or salt solution, the number of seeds, and the type of seeds can all affect the time it takes to reach equilibrium (within a sealed chamber). For a more precise relative humidity measurement, or if we are unsure about the equilibrium within a chamber, we use The Rotronic HygroPalm. This hygrometer is also useful for obtaining a baseline RH reading from fresh seeds. The HygroPalm uses a small cup to measure the relative humidity of a sample.We attach the probe to the connection port, and turn it on by pressing the red button. We fill this small cup to the line with seeds and the probe tightly covers the sample. It then slowly draws in air and in about 5 minutes gives a very accurate reading. Note some of the drawbacks of using this hygrometer. Sometimes we have very small amounts of seeds in many maternal lines that we can not combine. We don’t always have a large enough sample for this cup. We also have to open our chambers and disrupt equilibrium to test. For this reason, we use both the ibuttons and the hygrometer. We have designated desiccation chambers that are very stable and hold small accessions that stabilize quickly, and others for large bulked accessions. We always open and close these chambers as quickly as possible.  When we know that it is safe to freeze the seed, we place the accession into foil lined bags and heat seal them. The bags are now ready to freeze for long term storage, where they can survive for hundreds or even thousands of years, be preserved for generations to come and act as a safe guard against extinction.