Do seeds from different states have different responses to temperature? This is one of the questions I have been testing for the second half of my internship at the garden. In my last post, I talked about how we were scouting populations across a latitudinal gradient. When we got back to the lab, we began an experiment with common milkweed (Asclepias syriaca) seeds collected from those states last year.
With global climate change, we expect that seeds will experience shorter winters and warmer temperatures. My experiment simulates some possible climactic changes that will impact Asclepias syriaca germination. We used seeds from Minnesota, Illinois, and Missouri, which all experience different winter lengths and temperatures. The seeds were treated with different “winter” lengths, by placing them in the refrigerator for six, eight, and ten weeks. The seeds were placed on petri plates, with eight seeds per plate. Then, six plates of seeds from each treatment were placed in one of ten temperature zones on the thermogradient table. A thermogradient table is a unique piece of equipment that produces a continuous range of different temperatures. The table allows us to create temperature zones, so we can test how the different seeds will germinate when they experience different temperatures. The table was set to range from 15˚C to 30˚C, which means that the seeds in the lower zones experienced really cold temperatures!
One thing we had to account for with this table is that the two sides of a zone have different temperatures. To account for this, every day the plates were switched, so over two days they experienced the same average temperature. Every other day I collected data by looking at each plate and counting how many seeds had germinated, and recording the number for that day. Data was collected for three weeks, which allows us to see how the seed germinate over time. Data collection was straightforward, except when some of the plates molded, which made it hard to see the seedlings. Also, as the seedlings continued to grow, they became entangled around each other, so it was hard to tell them apart.
After only a few days on the table, many of the seeds in the warmer zones had already germinated. I was surprised that the seeds germinated so quickly. It did take a few more days for the seeds in the colder zones to germinate, but many did eventually. When a seed germinates, first its radicle emerges, the embryonic root of the plant. Then the cotyledons emerged, and some of them pushed up on the lid so much that it came off! One thing I noticed was that some of the seeds were mutants, so they were not exactly like the other ones. A couple had radicles that emerged out of the top instead of the bottom, and some had cotyledons that emerged out of the top of the seed, instead of following the radicle. We also had one twin, with two embryos in one seed and two seedlings emerging!
After three weeks of data collection, all the plates were opened up and the seedlings were counted a final time to make sure none were missed. One way to do this is to feel the seeds. If they germinated, the embryo will be gone and the seed will just be a flat empty shell. If the seed didn’t germinate, then it will be firm and feel like something is still inside. After the final data collection, some of the seeds still didn’t germinate. I was wondering if they didn’t germinate because of the treatments, or because they weren’t viable seeds. The garden has a seed x-ray machine, which can be used to see the embryo inside the seed coat. So far, some of the seeds have been x-rayed and most of them had full embryos. This means that the seeds could have germinated, but some part of the treatment was stopping them!