We can divide the early stage of plant life into two broad stages: the seed and the seedling. Both have multiple steps that were identified and have been thoroughly described by plant scientists. Here we are going to keep things simple and talk about the main aspects that have something to do with temperature during the two sub-stages. Actually, the temperature is considered the most important environmental factor with respect to seed dormancy and maturity so it is crucial to seek the best temperature conditions for your plants. The idea is that you can have a better understanding of what is happening inside your plants as they remain protected in the seed and when they germinate to bring their beauty into the world (I’m not saying that seed can’t be pretty). You will also find useful tips to help your photosynthetic friends remain healthy in the seed and grow strong as they begin to make their way up to the light and down deep into the soil.
This plant stage is composed of three parts:
Plants in the Embryo state need to survive in full desiccation while they stay in the seed, that is precisely the seed’s purpose, but how does it achieve it? It has a few allies, Late Embryogenesis Abundant proteins (LEAs), storage molecules such as sugars, lipids, and maturation (MAT) proteins, and a hormone, abscisic acid (ABA). The temperature has an important role here, low temperatures tend to lead to a long dormancy period, meaning the seed will take longer to germinate; warm temperatures have the opposite effect. This happens because of a gene called DELAY OF GERMINATION1 (DOG1) and plant hormones, mainly abscisic acid and gibberellins.
Let’s focus our attention on the hormonal regulation mentioned above. First, we are going to talk about abscisic acid (ABA), because it prevents the growth of the embryo, promotes the accumulation of storage molecules, inhibits germination, and induces desiccation tolerance. It is able to drive those mechanisms in the seed by promoting the accumulation of sugars and LEA proteins. ABA levels decrease with seed maturation so that the embryo can come out of its dormant state but temperature also plays a role in ABA accumulation in the seed. Basically, low temperatures increase the synthesis of ABA, in temperate zones, this process takes place during winter and declines as temperatures begin to rise in spring. The opposite occurs, ABA is “destroyed” by catabolism, and the hormone that influences germination starts to be synthesized instead, but we’ll talk about it in a moment.
Now you need to know about the secondary dormancy state. Seeds can enter secondary dormancy when conditions are very harsh and when unfavorable conditions prolong for long periods, this is a method of protection that allows the seed to remain viable for longer, in the wait for conditions to improve. High temperatures are one of the causes for seeds to enter this level of dormancy. Let’s assume some unforeseen event happened and now your seeds are in the secondary dormant state, you can recover them from that, let’s call it “deep sleep”, by returning them and ensuring they stay in favorable conditions, temperatures of around 23°C / 73.4°F, should be okay.
Now you know a little more about your seeds. All of this information will be helpful as you work to maintain your viable seed's health. You must consider keeping your seeds in cool (but not cold) and dry conditions. A good way to achieve this is to keep them in a room or cabinet that’s not exposed to the sun and to use a dehumidifier in the same space where you are keeping them.
Ultimately, you will want your seeds to germinate. At this point temperature is key, it is just as important as water, oxygen availability, and exposure to light. A new player comes into the field at this point too, another hormone, gibberellins (GA), ABA’s opponent for seed dormancy, instead of trying to keep the seed dormant, GA tries to “wake it up” by inducing germination. GA induces ethylene accumulation that blocks ABA synthesis but reinforces GA’s accumulation, this leads to a reduction of ABA’s concentration. The expression of DOG1 is then inhibited and so the transcriptomic gears that rule germination begin to move (Yan & Chen, 2020).
An increase in GA’s concentration also leads to germination because it activates hydrolytic enzymes. Such enzymes remained inactive in the aleurone layer cells during the seed’s dormant state. These enzymes are in charge of turning starch and the other macromolecules of the nutritive tissue into compounds that the embryo can assimilate as it begins to grow out of its “shell”. Temperature also plays an important role at this step, high temperatures inhibit germination, in a process known as thermoinhibition. The temperature threshold above which thermoinhibition is triggered varies between plant species, make sure to check if your plants are especially sensitive to heat and avoid temperatures higher than 28°C / 82.4°F. There’s an interplay too, this time with light, seeds under light conditions can germinate under a wide range of temperatures, but at dark, they will generally require higher temperatures.
Other hormones also influence the seed stage, ethylene is a small volatile molecule that counteracts ABA (the hormone in charge of keeping the seed dormant), therefore ethylene signaling in the seed will induce germination. Ethylene can enter the seed from an external source so it’s not always about the seed’s ethylene production; exposing your seeds to ethylene will help germination succeed under chill conditions or in case your seed entered a secondary dormancy state due to very high temperatures (Corbineau et al, 2014).
After you sow your seeds make sure to keep the right conditions for the seedlings as they germinate. For that remember that it can’t be too cold because ABA concentration will be too high in the seed under those conditions, it will promote dormancy and block the synthesis of ethylene and GA, all nourishment material will remain in a form that can’t be assimilated by the embryo. It can’t be too hot either, if temperatures are too high the seed will go into a “deep sleep”, the secondary dormancy and if the embryo had already managed to grow or if you are transplanting seedlings, they are prompt to enter a state of thermo-inhibition that will keep them from growing and can kill them if conditions don’t improve.
In case your conditions aren’t optimal you might want to try exposing the seeds to ethylene. This plant hormone will induce germination by flipping the balance between ABA and GA in the seed. It is a good method to bring your seeds back from the secondary dormancy, in case they ever enter it. There are other molecules that will induce germination, these will be of your interest if you’re struggling to make your seeds germinate. Nitric oxide (NO) is one of them, this is a small volatile molecule that promotes ABA catabolism in the seed, it is actually thought to be a natural regulator of seed dormancy. You can also get a bottle of GA and apply small concentrations to your seeds, this will help if your seeds aren’t in the secondary dormancy but struggling to germinate, it will be like giving them a little push in the right direction, ethylene will accumulate and GA concentrations will surpass those of ABA.
The journey of the plant from seed to seedling and from seedling to grown plant depends on environmental temperature during multiple steps. In order for it to be successful, temperatures must favor all the mechanisms involved in the process of germination and plant growth. Most species will do very well if you apply what you learned in this guide, but some exceptions need extreme conditions, for example, Ceanothus greggii seeds require wildfires to germinate (Moreno & Oechel, 1991), so before you apply these rules, make sure you’re not dealing with one of these fantastic fire-dependent seeds. Your plants will grow much better if you start to apply this knowledge in your garden.