Low temperature when sowing can result in poorer germination and thus prevent the early development of seedlings. To improve the germination ability of seeds, they can be primed before sowing. When primed the seeds are soaked in either water or a nutrient solution, this triggers the germination process, but it is interrupted and the seeds are dried before they can be sown. It can help speed up germination, improve tolerance to stress, such as low temperatures, and ultimately it can help increase establishment of the crop.

“You can choose to prime with water or with a solution of mineral nutrients such as zinc (Zn). Zn is taken up by the seed and improves germination ability and speed, just as it is also seen with water priming,” says senior researcher Birte Boelt from the Department of Agroecology at Aarhus University.

Spinach usually requires heat

In the experiment, it was investigated how water and zinc priming of spinach seeds, respectively, affect germination under low temperatures.

Spinach germinates more slowly and takes a long time to establish itself at low temperatures. At low temperatures there are e.g. increased risk of membrane damage when the seed starts to absorb water. If the membrane is damaged, the seed's storage nutrients, sugar, and amino acids will start to leak.

Spinach seeds germinate best between 15 and 24 degrees Celsius, and the germination rate is very slow at temperatures just above freezing. At 20 degrees it takes a week for germination, while it can take up to 5 weeks at 5 degrees. Priming is a way to kickstart the seeds, especially at low temperatures.

Better growth with Zn priming

Priming is not a new phenomenon, it has been used successfully on crops such as maize, barley, rice, and soybeans. Here, the process has improved germination and strengthened the resistance to e.g. cold, drought, and in some cases lack of nutrients.

"We can see that Zn priming improves the germination and development of the seedlings, but it is very important to determine the optimal concentration of Zn during priming to achieve the most beneficial effect," says Birte Boelt.

Therefore, the researchers studied solutions with 0 (deionized water), 1-, 2-, 4-, 6-, 8-, and 10-mM Zn. In addition, they used unprimed seeds as a control group.

“In this study, we found that the solutions at 6-mM and 10-mM had the best effect on germination at low temperature, we saw the most germinated seeds here. But Zn priming of spinach seeds at 6-mM Zn showed a significant increase in germination rate and total number of germinated seeds at low temperature - in this experiment with 8 degrees C, "says Birte Boelt.

Innovative technology

A Zn priming with a solution of 6-mM thus gives the greatest possible germination in the tested spinach seeds both in relation to the other Zn-primed, the water-primed, and non-primed spinach seeds. But the benefits of Zn priming do not stop here.

During the priming process, according to the researchers, a form of priming memory is created in the spinach seed. And it helps to create a greater stress tolerance.

“A seed usually has sufficient storage nutrients -equvalent to what we know as packed lunches - to complete germination, but we know that seeds grown on soils with a low Zn content germinate more slowly. Our studies showed that Zn is absorbed by the seed and transported to the actively developing roots, but we do not actually know the background for the positive effect of Zn priming,” explains Birte Boelt.

“Previous research has shown an increased yield in maize and wheat using Zn priming of the seeds, and much of it must be attributed to the headstart of the seeds as a result of the Zn reserve they carry with them. In this study, we found that Zn priming could increase the Zn concentration in the seed up to 48 times, which gives the seeds a somewhat better start than seeds that are not Zn primed,” explains Birte Boelt.

The researchers were able to determine the zinc content using a DTZ staining technique (1,5-diphenylthiocarbazone). Here the seeds are colored so that they develop a red color that becomes stronger the more zinc the seed contains. In other words, the concentration of the red color indicates the Zn level of the seed after priming with Zinc.

“Due to the thick and relatively dark coating of the spinach seeds, we could not use ordinary images to differentiate between the DTZ staining of water and Zn-primed seeds. However, because other researchers have previously been successful in using multispectral imaging to classify tomato seeds, we chose to use the same technique. And the pictures we got showed a clear difference between water- and Zn-primed seeds. We attribute the variation in color after the DTZ staining to increased Zn in the spinach seeds after Zn priming. In addition, we found a development of red color in the inner parts of Zn-primed seeds after DTZ staining; it reveals that zinc added via Zn-priming is not only absorbed into the outer tissues of the seed, but that it also accumulates in the inner parts of the seed,” explains Birte Boelt.

Potential for fewer pesticides

According to the researchers, it has beneficial effects to increase seed Zn levels via a priming of spinach seeds before sowing. It improves germination rate and ability just as it can improve establishment even under stressful conditions such as low temperature or lack of nutrients. This study shows that increased Zn level in spinach seeds using Zn priming can help increase the yield of spinach as it creates a better and faster establishment of viable plants in the field. According to the researchers, it will be beneficial to confirm the results of the study under field conditions, but it does not change the results of the study. Just as it does not change the possibilities that exist for further developing the priming technology in the future.

“Priming is a cost-effective method, but it is primarily used in connection with high-value crops. Should the method be assessed in a broader perspective, the potential could be better competition against weeds - and thus lower herbicide consumption. For cover crops, the potential for faster germination and establishment would be increased nitrogen uptake throughout the fall. Our results aim to test several applications of priming and to quantify the effects thereof,”explains Birte Boelt.