Leveling the Field
Photo by Gokhan Okur/sxc.hu.
We tried 60 different treatments before settling on one," says Dr. Rajeev Arora, professor of horticulture at Iowa State University, referring to a recent study that looked at the effects of priming on seed germination performance in spinach. The first phase of the study, which he started in 2008 with doctoral student Keting Chen (now Dr. Chen) is complete, and the results are surprising. The study only piqued Arora's interest, and he is now seeking funding to continue this work on a deeper level.
As a stress physiologist, Arora has a long-held interest in how plants respond to abiotic stresses, such as temperature fluctuations, flooding and drought. He had read anecdotal evidence that indicated seed priming and treatment could improve seed germination performance not only under optimal conditions, but also under stressful environments. The scientist wanted to conduct a systematic study where he and his colleagues controlled all the environmental parameters. He wanted to test hypotheses and ultimately reach a science-based conclusion.
A simple formula yields surprising results
Seed priming is not new. This pre-sowing partial hydration of seeds has been used for decades to improve crop performance. Methods include osmopriming, matric priming, halopriming, hydropriming and drum priming. Materials include distilled water, nutrients, salt and sand. Arora and Chen's simple formula uses a polymer of ethylene oxide called polyethylene glycol (PEG) dissolved in water. The key to success was to establish which method of application, concentration of the priming solution, temperature and time would work best.
Developing the study
Arora and his team started by developing a protocol for the study. They chose spinach (Spinacia oleracea) because of its value as a horticultural crop. The 'Bloomsdale' cultivar is standard in research and easy to access. They determined the optimum temperature for this cultivar's seed germination: 10 to 15 degrees Celsius.
With the basics settled, they created a protocol for priming. They chose to explore osmopriming, a method in which the seed is placed in an osmotic solution (a chemical dissolved in water) and stirred to create aeration. The seed's osmosis behavior when submerged in the chemical solution differs from its osmosis behavior in plain water.
The researchers began playing with various combinations of PEG. After trying 60 combinations, they came to the conclusion that a solution of 8,000 molecular weight PEG dissolved in water at a concentration of -0.6 megapascal (MPa) was the most effective when the seeds were primed for eight days at 15 degrees Celsius. After priming, Arora and Chen dried the seeds to their original level of dehydration to preserve shelf life.
The combination of these three factors yielded the highest-performing seeds when germinated at near-optimal germination temperature for this cultivar.
"This protocol, compared to control [unprimed], resulted in a 10 to 15 percent higher germination percentage, but even more significantly showed twice as fast a germination rate. We saved a full four to five days in germination time for the maximum germination, compared to controls, and germination was more uniform," reports Arora. "Synchronous germination happened in primed seeds much more than in the control as well."
Now the researcher wants to understand why he and his colleagues achieved these results.
Does priming affect physiology?
A seed needs moisture to germinate; this is not news. The reason a seed needs water to germinate is that in the dry state, enzymes that enable the seeds to utilize stored food for germination and growth remain inactive. Once the seed absorbs water, these enzymes are activated. The activated enzymes break down the complex food substances (proteins, oils, starch, complexed minerals) into their simpler components, such as sugars, amino acids, fats and minerals. The embryo in the seed uses the simple molecules released during this imbibing process to fuel the growth and development of the emerging root system and the sprout.
Osmopriming does not fully hydrate the seed. Partial hydration activates the seed's enzymes and other biochemistry, but prevents full germination. Arora calls this "giving the seeds a head start." The process brings all the seeds in a seed lot to roughly the same level of vigor. Arora wants to know what is happening at the cellular level to produce these effects.
"We found that the primed seeds of 'Bloomsdale' spinach not only had higher germination, faster germination and more synchronous germination at a more optimal temperature, but also at suboptimal [cold, 5 degrees Celsius] or supraoptimal [warm, 20 degrees Celsius] temperatures. In other words, even when subjected to environmental stress, they still outperformed unprimed seeds," says the researcher.
Arora and his team also discovered that their primed seeds have a memory, so the proteins that are activated during priming, which go dormant again during post-priming drying, are reactivated at germination at a higher level than unprimed seeds.
Some of those proteins help with stress tolerance. One family of proteins, called dehydrins, helps seeds tolerate dehydration caused by cold or drought. These proteins are hydrophilic, so they don't let the plant cells get desiccated beyond the cell membranes' and enzymes' threshold of dehydration tolerance. Dehydrins hold the plant cells more or less in a healthy state.
Learn more about Arora's work with priming in next month's column.
The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.