Researchers are one step closer to solving the mysterious case of alfalfa autotoxicity. Up until now, the chemical compound — or compounds — that causes autotoxicity has kept a secret identity, but recent findings suggest its production by plants may be linked to soil fertility.

Autotoxicity ultimately inhibits farmers from thickening thin stands by overseeding alfalfa, and it prevents the reseeding of alfalfa into a terminated stand without a rotation crop in between. Autotoxicity has also been shown to cause autoconditioning or autosuppressium, which occur when alfalfa seedlings are able to germinate and grow, but their root systems are permanently damaged, and plants never realize their full potential.

Several years ago, Kim Cassida started working to develop a bioassay test that would help farmers determine a soil’s autotoxicity risk level. In a recent article from Michigan State University Extension, however, the extension forage specialist explains some of the challenges with that research, which have ultimately pointed her team toward a new hypothesis.

Bioassay roadblocks

Cassida previously addressed the two major hurdles of the bioassay test were that the protocol had to prevent seedlings from damping off and that the control variety of alfalfa needed to consistently exhibit autotoxic symptoms. To date, studies have shown variation among alfalfa varieties in terms of their susceptibility to autotoxicity.

Her team initially set out to develop a fast, three-day bioassay that could be used in the Michigan State University Plant Diagnostic Lab as a tool for farmers to test and flag soils where alfalfa establishment was likely to fail.

“Unfortunately, while we were able to detect differences among soils with this short bioassay, validation tests showed it is not reliable at detecting potential for long-term autosupression,” Cassida writes. “We have not given up on the idea of a diagnostic test, but it will not be a fast turnover test, and it may take several weeks to provide an answer.”

So, now what?

Research redirection

In the midst of the bioassay research this summer, graduate student Paige Baisley was evaluating alfalfa planted into stands where existing alfalfa was terminated either two or 20 weeks prior. While Baisley expected stands in the two-week rotation to show more signs of autotoxicity, she also discovered the greatest instance of reduced seedling growth in soils that were deficient in essential nutrients like phosphorus and potassium.

Cassida explains how plant roots excrete chemical mixtures called exudates into the layer of soil immediately surrounding the roots, also known as the rhizosphere. These chemicals are necessary to facilitate plant nutrient uptake, and when plants are stressed, they may produce and excrete more exudates in attempt to scavenge for more nutrients.

If stressed alfalfa produces more root exudates to compensate for nutrient deficiencies, and these stands also exhibit a greater risk of autotoxicity, Cassida contends there may be a connection between poor soil fertility and autotoxicity. In other words, Baisley’s observation may explain why some failing alfalfa stands that overproduce and release plant chemicals appear to be more autotoxic than others.

Continuing efforts

Moving forward, Cassida and her team will continue to try to identify the compounds responsible for autotoxicity, but they have also resolved to determine how root function and soil microbiology interact with factors like soil fertility. The goal is to understand how these interactions influence the development and release of plant chemicals.

“Currently, we are working on categorizing possible autotoxins in root exudates, planning how to best evaluate possible interactions with potassium fertility, and analyzing microbial populations from alfalfa root microbiome,” she explains.

In addition, the researchers aim to screen alfalfa populations that are tolerant of autotoxicity and begin breeding for varieties that do not cause the harmful effects, communicating these findings with farmers through extension resources along the way.

“The end goal of the research is to pave the way for developing new varieties that either produce less toxins or are able to tolerate them more effectively,” Cassida asserts. “This research may also serve as a framework for other crops that exhibit autotoxicity.

“It is early in the project, but we are very excited about the results to date and look forward to having more to share with the industry in the future,” Cassida concludes. She notes the project is funded in part by a grant from the Alfalfa Seed and Forage Systems (ASAFS) research program.