Everything I just read at Royal Seed mentioned light and stress in general and genetics
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It is not uncommon for some auto flower cannabis plants to produce three leaves during their maturation process, instead of the typical five or seven leaves. This phenomenon is referred to as "trifoliate" growth and can be attributed to genetic mutations or environmental factors.
In some cases, genetic mutations can cause a cannabis plant to develop with fewer leaves than usual. This can occur naturally, or it can be the result of crossbreeding strains with different leaf structures. Some growers even intentionally breed plants for trifoliate growth because they believe it can lead to higher potency.
Environmental factors, such as changes in temperature or lighting, can also affect the number of leaves a cannabis plant produces. For example, a sudden change in light intensity or duration can cause a plant to develop with fewer leaves than usual.
Overall, while three-leaf growth in auto flower cannabis may not be the norm, it is not necessarily a cause for concern and may even be a desirable trait for some growers.