What cause foxtails?

Botanical panic mode, foxtailing, at its root, is a stress response: when a plant has way more light/heat than it can handle, it triggers NPQ and a slew of responsive hormones. To keep it short it's a developmental hormonal stress response from "burning out", way too much of a good thing. "Stress" for a plant is linked to ABA (abscisic acid). If the levels of ABA surpass the plant's protective thresholds she will have a stress response; stressors can come from anywhere, from roots to leaves, from heat to cold, from deficiency, or almost anything. Everything contributes to a "total stress". Abscisic acid (ABA) functions as the central "stress rheostat" of a plant, acting as an internal ledger that sums up every negative environmental input into a single "total stress load." When the cumulative concentration of ABA crosses a genetically predetermined protective threshold, the plant shifts energy away from vegetative growth and triggers survival-focused physiological changes. (hermie) Hope that helps. Best of luck.