Sustainable Use of Fresh and Lyophilized Banana Peel Extracts as Biostimulants to Modulate Stress Tolerance and Bioactive Phytochemicals in Broccoli Microgreens

With rising global temperatures, biostimulants might be a promising tool to alleviate plant stress and support adaptation. The potential of fresh (FBP) and lyophilized (LBP) banana peel aqueous extracts as biostimulants for protecting broccoli from high temperature (HT) stress was analyzed. Spectrophotometric and statistical analyses revealed that BP affected broccoli phytochemistry in a temperature-dependent manner. Under room temperature (RT), FBP and LBP decreased glucosinolates (−15% and −25%, respectively). Conversely, FBP increased flavonols and proanthocyanidins (141% and 202%, respectively). Under RT, LBP decreased sugars in broccoli (−27%). FBP had stronger effects at HT than at RT, further boosting phenolics (70%), flavonoids (89%), tannins (31%), and hydroxycinnamic acids (64%), and antioxidant capacity (FRAP) (10%). LBP also increased flavonoids (39%), flavonols (95%), and hydroxycinnamic acids (45%) under HT. Both FBP and LBP increased glucosinolates (47% and 46%, respectively) in HT-grown broccoli. HT significantly affected glucosinolates, decreased them in control plants, and increased them in BP-treated plants. All HT-grown plants had higher soluble sugars and lower hydrogen peroxide than RT-grown plants. Principal component analysis confirmed greater biochemical diversity under HT. Temperature–BP interaction significantly affected flavonoids and glucosinolates, highlighting the central role of environmental temperature in determining biostimulant outcomes. These findings suggest that global warming may markedly alter biostimulant efficacy and should be considered in their development.