Downregulation of an allene oxide synthase gene improves photosynthetic rate and alters phytohormone homeostasis in field-grown guayule

•A two-year field trial of transgenic guayule was conducted.•Constitutive gene overexpression/downregulation was confirmed at two years of growth.•Photosynthetic rate increased when allene oxide synthase was downregulated.•Plants with lower allene oxide synthase had altered phytohormone levels.•Plants with lower allene oxide synthase had more branches and thicker stembark.