The end-of-day far-red irradiation increases gibberellin A(1) content in cowpea (Vigna sinensis) epicotyls by reducing its inactivation

It has been shown previously that gibberellins (GAs) mediate the phytochrom e (Phy) control of cowpea (Vigna sinensis L,) epicotyl elongation induced b y end-of-day (EOD)-far-red light (FR), In the present work, the EOD-FR effe ct on GA metabolism and GA levels in cowpea has been investigated, GA(1), G A(8), GA(19) and GA(20) were identified in epicotyls, and GA(1), GA(19), GA (20) and GA(29)-catabolite in leaves of 6-day-old cowpea seedlings, The con tent of GA(1) in the epicotyl paralleled the decrease of its growth rate, s upporting the hypothesis that this is the GA bioactive in controlling cowpe a epicotyl elongation. FR enhanced both the amount of [H-3]GA(1) in the epi cotyl produced from applied [H-3]GA(20), and that of applied [H-3]GA(1) tha t remained unmetabolized in epicotyl explants, suggesting that Phy mag regu late the inactivation of GA(1). In agreement with this effect of light on G A(1) metabolism, the contents of GA(1) in the epicotyl remained higher in F R-treated than in R-treated explants, Moreover, in intact seedlings EOD-FR treatment increased both epicotyl elongation and GA(1) content in the respo nsive epicotyl, whereas it was not altered in the leaves, These results sho w, for the first time, that photostable Phys modulate the stem elongation i n light-grown plants by locally controlling the GA(1) levels through regula tion of its inactivation.