Light-controlled inhibition of hypocotyl growth inSinapis alba L. seedlings

Fluence rate-response curves were determined for the inhibition of hypocotyl growth in 54 h old dark-grownSinapis alba L. seedlings by continuous or hourly 5 min red light irradiation (24 h). In both cases a fluence rate-dependence was observed. More than 90% of the continuous light effect could be substituted for by hourly light pulses if the total fluence of the two different light regimes was the same. Measurements of the far red absorbing form of phytochrome ([P_fr]) and [P_fr]/[P tot] (total phytochrome) showed a strong fluence rate-dependence under continuous and pulsed light which partially paralleled the fluence rate-response curves for the inhibition of the hypocotyl growth.Abbreviations R red - HIR high irradiance response - P_rfr phytochrome in its red, far-red absorbing form - [P_tot]=[P_r]+[P_fr] =k₁/(k₁+k₂): photoequilibrium of phytochrome at wavelength , wherebyk_1,2 rate constants ofP_rP_fr,P_frP_r photoconversion - [P_fr]/[P_tot]