PHYTOCHROME‐MEDIATED THRESHOLD CONTROL OF HYPOCOTYL GROWTH IN PARTLY DE‐ETIOLATED MUSTARD (Sinapis alba L.) SEEDLINGS

Abstract— Hypocotyl elongation in etiolated mustard (Sinapis alba L.) seedlings is known to be controlled by phytochrome (Pfr) through a threshold mechanism. The Pfr threshold value required to suppress hypocotyl growth was low (3 times 10−2% Pfr, based on total phytochrome in the hypocotyl at 36 h after sowing = 100%). In the present study the question was addressed whether the threshold control by Pfr of hypocotyl elongation also operates in light‐pretreated, partly de‐etiolated seedlings after transfer to darkness. The experimental results show that this is the case. Calculation of the threshold level in far‐red light pretreated seedlings led to a very low value (3 times 10−7%) compared to etiolated seedlings (3 times 10−2%). In red light pretreated seedlings the threshold level was calculated to be 9 times 10−7%. Since the light pretreatment affected the rate of degradation of phytochrome strongly (half‐life of Ptot in continuous red light was found to be 35 min in far‐red pretreated instead of 47 min in etiolated material), the difference in threshold level between far‐red and red pretreated material cannot be interpreted unambiguously. However, the conclusion can be drawn that light nretreatment strongly increases the degradation rate of Pfr and decreases the threshold level.