Effects of Blue Light Pretreatments on Internode Extension Growth in Mustard Seedlings after the Transition to Darkness: Analysis of the Interaction with Phytochrome

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Abstract
The effects of blue light (B) pretreatments on internode extension growth and their possible interaction with phytochrome mediated responses were examined in Sinapis alba seedlings grown for 11 d under 280 μmol m−2 s−1 of continuous blue-deficient light from low pressure sodium lamps (SOX). Supplementary B (16 μmol m−2 s−1) caused no detectable inhibition of the first internode growth rate under continuous SOX, but growth rate was inhibited after transfer to darkness. This effect, and the growth promotion caused by far-red bend-of-day' light pulses were additive. The addition of B at 16 μmol m−2 s−1 during 11 d, or only during the first 9 or 10 d or the latest 0.75, 1 or 2 d of the SOX pretreatment caused approximately the same extent of inhibition after the transition to darkness. A single hour of supplementary B before darkness caused more than 50% of the maximum inhibition. However, 24 h of lower fluence rates of B (4 or 7 μmol m−2 s−1) were ineffective. Covering the internode during the supplementary B period did not prevent the response to B after the transition to darkness. Far-red light given simultaneously with B (instead of the SOX background) reduced the inhibitory effect of B. Above a given threshold fluence rate, B perceived mainly in the leaves inhibits extension growth in subsequent darkness, provided that high phytochrome photo-equilibria are present during the irradiation with B. Once triggered, this effect does not interact significantly with the ‘end-of-day’ phytochrome effect.