AbstractIn germinating seedlings ofSinapis albanitrate reductase activity as assayedin vivobecomes accessible to phytochrome control between 15 and 17 h after sowing. Phytochrome operates via the high irradiance reaction to control nitrate reductase activity in the period 15 to 20 h after sowing. Both continuous red light and far‐red light elicit this response with a strong fluence rate dependency being apparent in each case. The induction of nitrate reductase activity by light pulses at 20 h after sowing is greatly influenced by red light pre‐treatments (operating through phytochrome) given between 0 and 15 h after sowing. Low fluence rate pre‐treatments reduce the effectiveness of a subsequent pulse to below the level of a dark control whilst high fluence rate pre‐treatments greatly increase the effectiveness of a subsequen