THE CONTROL BY PHYTOCHROME OF NITRATE REDUCTASE IN THE CURD OF LIGHT‐GROWN CAULIFLOWER

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Abstract
Abstract— The activity of nitrate reductase from the curd of light‐grown cauliflower (Brassica oleracea (L) var botrytis (DC) ‘St. Hilary’) is modulated by nitrate and by light. Using broad‐band sources of equal photosynthetically active radiation but with different proportions of red and far‐red light, a linear relationship between nitrate reductase activity and ψ(Estimated phytochrome photoequilibrium) was obtained. This relationship, apparent after 8 h incubation, was maintained and little altered after 48 h incubation. The linearity was apparent between ψE 0.26 and ψE 0.69; ψE 0.26 being no more effective than a dark control. Far‐red reversibility confirmed the involvement of phytochrome. Brief pulses of red light were also used to establish a range of phytochrome photoequilibria within the tissue. Again a linear relationship between ψ and nitrate reductase activity was obtained with a threshold for the response at ψ 0.3. With both monochromatic and broad‐band sources it was seen that neither photon fluence rate nor duration of exposure affected the final activity of the enzyme and that phytochrome was acting solely through ψ (or [Pfr] since phytochrome is stable in this tissue) to bring about these responses.