FLUENCE RATE COMPENSATION OF THE PERCEPTION OF RED: FAR‐RED RATIO BY PHYTOCHROME IN LIGHT‐GROWN SEEDLINGS*

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Abstract
Abstract— The hypothesis that phytochrome functions as a sensor of vegetational shade through the perception of the red: far‐red photon fluence rate ratio requires that the mechanism of perception be compensated for wavelength‐independent fluctuations in fluence rate (Smith, 1982). This paper seeks to establish the lower limit of fluence‐rate compensation and to assess whether or not compensation is effective at the total fluence rates typical of herbaceous canopies. Using specially‐designed cabinets, Sinapis alba L. (white mustard) seedlings were grown from germination under a range of total photosynthetically‐active radiation (PAR = 400 to 700 nm) values and a range of red: far‐red ratios. The data indicate that fluence‐rate compensation is effective above a PAR value of ca. 60 μ.mol m2 s'. Pretreating seedlings at high red: far‐red ratio and a PAR level of 300 (μmol m2S‐1for increasing periods of time led to an extension of fluence rate compensation to lower fluence rates. The results are discussed in relation to the photosynthetic competence of the seedlings grown under these conditions.