The herbicide, 4-chloro-5-(methybano)-2-(aaa-trifluoro-m-tolyl)- In many plant species sublethal doses of 4-chloro-5-(methylamino)-2-(a,a,a-trifluoro-m-tolyl)-3(2H)-pyridazinone (Norflurazon; hereafter referred to as San 9789, manufacturer's code no.) inhibit carotenoid synthesis (2). When exposed to white light these plants do not accumulate Chl pigments, but appear to grow and develop as well as untreated plants for several days.Our interest in this herbicide evolved from the possibility of using it as a tool to study phytochrome spectrophotometrically in light-grown plants. Normally ChM fluorescence makes spectrophotometric measurements in light-grown plants impossible (18). Furthermore, R3/FR-reversible-fluorescence yield changes, unrelated to phytochrome, produce apparent A changes which could be confused with phytochrome (6

(14).Subsequent experiments with daylength extensions, rather than dark interruptions, found that a mixture of R and FR was more effective for promotion than R alone (7, 21, 31). The action spectrum for this response (27) shows a single peak in the R-FR region between 710 and 720 nm and some action in the blue at high irradiances. Such responses have been termed "high irradiance responses" (HIR) (28), to distinguish them from the low energy R/FR reversible responses, and are assumed to be a complex function of the photostationary state between Pr and Pfr (13).Although Hartmann (13)

The principle of equivalent light action predicts that two light treatments (wavelengths XI and X2) producing the same Pfr/P ratio (Vu = -P) and the same rate of phytochrome photoconversion (kX1 = kX2) are perceived by phytochrome as being the same and should produce the same effect. The results of experiments based on the principle of equivalent light action indicate that cryptochrome is involved in the photoregulation of anthocyanin production elicited by blue light in tomato seedlings. This was also the case for one strain of cabbage seedlings. For another strain ofcabbage seedlings, the results suggest that cryptochrome is either not involved or that the state of phytochrome is the principal limiting factor.

Photosynthesis is fundamentally driven by photon flux rather than energy flux, but not all absorbed photons yield equal amounts of photosynthesis. Thus, two measures of photosynthetically active radiation have emerged: photosynthetic photon flux (PPF), which values all photons from 400 to 700 nm equally, and yield photon flux (YPF), which weights photons in the range from 360 to 760 nm according to plant photosynthetic response. We selected seven common radiation sources and measured YPF and PPF from each source with a spectroradiometer. We then compared these measurements with measurements from three quantum sensors designed to measure YPF, and from six quantum sensors designed to measure PPF. There were few differences among sensors within a group (usually <5%), but YPF values from sensors were consistently lower (3 % to 20 %) than YPF values calculated from spectroradiometric measurements. Quantum sensor measurements of PPF also were consistently lower than PPF values calculated from spectroradiometric measurements, but the differences were <7% for all sources, except red-light-emitting diodes. The sensors were most accurate for broad-band sources and least accurate for narrow-band sources. According to spectroradiometric measurement, YPF sensors were significantly less accurate (>9% difference) than PPF sensors under metal halide, high-pressure sodium, and low-pressure sodium lamps. Both sensor types were inaccurate (>18% error) under red-light-emitting diodes. Because both YPF and PPF sensors are imperfect integrators, and because spectroradiometers can measure photosynthetically active radiation much more accurately, researchers should consider developing calibration factors from spectroradiometric data for some specific radiation sources to improve the accuracy of integrating sensors.

Phytochrome was studied spectrophotometrically in Avena sativa L. seedlings that had been grown for 6 d in continous white fluorescent light from lamps. Greening was prevented through the use of the herbicide San 9789. When placed in the light, phytochrome (Ptot) decreased with first order kinetics (τ1/2 ≈ 2 h) but reached a stable low level (≈2.5% of the dark level) after 36 h. This concentration of phytochrome remained constant in the light and during the initial hours of a subsequent dark period, but increased significantly after a prolonged dark period. Evidence suggests that the constant pool of phytochrome in the light is achieved through an equilibrium between synthesis of the red absorbing (Pr) and destruction of the far-red absorbing form (Pfr) of phytochrome. It is concluded that the phytochrome system in light-grown oat seedlings is qualitatively the same as that known from etiolated monocotyledonous seedlings, but different than that described for cauliflower florets.