Phytochrome (far red form) alone can mediate anthocyanin synthesis in the mustard seedling (Sinapis alba L.). Complete photoreversibility and reciprocity, for both red and far red light exposures over a period of at least 5 minutes, demonstrate this phytochrome involvement.The duration of the initial lag-phase is constant (about 3 hours at 25 C) for seedlings more than 30 hours old and is specific for the system, being independent of the dose or quality of light. Since a complete reversal by far red of a red light induction is possible only during a 5 minute period, phytochrome (far red form) obviously mediates anthocyanin synthesis during the lag-phase although the actual synthesis of pigment can proceed only after the lag-phase is overcome. We suggest that phytochrome (far red form) exerts a double function during the initial lag-phase. It mediates both the build up of a biosynthetic potential ("capacity") and anthocyanin synthesis.However, the sequence of events leading to anthocyanin is arrested at some intermediate stage until this "capacity" is built up after 3 hours. Once "capacity" is achieved it does not decay readily. Therefore, no significant "secondary lag-phase" occurs if the seedling, under appropriate conditions, is reirradiated after an intervening dark period. The rate or extent of synthesis for both anthocyanin and lipoxygenase, previously reported (32), are functions of the amount of phytochrome (far red form). No "phytochrome paradoxes," i.e., nonrational relationships between the amount of phytochrome (far red form) and rate or extent of response, were detected. This fact suggests that the mustard seedling is especially well suited for investigating the biophysical and molecular mechanisms of phytochrome action.Since the well known report by Arthur (1), the effect of light on anthocyanin synthesis has attracted the attention of many plant physiologists (5, 21, 37). However, the general mechanism of this light effect has not yet been formulated. This failure is due partly to the descriptive nature of many reports and, in the main, because light appears to exert its influence through different photochemical mechanisms which probably are mixed in the usual experimental approaches. Recently, (e.g.,2,6,7,9,10,11,35,40) anin synthesis be mediated exclusively without the interference of any other photochemical mechanism? These advantages are that (24) anthocyanin synthesis by light is predominantly controlled by phytochrome without the requirement of any prolonged "high energy pretreatment" (e.g., 2, 6, 10, 35), and the mustard seedling does not produce significant amounts of anthocyanin in complete darkness. Seedlings can easily be handled under standardized conditions, and a great amount of information, including spectrophotometric measurements of P,, is available on this system (26). While the mustard seedling forms five anthocyanins, the aglycon is always cyanidin (17). At 25 C the stored fat and protein of the cotyledons allow development in total darkness without any indications of ...

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In this paper measurements are presented of action spectra for the photoresponses of sporangiophores of Phycomyces an(d of the transmission spectra of the growing zone of single sporangiophores in stage IV b (7) and of the cell wall. Compared to other organisms, Phyconmyces offers several definite advantages for such measurements. There are two distinct photoresponses, the growvth response and the tropic response, and there are many indications, particularly the action spectra presented here, that these two responses are mediated by the same pigment. The growth response occurs under conditions of symmetric irradiation when the intensity varies with time. and the tropic response occurs with an irradiation program which may be constant in time, but asymmetric with respect to azimuth angle, i.e., the angle around the axis of the sporangiophore. Crudely speaking. the growth response measures the azimuthal summation of photoeffects, while the tropic response measures the azimuthal asymmetry. The combination of these two measurements permits an estimate of the influence of screening pigments lying betwzeen the receptors proximal to the incident light andl those distal to it.Similarly, the measurement of transmission spectra of the intact sporangiophore and of the cell wall permits some conclusions with respect to the absorbing materials present in various locations.Another advantage of Phycomyces is that the optics of the system are relatively simple. The sensitive region closely approximates a cylindrical lens with uniform refractive index. The importance of the dioptric properties of this lens for the tropic effect was clearly demonstrated 40 years ago by Buder (2), who showed that immersion of the specimens in a medium with refractive index higher than that of the cell contents reversed the sign of the tropic response. Thus, the dioptric properties, both refractive and reflective, and not scattering or absorption have the decisive influence in producing the azimuthal asymmetry in the sporangiophore.Two years ago, Curry and Gruen (10) discovered that the tropic sensitivity of the sporangiophores extends deep into the ultraviolet region and that the tropic response reverses sign aroundl 300 mu. The reversal has a simple explanation. There are screening substances in the growing zone which, for waveReceive ( June 22, 1959.

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