Blue-light reception in Phycomyces phototropism: evidence for two photosystems operating in low- and high-intensity ranges.

Phototropism in the fungus Phycomyces is mediated by two photosystems that are optimized for the low-intensity region (below 10(-6) W X m-2) and the high-intensity region (above 10(-6) W X m-2). These photosystems can be distinguished under special experimental conditions, in which sporangiophores grown in the dark are suddenly exposed to continuous unilateral light. With this treatment, the bending occurs in two steps. Below 10(-6) W X m-2, an early-response component (15-min latency) and a late-response component (50- to 70-min latency) are observed that are mediated by photosystem I. Above 10(-6) W X m-2, the early component is augmented by an intermediate component with a 40-min delay that is mediated by photosystem II. The two photosystems are distinguished further by their wavelength sensitivities and adaptation kinetics. Photosystem I is more effective at 334, 347, and 550 nm than photosystem II, but it is less effective at 383 nm. At wavelength 450 nm, the dark-adaptation kinetics associated with photosystem I are approximately half as fast as those associated with photosystem II. However, the light-adaptation kinetics of photosystem I are approximately equal to 3 times faster than the kinetics associated with photosystem II. The existence of two photosystems clarifies several behavioral features of Phycomyces and helps explain how the sporangiophore can manage the full range of 10 decades.