Interaction between gravitropism and phototropism in sporangiophores of Phycomyces blakesleeanus

The interaction between gravitropism and phototropism was analyzed for spor angiophores of Phycomyces blakesleeanus: Fluence rate-response curves for p hototropism were generated under three different conditions: (a) for statio nary sporangiophores, which reached photogravitropic equilibrium; (b) for s porangiophores, which were clinostated head-over during phototropic stimula tion; and (c) for sporangiophores, which were subjected to centrifugal acce lerations of 2.3g to 8.4g. For blue light (454 nm), clinostating caused an increase of the slope of the fluence rate-response curves and an increase o f the maximal bending angles at saturating fluence rates. The absolute thre shold remained, however, practically unaffected. In contrast to the results obtained with blue light, no increase of the slope of the fluence rate-res ponse curves was obtained with near-ultraviolet light at 369 nm. Bilateral irradiation with near-ultraviolet or blue light enhanced gravitropism, wher eas symmetric gravitropic stimulation caused a partial suppression of photo tropism. Gravitropism and phototropism appear to be tightly linked by a ton ic feedback loop that allows the respective transduction chains a mutual in fluence over each other. The use of tropism mutants allowed conclusions to be drawn about the tonic feedback loop with the gravitropic and phototropic transduction chains. The results from clinostating mutants that lack octah edral crystals (implicated as statoliths) showed that these crystals are no t involved in the tonic feedback loop. At elevated centrifugal acceleration s, the fluence-rate-response curves for photogravitropic equilibrium were d isplaced to higher fluence rates and the slope decreased. The results indic ate that light transduction possesses a logarithmic transducer, whereas gra vi-transduction uses a linear one.