Gravireception in <i>Phycomyces</i>: Threshold Determination on the Sounding Rocket TEXUS 50

Abstract

Under parabolic flight conditions microgravity is not lower than 3 to 5 times 10 −2 g. In contrast to parabolic flights, sounding rocket flights are virtually vibrational-free allowing microgravity as low as 10 −5 g. Thus, a rotating platform serving as centrifuge allows the precise generation of gravitational forces ranging from 5 to 100 mg (not possible during parabolic flights). On this basis we determined the threshold 1 for optical reflection/absorption changes in Phycomyces to be lower than 25 × 10 −3 g. This compares well with the threshold determination of gravitropism in Phycomyces on a clinostat centrifuge. Kinetics of gravity-induced absorption changes and gravity as generated by the on-board centrifuge do not coincide but show a distinctive hysteresis with a latency of 4 s (75 mg-ramp, pull-up).Keywords MDWS (Micro-Dual Wavelength Spectrophotometer), Δ GIAC (Differential Gravity-Induced Absorption Change), Phycomyces blakesleeanus, Sporangiophore, Micro-and Hypergravity, Texus 50, Sounding Rocket, Gravireception 1 The terminus "threshold" is adopted (i) from the biophysical theory of nerve signal propagation and (ii) from the physical theory of field-effect transistors (FETs). Both thresholds are typical all or none reactions; i.e. ion respectively electrical conductivity changes abruptly only if a well defined voltage, the "threshold voltage" is applied. In the theory of nerve conductivity the membrane potential must be depolarized to a precise extend in order to initiate an action potential. The threshold voltage of a field-effect transistor is the minimum gate-to-source voltage differential that is needed to create a conducting path between the source and drain terminals (e.g. representing a switch). Threshold potentials are necessary in order to regulate and propagate signals in both the central nervous system and the peripheral nervous system. However, so far it is common custom in plant physiology also to speak of "threshold of gravitropism" even if the questioned reaction is decreasing monotonously to zero upon decreasing gravi-stimuli and does not represent an all or none reaction. The "threshold" is the smallest stimulus where a response is observed. It depends on the sensitivity of the measuring set-up.

Hyper Gravity-Induced Transients in <i>Phycomyces</i> as Measured by Single Beam Spectrophotometer on the Sounding Rocket TEXUS 50

“…After "YoYo-despin" the rocket "falls" in a fixed direction through a narrow parabola (cf. Figure 1 in [1]) and is completely rotational-free as tested by a three-dimensional on-board magnetometer. The greatest height of 261.2 km is reached after 261.9 s. During the whole phase of microgravity (<10 −5 g, between 68.2 to 449.5 s after liftoff) the payload remains completely stable in space allowing the fine-tuned application of gravitational forces by the rotary platform, particularly during the subsequent downwards movement.…”

“…The greatest height of 261.2 km is reached after 261.9 s. During the whole phase of microgravity (<10 −5 g, between 68.2 to 449.5 s after liftoff) the payload remains completely stable in space allowing the fine-tuned application of gravitational forces by the rotary platform, particularly during the subsequent downwards movement. Gravitational ramps of 5, 25, 50, 75 and 100 mg were applied, giving rise to the accompanying GIACs as measured with the MDWS [1].…”

“…than common double and single beam spectrophotometers, depending on the measuring conditions [4]. More details are given in the first paper [1]. [4]).…”

“…The Micro Dual Wavelength Spectrometer (MDWS) only allows measurements at individual wavelengths of the spectrum defined by selected light emitting diodes (LEDs) [1]. It does not allow to measure complete spectra with a comparable high sensitivity.…”

“…However, due to its fundamentally much lower sensitivity of the SBS compared to the MDWS we were not able to detect any GIACs on a spectral basis during the microgravity phase. During our first evaluation of the TEXUS 50 data [1] we only focussed on the originally envisaged data obtained during microgravity, ignoring data obtained during the start phase, i.e. hypergravity.…”