Light and dark adaptation in Phycomyces light-growth response.

Area of Science:

• * Mycology
• * Photobiology
• * Cellular Physiology

Background:

• * Sporangiophores of the fungus *Phycomyces* exhibit remarkable adaptation to light stimuli across a 10^10 dynamic range.
• * This adaptation is crucial for both phototropism and the light-growth response, where elongation rate is modulated by light intensity.
• * Understanding these adaptation mechanisms is key to comprehending sensory responses in biological systems.

Purpose of the Study:

• * To investigate the light and dark adaptation processes in *Phycomyces* sporangiophores.
• * To analyze adaptation kinetics using an automated tracking system for continuous growth velocity measurement.
• * To evaluate the experimental results against the established Delbrück and Reichardt adaptation model.

Main Methods:

• * Utilized an automated tracking machine for continuous measurement of *Phycomyces* sporangiophore growth velocity.
• * Conducted light- and dark-adaptation experiments under controlled light conditions.
• * Inferred the level of adaptation (A) through responses to test light pulses and intensity-response curves.

Main Results:

• * Dark adaptation to decreasing light intensity in the normal range (10^-6–10^-2 W/m^2) showed exponential decay (time constant b = 6.1 ± 0.3 min), aligning with the model.
• * Higher-order kinetics were observed for dark adaptation in the high-intensity range (10^-2–1 W/m^2).
• * Light adaptation to increasing intensity revealed rapid increases in adaptation level (A), overshoot, and subsequent relaxation within 40 min, inconsistent with the Delbrück-Reichardt model.

Conclusions:

• * The Delbrück-Reichardt model accurately describes dark adaptation in the normal intensity range but requires modification for high intensities.
• * Light adaptation exhibits asymmetric behavior and overshoot, suggesting distinct underlying mechanisms from dark adaptation.
• * These findings indicate that light and dark adaptation in *Phycomyces* are likely mediated by separate biological processes.