Lipson E D, Block S M
J Gen Physiol. 1983 Jun;81(6):845-59. doi: 10.1085/jgp.81.6.845.
Sporangiophores of the fungus Phycomyces exhibit adaptation to light stimuli over a dynamic range of 10(10). This range applies to both phototropism and the closely related light-growth response; in the latter response, the elongation rate is modulated transiently by changes in the light intensity. We have performed light- and dark-adaptation experiments on growing sporangiophores using an automated tracking machine that allows a continuous measurement of growth velocity under controlled conditions. The results are examined in terms of the adaptation model of Delbrück and Reichardt (1956, Cellular Mechanisms in Differentiation and Growth, 3-44). The "level of adaptation," A, was inferred from responses to test pulses of light by means of a series of intensity-response curves. For dark adaptation to steps down in the normal intensity range (10(-6)-10(-2) W/m2), A decays exponentially with a time constant b = 6.1 +/- 0.3 min. This result is in agreement with the model. Higher-order kinetics are indicated, however, for dark adaptation in the high-intensity range (10(-2)-1 W/m2). Adaptation in this range is compared with predictions of a model relating changes in A to the inactivation and recovery of a receptor pigment. In response to steps up in intensity in the normal range, A was found to increase rapidly, overshoot the applied intensity level, and then relax to that level within 40 min. These results are incompatible with the Delbrück-Reichardt model or any simple generalizations of it. The asymmetry and overshoot are similar to adaptation phenomena observed in systems as diverse as bacterial chemotaxis and human vision. It appears likely that light and dark adaptation in Phycomyces are mediated by altogether different processes.
毛霉目真菌根霉的孢子囊柄在10的10次方的动态范围内表现出对光刺激的适应性。这个范围适用于向光性和与之密切相关的光生长反应；在后者的反应中，伸长率会因光强度的变化而瞬时调节。我们使用自动跟踪机器对生长中的孢子囊柄进行了光适应和暗适应实验，该机器能够在受控条件下连续测量生长速度。根据德尔布吕克和赖夏德（1956年，《分化与生长中的细胞机制》，第3 - 44页）的适应模型对结果进行了检验。“适应水平”A是通过一系列强度 - 反应曲线从对测试光脉冲的反应中推断出来的。对于在正常强度范围（10的 - 6次方 - 10的 - 2次方瓦/平方米）内强度下降的暗适应，A以时间常数b = 6.1 ± 0.3分钟呈指数衰减。这个结果与模型相符。然而，对于高强度范围（10的 - 2次方 - 1瓦/平方米）内的暗适应，表明存在高阶动力学。将该范围内的适应与一个将A的变化与受体色素的失活和恢复相关联的模型的预测进行了比较。在正常范围内强度增加时，发现A迅速增加，超过施加的强度水平，然后在40分钟内松弛到该水平。这些结果与德尔布吕克 - 赖夏德模型或其任何简单推广都不相符。这种不对称性和过冲类似于在诸如细菌趋化性和人类视觉等不同系统中观察到的适应现象。根霉中的光适应和暗适应似乎很可能是由完全不同的过程介导的。
