Passage:
In the 1930s and 1940s, plant physiologists sought to determine how flowering plants detect changes in seasonal daylength—a regulatory phenomenon known as photoperiodism. Early investigators observed that interrupting the nocturnal period with a brief pulse of light prevented short-day plants from flowering, demonstrating that the duration of uninterrupted darkness, rather than daylight exposure, served as the primary regulatory trigger. However, the biochemical sensor governing this response remained elusive until researchers at the United States Department of Agriculture’s Beltsville Agricultural Research Center initiated a series of systematic action-spectrum experiments.
Led by botanist Harry Borthwick and physical chemist Sterling Hendricks, the Beltsville team irradiated dark-grown seedlings of Xanthium strumarium (cocklebur) with precise wavelengths of light across the visible and near-infrared spectrum. In 1952, Borthwick and Hendricks documented that red light with a wavelength of approximately 660 nanometers was the most effective in suppressing flowering in short-day plants and promoting seed germination in lettuce (Lactuca sativa). Crucially, they discovered that if a flash of far-red light (peaking at approximately 730 nanometers) was administered immediately following the red light exposure, the inhibitory effect of the red light was completely canceled, restoring the plant's capacity to flower. Repeated alternation between red and far-red light pulses demonstrated that whichever wavelength was applied last determined the physiological outcome.
Based on this photoreversible reaction, Hendricks hypothesized that the underlying pigment existed in two interconvertible configurations: a red-absorbing inactive form (Pr) and a far-red-absorbing active form (Pfr). According to their model, exposure to 660-nanometer light rapidly converts Pr into Pfr, which initiates downstream developmental signaling, whereas 730-nanometer light converts Pfr back into the stable Pr form. In prolonged darkness, Pfr was postulated to undergo slow, non-photochemical reversion back to Pr or experience enzymatic degradation. This hypothesized chromoprotein, designated "phytochrome" in 1959, was subsequently isolated and verified by Warren Butler using dual-wavelength spectrophotometry, confirming that a single interconvertible molecule functioned as a reversible light switch regulating plant photomorphogenesis.
According to the passage, which of the following is true of the physiological response of plants subjected to alternating exposures of red and far-red light?
- AThe cumulative exposure to red light permanently suppressed flowering regardless of any subsequent far-red light pulses.
- The specific wavelength administered at the end of the sequence determined the plant's ultimate physiological outcome.Answer
- CThe alternating pulses caused rapid, irreversible enzymatic degradation of the underlying pigment molecule.
- DThe inhibitory effect of red light could be reversed only if far-red light was applied concurrently rather than sequentially.
- EThe sequential light pulses altered the overall duration of uninterrupted darkness required by the plants to initiate flowering.