Abstract: The persistence of the "Green Window"—the relative reflection and transmission of green light by terrestrial plants—remains a stubborn paradox in evolutionary biology. Despite the solar spectrum peaking in intensity around 500–550 nm, terrestrial flora never evolved supplementary primary pigments to completely close this epidermal absorption gap. Traditional explanations focusing on thermal regulation or canopy penetration fail to account for the immense evolutionary opportunity cost of rejecting a critical portion of the sun's most abundant photons. This hypothesis posits a novel biophysical constraint for this phenomenon based on recent discoveries of the photomolecular effect. Landmark studies (Chen et al., 2023) demonstrate that photons in the green spectrum (peaking at ~520 nm) induce a resonant vibrational coupling with interfacial water clusters, driving massive non-thermal evaporation. We propose that when aquatic plants transitioned to land, their ancestral marine "Green Window" became a critical exaptation. In a desiccating terrestrial environment, any mutation that closed the green gap to fully absorb peak 520 nm solar radiation at the surface would subject the plant's internal spongy mesophyll to catastrophic, non-thermal photomolecular water loss. Thus, the partial epidermal reflection of green light acts as a calibrated photonic "notch filter," preserving hydraulic stability. This hypothesis reframes the terrestrial persistence of green pigmentation as a strict biophysical adaptation against quantum desiccation. Version 2.0 Update Notes: Evolutionary Timeline Refinement: Reframed the persistence of the "Green Window" on dry land as a biophysical exaptation rather than a primary adaptation, clarifying the evolutionary timeline from marine to terrestrial environments. Leaf Optics Update: Acknowledged the "detour effect" (Terashima et al., 2009) wherein leaves absorb scattered green light internally. Reframed the epidermal green reflection as a calibrated "notch filter" rather than an absolute shield. Experimental Protocol Pivot: Overhauled the proposed experimental framework to emphasize metrological precision. The protocol now proposes utilizing flacca tomato mutants (to eliminate biological stomatal noise) or anthocyanin-rich Coleus to rigorously isolate the physical photomolecular effect from biological guard-cell responses.
Steve Kemp (Thu,) studied this question.