Abstract
Seeds beneath the soil sense the changing environment to time germination and seedling emergence with the optimum time of year for survival. Environmental signals first impact with the seed at the seed coat. To investigate whether seed coats have a role in environmental sensing we investigated their ultraweak photon emission (UPE) under the variable temperature, relative humidity and oxygen conditions they could experience in the soil seed bank. Using a custom‐built luminometer we measured UPE intensity and spectra (300–700 nm) from Phaseolus vulgaris seeds, seed coats and cotyledons. UPE was greatest from the internal surface of the seed coat. Seed coat UPE increased concomitantly with both increasing temperature and decreasing relative humidity. Emission was oxygen dependent and it was abolished by treatment with dinitrophenylhydrazine, demonstrating the key role of seed coat carbonyls in the phenomenon. We hypothesize that beneath the soil surface the attenuation of light (virtual darkness: low background noise) enables seeds to exploit UPE for transducing key environmental variables in the soil (temperature, humidity and oxygen) to inform them of seasonal and local temperature patterns. Overall, seed coats were found to have potential as effective transducers of key fluctuating environmental variables in the soil.
Highlights
Ultraweak photon emissions (UPEs) discovered by Gurwitsch in the 1920s are photons emitted during de-excitation of highenergy excited species back to the ground state as a direct result of a chemical reaction [1,2,3]
That reactive oxygen species (ROS) are a source of UPE is seen by a reduction in UPE when excited triplet carbonyls are quenched by sorbate anions [7] and when singlet oxygen is quenched by sodium azide [8]
At 25°C, UPE from seed coats increased as relative humidity (RH) decreased from 75% to 10% (Fig. 1) a range seeds would encounter in the soil
Summary
Ultraweak photon emissions (UPEs) ( termed low-level chemiluminescence or biophotons) discovered by Gurwitsch in the 1920s are photons emitted during de-excitation of highenergy excited species back to the ground state as a direct result of a chemical reaction [1,2,3]. Spontaneous UPEs are continuously emitted by almost all metabolically active microbial, plant and animal cells without any external stimuli They are generally considered to originate from the electronically excited species formed during oxidative metabolic reactions. Induced UPE is initiated by various biotic (e.g. infection) and abiotic (e.g. temperature, oxygen and humidity) stresses that lead to production of reactive oxygen species (ROS) and oxidative damage, resulting in ultraweak photon intensities higher than for spontaneous emissions [1]. Singlet oxygen is produced by interaction between hydroxyl ( ̇OH) and superoxide (O2 ̇À) radicals and between excited pigments and molecular oxygen [4] These ROS, triplet excited carbonyls and singlet oxygen, are produced in the oxidation of complex polysaccharides such as cellulose and during lipid peroxidation [5,6]. That ROS are a source of UPE is seen by a reduction in UPE when excited triplet carbonyls are quenched by sorbate anions [7] and when singlet oxygen is quenched by sodium azide [8]
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