Plant photosynthesis and photomorphogenesis are influenced by light wavelength, intensity, and photoperiod through photoreceptors sensitive to light and able to control plant growth. Electromagnetic radiation with wavelengths between 380 and 700 nm represents visible light and it can largely influence plant morphological traits, photosynthetic pigments, and osmolytes production.
Photosynthetic pigments such as chlorophylls among others, are capable of absorbing electromagnetic radiation to make it available to the photosynthetic system. Osmolytes are low molecular weight molecules capable of maintaining the integrity of cells by affecting the properties of biological fluids combating osmotic stress caused by salinity and high metal level stresses.
Optimizing light quality and composition is crucial to produce plants with desired phenotypes and better-yielding crops. Blue light (400-500 nm) has been shown to produce smaller leaves and shorter stems, green light (510-585 nm) can lead to a greater retention of lower leaves, and red light (620-700 nm) can prevent flowering. Nevertheless, combining red and far-red light (700-780 nm) can promote flowering and elongate leaves and stems in shade-avoiding plants in a process called shade avoidance syndrome. [1] This occurs because one plant senses the shade created by a neighboring plant, and tried to outcompete it.
A recent study reported the performance of hemp seedlings grown in aeroponic systems under ten different light-emitting diode (LED) light compositions in addition to sunlight. [2] Light treatments were performed in steel-made chambers on 25-day old plants for a total duration of 20 days. The number of photons between 400 and 700 nm reaching the surface of the plant (photosynthetic photon flux density, PPFD) was 300 μmol m−2s−1, the photoperiod was 16 hours, and the temperature was between 23 to 27 °C.
The composition of the LED light varied in color light ratio going from natural light to white light and other wavelength combinations in which red, blue, green, far-red, and ultraviolet light ratios were increased or decreased. Shoot and root lengths, the number of nodes and leaves, leaf length and width, in addition to plant fresh weight were measured on selected samples of the different aeroponic chambers.
In addition to the morphological traits, the content of photosynthetic pigments and osmolytes including proline, ascorbic acid, total soluble carbohydrates, and sucrose were measured. The study reported that light spectral quality largely influenced growth and morphology of plant shoots and roots. A negative correlation between the number of leaves and nodes with leaf length, node length, and leaf width was observed, in addition to a negative correlation between photosynthetic traits with osmolytes and root length.
From this LED light comparative study, several treatments seemed more suitable for hemp cultivation because of the plant’s bushy morphology, the potential for higher numbers of inflorescences, with a corresponding augmented production of active compounds. These higher performing treatments involved various proportions of red (R), green (G), blue (B), far-red (FR), ultraviolet (UV) and white light (W) content: 70:20:10 of R:G:B, 60:20:10:10 of R:B:G:FR, 50:20:10:10:10 of R:B:G:FR:UV and 20:20:20:20:10:10 of R:B:G:W:FR:UV ratios, respectively.
References:
[1] Wang Y, Folta KM. Contributions of green light to plant growth and development. Am J Bot. 2013;100(1):70-78. [Times cited = 160] [Journal impact factor = 3.844][2] Islam MJ, Ryu BR, Azad MOK, Rahman MH, Rana MS, Kang C-W, Lim J-D, Lim Y-S. Comparative growth, photosynthetic pigments, and osmolytes analysis of hemp (Cannabis sativa L.) seedlings under an aeroponics system with different LED light sources. Horticulturae. 2021; 7(8):239. [Times cited = 1] [Journal impact factor = 2.331]
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